summaryrefslogtreecommitdiff
path: root/doc/bison.texi
blob: 32d4192485c4b5bc4564fca9c084f0e1e5bdaf46 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
3360
3361
3362
3363
3364
3365
3366
3367
3368
3369
3370
3371
3372
3373
3374
3375
3376
3377
3378
3379
3380
3381
3382
3383
3384
3385
3386
3387
3388
3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401
3402
3403
3404
3405
3406
3407
3408
3409
3410
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
3433
3434
3435
3436
3437
3438
3439
3440
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
3452
3453
3454
3455
3456
3457
3458
3459
3460
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3478
3479
3480
3481
3482
3483
3484
3485
3486
3487
3488
3489
3490
3491
3492
3493
3494
3495
3496
3497
3498
3499
3500
3501
3502
3503
3504
3505
3506
3507
3508
3509
3510
3511
3512
3513
3514
3515
3516
3517
3518
3519
3520
3521
3522
3523
3524
3525
3526
3527
3528
3529
3530
3531
3532
3533
3534
3535
3536
3537
3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549
3550
3551
3552
3553
3554
3555
3556
3557
3558
3559
3560
3561
3562
3563
3564
3565
3566
3567
3568
3569
3570
3571
3572
3573
3574
3575
3576
3577
3578
3579
3580
3581
3582
3583
3584
3585
3586
3587
3588
3589
3590
3591
3592
3593
3594
3595
3596
3597
3598
3599
3600
3601
3602
3603
3604
3605
3606
3607
3608
3609
3610
3611
3612
3613
3614
3615
3616
3617
3618
3619
3620
3621
3622
3623
3624
3625
3626
3627
3628
3629
3630
3631
3632
3633
3634
3635
3636
3637
3638
3639
3640
3641
3642
3643
3644
3645
3646
3647
3648
3649
3650
3651
3652
3653
3654
3655
3656
3657
3658
3659
3660
3661
3662
3663
3664
3665
3666
3667
3668
3669
3670
3671
3672
3673
3674
3675
3676
3677
3678
3679
3680
3681
3682
3683
3684
3685
3686
3687
3688
3689
3690
3691
3692
3693
3694
3695
3696
3697
3698
3699
3700
3701
3702
3703
3704
3705
3706
3707
3708
3709
3710
3711
3712
3713
3714
3715
3716
3717
3718
3719
3720
3721
3722
3723
3724
3725
3726
3727
3728
3729
3730
3731
3732
3733
3734
3735
3736
3737
3738
3739
3740
3741
3742
3743
3744
3745
3746
3747
3748
3749
3750
3751
3752
3753
3754
3755
3756
3757
3758
3759
3760
3761
3762
3763
3764
3765
3766
3767
3768
3769
3770
3771
3772
3773
3774
3775
3776
3777
3778
3779
3780
3781
3782
3783
3784
3785
3786
3787
3788
3789
3790
3791
3792
3793
3794
3795
3796
3797
3798
3799
3800
3801
3802
3803
3804
3805
3806
3807
3808
3809
3810
3811
3812
3813
3814
3815
3816
3817
3818
3819
3820
3821
3822
3823
3824
3825
3826
3827
3828
3829
3830
3831
3832
3833
3834
3835
3836
3837
3838
3839
3840
3841
3842
3843
3844
3845
3846
3847
3848
3849
3850
3851
3852
3853
3854
3855
3856
3857
3858
3859
3860
3861
3862
3863
3864
3865
3866
3867
3868
3869
3870
3871
3872
3873
3874
3875
3876
3877
3878
3879
3880
3881
3882
3883
3884
3885
3886
3887
3888
3889
3890
3891
3892
3893
3894
3895
3896
3897
3898
3899
3900
3901
3902
3903
3904
3905
3906
3907
3908
3909
3910
3911
3912
3913
3914
3915
3916
3917
3918
3919
3920
3921
3922
3923
3924
3925
3926
3927
3928
3929
3930
3931
3932
3933
3934
3935
3936
3937
3938
3939
3940
3941
3942
3943
3944
3945
3946
3947
3948
3949
3950
3951
3952
3953
3954
3955
3956
3957
3958
3959
3960
3961
3962
3963
3964
3965
3966
3967
3968
3969
3970
3971
3972
3973
3974
3975
3976
3977
3978
3979
3980
3981
3982
3983
3984
3985
3986
3987
3988
3989
3990
3991
3992
3993
3994
3995
3996
3997
3998
3999
4000
4001
4002
4003
4004
4005
4006
4007
4008
4009
4010
4011
4012
4013
4014
4015
4016
4017
4018
4019
4020
4021
4022
4023
4024
4025
4026
4027
4028
4029
4030
4031
4032
4033
4034
4035
4036
4037
4038
4039
4040
4041
4042
4043
4044
4045
4046
4047
4048
4049
4050
4051
4052
4053
4054
4055
4056
4057
4058
4059
4060
4061
4062
4063
4064
4065
4066
4067
4068
4069
4070
4071
4072
4073
4074
4075
4076
4077
4078
4079
4080
4081
4082
4083
4084
4085
4086
4087
4088
4089
4090
4091
4092
4093
4094
4095
4096
4097
4098
4099
4100
4101
4102
4103
4104
4105
4106
4107
4108
4109
4110
4111
4112
4113
4114
4115
4116
4117
4118
4119
4120
4121
4122
4123
4124
4125
4126
4127
4128
4129
4130
4131
4132
4133
4134
4135
4136
4137
4138
4139
4140
4141
4142
4143
4144
4145
4146
4147
4148
4149
4150
4151
4152
4153
4154
4155
4156
4157
4158
4159
4160
4161
4162
4163
4164
4165
4166
4167
4168
4169
4170
4171
4172
4173
4174
4175
4176
4177
4178
4179
4180
4181
4182
4183
4184
4185
4186
4187
4188
4189
4190
4191
4192
4193
4194
4195
4196
4197
4198
4199
4200
4201
4202
4203
4204
4205
4206
4207
4208
4209
4210
4211
4212
4213
4214
4215
4216
4217
4218
4219
4220
4221
4222
4223
4224
4225
4226
4227
4228
4229
4230
4231
4232
4233
4234
4235
4236
4237
4238
4239
4240
4241
4242
4243
4244
4245
4246
4247
4248
4249
4250
4251
4252
4253
4254
4255
4256
4257
4258
4259
4260
4261
4262
4263
4264
4265
4266
4267
4268
4269
4270
4271
4272
4273
4274
4275
4276
4277
4278
4279
4280
4281
4282
4283
4284
4285
4286
4287
4288
4289
4290
4291
4292
4293
4294
4295
4296
4297
4298
4299
4300
4301
4302
4303
4304
4305
4306
4307
4308
4309
4310
4311
4312
4313
4314
4315
4316
4317
4318
4319
4320
4321
4322
4323
4324
4325
4326
4327
4328
4329
4330
4331
4332
4333
4334
4335
4336
4337
4338
4339
4340
4341
4342
4343
4344
4345
4346
4347
4348
4349
4350
4351
4352
4353
4354
4355
4356
4357
4358
4359
4360
4361
4362
4363
4364
4365
4366
4367
4368
4369
4370
4371
4372
4373
4374
4375
4376
4377
4378
4379
4380
4381
4382
4383
4384
4385
4386
4387
4388
4389
4390
4391
4392
4393
4394
4395
4396
4397
4398
4399
4400
4401
4402
4403
4404
4405
4406
4407
4408
4409
4410
4411
4412
4413
4414
4415
4416
4417
4418
4419
4420
4421
4422
4423
4424
4425
4426
4427
4428
4429
4430
4431
4432
4433
4434
4435
4436
4437
4438
4439
4440
4441
4442
4443
4444
4445
4446
4447
4448
4449
4450
4451
4452
4453
4454
4455
4456
4457
4458
4459
4460
4461
4462
4463
4464
4465
4466
4467
4468
4469
4470
4471
4472
4473
4474
4475
4476
4477
4478
4479
4480
4481
4482
4483
4484
4485
4486
4487
4488
4489
4490
4491
4492
4493
4494
4495
4496
4497
4498
4499
4500
4501
4502
4503
4504
4505
4506
4507
4508
4509
4510
4511
4512
4513
4514
4515
4516
4517
4518
4519
4520
4521
4522
4523
4524
4525
4526
4527
4528
4529
4530
4531
4532
4533
4534
4535
4536
4537
4538
4539
4540
4541
4542
4543
4544
4545
4546
4547
4548
4549
4550
4551
4552
4553
4554
4555
4556
4557
4558
4559
4560
4561
4562
4563
4564
4565
4566
4567
4568
4569
4570
4571
4572
4573
4574
4575
4576
4577
4578
4579
4580
4581
4582
4583
4584
4585
4586
4587
4588
4589
4590
4591
4592
4593
4594
4595
4596
4597
4598
4599
4600
4601
4602
4603
4604
4605
4606
4607
4608
4609
4610
4611
4612
4613
4614
4615
4616
4617
4618
4619
4620
4621
4622
4623
4624
4625
4626
4627
4628
4629
4630
4631
4632
4633
4634
4635
4636
4637
4638
4639
4640
4641
4642
4643
4644
4645
4646
4647
4648
4649
4650
4651
4652
4653
4654
4655
4656
4657
4658
4659
4660
4661
4662
4663
4664
4665
4666
4667
4668
4669
4670
4671
4672
4673
4674
4675
4676
4677
4678
4679
4680
4681
4682
4683
4684
4685
4686
4687
4688
4689
4690
4691
4692
4693
4694
4695
4696
4697
4698
4699
4700
4701
4702
4703
4704
4705
4706
4707
4708
4709
4710
4711
4712
4713
4714
4715
4716
4717
4718
4719
4720
4721
4722
4723
4724
4725
4726
4727
4728
4729
4730
4731
4732
4733
4734
4735
4736
4737
4738
4739
4740
4741
4742
4743
4744
4745
4746
4747
4748
4749
4750
4751
4752
4753
4754
4755
4756
4757
4758
4759
4760
4761
4762
4763
4764
4765
4766
4767
4768
4769
4770
4771
4772
4773
4774
4775
4776
4777
4778
4779
4780
4781
4782
4783
4784
4785
4786
4787
4788
4789
4790
4791
4792
4793
4794
4795
4796
4797
4798
4799
4800
4801
4802
4803
4804
4805
4806
4807
4808
4809
4810
4811
4812
4813
4814
4815
4816
4817
4818
4819
4820
4821
4822
4823
4824
4825
4826
4827
4828
4829
4830
4831
4832
4833
4834
4835
4836
4837
4838
4839
4840
4841
4842
4843
4844
4845
4846
4847
4848
4849
4850
4851
4852
4853
4854
4855
4856
4857
4858
4859
4860
4861
4862
4863
4864
4865
4866
4867
4868
4869
4870
4871
4872
4873
4874
4875
4876
4877
4878
4879
4880
4881
4882
4883
4884
4885
4886
4887
4888
4889
4890
4891
4892
4893
4894
4895
4896
4897
4898
4899
4900
4901
4902
4903
4904
4905
4906
4907
4908
4909
4910
4911
4912
4913
4914
4915
4916
4917
4918
4919
4920
4921
4922
4923
4924
4925
4926
4927
4928
4929
4930
4931
4932
4933
4934
4935
4936
4937
4938
4939
4940
4941
4942
4943
4944
4945
4946
4947
4948
4949
4950
4951
4952
4953
4954
4955
4956
4957
4958
4959
4960
4961
4962
4963
4964
4965
4966
4967
4968
4969
4970
4971
4972
4973
4974
4975
4976
4977
4978
4979
4980
4981
4982
4983
4984
4985
4986
4987
4988
4989
4990
4991
4992
4993
4994
4995
4996
4997
4998
4999
5000
5001
5002
5003
5004
5005
5006
5007
5008
5009
5010
5011
5012
5013
5014
5015
5016
5017
5018
5019
5020
5021
5022
5023
5024
5025
5026
5027
5028
5029
5030
5031
5032
5033
5034
5035
5036
5037
5038
5039
5040
5041
5042
5043
5044
5045
5046
5047
5048
5049
5050
5051
5052
5053
5054
5055
5056
5057
5058
5059
5060
5061
5062
5063
5064
5065
5066
5067
5068
5069
5070
5071
5072
5073
5074
5075
5076
5077
5078
5079
5080
5081
5082
5083
5084
5085
5086
5087
5088
5089
5090
5091
5092
5093
5094
5095
5096
5097
5098
5099
5100
5101
5102
5103
5104
5105
5106
5107
5108
5109
5110
5111
5112
5113
5114
5115
5116
5117
5118
5119
5120
5121
5122
5123
5124
5125
5126
5127
5128
5129
5130
5131
5132
5133
5134
5135
5136
5137
5138
5139
5140
5141
5142
5143
5144
5145
5146
5147
5148
5149
5150
5151
5152
5153
5154
5155
5156
5157
5158
5159
5160
5161
5162
5163
5164
5165
5166
5167
5168
5169
5170
5171
5172
5173
5174
5175
5176
5177
5178
5179
5180
5181
5182
5183
5184
5185
5186
5187
5188
5189
5190
5191
5192
5193
5194
5195
5196
5197
5198
5199
5200
5201
5202
5203
5204
5205
5206
5207
5208
5209
5210
5211
5212
5213
5214
5215
5216
5217
5218
5219
5220
5221
5222
5223
5224
5225
5226
5227
5228
5229
5230
5231
5232
5233
5234
5235
5236
5237
5238
5239
5240
5241
5242
5243
5244
5245
5246
5247
5248
5249
5250
5251
5252
5253
5254
5255
5256
5257
5258
5259
5260
5261
5262
5263
5264
5265
5266
5267
5268
5269
5270
5271
5272
5273
5274
5275
5276
5277
5278
5279
5280
5281
5282
5283
5284
5285
5286
5287
5288
5289
5290
5291
5292
5293
5294
5295
5296
5297
5298
5299
5300
5301
5302
5303
5304
5305
5306
5307
5308
5309
5310
5311
5312
5313
5314
5315
5316
5317
5318
5319
5320
5321
5322
5323
5324
5325
5326
5327
5328
5329
5330
5331
5332
5333
5334
5335
5336
5337
5338
5339
5340
5341
5342
5343
5344
5345
5346
5347
5348
5349
5350
5351
5352
5353
5354
5355
5356
5357
5358
5359
5360
5361
5362
5363
5364
5365
5366
5367
5368
5369
5370
5371
5372
5373
5374
5375
5376
5377
5378
5379
5380
5381
5382
5383
5384
5385
5386
5387
5388
5389
5390
5391
5392
5393
5394
5395
5396
5397
5398
5399
5400
5401
5402
5403
5404
5405
5406
5407
5408
5409
5410
5411
5412
5413
5414
5415
5416
5417
5418
5419
5420
5421
5422
5423
5424
5425
5426
5427
5428
5429
5430
5431
5432
5433
5434
5435
5436
5437
5438
5439
5440
5441
5442
5443
5444
5445
5446
5447
5448
5449
5450
5451
5452
5453
5454
5455
5456
5457
5458
5459
5460
5461
5462
5463
5464
5465
5466
5467
5468
5469
5470
5471
5472
5473
5474
5475
5476
5477
5478
5479
5480
5481
5482
5483
5484
5485
5486
5487
5488
5489
5490
5491
5492
5493
5494
5495
5496
5497
5498
5499
5500
5501
5502
5503
5504
5505
5506
5507
5508
5509
5510
5511
5512
5513
5514
5515
5516
5517
5518
5519
5520
5521
5522
5523
5524
5525
5526
5527
5528
5529
5530
5531
5532
5533
5534
5535
5536
5537
5538
5539
5540
5541
5542
5543
5544
5545
5546
5547
5548
5549
5550
5551
5552
5553
5554
5555
5556
5557
5558
5559
5560
5561
5562
5563
5564
5565
5566
5567
5568
5569
5570
5571
5572
5573
5574
5575
5576
5577
5578
5579
5580
5581
5582
5583
5584
5585
5586
5587
5588
5589
5590
5591
5592
5593
5594
5595
5596
5597
5598
5599
5600
5601
5602
5603
5604
5605
5606
5607
5608
5609
5610
5611
5612
5613
5614
5615
5616
5617
5618
5619
5620
5621
5622
5623
5624
5625
5626
5627
5628
5629
5630
5631
5632
5633
5634
5635
5636
5637
5638
5639
5640
5641
5642
5643
5644
5645
5646
5647
5648
5649
5650
5651
5652
5653
5654
5655
5656
5657
5658
5659
5660
5661
5662
5663
5664
5665
5666
5667
5668
5669
5670
5671
5672
5673
5674
5675
5676
5677
5678
5679
5680
5681
5682
5683
5684
5685
5686
5687
5688
5689
5690
5691
5692
5693
5694
5695
5696
5697
5698
5699
5700
5701
5702
5703
5704
5705
5706
5707
5708
5709
5710
5711
5712
5713
5714
5715
5716
5717
5718
5719
5720
5721
5722
5723
5724
5725
5726
5727
5728
5729
5730
5731
5732
5733
5734
5735
5736
5737
5738
5739
5740
5741
5742
5743
5744
5745
5746
5747
5748
5749
5750
5751
5752
5753
5754
5755
5756
5757
5758
5759
5760
5761
5762
5763
5764
5765
5766
5767
5768
5769
5770
5771
5772
5773
5774
5775
5776
5777
5778
5779
5780
5781
5782
5783
5784
5785
5786
5787
5788
5789
5790
5791
5792
5793
5794
5795
5796
5797
5798
5799
5800
5801
5802
5803
5804
5805
5806
5807
5808
5809
5810
5811
5812
5813
5814
5815
5816
5817
5818
5819
5820
5821
5822
5823
5824
5825
5826
5827
5828
5829
5830
5831
5832
5833
5834
5835
5836
5837
5838
5839
5840
5841
5842
5843
5844
5845
5846
5847
5848
5849
5850
5851
5852
5853
5854
5855
5856
5857
5858
5859
5860
5861
5862
5863
5864
5865
5866
5867
5868
5869
5870
5871
5872
5873
5874
5875
5876
5877
5878
5879
5880
5881
5882
5883
5884
5885
5886
5887
5888
5889
5890
5891
5892
5893
5894
5895
5896
5897
5898
5899
5900
5901
5902
5903
5904
5905
5906
5907
5908
5909
5910
5911
5912
5913
5914
5915
5916
5917
5918
5919
5920
5921
5922
5923
5924
5925
5926
5927
5928
5929
5930
5931
5932
5933
5934
5935
5936
5937
5938
5939
5940
5941
5942
5943
5944
5945
5946
5947
5948
5949
5950
5951
5952
5953
5954
5955
5956
5957
5958
5959
5960
5961
5962
5963
5964
5965
5966
5967
5968
5969
5970
5971
5972
5973
5974
5975
5976
5977
5978
5979
5980
5981
5982
5983
5984
5985
5986
5987
5988
5989
5990
5991
5992
5993
5994
5995
5996
5997
5998
5999
6000
6001
6002
6003
6004
6005
6006
6007
6008
6009
6010
6011
6012
6013
6014
6015
6016
6017
6018
6019
6020
6021
6022
6023
6024
6025
6026
6027
6028
6029
6030
6031
6032
6033
6034
6035
6036
6037
6038
6039
6040
6041
6042
6043
6044
6045
6046
6047
6048
6049
6050
6051
6052
6053
6054
6055
6056
6057
6058
6059
6060
6061
6062
6063
6064
6065
6066
6067
6068
6069
6070
6071
6072
6073
6074
6075
6076
6077
6078
6079
6080
6081
6082
6083
6084
6085
6086
6087
6088
6089
6090
6091
6092
6093
6094
6095
6096
6097
6098
6099
6100
6101
6102
6103
6104
6105
6106
6107
6108
6109
6110
6111
6112
6113
6114
6115
6116
6117
6118
6119
6120
6121
6122
6123
6124
6125
6126
6127
6128
6129
6130
6131
6132
6133
6134
6135
6136
6137
6138
6139
6140
6141
6142
6143
6144
6145
6146
6147
6148
6149
6150
6151
6152
6153
6154
6155
6156
6157
6158
6159
6160
6161
6162
6163
6164
6165
6166
6167
6168
6169
6170
6171
6172
6173
6174
6175
6176
6177
6178
6179
6180
6181
6182
6183
6184
6185
6186
6187
6188
6189
6190
6191
6192
6193
6194
6195
6196
6197
6198
6199
6200
6201
6202
6203
6204
6205
6206
6207
6208
6209
6210
6211
6212
6213
6214
6215
6216
6217
6218
6219
6220
6221
6222
6223
6224
6225
6226
6227
6228
6229
6230
6231
6232
6233
6234
6235
6236
6237
6238
6239
6240
6241
6242
6243
6244
6245
6246
6247
6248
6249
6250
6251
6252
6253
6254
6255
6256
6257
6258
6259
6260
6261
6262
6263
6264
6265
6266
6267
6268
6269
6270
6271
6272
6273
6274
6275
6276
6277
6278
6279
6280
6281
6282
6283
6284
6285
6286
6287
6288
6289
6290
6291
6292
6293
6294
6295
6296
6297
6298
6299
6300
6301
6302
6303
6304
6305
6306
6307
6308
6309
6310
6311
6312
6313
6314
6315
6316
6317
6318
6319
6320
6321
6322
6323
6324
6325
6326
6327
6328
6329
6330
6331
6332
6333
6334
6335
6336
6337
6338
6339
6340
6341
6342
6343
6344
6345
6346
6347
6348
6349
6350
6351
6352
6353
6354
6355
6356
6357
6358
6359
6360
6361
6362
6363
6364
6365
6366
6367
6368
6369
6370
6371
6372
6373
6374
6375
6376
6377
6378
6379
6380
6381
6382
6383
6384
6385
6386
6387
6388
6389
6390
6391
6392
6393
6394
6395
6396
6397
6398
6399
6400
6401
6402
6403
6404
6405
6406
6407
6408
6409
6410
6411
6412
6413
6414
6415
6416
6417
6418
6419
6420
6421
6422
6423
6424
6425
6426
6427
6428
6429
6430
6431
6432
6433
6434
6435
6436
6437
6438
6439
6440
6441
6442
6443
6444
6445
6446
6447
6448
6449
6450
6451
6452
6453
6454
6455
6456
6457
6458
6459
6460
6461
6462
6463
6464
6465
6466
6467
6468
6469
6470
6471
6472
6473
6474
6475
6476
6477
6478
6479
6480
6481
6482
6483
6484
6485
6486
6487
6488
6489
6490
6491
6492
6493
6494
6495
6496
6497
6498
6499
6500
6501
6502
6503
6504
6505
6506
6507
6508
6509
6510
6511
6512
6513
6514
6515
6516
6517
6518
6519
6520
6521
6522
6523
6524
6525
6526
6527
6528
6529
6530
6531
6532
6533
6534
6535
6536
6537
6538
6539
6540
6541
6542
6543
6544
6545
6546
6547
6548
6549
6550
6551
6552
6553
6554
6555
6556
6557
6558
6559
6560
6561
6562
6563
6564
6565
6566
6567
6568
6569
6570
6571
6572
6573
6574
6575
6576
6577
6578
6579
6580
6581
6582
6583
6584
6585
6586
6587
6588
6589
6590
6591
6592
6593
6594
6595
6596
6597
6598
6599
6600
6601
6602
6603
6604
6605
6606
6607
6608
6609
6610
6611
6612
6613
6614
6615
6616
6617
6618
6619
6620
6621
6622
6623
6624
6625
6626
6627
6628
6629
6630
6631
6632
6633
6634
6635
6636
6637
6638
6639
6640
6641
6642
6643
6644
6645
6646
6647
6648
6649
6650
6651
6652
6653
6654
6655
6656
6657
6658
6659
6660
6661
6662
6663
6664
6665
6666
6667
6668
6669
6670
6671
6672
6673
6674
6675
6676
6677
6678
6679
6680
6681
6682
6683
6684
6685
6686
6687
6688
6689
6690
6691
6692
6693
6694
6695
6696
6697
6698
6699
6700
6701
6702
6703
6704
6705
6706
6707
6708
6709
6710
6711
6712
6713
6714
6715
6716
6717
6718
6719
6720
6721
6722
6723
6724
6725
6726
6727
6728
6729
6730
6731
6732
6733
6734
6735
6736
6737
6738
6739
6740
6741
6742
6743
6744
6745
6746
6747
6748
6749
6750
6751
6752
6753
6754
6755
6756
6757
6758
6759
6760
6761
6762
6763
6764
6765
6766
6767
6768
6769
6770
6771
6772
6773
6774
6775
6776
6777
6778
6779
6780
6781
6782
6783
6784
6785
6786
6787
6788
6789
6790
6791
6792
6793
6794
6795
6796
6797
6798
6799
6800
6801
6802
6803
6804
6805
6806
6807
6808
6809
6810
6811
6812
6813
6814
6815
6816
6817
6818
6819
6820
6821
6822
6823
6824
6825
6826
6827
6828
6829
6830
6831
6832
6833
6834
6835
6836
6837
6838
6839
6840
6841
6842
6843
6844
6845
6846
6847
6848
6849
6850
6851
6852
6853
6854
6855
6856
6857
6858
6859
6860
6861
6862
6863
6864
6865
6866
6867
6868
6869
6870
6871
6872
6873
6874
6875
6876
6877
6878
6879
6880
6881
6882
6883
6884
6885
6886
6887
6888
6889
6890
6891
6892
6893
6894
6895
6896
6897
6898
6899
6900
6901
6902
6903
6904
6905
6906
6907
6908
6909
6910
6911
6912
6913
6914
6915
6916
6917
6918
6919
6920
6921
6922
6923
6924
6925
6926
6927
6928
6929
6930
6931
6932
6933
6934
6935
6936
6937
6938
6939
6940
6941
6942
6943
6944
6945
6946
6947
6948
6949
6950
6951
6952
6953
6954
6955
6956
6957
6958
6959
6960
6961
6962
6963
6964
6965
6966
6967
6968
6969
6970
6971
6972
6973
6974
6975
6976
6977
6978
6979
6980
6981
6982
6983
6984
6985
6986
6987
6988
6989
6990
6991
6992
6993
6994
6995
6996
6997
6998
6999
7000
7001
7002
7003
7004
7005
7006
7007
7008
7009
7010
7011
7012
7013
7014
7015
7016
7017
7018
7019
7020
7021
7022
7023
7024
7025
7026
7027
7028
7029
7030
7031
7032
7033
7034
7035
7036
7037
7038
7039
7040
7041
7042
7043
7044
7045
7046
7047
7048
7049
7050
7051
7052
7053
7054
7055
7056
7057
7058
7059
7060
7061
7062
7063
7064
7065
7066
7067
7068
7069
7070
7071
7072
7073
7074
7075
7076
7077
7078
7079
7080
7081
7082
7083
7084
7085
7086
7087
7088
7089
7090
7091
7092
7093
7094
7095
7096
7097
7098
7099
7100
7101
7102
7103
7104
7105
7106
7107
7108
7109
7110
7111
7112
7113
7114
7115
7116
7117
7118
7119
7120
7121
7122
7123
7124
7125
7126
7127
7128
7129
7130
7131
7132
7133
7134
7135
7136
7137
7138
7139
7140
7141
7142
7143
7144
7145
7146
7147
7148
7149
7150
7151
7152
7153
7154
7155
7156
7157
7158
7159
7160
7161
7162
7163
7164
7165
7166
7167
7168
7169
7170
7171
7172
7173
7174
7175
7176
7177
7178
7179
7180
7181
7182
7183
7184
7185
7186
7187
7188
7189
7190
7191
7192
7193
7194
7195
7196
7197
7198
7199
7200
7201
7202
7203
7204
7205
7206
7207
7208
7209
7210
7211
7212
7213
7214
7215
7216
7217
7218
7219
7220
7221
7222
7223
7224
7225
7226
7227
7228
7229
7230
7231
7232
7233
7234
7235
7236
7237
7238
7239
7240
7241
7242
7243
7244
7245
7246
7247
7248
7249
7250
7251
7252
7253
7254
7255
7256
7257
7258
7259
7260
7261
7262
7263
7264
7265
7266
7267
7268
7269
7270
7271
7272
7273
7274
7275
7276
7277
7278
7279
7280
7281
7282
7283
7284
7285
7286
7287
7288
7289
7290
7291
7292
7293
7294
7295
7296
7297
7298
7299
7300
7301
7302
7303
7304
7305
7306
7307
7308
7309
7310
7311
7312
7313
7314
7315
7316
7317
7318
7319
7320
7321
7322
7323
7324
7325
7326
7327
7328
7329
7330
7331
7332
7333
7334
7335
7336
7337
7338
7339
7340
7341
7342
7343
7344
7345
7346
7347
7348
7349
7350
7351
7352
7353
7354
7355
7356
7357
7358
7359
7360
7361
7362
7363
7364
7365
7366
7367
7368
7369
7370
7371
7372
7373
7374
7375
7376
7377
7378
7379
7380
7381
7382
7383
7384
7385
7386
7387
7388
7389
7390
7391
7392
7393
7394
7395
7396
7397
7398
7399
7400
7401
7402
7403
7404
7405
7406
7407
7408
7409
7410
7411
7412
7413
7414
7415
7416
7417
7418
7419
7420
7421
7422
7423
7424
7425
7426
7427
7428
7429
7430
7431
7432
7433
7434
7435
7436
7437
7438
7439
7440
7441
7442
7443
7444
7445
7446
7447
7448
7449
7450
7451
7452
7453
7454
7455
7456
7457
7458
7459
7460
7461
7462
7463
7464
7465
7466
7467
7468
7469
7470
7471
7472
7473
7474
7475
7476
7477
7478
7479
7480
7481
7482
7483
7484
7485
7486
7487
7488
7489
7490
7491
7492
7493
7494
7495
7496
7497
7498
7499
7500
7501
7502
7503
7504
7505
7506
7507
7508
7509
7510
7511
7512
7513
7514
7515
7516
7517
7518
7519
7520
7521
7522
7523
7524
7525
7526
7527
7528
7529
7530
7531
7532
7533
7534
7535
7536
7537
7538
7539
7540
7541
7542
7543
7544
7545
7546
7547
7548
7549
7550
7551
7552
7553
7554
7555
7556
7557
7558
7559
7560
7561
7562
7563
7564
7565
7566
7567
7568
7569
7570
7571
7572
7573
7574
7575
7576
7577
7578
7579
7580
7581
7582
7583
7584
7585
7586
7587
7588
7589
7590
7591
7592
7593
7594
7595
7596
7597
7598
7599
7600
7601
7602
7603
7604
7605
7606
7607
7608
7609
7610
7611
7612
7613
7614
7615
7616
7617
7618
7619
7620
7621
7622
7623
7624
7625
7626
7627
7628
7629
7630
7631
7632
7633
7634
7635
7636
7637
7638
7639
7640
7641
7642
7643
7644
7645
7646
7647
7648
7649
7650
7651
7652
7653
7654
7655
7656
7657
7658
7659
7660
7661
7662
7663
7664
7665
7666
7667
7668
7669
7670
7671
7672
7673
7674
7675
7676
7677
7678
7679
7680
7681
7682
7683
7684
7685
7686
7687
7688
7689
7690
7691
7692
7693
7694
7695
7696
7697
7698
7699
7700
7701
7702
7703
7704
7705
7706
7707
7708
7709
7710
7711
7712
7713
7714
7715
7716
7717
7718
7719
7720
7721
7722
7723
7724
7725
7726
7727
7728
7729
7730
7731
7732
7733
7734
7735
7736
7737
7738
7739
7740
7741
7742
7743
7744
7745
7746
7747
7748
7749
7750
7751
7752
7753
7754
7755
7756
7757
7758
7759
7760
7761
7762
7763
7764
7765
7766
7767
7768
7769
7770
7771
7772
7773
7774
7775
7776
7777
7778
7779
7780
7781
7782
7783
7784
7785
7786
7787
7788
7789
7790
7791
7792
7793
7794
7795
7796
7797
7798
7799
7800
7801
7802
7803
7804
7805
7806
7807
7808
7809
7810
7811
7812
7813
7814
7815
7816
7817
7818
7819
7820
7821
7822
7823
7824
7825
7826
7827
7828
7829
7830
7831
7832
7833
7834
7835
7836
7837
7838
7839
7840
7841
7842
7843
7844
7845
7846
7847
7848
7849
7850
7851
7852
7853
7854
7855
7856
7857
7858
7859
7860
7861
7862
7863
7864
7865
7866
7867
7868
7869
7870
7871
7872
7873
7874
7875
7876
7877
7878
7879
7880
7881
7882
7883
7884
7885
7886
7887
7888
7889
7890
7891
7892
7893
7894
7895
7896
7897
7898
7899
7900
7901
7902
7903
7904
7905
7906
7907
7908
7909
7910
7911
7912
7913
7914
7915
7916
7917
7918
7919
7920
7921
7922
7923
7924
7925
7926
7927
7928
7929
7930
7931
7932
7933
7934
7935
7936
7937
7938
7939
7940
7941
7942
7943
7944
7945
7946
7947
7948
7949
7950
7951
7952
7953
7954
7955
7956
7957
7958
7959
7960
7961
7962
7963
7964
7965
7966
7967
7968
7969
7970
7971
7972
7973
7974
7975
7976
7977
7978
7979
7980
7981
7982
7983
7984
7985
7986
7987
7988
7989
7990
7991
7992
7993
7994
7995
7996
7997
7998
7999
8000
8001
8002
8003
8004
8005
8006
8007
8008
8009
8010
8011
8012
8013
8014
8015
8016
8017
8018
8019
8020
8021
8022
8023
8024
8025
8026
8027
8028
8029
8030
8031
8032
8033
8034
8035
8036
8037
8038
8039
8040
8041
8042
8043
8044
8045
8046
8047
8048
8049
8050
8051
8052
8053
8054
8055
8056
8057
8058
8059
8060
8061
8062
8063
8064
8065
8066
8067
8068
8069
8070
8071
8072
8073
8074
8075
8076
8077
8078
8079
8080
8081
8082
8083
8084
8085
8086
8087
8088
8089
8090
8091
8092
8093
8094
8095
8096
8097
8098
8099
8100
8101
8102
8103
8104
8105
8106
8107
8108
8109
8110
8111
8112
8113
8114
8115
8116
8117
8118
8119
8120
8121
8122
8123
8124
8125
8126
8127
8128
8129
8130
8131
8132
8133
8134
8135
8136
8137
8138
8139
8140
8141
8142
8143
8144
8145
8146
8147
8148
8149
8150
8151
8152
8153
8154
8155
8156
8157
8158
8159
8160
8161
8162
8163
8164
8165
8166
8167
8168
8169
8170
8171
8172
8173
8174
8175
8176
8177
8178
8179
8180
8181
8182
8183
8184
8185
8186
8187
8188
8189
8190
8191
8192
8193
8194
8195
8196
8197
8198
8199
8200
8201
8202
8203
8204
8205
8206
8207
8208
8209
8210
8211
8212
8213
8214
8215
8216
8217
8218
8219
8220
8221
8222
8223
8224
8225
8226
8227
8228
8229
8230
8231
8232
8233
8234
8235
8236
8237
8238
8239
8240
8241
8242
8243
8244
8245
8246
8247
8248
8249
8250
8251
8252
8253
8254
8255
8256
8257
8258
8259
8260
8261
8262
8263
8264
8265
8266
8267
8268
8269
8270
8271
8272
8273
8274
8275
8276
8277
8278
8279
8280
8281
8282
8283
8284
8285
8286
8287
8288
8289
8290
8291
8292
8293
8294
8295
8296
8297
8298
8299
8300
8301
8302
8303
8304
8305
8306
8307
8308
8309
8310
8311
8312
8313
8314
8315
8316
8317
8318
8319
8320
8321
8322
8323
8324
8325
8326
8327
8328
8329
8330
8331
8332
8333
8334
8335
8336
8337
8338
8339
8340
8341
8342
8343
8344
8345
8346
8347
8348
8349
8350
8351
8352
8353
8354
8355
8356
8357
8358
8359
8360
8361
8362
8363
8364
8365
8366
8367
8368
8369
8370
8371
8372
8373
8374
8375
8376
8377
8378
8379
8380
8381
8382
8383
8384
8385
8386
8387
8388
8389
8390
8391
8392
8393
8394
8395
8396
8397
8398
8399
8400
8401
8402
8403
8404
8405
8406
8407
8408
8409
8410
8411
8412
8413
8414
8415
8416
8417
8418
8419
8420
8421
8422
8423
8424
8425
8426
8427
8428
8429
8430
8431
8432
8433
8434
8435
8436
8437
8438
8439
8440
8441
8442
8443
8444
8445
8446
8447
8448
8449
8450
8451
8452
8453
8454
8455
8456
8457
8458
8459
8460
8461
8462
8463
8464
8465
8466
8467
8468
8469
8470
8471
8472
8473
8474
8475
8476
8477
8478
8479
8480
8481
8482
8483
8484
8485
8486
8487
8488
8489
8490
8491
8492
8493
8494
8495
8496
8497
8498
8499
8500
8501
8502
8503
8504
8505
8506
8507
8508
8509
8510
8511
8512
8513
8514
8515
8516
8517
8518
8519
8520
8521
8522
8523
8524
8525
8526
8527
8528
8529
8530
8531
8532
8533
8534
8535
8536
8537
8538
8539
8540
8541
8542
8543
8544
8545
8546
8547
8548
8549
8550
8551
8552
8553
8554
8555
8556
8557
8558
8559
8560
8561
8562
8563
8564
8565
8566
8567
8568
8569
8570
8571
8572
8573
8574
8575
8576
8577
8578
8579
8580
8581
8582
8583
8584
8585
8586
8587
8588
8589
8590
8591
8592
8593
8594
8595
8596
8597
8598
8599
8600
8601
8602
8603
8604
8605
8606
8607
8608
8609
8610
8611
8612
8613
8614
8615
8616
8617
8618
8619
8620
8621
8622
8623
8624
8625
8626
8627
8628
8629
8630
8631
8632
8633
8634
8635
8636
8637
8638
8639
8640
8641
8642
8643
8644
8645
8646
8647
8648
8649
8650
8651
8652
8653
8654
8655
8656
8657
8658
8659
8660
8661
8662
8663
8664
8665
8666
8667
8668
8669
8670
8671
8672
8673
8674
8675
8676
8677
8678
8679
8680
8681
8682
8683
8684
8685
8686
8687
8688
8689
8690
8691
8692
8693
8694
8695
8696
8697
8698
8699
8700
8701
8702
8703
8704
8705
8706
8707
8708
8709
8710
8711
8712
8713
8714
8715
8716
8717
8718
8719
8720
8721
8722
8723
8724
8725
8726
8727
8728
8729
8730
8731
8732
8733
8734
8735
8736
8737
8738
8739
8740
8741
8742
8743
8744
8745
8746
8747
8748
8749
8750
8751
8752
8753
8754
8755
8756
8757
8758
8759
8760
8761
8762
8763
8764
8765
8766
8767
8768
8769
8770
8771
8772
8773
8774
8775
8776
8777
8778
8779
8780
8781
8782
8783
8784
8785
8786
8787
8788
8789
8790
8791
8792
8793
8794
8795
8796
8797
8798
8799
8800
8801
8802
8803
8804
8805
8806
8807
8808
8809
8810
8811
8812
8813
8814
8815
8816
8817
8818
8819
8820
8821
8822
8823
8824
8825
8826
8827
8828
8829
8830
8831
8832
8833
8834
8835
8836
8837
8838
8839
8840
8841
8842
8843
8844
8845
8846
8847
8848
8849
8850
8851
8852
8853
8854
8855
8856
8857
8858
8859
8860
8861
8862
8863
8864
8865
8866
8867
8868
8869
8870
8871
8872
8873
8874
8875
8876
8877
8878
8879
8880
8881
8882
8883
8884
8885
8886
8887
8888
8889
8890
8891
8892
8893
8894
8895
8896
8897
8898
8899
8900
8901
8902
8903
8904
8905
8906
8907
8908
8909
8910
8911
8912
8913
8914
8915
8916
8917
8918
8919
8920
8921
8922
8923
8924
8925
8926
8927
8928
8929
8930
8931
8932
8933
8934
8935
8936
8937
8938
8939
8940
8941
8942
8943
8944
8945
8946
8947
8948
8949
8950
8951
8952
8953
8954
8955
8956
8957
8958
8959
8960
8961
8962
8963
8964
8965
8966
8967
8968
8969
8970
8971
8972
8973
8974
8975
8976
8977
8978
8979
8980
8981
8982
8983
8984
8985
8986
8987
8988
8989
8990
8991
8992
8993
8994
8995
8996
8997
8998
8999
9000
9001
9002
9003
9004
9005
9006
9007
9008
9009
9010
9011
9012
9013
9014
9015
9016
9017
9018
9019
9020
9021
9022
9023
9024
9025
9026
9027
9028
9029
9030
9031
9032
9033
9034
9035
9036
9037
9038
9039
9040
9041
9042
9043
9044
9045
9046
9047
9048
9049
9050
9051
9052
9053
9054
9055
9056
9057
9058
9059
9060
9061
9062
9063
9064
9065
9066
9067
9068
9069
9070
9071
9072
9073
9074
9075
9076
9077
9078
9079
9080
9081
9082
9083
9084
9085
9086
9087
9088
9089
9090
9091
9092
9093
9094
9095
9096
9097
9098
9099
9100
9101
9102
9103
9104
9105
9106
9107
9108
9109
9110
9111
9112
9113
9114
9115
9116
9117
9118
9119
9120
9121
9122
9123
9124
9125
9126
9127
9128
9129
9130
9131
9132
9133
9134
9135
9136
9137
9138
9139
9140
9141
9142
9143
9144
9145
9146
9147
9148
9149
9150
9151
9152
9153
9154
9155
9156
9157
9158
9159
9160
9161
9162
9163
9164
9165
9166
9167
9168
9169
9170
9171
9172
9173
9174
9175
9176
9177
9178
9179
9180
9181
9182
9183
9184
9185
9186
9187
9188
9189
9190
9191
9192
9193
9194
9195
9196
9197
9198
9199
9200
9201
9202
9203
9204
9205
9206
9207
9208
9209
9210
9211
9212
9213
9214
9215
9216
9217
9218
9219
9220
9221
9222
9223
9224
9225
9226
9227
9228
9229
9230
9231
9232
9233
9234
9235
9236
9237
9238
9239
9240
9241
9242
9243
9244
9245
9246
9247
9248
9249
9250
9251
9252
9253
9254
9255
9256
9257
9258
9259
9260
9261
9262
9263
9264
9265
9266
9267
9268
9269
9270
9271
9272
9273
9274
9275
9276
9277
9278
9279
9280
9281
9282
9283
9284
9285
9286
9287
9288
9289
9290
9291
9292
9293
9294
9295
9296
9297
9298
9299
9300
9301
9302
9303
9304
9305
9306
9307
9308
9309
9310
9311
9312
9313
9314
9315
9316
9317
9318
9319
9320
9321
9322
9323
9324
9325
9326
9327
9328
9329
9330
9331
9332
9333
9334
9335
9336
9337
9338
9339
9340
9341
9342
9343
9344
9345
9346
9347
9348
9349
9350
9351
9352
9353
9354
9355
9356
9357
9358
9359
9360
9361
9362
9363
9364
9365
9366
9367
9368
9369
9370
9371
9372
9373
9374
9375
9376
9377
9378
9379
9380
9381
9382
9383
9384
9385
9386
9387
9388
9389
9390
9391
9392
9393
9394
9395
9396
9397
9398
9399
9400
9401
9402
9403
9404
9405
9406
9407
9408
9409
9410
9411
9412
9413
9414
9415
9416
9417
9418
9419
9420
9421
9422
9423
9424
9425
9426
9427
9428
9429
9430
9431
9432
9433
9434
9435
9436
9437
9438
9439
9440
9441
9442
9443
9444
9445
9446
9447
9448
9449
9450
9451
9452
9453
9454
9455
9456
9457
9458
9459
9460
9461
9462
9463
9464
9465
9466
9467
9468
9469
9470
9471
9472
9473
9474
9475
9476
9477
9478
9479
9480
9481
9482
9483
9484
9485
9486
9487
9488
9489
9490
9491
9492
9493
9494
9495
9496
9497
9498
9499
9500
9501
9502
9503
9504
9505
9506
9507
9508
9509
9510
9511
9512
9513
9514
9515
9516
9517
9518
9519
9520
9521
9522
9523
9524
9525
9526
9527
9528
9529
9530
9531
9532
9533
9534
9535
9536
9537
9538
9539
9540
9541
9542
9543
9544
9545
9546
9547
9548
9549
9550
9551
9552
9553
9554
9555
9556
9557
9558
9559
9560
9561
9562
9563
9564
9565
9566
9567
9568
9569
9570
9571
9572
9573
9574
9575
9576
9577
9578
9579
9580
9581
9582
9583
9584
9585
9586
9587
9588
9589
9590
9591
9592
9593
9594
9595
9596
9597
9598
9599
9600
9601
9602
9603
9604
9605
9606
9607
9608
9609
9610
9611
9612
9613
9614
9615
9616
9617
9618
9619
9620
9621
9622
9623
9624
9625
9626
9627
9628
9629
9630
9631
9632
9633
9634
9635
9636
9637
9638
9639
9640
9641
9642
9643
9644
9645
9646
9647
9648
9649
9650
9651
9652
9653
9654
9655
9656
9657
9658
9659
9660
9661
9662
9663
9664
9665
9666
9667
9668
9669
9670
9671
9672
9673
9674
9675
9676
9677
9678
9679
9680
9681
9682
9683
9684
9685
9686
9687
9688
9689
9690
9691
9692
9693
9694
9695
9696
9697
9698
9699
9700
9701
9702
9703
9704
9705
9706
9707
9708
9709
9710
9711
9712
9713
9714
9715
9716
9717
9718
9719
9720
9721
9722
9723
9724
9725
9726
9727
9728
9729
9730
9731
9732
9733
9734
9735
9736
9737
9738
9739
9740
9741
9742
9743
9744
9745
9746
9747
9748
9749
9750
9751
9752
9753
9754
9755
9756
9757
9758
9759
9760
9761
9762
9763
9764
9765
9766
9767
9768
9769
9770
9771
9772
9773
9774
9775
9776
9777
9778
9779
9780
9781
9782
9783
9784
9785
9786
9787
9788
9789
9790
9791
9792
9793
9794
9795
9796
9797
9798
9799
9800
9801
9802
9803
9804
9805
9806
9807
9808
9809
9810
9811
9812
9813
9814
9815
9816
9817
9818
9819
9820
9821
9822
9823
9824
9825
9826
9827
9828
9829
9830
9831
9832
9833
9834
9835
9836
9837
9838
9839
9840
9841
9842
9843
9844
9845
9846
9847
9848
9849
9850
9851
9852
9853
9854
9855
9856
9857
9858
9859
9860
9861
9862
9863
9864
9865
9866
9867
9868
9869
9870
9871
9872
9873
9874
9875
9876
9877
9878
9879
9880
9881
9882
9883
9884
9885
9886
9887
9888
9889
9890
9891
9892
9893
9894
9895
9896
9897
9898
9899
9900
9901
9902
9903
9904
9905
9906
9907
9908
9909
9910
9911
9912
9913
9914
9915
9916
9917
9918
9919
9920
9921
9922
9923
9924
9925
9926
9927
9928
9929
9930
9931
9932
9933
9934
9935
9936
9937
9938
9939
9940
9941
9942
9943
9944
9945
9946
9947
9948
9949
9950
9951
9952
9953
9954
9955
9956
9957
9958
9959
9960
9961
9962
9963
9964
9965
9966
9967
9968
9969
9970
9971
9972
9973
9974
9975
9976
9977
9978
9979
9980
9981
9982
9983
9984
9985
9986
9987
9988
9989
9990
9991
9992
9993
9994
9995
9996
9997
9998
9999
10000
10001
10002
10003
10004
10005
10006
10007
10008
10009
10010
10011
10012
10013
10014
10015
10016
10017
10018
10019
10020
10021
10022
10023
10024
10025
10026
10027
10028
10029
10030
10031
10032
10033
10034
10035
10036
10037
10038
10039
10040
10041
10042
10043
10044
10045
10046
10047
10048
10049
10050
10051
10052
10053
10054
10055
10056
10057
10058
10059
10060
10061
10062
10063
10064
10065
10066
10067
10068
10069
10070
10071
10072
10073
10074
10075
10076
10077
10078
10079
10080
10081
10082
10083
10084
10085
10086
10087
10088
10089
10090
10091
10092
10093
10094
10095
10096
10097
10098
10099
10100
10101
10102
10103
10104
10105
10106
10107
10108
10109
10110
10111
10112
10113
10114
10115
10116
10117
10118
10119
10120
10121
10122
10123
10124
10125
10126
10127
10128
10129
10130
10131
10132
10133
10134
10135
10136
10137
10138
10139
10140
10141
10142
10143
10144
10145
10146
10147
10148
10149
10150
10151
10152
10153
10154
10155
10156
10157
10158
10159
10160
10161
10162
10163
10164
10165
10166
10167
10168
10169
10170
10171
10172
10173
10174
10175
10176
10177
10178
10179
10180
10181
10182
10183
10184
10185
10186
10187
10188
10189
10190
10191
10192
10193
10194
10195
10196
10197
10198
10199
10200
10201
10202
10203
10204
10205
10206
10207
10208
10209
10210
10211
10212
10213
10214
10215
10216
10217
10218
10219
10220
10221
10222
10223
10224
10225
10226
10227
10228
10229
10230
10231
10232
10233
10234
10235
10236
10237
10238
10239
10240
10241
10242
10243
10244
10245
10246
10247
10248
10249
10250
10251
10252
10253
10254
10255
10256
10257
10258
10259
10260
10261
10262
10263
10264
10265
10266
10267
10268
10269
10270
10271
10272
10273
10274
10275
10276
10277
10278
10279
10280
10281
10282
10283
10284
10285
10286
10287
10288
10289
10290
10291
10292
10293
10294
10295
10296
10297
10298
10299
10300
10301
10302
10303
10304
10305
10306
10307
10308
10309
10310
10311
10312
10313
10314
10315
10316
10317
10318
10319
10320
10321
10322
10323
10324
10325
10326
10327
10328
10329
10330
10331
10332
10333
10334
10335
10336
10337
10338
10339
10340
10341
10342
10343
10344
10345
10346
10347
10348
10349
10350
10351
10352
10353
10354
10355
10356
10357
10358
10359
10360
10361
10362
10363
10364
10365
10366
10367
10368
10369
10370
10371
10372
10373
10374
10375
10376
10377
10378
10379
10380
10381
10382
10383
10384
10385
10386
10387
10388
10389
10390
10391
10392
10393
10394
10395
10396
10397
10398
10399
10400
10401
10402
10403
10404
10405
10406
10407
10408
10409
10410
10411
10412
10413
10414
10415
10416
10417
10418
10419
10420
10421
10422
10423
10424
10425
10426
10427
10428
10429
10430
10431
10432
10433
10434
10435
10436
10437
10438
10439
10440
10441
10442
10443
10444
10445
10446
10447
10448
10449
10450
10451
10452
10453
10454
10455
10456
10457
10458
10459
10460
10461
10462
10463
10464
10465
10466
10467
10468
10469
10470
10471
10472
10473
10474
10475
10476
10477
10478
10479
10480
10481
10482
10483
10484
10485
10486
10487
10488
10489
10490
10491
10492
10493
10494
10495
10496
10497
10498
10499
10500
10501
10502
10503
10504
10505
10506
10507
10508
10509
10510
10511
10512
10513
10514
10515
10516
10517
10518
10519
10520
10521
10522
10523
10524
10525
10526
10527
10528
10529
10530
10531
10532
10533
10534
10535
10536
10537
10538
10539
10540
10541
10542
10543
10544
10545
10546
10547
10548
10549
10550
10551
10552
10553
10554
10555
10556
10557
10558
10559
10560
10561
10562
10563
10564
10565
10566
10567
10568
10569
10570
10571
10572
10573
10574
10575
10576
10577
10578
10579
10580
10581
10582
10583
10584
10585
10586
10587
10588
10589
10590
10591
10592
10593
10594
10595
10596
10597
10598
10599
10600
10601
10602
10603
10604
10605
10606
10607
10608
10609
10610
10611
10612
10613
10614
10615
10616
10617
10618
10619
10620
10621
10622
10623
10624
10625
10626
10627
10628
10629
10630
10631
10632
10633
10634
10635
10636
10637
10638
10639
10640
10641
10642
10643
10644
10645
10646
10647
10648
10649
10650
10651
10652
10653
10654
10655
10656
10657
10658
10659
10660
10661
10662
10663
10664
10665
10666
10667
10668
10669
10670
10671
10672
10673
10674
10675
10676
10677
10678
10679
10680
10681
10682
10683
10684
10685
10686
10687
10688
10689
10690
10691
10692
10693
10694
10695
10696
10697
10698
10699
10700
10701
10702
10703
10704
10705
10706
10707
10708
10709
10710
10711
10712
10713
10714
10715
10716
10717
10718
10719
10720
10721
10722
10723
10724
10725
10726
10727
10728
10729
10730
10731
10732
10733
10734
10735
10736
10737
10738
10739
10740
10741
10742
10743
10744
10745
10746
10747
10748
10749
10750
10751
10752
10753
10754
10755
10756
10757
10758
10759
10760
10761
10762
10763
10764
10765
10766
10767
10768
10769
10770
10771
10772
10773
10774
10775
10776
10777
10778
10779
10780
10781
10782
10783
10784
10785
10786
10787
10788
10789
10790
10791
10792
10793
10794
10795
10796
10797
10798
10799
10800
10801
10802
10803
10804
10805
10806
10807
10808
10809
10810
10811
10812
10813
10814
10815
10816
10817
10818
10819
10820
10821
10822
10823
10824
10825
10826
10827
10828
10829
10830
10831
10832
10833
10834
10835
10836
10837
10838
10839
10840
10841
10842
10843
10844
10845
10846
10847
10848
10849
10850
10851
10852
10853
10854
10855
10856
10857
10858
10859
10860
10861
10862
10863
10864
10865
10866
10867
10868
10869
10870
10871
10872
10873
10874
10875
10876
10877
10878
10879
10880
10881
10882
10883
10884
10885
10886
10887
10888
10889
10890
10891
10892
10893
10894
10895
10896
10897
10898
10899
10900
10901
10902
10903
10904
10905
10906
10907
10908
10909
10910
10911
10912
10913
10914
10915
10916
10917
10918
10919
10920
10921
10922
10923
10924
10925
10926
10927
10928
10929
10930
10931
10932
10933
10934
10935
10936
10937
10938
10939
10940
10941
10942
10943
10944
10945
10946
10947
10948
10949
10950
10951
10952
10953
10954
10955
10956
10957
10958
10959
10960
10961
10962
10963
10964
10965
10966
10967
10968
10969
10970
10971
10972
10973
10974
10975
10976
10977
10978
10979
10980
10981
10982
10983
10984
10985
10986
10987
10988
10989
10990
10991
10992
10993
10994
10995
10996
10997
10998
10999
11000
11001
11002
11003
11004
11005
11006
11007
11008
11009
11010
11011
11012
11013
11014
11015
11016
11017
11018
11019
11020
11021
11022
11023
11024
11025
11026
11027
11028
11029
11030
11031
11032
11033
11034
11035
11036
11037
11038
11039
11040
11041
11042
11043
11044
11045
11046
11047
11048
11049
11050
11051
11052
11053
11054
11055
11056
11057
11058
11059
11060
11061
11062
11063
11064
11065
11066
11067
11068
11069
11070
11071
11072
11073
11074
11075
11076
11077
11078
11079
11080
11081
11082
11083
11084
11085
11086
11087
11088
11089
11090
11091
11092
11093
11094
11095
11096
11097
11098
11099
11100
11101
11102
11103
11104
11105
11106
11107
11108
11109
11110
11111
11112
11113
11114
11115
11116
11117
11118
11119
11120
11121
11122
11123
11124
11125
11126
11127
11128
11129
11130
11131
11132
11133
11134
11135
11136
11137
11138
11139
11140
11141
11142
11143
11144
11145
11146
11147
11148
11149
11150
11151
11152
11153
11154
11155
11156
11157
11158
11159
11160
11161
11162
11163
11164
11165
11166
11167
11168
11169
11170
11171
11172
11173
11174
11175
11176
11177
11178
11179
11180
11181
11182
11183
11184
11185
11186
11187
11188
11189
11190
11191
11192
11193
11194
11195
11196
11197
11198
11199
11200
11201
11202
11203
11204
11205
11206
11207
11208
11209
11210
11211
11212
11213
11214
11215
11216
11217
11218
11219
11220
11221
11222
11223
11224
11225
11226
11227
11228
11229
11230
11231
11232
11233
11234
11235
11236
11237
11238
11239
11240
11241
11242
11243
11244
11245
11246
11247
11248
11249
11250
11251
11252
11253
11254
11255
11256
11257
11258
11259
11260
11261
11262
11263
11264
11265
11266
11267
11268
11269
11270
11271
11272
11273
11274
11275
11276
11277
11278
11279
11280
11281
11282
11283
11284
11285
11286
11287
11288
11289
11290
11291
11292
11293
11294
11295
11296
11297
11298
11299
11300
11301
11302
11303
11304
11305
11306
11307
11308
11309
11310
11311
11312
11313
11314
11315
11316
11317
11318
11319
11320
11321
11322
11323
11324
11325
11326
11327
11328
11329
11330
11331
11332
11333
11334
11335
11336
11337
11338
11339
11340
11341
11342
11343
11344
11345
11346
11347
11348
11349
11350
11351
11352
11353
11354
11355
11356
11357
11358
11359
11360
11361
11362
11363
11364
11365
11366
11367
11368
11369
11370
11371
11372
11373
11374
11375
11376
11377
11378
11379
11380
11381
11382
11383
11384
11385
11386
11387
11388
11389
11390
11391
11392
11393
11394
11395
11396
11397
11398
11399
11400
11401
11402
11403
11404
11405
11406
11407
11408
11409
11410
11411
11412
11413
11414
11415
11416
11417
11418
11419
11420
11421
11422
11423
11424
11425
11426
11427
11428
11429
11430
11431
11432
11433
11434
11435
11436
11437
11438
11439
11440
11441
11442
11443
11444
11445
11446
11447
11448
11449
11450
11451
11452
11453
11454
11455
11456
11457
11458
11459
11460
11461
11462
11463
11464
11465
11466
11467
11468
11469
11470
11471
11472
11473
11474
11475
11476
11477
11478
11479
11480
11481
11482
11483
11484
11485
11486
11487
11488
11489
11490
11491
11492
11493
11494
11495
11496
11497
11498
11499
11500
11501
11502
11503
11504
11505
11506
11507
11508
11509
11510
11511
11512
11513
11514
11515
11516
11517
11518
11519
11520
11521
11522
11523
11524
11525
11526
11527
11528
11529
11530
11531
11532
11533
11534
11535
11536
11537
11538
11539
11540
11541
11542
11543
11544
11545
11546
11547
11548
11549
11550
11551
11552
11553
11554
11555
11556
11557
11558
11559
11560
11561
11562
11563
11564
11565
11566
11567
11568
11569
11570
11571
11572
11573
11574
11575
11576
11577
11578
11579
11580
11581
11582
11583
11584
11585
11586
11587
11588
11589
11590
11591
11592
11593
11594
11595
11596
11597
11598
11599
11600
11601
11602
11603
11604
11605
11606
11607
11608
11609
11610
11611
11612
11613
11614
11615
11616
11617
11618
11619
11620
11621
11622
11623
11624
11625
11626
11627
11628
11629
11630
11631
11632
11633
11634
11635
11636
11637
11638
11639
11640
11641
11642
11643
11644
11645
11646
11647
11648
11649
11650
11651
11652
11653
11654
11655
11656
11657
11658
11659
11660
11661
11662
11663
11664
11665
11666
11667
11668
11669
11670
11671
11672
11673
11674
11675
11676
11677
11678
11679
11680
11681
11682
11683
11684
11685
11686
11687
11688
11689
11690
11691
11692
11693
11694
11695
11696
11697
11698
11699
11700
11701
11702
11703
11704
11705
11706
11707
11708
11709
11710
11711
11712
11713
11714
11715
11716
11717
11718
11719
11720
11721
11722
11723
11724
11725
11726
11727
11728
11729
11730
11731
11732
11733
11734
11735
11736
11737
11738
11739
11740
11741
11742
11743
11744
11745
11746
11747
11748
11749
11750
11751
11752
11753
11754
11755
11756
11757
11758
11759
11760
11761
11762
11763
11764
11765
11766
11767
11768
11769
11770
11771
11772
11773
11774
11775
11776
11777
11778
11779
11780
11781
11782
11783
11784
11785
11786
11787
11788
11789
11790
11791
11792
11793
11794
11795
11796
11797
11798
11799
11800
11801
11802
11803
11804
11805
11806
11807
11808
11809
11810
11811
11812
11813
11814
11815
11816
11817
11818
11819
11820
11821
11822
11823
11824
11825
11826
11827
11828
11829
11830
11831
11832
11833
11834
11835
11836
11837
11838
11839
11840
11841
11842
11843
11844
11845
11846
11847
11848
11849
11850
11851
11852
11853
11854
11855
11856
11857
11858
11859
11860
11861
11862
11863
11864
11865
11866
11867
11868
11869
11870
11871
11872
11873
11874
11875
11876
11877
11878
11879
11880
11881
11882
11883
11884
11885
11886
11887
11888
11889
11890
11891
11892
11893
11894
11895
11896
11897
11898
11899
11900
11901
11902
11903
11904
11905
11906
11907
11908
11909
11910
11911
11912
11913
11914
11915
11916
11917
11918
11919
11920
11921
11922
11923
11924
11925
11926
11927
11928
11929
11930
11931
11932
11933
11934
11935
11936
11937
11938
11939
11940
11941
11942
11943
11944
11945
11946
11947
11948
11949
11950
11951
11952
11953
11954
11955
11956
11957
11958
11959
11960
11961
11962
11963
11964
11965
11966
11967
11968
11969
11970
11971
11972
11973
11974
11975
11976
11977
11978
11979
11980
11981
11982
11983
11984
11985
11986
11987
11988
11989
11990
11991
11992
11993
11994
11995
11996
11997
11998
11999
12000
12001
12002
12003
12004
12005
12006
12007
12008
12009
12010
12011
12012
12013
12014
12015
12016
12017
12018
12019
12020
12021
12022
12023
12024
12025
12026
12027
12028
12029
12030
12031
12032
12033
12034
12035
12036
12037
12038
12039
12040
12041
12042
12043
12044
12045
12046
12047
12048
12049
12050
12051
12052
12053
12054
12055
12056
12057
12058
12059
12060
12061
12062
12063
12064
12065
12066
12067
12068
12069
12070
12071
12072
12073
12074
12075
12076
12077
12078
12079
12080
12081
12082
12083
12084
12085
12086
12087
12088
12089
12090
12091
12092
12093
12094
12095
12096
12097
12098
12099
12100
12101
12102
12103
12104
12105
12106
12107
12108
12109
12110
12111
12112
12113
12114
12115
12116
12117
12118
12119
12120
12121
12122
12123
12124
12125
12126
12127
12128
12129
12130
12131
12132
12133
12134
12135
12136
12137
12138
12139
12140
12141
12142
12143
12144
12145
12146
12147
12148
12149
12150
12151
12152
12153
12154
12155
12156
12157
12158
12159
12160
12161
12162
12163
12164
12165
12166
12167
12168
12169
12170
12171
12172
12173
12174
12175
12176
12177
12178
12179
12180
12181
12182
12183
12184
12185
12186
12187
12188
12189
12190
12191
12192
12193
12194
12195
12196
12197
12198
12199
12200
12201
12202
12203
12204
12205
12206
12207
12208
12209
12210
12211
12212
12213
12214
12215
12216
12217
12218
12219
12220
12221
12222
12223
12224
12225
12226
12227
12228
12229
12230
12231
12232
12233
12234
12235
12236
12237
12238
12239
12240
12241
12242
12243
12244
12245
12246
12247
12248
12249
12250
12251
12252
12253
12254
12255
12256
12257
12258
12259
12260
12261
12262
12263
12264
12265
12266
12267
12268
12269
12270
12271
12272
12273
12274
12275
12276
12277
12278
12279
12280
12281
12282
12283
12284
12285
12286
12287
12288
12289
12290
12291
12292
12293
12294
12295
12296
12297
12298
12299
12300
12301
12302
12303
12304
12305
12306
12307
12308
12309
12310
12311
12312
12313
12314
12315
12316
12317
12318
12319
12320
12321
12322
12323
12324
12325
12326
12327
12328
12329
12330
12331
12332
12333
12334
12335
12336
12337
12338
12339
12340
12341
12342
12343
12344
12345
12346
12347
12348
12349
12350
12351
12352
12353
12354
12355
12356
12357
12358
12359
12360
12361
12362
12363
12364
12365
12366
12367
12368
12369
12370
12371
12372
12373
12374
12375
12376
12377
12378
12379
12380
12381
12382
12383
12384
12385
12386
12387
12388
12389
12390
12391
12392
12393
12394
12395
12396
12397
12398
12399
12400
12401
12402
12403
12404
12405
12406
12407
12408
12409
12410
12411
12412
12413
12414
12415
12416
12417
12418
12419
12420
12421
12422
12423
12424
12425
12426
12427
12428
12429
12430
12431
12432
12433
12434
12435
12436
12437
12438
12439
12440
12441
12442
12443
12444
12445
12446
12447
12448
12449
12450
12451
12452
12453
12454
12455
12456
12457
12458
12459
12460
12461
12462
12463
12464
12465
12466
12467
12468
12469
12470
12471
12472
12473
12474
12475
12476
12477
12478
12479
12480
12481
12482
12483
12484
12485
12486
12487
12488
12489
12490
12491
12492
12493
12494
12495
12496
12497
12498
12499
12500
12501
12502
12503
12504
12505
12506
12507
12508
12509
12510
12511
12512
12513
12514
12515
12516
12517
12518
12519
12520
12521
12522
12523
12524
12525
12526
12527
12528
12529
12530
12531
12532
12533
12534
12535
12536
12537
12538
12539
12540
12541
12542
12543
12544
12545
12546
12547
12548
12549
12550
12551
12552
12553
12554
12555
12556
12557
12558
12559
12560
12561
12562
12563
12564
12565
12566
12567
12568
12569
12570
12571
12572
12573
12574
12575
12576
12577
12578
12579
12580
12581
12582
12583
12584
12585
12586
12587
12588
12589
12590
12591
12592
12593
12594
12595
12596
12597
12598
12599
12600
12601
12602
12603
12604
12605
12606
12607
12608
12609
12610
12611
12612
12613
12614
12615
12616
12617
12618
12619
12620
12621
12622
12623
12624
12625
12626
12627
12628
12629
12630
12631
12632
12633
12634
12635
12636
12637
12638
12639
12640
12641
12642
12643
12644
12645
12646
12647
12648
12649
12650
12651
12652
12653
12654
12655
12656
12657
12658
12659
12660
12661
12662
12663
12664
12665
12666
12667
12668
12669
12670
12671
12672
12673
12674
12675
12676
12677
12678
12679
12680
12681
12682
12683
12684
12685
12686
12687
12688
12689
12690
12691
12692
12693
12694
12695
12696
12697
12698
12699
12700
12701
12702
12703
12704
12705
12706
12707
12708
12709
12710
12711
12712
12713
12714
12715
12716
12717
12718
12719
12720
12721
12722
12723
12724
12725
12726
12727
12728
12729
12730
12731
12732
12733
12734
12735
12736
12737
12738
12739
12740
12741
12742
12743
12744
12745
12746
12747
12748
12749
12750
12751
12752
12753
12754
12755
12756
12757
12758
12759
12760
12761
12762
12763
12764
12765
12766
12767
12768
12769
12770
12771
12772
12773
12774
12775
12776
12777
12778
12779
12780
12781
12782
12783
12784
12785
12786
12787
12788
12789
12790
12791
12792
12793
12794
12795
12796
12797
12798
12799
12800
12801
12802
12803
12804
12805
12806
12807
12808
12809
12810
12811
12812
12813
12814
12815
12816
12817
12818
12819
12820
12821
12822
12823
12824
12825
12826
12827
12828
12829
12830
12831
12832
12833
12834
12835
12836
12837
12838
12839
12840
12841
12842
12843
12844
12845
12846
12847
12848
12849
12850
12851
12852
12853
12854
12855
12856
12857
12858
12859
12860
12861
12862
12863
12864
12865
12866
12867
12868
12869
12870
12871
12872
12873
12874
12875
12876
12877
12878
12879
12880
12881
12882
12883
12884
12885
12886
12887
12888
12889
12890
12891
12892
12893
12894
12895
12896
12897
12898
12899
12900
12901
12902
12903
12904
12905
12906
12907
12908
12909
12910
12911
12912
12913
12914
12915
12916
12917
12918
12919
12920
12921
12922
12923
12924
12925
12926
12927
12928
12929
12930
12931
12932
12933
12934
12935
12936
12937
12938
12939
12940
12941
12942
12943
12944
12945
12946
12947
12948
12949
12950
12951
12952
12953
12954
12955
12956
12957
12958
12959
12960
12961
12962
12963
12964
12965
12966
12967
12968
12969
12970
12971
12972
12973
12974
12975
12976
12977
12978
12979
12980
12981
12982
12983
12984
12985
12986
12987
12988
12989
12990
12991
12992
12993
12994
12995
12996
12997
12998
12999
13000
13001
13002
13003
13004
13005
13006
13007
13008
13009
13010
13011
13012
13013
13014
13015
13016
13017
13018
13019
13020
13021
13022
13023
13024
13025
13026
13027
13028
13029
13030
13031
13032
13033
13034
13035
13036
13037
13038
13039
13040
13041
13042
13043
13044
13045
13046
13047
13048
13049
13050
13051
13052
13053
13054
13055
13056
13057
13058
13059
13060
13061
13062
13063
13064
13065
13066
13067
13068
13069
13070
13071
13072
13073
13074
13075
13076
13077
13078
13079
13080
13081
13082
13083
13084
13085
13086
13087
13088
13089
13090
13091
13092
13093
13094
13095
13096
13097
13098
13099
13100
13101
13102
13103
13104
13105
13106
13107
13108
13109
13110
13111
13112
13113
13114
13115
13116
13117
13118
13119
13120
13121
13122
13123
13124
13125
13126
13127
13128
13129
13130
13131
13132
13133
13134
13135
13136
13137
13138
13139
13140
13141
13142
13143
13144
13145
13146
13147
13148
13149
13150
13151
13152
13153
13154
13155
13156
13157
13158
13159
13160
13161
13162
13163
13164
13165
13166
13167
13168
13169
13170
13171
13172
13173
13174
13175
13176
13177
13178
13179
13180
13181
13182
13183
13184
13185
13186
13187
13188
13189
13190
13191
13192
13193
13194
13195
13196
13197
13198
13199
13200
13201
13202
13203
13204
13205
13206
13207
13208
13209
13210
13211
13212
13213
13214
13215
13216
13217
13218
13219
13220
13221
13222
13223
13224
13225
13226
13227
13228
13229
13230
13231
13232
13233
13234
13235
13236
13237
13238
13239
13240
13241
13242
13243
13244
13245
13246
13247
13248
13249
13250
13251
13252
13253
13254
13255
13256
13257
13258
13259
13260
13261
13262
13263
13264
13265
13266
13267
13268
13269
13270
13271
13272
13273
13274
13275
13276
13277
13278
13279
13280
13281
13282
13283
13284
13285
13286
13287
13288
13289
13290
13291
13292
13293
13294
13295
13296
13297
13298
13299
13300
13301
13302
13303
13304
13305
13306
13307
13308
13309
13310
13311
13312
13313
13314
13315
13316
13317
13318
13319
13320
13321
13322
13323
13324
13325
13326
13327
13328
13329
13330
13331
13332
13333
13334
13335
13336
13337
13338
13339
13340
13341
13342
13343
13344
13345
13346
13347
13348
13349
13350
13351
13352
13353
13354
13355
13356
13357
13358
13359
13360
13361
13362
13363
13364
13365
13366
13367
13368
13369
13370
13371
13372
13373
13374
13375
13376
13377
13378
13379
13380
13381
13382
13383
13384
13385
13386
13387
13388
13389
13390
13391
13392
13393
13394
13395
13396
13397
13398
13399
13400
13401
13402
13403
13404
13405
13406
13407
13408
13409
13410
13411
13412
13413
13414
13415
13416
13417
13418
13419
13420
13421
13422
13423
13424
13425
13426
13427
13428
13429
13430
13431
13432
13433
13434
13435
13436
13437
13438
13439
13440
13441
13442
13443
13444
13445
13446
13447
13448
13449
13450
13451
13452
13453
13454
13455
13456
13457
13458
13459
13460
13461
13462
13463
13464
13465
13466
13467
13468
13469
13470
13471
13472
13473
13474
13475
13476
13477
13478
13479
13480
13481
13482
13483
13484
13485
13486
13487
13488
13489
13490
13491
13492
13493
13494
13495
13496
13497
13498
13499
13500
13501
13502
13503
13504
13505
13506
13507
13508
13509
13510
13511
13512
13513
13514
13515
13516
13517
13518
13519
13520
13521
13522
13523
13524
13525
13526
13527
13528
13529
13530
13531
13532
13533
13534
13535
13536
13537
13538
13539
13540
13541
13542
13543
13544
13545
13546
13547
13548
13549
13550
13551
13552
13553
13554
13555
13556
13557
13558
13559
13560
13561
13562
13563
13564
13565
13566
13567
13568
13569
13570
13571
13572
13573
13574
13575
13576
13577
13578
13579
13580
13581
13582
13583
13584
13585
13586
13587
13588
13589
13590
13591
13592
13593
13594
13595
13596
13597
13598
13599
13600
13601
13602
13603
13604
13605
13606
13607
13608
13609
13610
13611
13612
13613
13614
13615
13616
13617
13618
13619
13620
13621
13622
13623
13624
13625
13626
13627
13628
13629
13630
13631
13632
13633
13634
13635
13636
13637
13638
13639
13640
13641
13642
13643
13644
13645
13646
13647
13648
13649
13650
13651
13652
13653
13654
13655
13656
13657
13658
13659
13660
13661
13662
13663
13664
13665
13666
13667
13668
13669
13670
13671
13672
13673
13674
13675
13676
13677
13678
13679
13680
13681
13682
13683
13684
13685
13686
13687
13688
13689
13690
13691
13692
13693
13694
13695
13696
13697
13698
13699
13700
13701
13702
13703
13704
13705
13706
13707
13708
13709
13710
13711
13712
13713
13714
13715
13716
13717
13718
13719
13720
13721
13722
13723
13724
13725
13726
13727
13728
13729
13730
13731
13732
13733
13734
13735
13736
13737
13738
13739
13740
13741
13742
13743
13744
13745
13746
13747
13748
13749
13750
13751
13752
13753
13754
13755
13756
13757
13758
13759
13760
13761
13762
13763
13764
13765
13766
13767
13768
13769
13770
13771
13772
13773
13774
13775
13776
13777
13778
13779
13780
13781
13782
13783
13784
13785
13786
13787
13788
13789
13790
13791
13792
13793
13794
13795
13796
13797
13798
13799
13800
13801
13802
13803
13804
13805
13806
13807
13808
13809
13810
13811
13812
13813
13814
13815
13816
13817
13818
13819
13820
13821
13822
13823
13824
13825
13826
13827
13828
13829
13830
13831
13832
13833
13834
13835
13836
13837
13838
13839
13840
13841
13842
13843
13844
13845
13846
13847
13848
13849
13850
13851
13852
13853
13854
13855
13856
13857
13858
13859
13860
13861
13862
13863
13864
13865
13866
13867
13868
13869
13870
13871
13872
13873
13874
13875
13876
13877
13878
13879
13880
13881
13882
13883
13884
13885
13886
13887
13888
13889
13890
13891
13892
13893
13894
13895
13896
13897
13898
13899
13900
13901
13902
13903
13904
13905
13906
13907
13908
13909
13910
13911
13912
13913
13914
13915
13916
13917
13918
13919
13920
13921
13922
13923
13924
13925
13926
13927
13928
13929
13930
13931
13932
13933
13934
13935
13936
13937
13938
13939
13940
13941
13942
13943
13944
13945
13946
13947
13948
13949
13950
13951
13952
13953
13954
13955
13956
13957
13958
13959
13960
13961
13962
13963
13964
13965
13966
13967
13968
13969
13970
13971
13972
13973
13974
13975
13976
13977
13978
13979
13980
13981
13982
13983
13984
13985
13986
13987
13988
13989
13990
13991
13992
13993
13994
13995
13996
13997
13998
13999
14000
14001
14002
14003
14004
14005
14006
14007
14008
14009
14010
14011
14012
14013
14014
14015
14016
14017
14018
14019
14020
14021
14022
14023
14024
14025
14026
14027
14028
14029
14030
14031
14032
14033
14034
14035
14036
14037
14038
14039
14040
14041
14042
14043
14044
14045
14046
14047
14048
14049
14050
14051
14052
14053
14054
14055
14056
14057
14058
14059
14060
14061
14062
14063
14064
14065
14066
14067
14068
14069
14070
14071
14072
14073
14074
14075
14076
14077
14078
14079
14080
14081
14082
14083
14084
14085
14086
14087
14088
14089
14090
14091
14092
14093
14094
14095
14096
14097
14098
14099
14100
14101
14102
14103
14104
14105
14106
14107
14108
14109
14110
14111
14112
14113
14114
14115
14116
14117
14118
14119
14120
14121
14122
14123
14124
14125
14126
14127
14128
14129
14130
14131
14132
14133
14134
14135
14136
14137
14138
14139
14140
14141
14142
14143
14144
14145
14146
14147
14148
14149
14150
14151
14152
14153
14154
14155
14156
14157
14158
14159
14160
14161
14162
14163
14164
14165
14166
14167
14168
14169
14170
14171
14172
14173
14174
14175
14176
14177
14178
14179
14180
14181
14182
14183
14184
14185
14186
14187
14188
14189
14190
14191
14192
14193
14194
14195
14196
14197
14198
14199
14200
14201
14202
14203
14204
14205
14206
14207
14208
14209
14210
14211
14212
14213
14214
14215
14216
14217
14218
14219
14220
14221
14222
14223
14224
14225
14226
14227
14228
14229
14230
14231
14232
14233
14234
14235
14236
14237
14238
14239
14240
14241
14242
14243
14244
14245
14246
14247
14248
14249
14250
14251
14252
14253
14254
14255
14256
14257
14258
14259
14260
14261
14262
14263
14264
14265
14266
14267
14268
14269
14270
14271
14272
14273
14274
14275
14276
14277
14278
14279
14280
14281
14282
14283
14284
14285
14286
14287
14288
14289
14290
14291
14292
14293
14294
14295
14296
14297
14298
14299
14300
14301
14302
14303
14304
14305
14306
14307
14308
14309
14310
14311
14312
14313
14314
14315
14316
14317
14318
14319
14320
14321
14322
14323
14324
14325
14326
14327
14328
14329
14330
14331
14332
14333
14334
14335
14336
14337
14338
14339
14340
14341
14342
14343
14344
14345
14346
14347
14348
14349
14350
14351
14352
14353
14354
14355
14356
14357
14358
14359
14360
14361
14362
14363
14364
14365
14366
14367
14368
14369
14370
14371
14372
14373
14374
14375
14376
14377
14378
14379
14380
14381
14382
14383
14384
14385
14386
14387
14388
14389
14390
14391
14392
14393
14394
14395
14396
14397
14398
14399
14400
14401
14402
14403
14404
14405
14406
14407
14408
14409
14410
14411
14412
14413
14414
14415
14416
14417
14418
14419
14420
14421
14422
14423
14424
14425
14426
14427
14428
14429
14430
14431
14432
14433
14434
14435
14436
14437
14438
14439
14440
14441
14442
14443
14444
14445
14446
14447
14448
14449
14450
14451
14452
14453
14454
14455
14456
14457
14458
14459
14460
14461
14462
14463
14464
14465
14466
14467
14468
14469
14470
14471
14472
14473
14474
14475
14476
14477
14478
14479
14480
14481
14482
14483
14484
14485
14486
14487
14488
14489
14490
14491
14492
14493
14494
14495
14496
14497
14498
14499
14500
14501
14502
14503
14504
14505
14506
14507
14508
14509
14510
14511
14512
14513
14514
14515
14516
14517
14518
14519
14520
14521
14522
14523
14524
14525
14526
14527
14528
14529
14530
14531
14532
14533
14534
14535
14536
14537
14538
14539
14540
14541
14542
14543
14544
14545
14546
14547
14548
14549
14550
14551
14552
14553
14554
14555
14556
14557
14558
14559
14560
14561
14562
14563
14564
14565
14566
14567
14568
14569
14570
14571
14572
14573
14574
14575
14576
14577
14578
14579
14580
14581
14582
14583
14584
14585
14586
14587
14588
14589
14590
14591
14592
14593
14594
14595
14596
14597
14598
14599
14600
14601
14602
14603
14604
14605
14606
14607
14608
14609
14610
14611
14612
14613
14614
14615
14616
14617
14618
14619
14620
14621
14622
14623
14624
14625
14626
14627
14628
14629
14630
14631
14632
14633
14634
14635
14636
14637
14638
14639
14640
14641
14642
14643
14644
14645
14646
14647
14648
14649
14650
14651
14652
14653
14654
14655
14656
14657
14658
14659
14660
14661
14662
14663
14664
14665
14666
14667
14668
14669
14670
14671
14672
14673
14674
14675
14676
14677
14678
14679
14680
14681
14682
14683
14684
14685
14686
14687
14688
14689
14690
14691
14692
14693
14694
14695
14696
14697
14698
14699
14700
14701
14702
14703
14704
14705
14706
14707
14708
14709
14710
14711
14712
14713
14714
14715
14716
14717
14718
14719
14720
14721
14722
14723
14724
14725
14726
14727
14728
14729
14730
14731
14732
14733
14734
14735
14736
14737
14738
14739
14740
14741
14742
14743
14744
14745
14746
14747
14748
14749
14750
14751
14752
14753
14754
14755
14756
14757
14758
14759
14760
14761
14762
14763
14764
14765
14766
14767
14768
14769
14770
14771
14772
14773
14774
14775
14776
14777
14778
14779
14780
14781
14782
14783
14784
14785
14786
14787
14788
14789
14790
14791
14792
14793
14794
14795
14796
14797
14798
14799
14800
14801
14802
14803
14804
14805
14806
14807
14808
14809
14810
14811
14812
14813
14814
14815
14816
14817
14818
14819
14820
14821
14822
14823
14824
14825
14826
14827
14828
14829
14830
14831
14832
14833
14834
14835
14836
14837
14838
14839
14840
14841
14842
14843
14844
14845
14846
14847
14848
14849
14850
14851
14852
14853
14854
14855
14856
14857
14858
14859
14860
14861
14862
14863
14864
14865
14866
14867
14868
14869
14870
14871
14872
14873
14874
14875
14876
14877
14878
14879
14880
14881
14882
14883
14884
14885
14886
14887
14888
14889
14890
14891
14892
14893
14894
14895
14896
14897
14898
14899
14900
14901
14902
14903
14904
14905
14906
14907
14908
14909
14910
14911
14912
14913
14914
14915
14916
14917
14918
14919
14920
14921
14922
14923
14924
14925
14926
14927
14928
14929
14930
14931
14932
14933
14934
14935
14936
14937
14938
14939
14940
14941
14942
14943
14944
14945
14946
14947
14948
14949
14950
14951
14952
14953
14954
14955
14956
14957
14958
14959
14960
14961
14962
14963
14964
14965
14966
14967
14968
14969
14970
14971
14972
14973
14974
14975
14976
14977
14978
14979
14980
14981
14982
14983
14984
14985
14986
14987
14988
14989
14990
14991
14992
14993
14994
14995
14996
14997
14998
14999
15000
15001
15002
15003
15004
15005
15006
15007
15008
15009
15010
15011
15012
15013
15014
15015
15016
15017
15018
15019
15020
15021
15022
15023
15024
15025
15026
15027
15028
15029
15030
15031
15032
15033
15034
15035
15036
15037
15038
15039
15040
15041
15042
15043
15044
15045
15046
15047
15048
15049
15050
15051
15052
15053
15054
15055
15056
15057
15058
15059
15060
15061
15062
15063
15064
15065
15066
15067
15068
15069
15070
15071
15072
15073
15074
15075
15076
15077
15078
15079
15080
15081
15082
15083
15084
15085
15086
15087
15088
15089
15090
15091
15092
15093
15094
15095
15096
15097
15098
15099
15100
15101
15102
15103
15104
15105
15106
15107
15108
15109
15110
15111
15112
15113
15114
15115
15116
15117
15118
15119
15120
15121
15122
15123
15124
15125
15126
15127
15128
15129
15130
15131
15132
15133
15134
15135
15136
15137
15138
15139
15140
15141
15142
15143
15144
15145
15146
15147
15148
15149
15150
15151
15152
15153
15154
15155
15156
15157
15158
15159
15160
15161
15162
15163
15164
15165
15166
15167
15168
15169
15170
15171
15172
15173
15174
15175
15176
15177
15178
15179
15180
15181
15182
15183
15184
15185
15186
15187
15188
15189
15190
15191
15192
15193
15194
15195
15196
15197
15198
15199
15200
15201
15202
15203
15204
15205
15206
15207
15208
15209
15210
15211
15212
15213
15214
15215
15216
15217
15218
15219
15220
15221
15222
15223
15224
15225
15226
15227
15228
15229
15230
15231
\input texinfo @c -*-texinfo-*-
@comment %**start of header
@setfilename bison.info
@documentencoding UTF-8
@include version.texi
@settitle Bison @value{VERSION}
@xrefautomaticsectiontitle on

@tex
\gdef\rgbWarning{0.50 0 0.50}
\gdef\colorWarning{%
 \setcolor{\rgbWarning}%
}
\gdef\rgbError{0.80 0 0}
\gdef\colorError{%
 \setcolor{\rgbError}%
}
\gdef\rgbNotice{0 0 0.80}
\gdef\colorNotice{%
 \setcolor{\rgbNotice}%
}
\gdef\colorOff{%
 \setcolor{\maincolor}%
}
@end tex

@ifnottex
@macro colorWarning
@inlineraw{html, <b style="color:darkviolet">}
@end macro
@macro colorError
@inlineraw{html, <b style="color:red">}
@end macro
@macro colorNotice
@inlineraw{html, <b style="color:darkcyan">}
@end macro
@macro colorOff
@inlineraw{html, </b>}
@end macro
@end ifnottex

@macro dwarning{text}
@colorWarning{}\text\@colorOff{}
@end macro

@macro derror{text}
@colorError{}\text\@colorOff{}
@end macro

@macro dnotice{text}
@colorNotice{}\text\@colorOff{}
@end macro

@finalout

@c SMALL BOOK version
@c This edition has been formatted so that you can format and print it in
@c the smallbook format.
@c @smallbook
@c @setchapternewpage odd

@c Set following if you want to document %default-prec and %no-default-prec.
@c This feature is experimental and may change in future Bison versions.
@c @set defaultprec

@ifnotinfo
@syncodeindex fn cp
@syncodeindex vr cp
@syncodeindex tp cp
@end ifnotinfo
@ifinfo
@synindex fn cp
@synindex vr cp
@synindex tp cp
@end ifinfo
@comment %**end of header

@copying

This manual (@value{UPDATED}) is for GNU Bison (version @value{VERSION}),
the GNU parser generator.

Copyright @copyright{} 1988--1993, 1995, 1998--2015, 2018--2020 Free
Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document under
the terms of the GNU Free Documentation License, Version 1.3 or any later
version published by the Free Software Foundation; with no Invariant
Sections, with the Front-Cover texts being ``A GNU Manual,'' and with the
Back-Cover Texts as in (a) below.  A copy of the license is included in the
section entitled ``GNU Free Documentation License.''

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and modify
this GNU manual.  Buying copies from the FSF supports it in developing GNU
and promoting software freedom.''
@end quotation
@end copying

@dircategory Software development
@direntry
* bison: (bison).       GNU parser generator (Yacc replacement).
@end direntry

@titlepage
@title Bison
@subtitle The Yacc-compatible Parser Generator
@subtitle @value{UPDATED}, Bison Version @value{VERSION}

@author by Charles Donnelly and Richard Stallman

@page
@vskip 0pt plus 1filll
@insertcopying
@sp 2
Published by the Free Software Foundation @*
51 Franklin Street, Fifth Floor @*
Boston, MA  02110-1301  USA @*
Printed copies are available from the Free Software Foundation.@*
ISBN 1-882114-44-2
@sp 2
Cover art by Etienne Suvasa.
@end titlepage

@contents

@ifnottex
@node Top
@top Bison
@insertcopying
@end ifnottex

@menu
* Introduction::        What GNU Bison is.
* Conditions::          Conditions for using Bison and its output.
* Copying::             The GNU General Public License says
                          how you can copy and share Bison.

Tutorial sections:
* Concepts::            Basic concepts for understanding Bison.
* Examples::            Three simple explained examples of using Bison.

Reference sections:
* Grammar File::        Writing Bison declarations and rules.
* Interface::           C-language interface to the parser function @code{yyparse}.
* Algorithm::           How the Bison parser works at run-time.
* Error Recovery::      Writing rules for error recovery.
* Context Dependency::  What to do if your language syntax is too
                          messy for Bison to handle straightforwardly.
* Debugging::           Understanding or debugging Bison parsers.
* Invocation::          How to run Bison (to produce the parser implementation).
* Other Languages::     Creating C++ and Java parsers.
* History::             How Bison came to be
* FAQ::                 Frequently Asked Questions
* Table of Symbols::    All the keywords of the Bison language are explained.
* Glossary::            Basic concepts are explained.
* GNU Free Documentation License:: Copying and sharing this manual
* Bibliography::        Publications cited in this manual.
* Index of Terms::      Cross-references to the text.

@detailmenu
 --- The Detailed Node Listing ---

The Concepts of Bison

* Language and Grammar:: Languages and context-free grammars,
                           as mathematical ideas.
* Grammar in Bison::     How we represent grammars for Bison's sake.
* Semantic Values::      Each token or syntactic grouping can have
                           a semantic value (the value of an integer,
                           the name of an identifier, etc.).
* Semantic Actions::     Each rule can have an action containing C code.
* GLR Parsers::          Writing parsers for general context-free languages.
* Locations::            Overview of location tracking.
* Bison Parser::         What are Bison's input and output,
                           how is the output used?
* Stages::               Stages in writing and running Bison grammars.
* Grammar Layout::       Overall structure of a Bison grammar file.

Writing GLR Parsers

* Simple GLR Parsers::     Using GLR parsers on unambiguous grammars.
* Merging GLR Parses::     Using GLR parsers to resolve ambiguities.
* GLR Semantic Actions::   Considerations for semantic values and deferred actions.
* Semantic Predicates::    Controlling a parse with arbitrary computations.
* Compiler Requirements for GLR::  GLR parsers require a modern C compiler.

Examples

* RPN Calc::               Reverse Polish Notation Calculator;
                             a first example with no operator precedence.
* Infix Calc::             Infix (algebraic) notation calculator.
                             Operator precedence is introduced.
* Simple Error Recovery::  Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
* Multi-function Calc::    Calculator with memory and trig functions.
                             It uses multiple data-types for semantic values.
* Exercises::              Ideas for improving the multi-function calculator.

Reverse Polish Notation Calculator

* Rpcalc Declarations::    Prologue (declarations) for rpcalc.
* Rpcalc Rules::           Grammar Rules for rpcalc, with explanation.
* Rpcalc Lexer::           The lexical analyzer.
* Rpcalc Main::            The controlling function.
* Rpcalc Error::           The error reporting function.
* Rpcalc Generate::        Running Bison on the grammar file.
* Rpcalc Compile::         Run the C compiler on the output code.

Grammar Rules for @code{rpcalc}

* Rpcalc Input::            Explanation of the @code{input} nonterminal
* Rpcalc Line::             Explanation of the @code{line} nonterminal
* Rpcalc Expr::             Explanation of the @code{expr} nonterminal

Location Tracking Calculator: @code{ltcalc}

* Ltcalc Declarations::    Bison and C declarations for ltcalc.
* Ltcalc Rules::           Grammar rules for ltcalc, with explanations.
* Ltcalc Lexer::           The lexical analyzer.

Multi-Function Calculator: @code{mfcalc}

* Mfcalc Declarations::    Bison declarations for multi-function calculator.
* Mfcalc Rules::           Grammar rules for the calculator.
* Mfcalc Symbol Table::    Symbol table management subroutines.
* Mfcalc Lexer::           The lexical analyzer.
* Mfcalc Main::            The controlling function.

Bison Grammar Files

* Grammar Outline::    Overall layout of the grammar file.
* Symbols::            Terminal and nonterminal symbols.
* Rules::              How to write grammar rules.
* Semantics::          Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References::   Using named references in actions.
* Declarations::       All kinds of Bison declarations are described here.
* Multiple Parsers::   Putting more than one Bison parser in one program.

Outline of a Bison Grammar

* Prologue::              Syntax and usage of the prologue.
* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations::    Syntax and usage of the Bison declarations section.
* Grammar Rules::         Syntax and usage of the grammar rules section.
* Epilogue::              Syntax and usage of the epilogue.

Grammar Rules

* Rules Syntax::   Syntax of the rules.
* Empty Rules::    Symbols that can match the empty string.
* Recursion::      Writing recursive rules.


Defining Language Semantics

* Value Type::        Specifying one data type for all semantic values.
* Multiple Types::    Specifying several alternative data types.
* Type Generation::   Generating the semantic value type.
* Union Decl::        Declaring the set of all semantic value types.
* Structured Value Type::  Providing a structured semantic value type.
* Actions::           An action is the semantic definition of a grammar rule.
* Action Types::      Specifying data types for actions to operate on.
* Midrule Actions::   Most actions go at the end of a rule.
                      This says when, why and how to use the exceptional
                        action in the middle of a rule.

Actions in Midrule

* Using Midrule Actions::       Putting an action in the middle of a rule.
* Typed Midrule Actions::       Specifying the semantic type of their values.
* Midrule Action Translation::  How midrule actions are actually processed.
* Midrule Conflicts::           Midrule actions can cause conflicts.

Tracking Locations

* Location Type::               Specifying a data type for locations.
* Actions and Locations::       Using locations in actions.
* Location Default Action::     Defining a general way to compute locations.

Bison Declarations

* Require Decl::      Requiring a Bison version.
* Token Decl::        Declaring terminal symbols.
* Precedence Decl::   Declaring terminals with precedence and associativity.
* Type Decl::         Declaring the choice of type for a nonterminal symbol.
* Symbol Decls::      Summary of the Syntax of Symbol Declarations.
* Initial Action Decl::  Code run before parsing starts.
* Destructor Decl::   Declaring how symbols are freed.
* Printer Decl::      Declaring how symbol values are displayed.
* Expect Decl::       Suppressing warnings about parsing conflicts.
* Start Decl::        Specifying the start symbol.
* Pure Decl::         Requesting a reentrant parser.
* Push Decl::         Requesting a push parser.
* Decl Summary::      Table of all Bison declarations.
* %define Summary::   Defining variables to adjust Bison's behavior.
* %code Summary::     Inserting code into the parser source.

Parser C-Language Interface

* Parser Function::         How to call @code{yyparse} and what it returns.
* Push Parser Function::    How to call @code{yypush_parse} and what it returns.
* Pull Parser Function::    How to call @code{yypull_parse} and what it returns.
* Parser Create Function::  How to call @code{yypstate_new} and what it returns.
* Parser Delete Function::  How to call @code{yypstate_delete} and what it returns.
* Lexical::                 You must supply a function @code{yylex}
                              which reads tokens.
* Error Reporting::         Passing error messages to the user.
* Action Features::         Special features for use in actions.
* Internationalization::    How to let the parser speak in the user's
                              native language.

The Lexical Analyzer Function @code{yylex}

* Calling Convention::  How @code{yyparse} calls @code{yylex}.
* Special Tokens::      Signaling end-of-file and errors to the parser.
* Tokens from Literals:: Finding token kinds from string aliases.
* Token Values::        How @code{yylex} must return the semantic value
                          of the token it has read.
* Token Locations::     How @code{yylex} must return the text location
                          (line number, etc.) of the token, if the
                          actions want that.
* Pure Calling::        How the calling convention differs in a pure parser
                          (@pxref{Pure Decl}).

Error Reporting

* Error Reporting Function::         You must supply a @code{yyerror} function.
* Syntax Error Reporting Function::  You can supply a @code{yyreport_syntax_error} function.

Parser Internationalization

* Enabling I18n::    Preparing your project to support internationalization.
* Token I18n::       Preparing tokens for internationalization in error messages.

The Bison Parser Algorithm

* Lookahead::         Parser looks one token ahead when deciding what to do.
* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
* Precedence::        Operator precedence works by resolving conflicts.
* Contextual Precedence::  When an operator's precedence depends on context.
* Parser States::     The parser is a finite-state-machine with stack.
* Reduce/Reduce::     When two rules are applicable in the same situation.
* Mysterious Conflicts:: Conflicts that look unjustified.
* Tuning LR::         How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted.  How to avoid it.

Operator Precedence

* Why Precedence::    An example showing why precedence is needed.
* Using Precedence::  How to specify precedence and associativity.
* Precedence Only::   How to specify precedence only.
* Precedence Examples::  How these features are used in the previous example.
* How Precedence::    How they work.
* Non Operators::     Using precedence for general conflicts.

Tuning LR

* LR Table Construction:: Choose a different construction algorithm.
* Default Reductions::    Disable default reductions.
* LAC::                   Correct lookahead sets in the parser states.
* Unreachable States::    Keep unreachable parser states for debugging.

Handling Context Dependencies

* Semantic Tokens::   Token parsing can depend on the semantic context.
* Lexical Tie-ins::   Token parsing can depend on the syntactic context.
* Tie-in Recovery::   Lexical tie-ins have implications for how
                        error recovery rules must be written.

Debugging Your Parser

* Understanding::     Understanding the structure of your parser.
* Graphviz::          Getting a visual representation of the parser.
* Xml::               Getting a markup representation of the parser.
* Tracing::           Tracing the execution of your parser.

Tracing Your Parser

* Enabling Traces::    Activating run-time trace support
* Mfcalc Traces::      Extending @code{mfcalc} to support traces
* The YYPRINT Macro::  Obsolete interface for semantic value reports

Invoking Bison

* Bison Options::     All the options described in detail,
                        in alphabetical order by short options.
* Option Cross Key::  Alphabetical list of long options.
* Yacc Library::      Yacc-compatible @code{yylex} and @code{main}.

Bison Options

* Operation Modes::    Options controlling the global behavior of @command{bison}
* Diagnostics::        Options controlling the diagnostics
* Tuning the Parser::  Options changing the generated parsers
* Output Files::       Options controlling the output

Parsers Written In Other Languages

* C++ Parsers::                 The interface to generate C++ parser classes
* Java Parsers::                The interface to generate Java parser classes

C++ Parsers

* A Simple C++ Example::        A short introduction to C++ parsers
* C++ Bison Interface::         Asking for C++ parser generation
* C++ Parser Interface::        Instantiating and running the parser
* C++ Semantic Values::         %union vs. C++
* C++ Location Values::         The position and location classes
* C++ Parser Context::          You can supply a @code{report_syntax_error} function.
* C++ Scanner Interface::       Exchanges between yylex and parse
* A Complete C++ Example::      Demonstrating their use

C++ Location Values

* C++ position::                  One point in the source file
* C++ location::                  Two points in the source file
* Exposing the Location Classes:: Using the Bison location class in your
                                  project
* User Defined Location Type::    Required interface for locations

A Complete C++ Example

* Calc++ --- C++ Calculator::   The specifications
* Calc++ Parsing Driver::       An active parsing context
* Calc++ Parser::               A parser class
* Calc++ Scanner::              A pure C++ Flex scanner
* Calc++ Top Level::            Conducting the band

Java Parsers

* Java Bison Interface::        Asking for Java parser generation
* Java Semantic Values::        %token and %nterm vs. Java
* Java Location Values::        The position and location classes
* Java Parser Interface::       Instantiating and running the parser
* Java Parser Context Interface:: Circumstances of a syntax error
* Java Scanner Interface::      Specifying the scanner for the parser
* Java Action Features::        Special features for use in actions
* Java Push Parser Interface::  Instantiating and running the a push parser
* Java Differences::            Differences between C/C++ and Java Grammars
* Java Declarations Summary::   List of Bison declarations used with Java

A Brief History of the Greater Ungulates

* Yacc::                        The original Yacc
* yacchack::                    An obscure early implementation of reentrancy
* Byacc::                       Berkeley Yacc
* Bison::                       This program
* Other Ungulates::             Similar programs

Frequently Asked Questions

* Memory Exhausted::            Breaking the Stack Limits
* How Can I Reset the Parser::  @code{yyparse} Keeps some State
* Strings are Destroyed::       @code{yylval} Loses Track of Strings
* Implementing Gotos/Loops::    Control Flow in the Calculator
* Multiple start-symbols::      Factoring closely related grammars
* Enabling Relocatability::     Moving Bison/using it through network shares
* Secure?  Conform?::           Is Bison POSIX safe?
* I can't build Bison::         Troubleshooting
* Where can I find help?::      Troubleshouting
* Bug Reports::                 Troublereporting
* More Languages::              Parsers in C++, Java, and so on
* Beta Testing::                Experimenting development versions
* Mailing Lists::               Meeting other Bison users

Copying This Manual

* GNU Free Documentation License:: Copying and sharing this manual

@end detailmenu
@end menu

@node Introduction
@unnumbered Introduction
@cindex introduction

@dfn{Bison} is a general-purpose parser generator that converts an annotated
context-free grammar into a deterministic LR or generalized LR (GLR) parser
employing LALR(1), IELR(1) or canonical LR(1) parser tables.  Once you are
proficient with Bison, you can use it to develop a wide range of language
parsers, from those used in simple desk calculators to complex programming
languages.

Bison is upward compatible with Yacc: all properly-written Yacc grammars
ought to work with Bison with no change.  Anyone familiar with Yacc should
be able to use Bison with little trouble.  You need to be fluent in C, C++
or Java programming in order to use Bison or to understand this manual.

We begin with tutorial chapters that explain the basic concepts of
using Bison and show three explained examples, each building on the
last.  If you don't know Bison or Yacc, start by reading these
chapters.  Reference chapters follow, which describe specific aspects
of Bison in detail.

Bison was written originally by Robert Corbett.  Richard Stallman made
it Yacc-compatible.  Wilfred Hansen of Carnegie Mellon University
added multi-character string literals and other features.  Since then,
Bison has grown more robust and evolved many other new features thanks
to the hard work of a long list of volunteers.  For details, see the
@file{THANKS} and @file{ChangeLog} files included in the Bison
distribution.

This edition corresponds to version @value{VERSION} of Bison.

@node Conditions
@unnumbered Conditions for Using Bison

The distribution terms for Bison-generated parsers permit using the parsers
in nonfree programs.  Before Bison version 2.2, these extra permissions
applied only when Bison was generating LALR(1) parsers in C@.  And before
Bison version 1.24, Bison-generated parsers could be used only in programs
that were free software.

The other GNU programming tools, such as the GNU C compiler, have never had
such a requirement.  They could always be used for nonfree software.  The
reason Bison was different was not due to a special policy decision; it
resulted from applying the usual General Public License to all of the Bison
source code.

The main output of the Bison utility---the Bison parser implementation
file---contains a verbatim copy of a sizable piece of Bison, which is the
code for the parser's implementation.  (The actions from your grammar are
inserted into this implementation at one point, but most of the rest of the
implementation is not changed.)  When we applied the GPL terms to the
skeleton code for the parser's implementation, the effect was to restrict
the use of Bison output to free software.

We didn't change the terms because of sympathy for people who want to make
software proprietary.  @strong{Software should be free.}  But we concluded
that limiting Bison's use to free software was doing little to encourage
people to make other software free.  So we decided to make the practical
conditions for using Bison match the practical conditions for using the
other GNU tools.

This exception applies when Bison is generating code for a parser.  You can
tell whether the exception applies to a Bison output file by inspecting the
file for text beginning with ``As a special exception@dots{}''.  The text
spells out the exact terms of the exception.

@node Copying
@unnumbered GNU GENERAL PUBLIC LICENSE
@include gpl-3.0.texi

@node Concepts
@chapter The Concepts of Bison

This chapter introduces many of the basic concepts without which the details
of Bison will not make sense.  If you do not already know how to use Bison
or Yacc, we suggest you start by reading this chapter carefully.

@menu
* Language and Grammar:: Languages and context-free grammars,
                           as mathematical ideas.
* Grammar in Bison::     How we represent grammars for Bison's sake.
* Semantic Values::      Each token or syntactic grouping can have
                           a semantic value (the value of an integer,
                           the name of an identifier, etc.).
* Semantic Actions::     Each rule can have an action containing C code.
* GLR Parsers::          Writing parsers for general context-free languages.
* Locations::            Overview of location tracking.
* Bison Parser::         What are Bison's input and output,
                           how is the output used?
* Stages::               Stages in writing and running Bison grammars.
* Grammar Layout::       Overall structure of a Bison grammar file.
@end menu

@node Language and Grammar
@section Languages and Context-Free Grammars

@cindex context-free grammar
@cindex grammar, context-free
In order for Bison to parse a language, it must be described by a
@dfn{context-free grammar}.  This means that you specify one or more
@dfn{syntactic groupings} and give rules for constructing them from their
parts.  For example, in the C language, one kind of grouping is called an
`expression'.  One rule for making an expression might be, ``An expression
can be made of a minus sign and another expression''.  Another would be,
``An expression can be an integer''.  As you can see, rules are often
recursive, but there must be at least one rule which leads out of the
recursion.

@cindex BNF
@cindex Backus-Naur form
The most common formal system for presenting such rules for humans to read
is @dfn{Backus-Naur Form} or ``BNF'', which was developed in
order to specify the language Algol 60.  Any grammar expressed in
BNF is a context-free grammar.  The input to Bison is
essentially machine-readable BNF.

@cindex LALR grammars
@cindex IELR grammars
@cindex LR grammars
There are various important subclasses of context-free grammars.  Although
it can handle almost all context-free grammars, Bison is optimized for what
are called LR(1) grammars.  In brief, in these grammars, it must be possible
to tell how to parse any portion of an input string with just a single token
of lookahead.  For historical reasons, Bison by default is limited by the
additional restrictions of LALR(1), which is hard to explain simply.
@xref{Mysterious Conflicts}, for more information on this.  You can escape
these additional restrictions by requesting IELR(1) or canonical LR(1)
parser tables.  @xref{LR Table Construction}, to learn how.

@cindex GLR parsing
@cindex generalized LR (GLR) parsing
@cindex ambiguous grammars
@cindex nondeterministic parsing

Parsers for LR(1) grammars are @dfn{deterministic}, meaning
roughly that the next grammar rule to apply at any point in the input is
uniquely determined by the preceding input and a fixed, finite portion
(called a @dfn{lookahead}) of the remaining input.  A context-free
grammar can be @dfn{ambiguous}, meaning that there are multiple ways to
apply the grammar rules to get the same inputs.  Even unambiguous
grammars can be @dfn{nondeterministic}, meaning that no fixed
lookahead always suffices to determine the next grammar rule to apply.
With the proper declarations, Bison is also able to parse these more
general context-free grammars, using a technique known as GLR
parsing (for Generalized LR).  Bison's GLR parsers
are able to handle any context-free grammar for which the number of
possible parses of any given string is finite.

@cindex symbols (abstract)
@cindex token
@cindex syntactic grouping
@cindex grouping, syntactic
In the formal grammatical rules for a language, each kind of syntactic unit
or grouping is named by a @dfn{symbol}.  Those which are built by grouping
smaller constructs according to grammatical rules are called
@dfn{nonterminal symbols}; those which can't be subdivided are called
@dfn{terminal symbols} or @dfn{token kinds}.  We call a piece of input
corresponding to a single terminal symbol a @dfn{token}, and a piece
corresponding to a single nonterminal symbol a @dfn{grouping}.

We can use the C language as an example of what symbols, terminal and
nonterminal, mean.  The tokens of C are identifiers, constants (numeric
and string), and the various keywords, arithmetic operators and
punctuation marks.  So the terminal symbols of a grammar for C include
`identifier', `number', `string', plus one symbol for each keyword,
operator or punctuation mark: `if', `return', `const', `static', `int',
`char', `plus-sign', `open-brace', `close-brace', `comma' and many more.
(These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)

Here is a simple C function subdivided into tokens:

@example
int             /* @r{keyword `int'} */
square (int x)  /* @r{identifier, open-paren, keyword `int',}
                   @r{identifier, close-paren} */
@{               /* @r{open-brace} */
  return x * x; /* @r{keyword `return', identifier, asterisk,}
                   @r{identifier, semicolon} */
@}               /* @r{close-brace} */
@end example

The syntactic groupings of C include the expression, the statement, the
declaration, and the function definition.  These are represented in the
grammar of C by nonterminal symbols `expression', `statement',
`declaration' and `function definition'.  The full grammar uses dozens of
additional language constructs, each with its own nonterminal symbol, in
order to express the meanings of these four.  The example above is a
function definition; it contains one declaration, and one statement.  In
the statement, each @samp{x} is an expression and so is @samp{x * x}.

Each nonterminal symbol must have grammatical rules showing how it is made
out of simpler constructs.  For example, one kind of C statement is the
@code{return} statement; this would be described with a grammar rule which
reads informally as follows:

@quotation
A `statement' can be made of a `return' keyword, an `expression' and a
`semicolon'.
@end quotation

@noindent
There would be many other rules for `statement', one for each kind of
statement in C.

@cindex start symbol
One nonterminal symbol must be distinguished as the special one which
defines a complete utterance in the language.  It is called the @dfn{start
symbol}.  In a compiler, this means a complete input program.  In the C
language, the nonterminal symbol `sequence of definitions and declarations'
plays this role.

For example, @samp{1 + 2} is a valid C expression---a valid part of a C
program---but it is not valid as an @emph{entire} C program.  In the
context-free grammar of C, this follows from the fact that `expression' is
not the start symbol.

The Bison parser reads a sequence of tokens as its input, and groups the
tokens using the grammar rules.  If the input is valid, the end result is
that the entire token sequence reduces to a single grouping whose symbol is
the grammar's start symbol.  If we use a grammar for C, the entire input
must be a `sequence of definitions and declarations'.  If not, the parser
reports a syntax error.

@node Grammar in Bison
@section From Formal Rules to Bison Input
@cindex Bison grammar
@cindex grammar, Bison
@cindex formal grammar

A formal grammar is a mathematical construct.  To define the language
for Bison, you must write a file expressing the grammar in Bison syntax:
a @dfn{Bison grammar} file.  @xref{Grammar File}.

A nonterminal symbol in the formal grammar is represented in Bison input
as an identifier, like an identifier in C@.  By convention, it should be
in lower case, such as @code{expr}, @code{stmt} or @code{declaration}.

The Bison representation for a terminal symbol is also called a @dfn{token
kind}.  Token kinds as well can be represented as C-like identifiers.  By
convention, these identifiers should be upper case to distinguish them from
nonterminals: for example, @code{INTEGER}, @code{IDENTIFIER}, @code{IF} or
@code{RETURN}.  A terminal symbol that stands for a particular keyword in
the language should be named after that keyword converted to upper case.
The terminal symbol @code{error} is reserved for error recovery.
@xref{Symbols}.

A terminal symbol can also be represented as a character literal, just like
a C character constant.  You should do this whenever a token is just a
single character (parenthesis, plus-sign, etc.): use that same character in
a literal as the terminal symbol for that token.

A third way to represent a terminal symbol is with a C string constant
containing several characters.  @xref{Symbols}, for more information.

The grammar rules also have an expression in Bison syntax.  For example,
here is the Bison rule for a C @code{return} statement.  The semicolon in
quotes is a literal character token, representing part of the C syntax for
the statement; the naked semicolon, and the colon, are Bison punctuation
used in every rule.

@example
stmt: RETURN expr ';' ;
@end example

@noindent
@xref{Rules}.

@node Semantic Values
@section Semantic Values
@cindex semantic value
@cindex value, semantic

A formal grammar selects tokens only by their classifications: for example,
if a rule mentions the terminal symbol `integer constant', it means that
@emph{any} integer constant is grammatically valid in that position.  The
precise value of the constant is irrelevant to how to parse the input: if
@samp{x+4} is grammatical then @samp{x+1} or @samp{x+3989} is equally
grammatical.

But the precise value is very important for what the input means once it is
parsed.  A compiler is useless if it fails to distinguish between 4, 1 and
3989 as constants in the program!  Therefore, each token in a Bison grammar
has both a token kind and a @dfn{semantic value}.  @xref{Semantics}, for
details.

The token kind is a terminal symbol defined in the grammar, such as
@code{INTEGER}, @code{IDENTIFIER} or @code{','}.  It tells everything you
need to know to decide where the token may validly appear and how to group
it with other tokens.  The grammar rules know nothing about tokens except
their kinds.

The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
identifier.  (A token such as @code{','} which is just punctuation doesn't
need to have any semantic value.)

For example, an input token might be classified as token kind @code{INTEGER}
and have the semantic value 4.  Another input token might have the same
token kind @code{INTEGER} but value 3989.  When a grammar rule says that
@code{INTEGER} is allowed, either of these tokens is acceptable because each
is an @code{INTEGER}.  When the parser accepts the token, it keeps track of
the token's semantic value.

Each grouping can also have a semantic value as well as its nonterminal
symbol.  For example, in a calculator, an expression typically has a
semantic value that is a number.  In a compiler for a programming
language, an expression typically has a semantic value that is a tree
structure describing the meaning of the expression.

@node Semantic Actions
@section Semantic Actions
@cindex semantic actions
@cindex actions, semantic

In order to be useful, a program must do more than parse input; it must
also produce some output based on the input.  In a Bison grammar, a grammar
rule can have an @dfn{action} made up of C statements.  Each time the
parser recognizes a match for that rule, the action is executed.
@xref{Actions}.

Most of the time, the purpose of an action is to compute the semantic value
of the whole construct from the semantic values of its parts.  For example,
suppose we have a rule which says an expression can be the sum of two
expressions.  When the parser recognizes such a sum, each of the
subexpressions has a semantic value which describes how it was built up.
The action for this rule should create a similar sort of value for the
newly recognized larger expression.

For example, here is a rule that says an expression can be the sum of
two subexpressions:

@example
expr: expr '+' expr   @{ $$ = $1 + $3; @} ;
@end example

@noindent
The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.

@node GLR Parsers
@section Writing GLR Parsers
@cindex GLR parsing
@cindex generalized LR (GLR) parsing
@findex %glr-parser
@cindex conflicts
@cindex shift/reduce conflicts
@cindex reduce/reduce conflicts

In some grammars, Bison's deterministic
LR(1) parsing algorithm cannot decide whether to apply a
certain grammar rule at a given point.  That is, it may not be able to
decide (on the basis of the input read so far) which of two possible
reductions (applications of a grammar rule) applies, or whether to apply
a reduction or read more of the input and apply a reduction later in the
input.  These are known respectively as @dfn{reduce/reduce} conflicts
(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts
(@pxref{Shift/Reduce}).

To use a grammar that is not easily modified to be LR(1), a more general
parsing algorithm is sometimes necessary.  If you include @code{%glr-parser}
among the Bison declarations in your file (@pxref{Grammar Outline}), the
result is a Generalized LR (GLR) parser.  These parsers handle Bison
grammars that contain no unresolved conflicts (i.e., after applying
precedence declarations) identically to deterministic parsers.  However,
when faced with unresolved shift/reduce and reduce/reduce conflicts, GLR
parsers use the simple expedient of doing both, effectively cloning the
parser to follow both possibilities.  Each of the resulting parsers can
again split, so that at any given time, there can be any number of possible
parses being explored.  The parsers proceed in lockstep; that is, all of
them consume (shift) a given input symbol before any of them proceed to the
next.  Each of the cloned parsers eventually meets one of two possible
fates: either it runs into a parsing error, in which case it simply
vanishes, or it merges with another parser, because the two of them have
reduced the input to an identical set of symbols.

During the time that there are multiple parsers, semantic actions are
recorded, but not performed.  When a parser disappears, its recorded
semantic actions disappear as well, and are never performed.  When a
reduction makes two parsers identical, causing them to merge, Bison records
both sets of semantic actions.  Whenever the last two parsers merge,
reverting to the single-parser case, Bison resolves all the outstanding
actions either by precedences given to the grammar rules involved, or by
performing both actions, and then calling a designated user-defined function
on the resulting values to produce an arbitrary merged result.

@menu
* Simple GLR Parsers::     Using GLR parsers on unambiguous grammars.
* Merging GLR Parses::     Using GLR parsers to resolve ambiguities.
* GLR Semantic Actions::   Considerations for semantic values and deferred actions.
* Semantic Predicates::    Controlling a parse with arbitrary computations.
* Compiler Requirements for GLR::  GLR parsers require a modern C compiler.
@end menu

@node Simple GLR Parsers
@subsection Using GLR on Unambiguous Grammars
@cindex GLR parsing, unambiguous grammars
@cindex generalized LR (GLR) parsing, unambiguous grammars
@findex %glr-parser
@findex %expect-rr
@cindex conflicts
@cindex reduce/reduce conflicts
@cindex shift/reduce conflicts

In the simplest cases, you can use the GLR algorithm
to parse grammars that are unambiguous but fail to be LR(1).
Such grammars typically require more than one symbol of lookahead.

Consider a problem that
arises in the declaration of enumerated and subrange types in the
programming language Pascal.  Here are some examples:

@example
type subrange = lo .. hi;
type enum = (a, b, c);
@end example

@noindent
The original language standard allows only numeric literals and constant
identifiers for the subrange bounds (@samp{lo} and @samp{hi}), but Extended
Pascal (ISO/IEC 10206) and many other Pascal implementations allow arbitrary
expressions there.  This gives rise to the following situation, containing a
superfluous pair of parentheses:

@example
type subrange = (a) .. b;
@end example

@noindent
Compare this to the following declaration of an enumerated
type with only one value:

@example
type enum = (a);
@end example

@noindent
(These declarations are contrived, but they are syntactically valid, and
more-complicated cases can come up in practical programs.)

These two declarations look identical until the @samp{..} token.  With
normal LR(1) one-token lookahead it is not possible to decide between the
two forms when the identifier @samp{a} is parsed.  It is, however, desirable
for a parser to decide this, since in the latter case @samp{a} must become a
new identifier to represent the enumeration value, while in the former case
@samp{a} must be evaluated with its current meaning, which may be a constant
or even a function call.

You could parse @samp{(a)} as an ``unspecified identifier in parentheses'',
to be resolved later, but this typically requires substantial contortions in
both semantic actions and large parts of the grammar, where the parentheses
are nested in the recursive rules for expressions.

You might think of using the lexer to distinguish between the two forms by
returning different tokens for currently defined and undefined identifiers.
But if these declarations occur in a local scope, and @samp{a} is defined in
an outer scope, then both forms are possible---either locally redefining
@samp{a}, or using the value of @samp{a} from the outer scope.  So this
approach cannot work.

A simple solution to this problem is to declare the parser to use the GLR
algorithm.  When the GLR parser reaches the critical state, it merely splits
into two branches and pursues both syntax rules simultaneously.  Sooner or
later, one of them runs into a parsing error.  If there is a @samp{..} token
before the next @samp{;}, the rule for enumerated types fails since it
cannot accept @samp{..} anywhere; otherwise, the subrange type rule fails
since it requires a @samp{..} token.  So one of the branches fails silently,
and the other one continues normally, performing all the intermediate
actions that were postponed during the split.

If the input is syntactically incorrect, both branches fail and the parser
reports a syntax error as usual.

The effect of all this is that the parser seems to ``guess'' the correct
branch to take, or in other words, it seems to use more lookahead than the
underlying LR(1) algorithm actually allows for.  In this example, LR(2)
would suffice, but also some cases that are not LR(@math{k}) for any
@math{k} can be handled this way.

In general, a GLR parser can take quadratic or cubic worst-case time, and
the current Bison parser even takes exponential time and space for some
grammars.  In practice, this rarely happens, and for many grammars it is
possible to prove that it cannot happen.  The present example contains only
one conflict between two rules, and the type-declaration context containing
the conflict cannot be nested.  So the number of branches that can exist at
any time is limited by the constant 2, and the parsing time is still linear.

Here is a Bison grammar corresponding to the example above.  It
parses a vastly simplified form of Pascal type declarations.

@example
%token TYPE DOTDOT ID

@group
%left '+' '-'
%left '*' '/'
@end group

%%
type_decl: TYPE ID '=' type ';' ;

@group
type:
  '(' id_list ')'
| expr DOTDOT expr
;
@end group

@group
id_list:
  ID
| id_list ',' ID
;
@end group

@group
expr:
  '(' expr ')'
| expr '+' expr
| expr '-' expr
| expr '*' expr
| expr '/' expr
| ID
;
@end group
@end example

When used as a normal LR(1) grammar, Bison correctly complains
about one reduce/reduce conflict.  In the conflicting situation the
parser chooses one of the alternatives, arbitrarily the one
declared first.  Therefore the following correct input is not
recognized:

@example
type t = (a) .. b;
@end example

The parser can be turned into a GLR parser, while also telling Bison
to be silent about the one known reduce/reduce conflict, by adding
these two declarations to the Bison grammar file (before the first
@samp{%%}):

@example
%glr-parser
%expect-rr 1
@end example

@noindent
No change in the grammar itself is required.  Now the parser recognizes all
valid declarations, according to the limited syntax above, transparently.
In fact, the user does not even notice when the parser splits.

So here we have a case where we can use the benefits of GLR, almost without
disadvantages.  Even in simple cases like this, however, there are at least
two potential problems to beware.  First, always analyze the conflicts
reported by Bison to make sure that GLR splitting is only done where it is
intended.  A GLR parser splitting inadvertently may cause problems less
obvious than an LR parser statically choosing the wrong alternative in a
conflict.  Second, consider interactions with the lexer (@pxref{Semantic
Tokens}) with great care.  Since a split parser consumes tokens without
performing any actions during the split, the lexer cannot obtain information
via parser actions.  Some cases of lexer interactions can be eliminated by
using GLR to shift the complications from the lexer to the parser.  You must
check the remaining cases for correctness.

In our example, it would be safe for the lexer to return tokens based on
their current meanings in some symbol table, because no new symbols are
defined in the middle of a type declaration.  Though it is possible for a
parser to define the enumeration constants as they are parsed, before the
type declaration is completed, it actually makes no difference since they
cannot be used within the same enumerated type declaration.

@node Merging GLR Parses
@subsection Using GLR to Resolve Ambiguities
@cindex GLR parsing, ambiguous grammars
@cindex generalized LR (GLR) parsing, ambiguous grammars
@findex %dprec
@findex %merge
@cindex conflicts
@cindex reduce/reduce conflicts

Let's consider an example, vastly simplified from a C++ grammar.

@example
%@{
  #include <stdio.h>
  #define YYSTYPE char const *
  int yylex (void);
  void yyerror (char const *);
%@}

%token TYPENAME ID

%right '='
%left '+'

%glr-parser

%%

prog:
  %empty
| prog stmt   @{ printf ("\n"); @}
;

stmt:
  expr ';'  %dprec 1
| decl      %dprec 2
;

expr:
  ID               @{ printf ("%s ", $$); @}
| TYPENAME '(' expr ')'
                   @{ printf ("%s <cast> ", $1); @}
| expr '+' expr    @{ printf ("+ "); @}
| expr '=' expr    @{ printf ("= "); @}
;

decl:
  TYPENAME declarator ';'
                   @{ printf ("%s <declare> ", $1); @}
| TYPENAME declarator '=' expr ';'
                   @{ printf ("%s <init-declare> ", $1); @}
;

declarator:
  ID               @{ printf ("\"%s\" ", $1); @}
| '(' declarator ')'
;
@end example

@noindent
This models a problematic part of the C++ grammar---the ambiguity between
certain declarations and statements.  For example,

@example
T (x) = y+z;
@end example

@noindent
parses as either an @code{expr} or a @code{stmt}
(assuming that @samp{T} is recognized as a @code{TYPENAME} and
@samp{x} as an @code{ID}).
Bison detects this as a reduce/reduce conflict between the rules
@code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the
time it encounters @code{x} in the example above.  Since this is a
GLR parser, it therefore splits the problem into two parses, one for
each choice of resolving the reduce/reduce conflict.
Unlike the example from the previous section (@pxref{Simple GLR Parsers}),
however, neither of these parses ``dies,'' because the grammar as it stands is
ambiguous.  One of the parsers eventually reduces @code{stmt : expr ';'} and
the other reduces @code{stmt : decl}, after which both parsers are in an
identical state: they've seen @samp{prog stmt} and have the same unprocessed
input remaining.  We say that these parses have @dfn{merged.}

At this point, the GLR parser requires a specification in the
grammar of how to choose between the competing parses.
In the example above, the two @code{%dprec}
declarations specify that Bison is to give precedence
to the parse that interprets the example as a
@code{decl}, which implies that @code{x} is a declarator.
The parser therefore prints

@example
"x" y z + T <init-declare>
@end example

The @code{%dprec} declarations only come into play when more than one
parse survives.  Consider a different input string for this parser:

@example
T (x) + y;
@end example

@noindent
This is another example of using GLR to parse an unambiguous
construct, as shown in the previous section (@pxref{Simple GLR Parsers}).
Here, there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters @code{x}, it does not
have enough information to resolve the reduce/reduce conflict (again,
between @code{x} as an @code{expr} or a @code{declarator}).  In this
case, no precedence declaration is used.  Again, the parser splits
into two, one assuming that @code{x} is an @code{expr}, and the other
assuming @code{x} is a @code{declarator}.  The second of these parsers
then vanishes when it sees @code{+}, and the parser prints

@example
x T <cast> y +
@end example

Suppose that instead of resolving the ambiguity, you wanted to see all
the possibilities.  For this purpose, you must merge the semantic
actions of the two possible parsers, rather than choosing one over the
other.  To do so, you could change the declaration of @code{stmt} as
follows:

@example
stmt:
  expr ';'  %merge <stmtMerge>
| decl      %merge <stmtMerge>
;
@end example

@noindent
and define the @code{stmtMerge} function as:

@example
static YYSTYPE
stmtMerge (YYSTYPE x0, YYSTYPE x1)
@{
  printf ("<OR> ");
  return "";
@}
@end example

@noindent
with an accompanying forward declaration
in the C declarations at the beginning of the file:

@example
%@{
  #define YYSTYPE char const *
  static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
%@}
@end example

@noindent
With these declarations, the resulting parser parses the first example
as both an @code{expr} and a @code{decl}, and prints

@example
"x" y z + T <init-declare> x T <cast> y z + = <OR>
@end example

Bison requires that all of the
productions that participate in any particular merge have identical
@samp{%merge} clauses.  Otherwise, the ambiguity would be unresolvable,
and the parser will report an error during any parse that results in
the offending merge.

@node GLR Semantic Actions
@subsection GLR Semantic Actions

The nature of GLR parsing and the structure of the generated
parsers give rise to certain restrictions on semantic values and actions.

@subsubsection Deferred semantic actions
@cindex deferred semantic actions
By definition, a deferred semantic action is not performed at the same time as
the associated reduction.
This raises caveats for several Bison features you might use in a semantic
action in a GLR parser.

@vindex yychar
@cindex GLR parsers and @code{yychar}
@vindex yylval
@cindex GLR parsers and @code{yylval}
@vindex yylloc
@cindex GLR parsers and @code{yylloc}
In any semantic action, you can examine @code{yychar} to determine the kind
of the lookahead token present at the time of the associated reduction.
After checking that @code{yychar} is not set to @code{YYEMPTY} or
@code{YYEOF}, you can then examine @code{yylval} and @code{yylloc} to
determine the lookahead token's semantic value and location, if any.  In a
nondeferred semantic action, you can also modify any of these variables to
influence syntax analysis.  @xref{Lookahead}.

@findex yyclearin
@cindex GLR parsers and @code{yyclearin}
In a deferred semantic action, it's too late to influence syntax analysis.
In this case, @code{yychar}, @code{yylval}, and @code{yylloc} are set to
shallow copies of the values they had at the time of the associated reduction.
For this reason alone, modifying them is dangerous.
Moreover, the result of modifying them is undefined and subject to change with
future versions of Bison.
For example, if a semantic action might be deferred, you should never write it
to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free
memory referenced by @code{yylval}.

@subsubsection YYERROR
@findex YYERROR
@cindex GLR parsers and @code{YYERROR}
Another Bison feature requiring special consideration is @code{YYERROR}
(@pxref{Action Features}), which you can invoke in a semantic action to
initiate error recovery.
During deterministic GLR operation, the effect of @code{YYERROR} is
the same as its effect in a deterministic parser.
The effect in a deferred action is similar, but the precise point of the
error is undefined;  instead, the parser reverts to deterministic operation,
selecting an unspecified stack on which to continue with a syntax error.
In a semantic predicate (see @ref{Semantic Predicates}) during nondeterministic
parsing, @code{YYERROR} silently prunes
the parse that invoked the test.

@subsubsection Restrictions on semantic values and locations
GLR parsers require that you use POD (Plain Old Data) types for
semantic values and location types when using the generated parsers as
C++ code.

@node Semantic Predicates
@subsection Controlling a Parse with Arbitrary Predicates
@findex %?
@cindex Semantic predicates in GLR parsers

In addition to the @code{%dprec} and @code{%merge} directives,
GLR parsers
allow you to reject parses on the basis of arbitrary computations executed
in user code, without having Bison treat this rejection as an error
if there are alternative parses.  For example,

@example
widget:
  %?@{  new_syntax @} "widget" id new_args  @{ $$ = f($3, $4); @}
| %?@{ !new_syntax @} "widget" id old_args  @{ $$ = f($3, $4); @}
;
@end example

@noindent
is one way to allow the same parser to handle two different syntaxes for
widgets.  The clause preceded by @code{%?} is treated like an ordinary
action, except that its text is treated as an expression and is always
evaluated immediately (even when in nondeterministic mode).  If the
expression yields 0 (false), the clause is treated as a syntax error,
which, in a nondeterministic parser, causes the stack in which it is reduced
to die.  In a deterministic parser, it acts like YYERROR.

As the example shows, predicates otherwise look like semantic actions, and
therefore you must be take them into account when determining the numbers
to use for denoting the semantic values of right-hand side symbols.
Predicate actions, however, have no defined value, and may not be given
labels.

There is a subtle difference between semantic predicates and ordinary
actions in nondeterministic mode, since the latter are deferred.
For example, we could try to rewrite the previous example as

@example
widget:
  @{ if (!new_syntax) YYERROR; @}
    "widget" id new_args  @{ $$ = f($3, $4); @}
|  @{ if (new_syntax) YYERROR; @}
    "widget" id old_args   @{ $$ = f($3, $4); @}
;
@end example

@noindent
(reversing the sense of the predicate tests to cause an error when they are
false).  However, this
does @emph{not} have the same effect if @code{new_args} and @code{old_args}
have overlapping syntax.
Since the midrule actions testing @code{new_syntax} are deferred,
a GLR parser first encounters the unresolved ambiguous reduction
for cases where @code{new_args} and @code{old_args} recognize the same string
@emph{before} performing the tests of @code{new_syntax}.  It therefore
reports an error.

Finally, be careful in writing predicates: deferred actions have not been
evaluated, so that using them in a predicate will have undefined effects.

@node Compiler Requirements for GLR
@subsection Considerations when Compiling GLR Parsers
@cindex @code{inline}
@cindex GLR parsers and @code{inline}

The GLR parsers require a compiler for ISO C89 or
later.  In addition, they use the @code{inline} keyword, which is not
C89, but is C99 and is a common extension in pre-C99 compilers.  It is
up to the user of these parsers to handle
portability issues.  For instance, if using Autoconf and the Autoconf
macro @code{AC_C_INLINE}, a mere

@example
%@{
  #include <config.h>
%@}
@end example

@noindent
will suffice.  Otherwise, we suggest

@example
%@{
  #if (__STDC_VERSION__ < 199901 && ! defined __GNUC__ \
       && ! defined inline)
  # define inline
  #endif
%@}
@end example

@node Locations
@section Locations
@cindex location
@cindex textual location
@cindex location, textual

Many applications, like interpreters or compilers, have to produce verbose
and useful error messages.  To achieve this, one must be able to keep track of
the @dfn{textual location}, or @dfn{location}, of each syntactic construct.
Bison provides a mechanism for handling these locations.

Each token has a semantic value.  In a similar fashion, each token has an
associated location, but the type of locations is the same for all tokens
and groupings.  Moreover, the output parser is equipped with a default data
structure for storing locations (@pxref{Tracking Locations}, for more
details).

Like semantic values, locations can be reached in actions using a dedicated
set of constructs.  In the example above, the location of the whole grouping
is @code{@@$}, while the locations of the subexpressions are @code{@@1} and
@code{@@3}.

When a rule is matched, a default action is used to compute the semantic value
of its left hand side (@pxref{Actions}).  In the same way, another default
action is used for locations.  However, the action for locations is general
enough for most cases, meaning there is usually no need to describe for each
rule how @code{@@$} should be formed.  When building a new location for a given
grouping, the default behavior of the output parser is to take the beginning
of the first symbol, and the end of the last symbol.

@node Bison Parser
@section Bison Output: the Parser Implementation File
@cindex Bison parser
@cindex Bison utility
@cindex lexical analyzer, purpose
@cindex parser

When you run Bison, you give it a Bison grammar file as input.  The
most important output is a C source file that implements a parser for
the language described by the grammar.  This parser is called a
@dfn{Bison parser}, and this file is called a @dfn{Bison parser
implementation file}.  Keep in mind that the Bison utility and the
Bison parser are two distinct programs: the Bison utility is a program
whose output is the Bison parser implementation file that becomes part
of your program.

The job of the Bison parser is to group tokens into groupings according to
the grammar rules---for example, to build identifiers and operators into
expressions.  As it does this, it runs the actions for the grammar rules it
uses.

The tokens come from a function called the @dfn{lexical analyzer} that
you must supply in some fashion (such as by writing it in C).  The Bison
parser calls the lexical analyzer each time it wants a new token.  It
doesn't know what is ``inside'' the tokens (though their semantic values
may reflect this).  Typically the lexical analyzer makes the tokens by
parsing characters of text, but Bison does not depend on this.
@xref{Lexical}.

The Bison parser implementation file is C code which defines a
function named @code{yyparse} which implements that grammar.  This
function does not make a complete C program: you must supply some
additional functions.  One is the lexical analyzer.  Another is an
error-reporting function which the parser calls to report an error.
In addition, a complete C program must start with a function called
@code{main}; you have to provide this, and arrange for it to call
@code{yyparse} or the parser will never run.  @xref{Interface}.

Aside from the token kind names and the symbols in the actions you
write, all symbols defined in the Bison parser implementation file
itself begin with @samp{yy} or @samp{YY}.  This includes interface
functions such as the lexical analyzer function @code{yylex}, the
error reporting function @code{yyerror} and the parser function
@code{yyparse} itself.  This also includes numerous identifiers used
for internal purposes.  Therefore, you should avoid using C
identifiers starting with @samp{yy} or @samp{YY} in the Bison grammar
file except for the ones defined in this manual.  Also, you should
avoid using the C identifiers @samp{malloc} and @samp{free} for
anything other than their usual meanings.

In some cases the Bison parser implementation file includes system
headers, and in those cases your code should respect the identifiers
reserved by those headers.  On some non-GNU hosts, @code{<limits.h>},
@code{<stddef.h>}, @code{<stdint.h>} (if available), and @code{<stdlib.h>}
are included to declare memory allocators and integer types and constants.
@code{<libintl.h>} is included if message translation is in use
(@pxref{Internationalization}).  Other system headers may be included
if you define @code{YYDEBUG} (@pxref{Tracing}) or
@code{YYSTACK_USE_ALLOCA} (@pxref{Table of Symbols}) to a nonzero value.

@node Stages
@section Stages in Using Bison
@cindex stages in using Bison
@cindex using Bison

The actual language-design process using Bison, from grammar specification
to a working compiler or interpreter, has these parts:

@enumerate
@item
Formally specify the grammar in a form recognized by Bison
(@pxref{Grammar File}).  For each grammatical rule
in the language, describe the action that is to be taken when an
instance of that rule is recognized.  The action is described by a
sequence of C statements.

@item
Write a lexical analyzer to process input and pass tokens to the parser.
The lexical analyzer may be written by hand in C (@pxref{Lexical}).  It
could also be produced using Lex, but the use of Lex is not discussed in
this manual.

@item
Write a controlling function that calls the Bison-produced parser.

@item
Write error-reporting routines.
@end enumerate

To turn this source code as written into a runnable program, you
must follow these steps:

@enumerate
@item
Run Bison on the grammar to produce the parser.

@item
Compile the code output by Bison, as well as any other source files.

@item
Link the object files to produce the finished product.
@end enumerate

@node Grammar Layout
@section The Overall Layout of a Bison Grammar
@cindex grammar file
@cindex file format
@cindex format of grammar file
@cindex layout of Bison grammar

The input file for the Bison utility is a @dfn{Bison grammar file}.  The
general form of a Bison grammar file is as follows:

@example
%@{
@var{Prologue}
%@}

@var{Bison declarations}

%%
@var{Grammar rules}
%%
@var{Epilogue}
@end example

@noindent
The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears
in every Bison grammar file to separate the sections.

The prologue may define types and variables used in the actions.  You can
also use preprocessor commands to define macros used there, and use
@code{#include} to include header files that do any of these things.
You need to declare the lexical analyzer @code{yylex} and the error
printer @code{yyerror} here, along with any other global identifiers
used by the actions in the grammar rules.

The Bison declarations declare the names of the terminal and nonterminal
symbols, and may also describe operator precedence and the data types of
semantic values of various symbols.

The grammar rules define how to construct each nonterminal symbol from its
parts.

The epilogue can contain any code you want to use.  Often the
definitions of functions declared in the prologue go here.  In a
simple program, all the rest of the program can go here.

@node Examples
@chapter Examples
@cindex simple examples
@cindex examples, simple

Now we show and explain several sample programs written using Bison: a
Reverse Polish Notation calculator, an algebraic (infix) notation
calculator --- later extended to track ``locations'' ---
and a multi-function calculator.  All
produce usable, though limited, interactive desk-top calculators.

These examples are simple, but Bison grammars for real programming
languages are written the same way.  You can copy these examples into a
source file to try them.

@menu
* RPN Calc::               Reverse Polish Notation Calculator;
                             a first example with no operator precedence.
* Infix Calc::             Infix (algebraic) notation calculator.
                             Operator precedence is introduced.
* Simple Error Recovery::  Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
* Multi-function Calc::    Calculator with memory and trig functions.
                             It uses multiple data-types for semantic values.
* Exercises::              Ideas for improving the multi-function calculator.
@end menu

@node RPN Calc
@section Reverse Polish Notation Calculator
@cindex Reverse Polish Notation
@cindex @code{rpcalc}
@cindex calculator, simple

The first example is that of a simple double-precision @dfn{Reverse Polish
Notation} calculator (a calculator using postfix operators).  This example
provides a good starting point, since operator precedence is not an issue.
The second example will illustrate how operator precedence is handled.

The source code for this calculator is named @file{rpcalc.y}.  The
@samp{.y} extension is a convention used for Bison grammar files.

@menu
* Rpcalc Declarations::    Prologue (declarations) for rpcalc.
* Rpcalc Rules::           Grammar Rules for rpcalc, with explanation.
* Rpcalc Lexer::           The lexical analyzer.
* Rpcalc Main::            The controlling function.
* Rpcalc Error::           The error reporting function.
* Rpcalc Generate::        Running Bison on the grammar file.
* Rpcalc Compile::         Run the C compiler on the output code.
@end menu

@node Rpcalc Declarations
@subsection Declarations for @code{rpcalc}

Here are the C and Bison declarations for the Reverse Polish Notation
calculator.  As in C, comments are placed between @samp{/*@dots{}*/} or
after @samp{//}.

@comment file: rpcalc.y
@example
/* Reverse Polish Notation calculator. */

@group
%@{
  #include <stdio.h>
  #include <math.h>
  int yylex (void);
  void yyerror (char const *);
%@}
@end group

%define api.value.type @{double@}
%token NUM

%% /* Grammar rules and actions follow. */
@end example

The declarations section (@pxref{Prologue}) contains two
preprocessor directives and two forward declarations.

The @code{#include} directive is used to declare the exponentiation
function @code{pow}.

The forward declarations for @code{yylex} and @code{yyerror} are
needed because the C language requires that functions be declared
before they are used.  These functions will be defined in the
epilogue, but the parser calls them so they must be declared in the
prologue.

The second section, Bison declarations, provides information to Bison about
the tokens and their types (@pxref{Bison Declarations}).

The @code{%define} directive defines the variable @code{api.value.type},
thus specifying the C data type for semantic values of both tokens and
groupings (@pxref{Value Type}).  The Bison
parser will use whatever type @code{api.value.type} is defined as; if you
don't define it, @code{int} is the default.  Because we specify
@samp{@{double@}}, each token and each expression has an associated value,
which is a floating point number.  C code can use @code{YYSTYPE} to refer to
the value @code{api.value.type}.

Each terminal symbol that is not a single-character literal must be
declared.  (Single-character literals normally don't need to be declared.)
In this example, all the arithmetic operators are designated by
single-character literals, so the only terminal symbol that needs to be
declared is @code{NUM}, the token kind for numeric constants.

@node Rpcalc Rules
@subsection Grammar Rules for @code{rpcalc}

Here are the grammar rules for the Reverse Polish Notation calculator.

@comment file: rpcalc.y
@example
@group
input:
  %empty
| input line
;
@end group

@group
line:
  '\n'
| exp '\n'      @{ printf ("%.10g\n", $1); @}
;
@end group

@group
exp:
  NUM
| exp exp '+'   @{ $$ = $1 + $2;      @}
| exp exp '-'   @{ $$ = $1 - $2;      @}
| exp exp '*'   @{ $$ = $1 * $2;      @}
| exp exp '/'   @{ $$ = $1 / $2;      @}
| exp exp '^'   @{ $$ = pow ($1, $2); @}  /* Exponentiation */
| exp 'n'       @{ $$ = -$1;          @}  /* Unary minus   */
;
@end group
%%
@end example

The groupings of the rpcalc ``language'' defined here are the expression
(given the name @code{exp}), the line of input (@code{line}), and the
complete input transcript (@code{input}).  Each of these nonterminal
symbols has several alternate rules, joined by the vertical bar @samp{|}
which is read as ``or''.  The following sections explain what these rules
mean.

The semantics of the language is determined by the actions taken when a
grouping is recognized.  The actions are the C code that appears inside
braces.  @xref{Actions}.

You must specify these actions in C, but Bison provides the means for
passing semantic values between the rules.  In each action, the
pseudo-variable @code{$$} stands for the semantic value for the grouping
that the rule is going to construct.  Assigning a value to @code{$$} is the
main job of most actions.  The semantic values of the components of the
rule are referred to as @code{$1}, @code{$2}, and so on.

@menu
* Rpcalc Input::            Explanation of the @code{input} nonterminal
* Rpcalc Line::             Explanation of the @code{line} nonterminal
* Rpcalc Expr::             Explanation of the @code{expr} nonterminal
@end menu

@node Rpcalc Input
@subsubsection Explanation of @code{input}

Consider the definition of @code{input}:

@example
input:
  %empty
| input line
;
@end example

This definition reads as follows: ``A complete input is either an empty
string, or a complete input followed by an input line''.  Notice that
``complete input'' is defined in terms of itself.  This definition is said
to be @dfn{left recursive} since @code{input} appears always as the
leftmost symbol in the sequence.  @xref{Recursion}.

The first alternative is empty because there are no symbols between the
colon and the first @samp{|}; this means that @code{input} can match an
empty string of input (no tokens).  We write the rules this way because it
is legitimate to type @kbd{Ctrl-d} right after you start the calculator.
It's conventional to put an empty alternative first and to use the
(optional) @code{%empty} directive, or to write the comment @samp{/* empty
*/} in it (@pxref{Empty Rules}).

The second alternate rule (@code{input line}) handles all nontrivial input.
It means, ``After reading any number of lines, read one more line if
possible.''  The left recursion makes this rule into a loop.  Since the
first alternative matches empty input, the loop can be executed zero or
more times.

The parser function @code{yyparse} continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end-of-input.

@node Rpcalc Line
@subsubsection Explanation of @code{line}

Now consider the definition of @code{line}:

@example
line:
  '\n'
| exp '\n'  @{ printf ("%.10g\n", $1); @}
;
@end example

The first alternative is a token which is a newline character; this means
that rpcalc accepts a blank line (and ignores it, since there is no
action).  The second alternative is an expression followed by a newline.
This is the alternative that makes rpcalc useful.  The semantic value of
the @code{exp} grouping is the value of @code{$1} because the @code{exp} in
question is the first symbol in the alternative.  The action prints this
value, which is the result of the computation the user asked for.

This action is unusual because it does not assign a value to @code{$$}.  As
a consequence, the semantic value associated with the @code{line} is
uninitialized (its value will be unpredictable).  This would be a bug if
that value were ever used, but we don't use it: once rpcalc has printed the
value of the user's input line, that value is no longer needed.

@node Rpcalc Expr
@subsubsection Explanation of @code{expr}

The @code{exp} grouping has several rules, one for each kind of expression.
The first rule handles the simplest expressions: those that are just
numbers.  The second handles an addition-expression, which looks like two
expressions followed by a plus-sign.  The third handles subtraction, and so
on.

@example
exp:
  NUM
| exp exp '+'     @{ $$ = $1 + $2;    @}
| exp exp '-'     @{ $$ = $1 - $2;    @}
@dots{}
;
@end example

We have used @samp{|} to join all the rules for @code{exp}, but we could
equally well have written them separately:

@example
exp: NUM;
exp: exp exp '+'  @{ $$ = $1 + $2; @};
exp: exp exp '-'  @{ $$ = $1 - $2; @};
@dots{}
@end example

Most of the rules have actions that compute the value of the expression in
terms of the value of its parts.  For example, in the rule for addition,
@code{$1} refers to the first component @code{exp} and @code{$2} refers to
the second one.  The third component, @code{'+'}, has no meaningful
associated semantic value, but if it had one you could refer to it as
@code{$3}.  The first rule relies on the implicit default action: @samp{@{
$$ = $1; @}}.


When @code{yyparse} recognizes a sum expression using this rule, the sum of
the two subexpressions' values is produced as the value of the entire
expression.  @xref{Actions}.

You don't have to give an action for every rule.  When a rule has no action,
Bison by default copies the value of @code{$1} into @code{$$}.  This is what
happens in the first rule (the one that uses @code{NUM}).

The formatting shown here is the recommended convention, but Bison does not
require it.  You can add or change white space as much as you wish.  For
example, this:

@example
exp: NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
@end example

@noindent
means the same thing as this:

@example
exp:
  NUM
| exp exp '+'    @{ $$ = $1 + $2; @}
| @dots{}
;
@end example

@noindent
The latter, however, is much more readable.

@node Rpcalc Lexer
@subsection The @code{rpcalc} Lexical Analyzer
@cindex writing a lexical analyzer
@cindex lexical analyzer, writing

The lexical analyzer's job is low-level parsing: converting characters
or sequences of characters into tokens.  The Bison parser gets its
tokens by calling the lexical analyzer.  @xref{Lexical}.

Only a simple lexical analyzer is needed for the RPN
calculator.  This
lexical analyzer skips blanks and tabs, then reads in numbers as
@code{double} and returns them as @code{NUM} tokens.  Any other character
that isn't part of a number is a separate token.  Note that the token-code
for such a single-character token is the character itself.

The return value of the lexical analyzer function is a numeric code which
represents a token kind.  The same text used in Bison rules to stand for
this token kind is also a C expression for the numeric code of the kind.
This works in two ways.  If the token kind is a character literal, then its
numeric code is that of the character; you can use the same character
literal in the lexical analyzer to express the number.  If the token kind is
an identifier, that identifier is defined by Bison as a C enum whose
definition is the appropriate code.  In this example, therefore, @code{NUM}
becomes an enum for @code{yylex} to use.

The semantic value of the token (if it has one) is stored into the global
variable @code{yylval}, which is where the Bison parser will look for it.
(The C data type of @code{yylval} is @code{YYSTYPE}, whose value was defined
at the beginning of the grammar via @samp{%define api.value.type
@{double@}}; @pxref{Rpcalc Declarations}.)

A token kind code of zero is returned if the end-of-input is encountered.
(Bison recognizes any nonpositive value as indicating end-of-input.)

Here is the code for the lexical analyzer:

@comment file: rpcalc.y
@example
@group
/* The lexical analyzer returns a double floating point
   number on the stack and the token NUM, or the numeric code
   of the character read if not a number.  It skips all blanks
   and tabs, and returns 0 for end-of-input. */

#include <ctype.h>
@end group

@group
int
yylex (void)
@{
  int c = getchar ();
  /* Skip white space. */
  while (c == ' ' || c == '\t')
    c = getchar ();
@end group
@group
  /* Process numbers. */
  if (c == '.' || isdigit (c))
    @{
      ungetc (c, stdin);
      scanf ("%lf", &yylval);
      return NUM;
    @}
@end group
@group
  /* Return end-of-input. */
  else if (c == EOF)
    return YYEOF;
  /* Return a single char. */
  else
    return c;
@}
@end group
@end example

@node Rpcalc Main
@subsection The Controlling Function
@cindex controlling function
@cindex main function in simple example

In keeping with the spirit of this example, the controlling function is
kept to the bare minimum.  The only requirement is that it call
@code{yyparse} to start the process of parsing.

@comment file: rpcalc.y
@example
@group
int
main (void)
@{
  return yyparse ();
@}
@end group
@end example

@node Rpcalc Error
@subsection The Error Reporting Routine
@cindex error reporting routine

When @code{yyparse} detects a syntax error, it calls the error reporting
function @code{yyerror} to print an error message (usually but not
always @code{"syntax error"}).  It is up to the programmer to supply
@code{yyerror} (@pxref{Interface}), so
here is the definition we will use:

@comment file: rpcalc.y
@example
#include <stdio.h>

@group
/* Called by yyparse on error. */
void
yyerror (char const *s)
@{
  fprintf (stderr, "%s\n", s);
@}
@end group
@end example

After @code{yyerror} returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(@pxref{Error Recovery}).  Otherwise, @code{yyparse} returns nonzero.  We
have not written any error rules in this example, so any invalid input will
cause the calculator program to exit.  This is not clean behavior for a
real calculator, but it is adequate for the first example.

@node Rpcalc Generate
@subsection Running Bison to Make the Parser
@cindex running Bison (introduction)

Before running Bison to produce a parser, we need to decide how to
arrange all the source code in one or more source files.  For such a
simple example, the easiest thing is to put everything in one file,
the grammar file.  The definitions of @code{yylex}, @code{yyerror} and
@code{main} go at the end, in the epilogue of the grammar file
(@pxref{Grammar Layout}).

For a large project, you would probably have several source files, and use
@code{make} to arrange to recompile them.

With all the source in the grammar file, you use the following command
to convert it into a parser implementation file:

@example
$ @kbd{bison @var{file}.y}
@end example

@noindent
In this example, the grammar file is called @file{rpcalc.y} (for
``Reverse Polish @sc{calc}ulator'').  Bison produces a parser
implementation file named @file{@var{file}.tab.c}, removing the
@samp{.y} from the grammar file name.  The parser implementation file
contains the source code for @code{yyparse}.  The additional functions
in the grammar file (@code{yylex}, @code{yyerror} and @code{main}) are
copied verbatim to the parser implementation file.

@node Rpcalc Compile
@subsection Compiling the Parser Implementation File
@cindex compiling the parser

Here is how to compile and run the parser implementation file:

@example
@group
# @r{List files in current directory.}
$ @kbd{ls}
rpcalc.tab.c  rpcalc.y
@end group

@group
# @r{Compile the Bison parser.}
# @r{@samp{-lm} tells compiler to search math library for @code{pow}.}
$ @kbd{cc -lm -o rpcalc rpcalc.tab.c}
@end group

@group
# @r{List files again.}
$ @kbd{ls}
rpcalc  rpcalc.tab.c  rpcalc.y
@end group
@end example

The file @file{rpcalc} now contains the executable code.  Here is an
example session using @code{rpcalc}.

@example
$ @kbd{rpcalc}
@kbd{4 9 +}
@result{} 13
@kbd{3 7 + 3 4 5 *+-}
@result{} -13
@kbd{3 7 + 3 4 5 * + - n}              @r{Note the unary minus, @samp{n}}
@result{} 13
@kbd{5 6 / 4 n +}
@result{} -3.166666667
@kbd{3 4 ^}                            @r{Exponentiation}
@result{} 81
@kbd{^D}                               @r{End-of-file indicator}
$
@end example

@node Infix Calc
@section Infix Notation Calculator: @code{calc}
@cindex infix notation calculator
@cindex @code{calc}
@cindex calculator, infix notation

We now modify rpcalc to handle infix operators instead of postfix.  Infix
notation involves the concept of operator precedence and the need for
parentheses nested to arbitrary depth.  Here is the Bison code for
@file{calc.y}, an infix desk-top calculator.

@example
/* Infix notation calculator. */

@group
%@{
  #include <math.h>
  #include <stdio.h>
  int yylex (void);
  void yyerror (char const *);
%@}
@end group

@group
/* Bison declarations. */
%define api.value.type @{double@}
%token NUM
%left '-' '+'
%left '*' '/'
%precedence NEG   /* negation--unary minus */
%right '^'        /* exponentiation */
@end group

%% /* The grammar follows. */
@group
input:
  %empty
| input line
;
@end group

@group
line:
  '\n'
| exp '\n'  @{ printf ("\t%.10g\n", $1); @}
;
@end group

@group
exp:
  NUM
| exp '+' exp        @{ $$ = $1 + $3;      @}
| exp '-' exp        @{ $$ = $1 - $3;      @}
| exp '*' exp        @{ $$ = $1 * $3;      @}
| exp '/' exp        @{ $$ = $1 / $3;      @}
| '-' exp  %prec NEG @{ $$ = -$2;          @}
| exp '^' exp        @{ $$ = pow ($1, $3); @}
| '(' exp ')'        @{ $$ = $2;           @}
;
@end group
%%
@end example

@noindent
The functions @code{yylex}, @code{yyerror} and @code{main} can be the
same as before.

There are two important new features shown in this code.

In the second section (Bison declarations), @code{%left} declares token
kinds and says they are left-associative operators.  The declarations
@code{%left} and @code{%right} (right associativity) take the place of
@code{%token} which is used to declare a token kind name without
associativity/precedence.  (These tokens are single-character literals,
which ordinarily don't need to be declared.  We declare them here to specify
the associativity/precedence.)

Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence.  Hence, exponentiation
has the highest precedence, unary minus (@code{NEG}) is next, followed
by @samp{*} and @samp{/}, and so on.  Unary minus is not associative,
only precedence matters (@code{%precedence}. @xref{Precedence}.

The other important new feature is the @code{%prec} in the grammar
section for the unary minus operator.  The @code{%prec} simply instructs
Bison that the rule @samp{| '-' exp} has the same precedence as
@code{NEG}---in this case the next-to-highest.  @xref{Contextual
Precedence}.

Here is a sample run of @file{calc.y}:

@need 500
@example
$ @kbd{calc}
@kbd{4 + 4.5 - (34/(8*3+-3))}
6.880952381
@kbd{-56 + 2}
-54
@kbd{3 ^ 2}
9
@end example

@node Simple Error Recovery
@section Simple Error Recovery
@cindex error recovery, simple

Up to this point, this manual has not addressed the issue of @dfn{error
recovery}---how to continue parsing after the parser detects a syntax
error.  All we have handled is error reporting with @code{yyerror}.
Recall that by default @code{yyparse} returns after calling
@code{yyerror}.  This means that an erroneous input line causes the
calculator program to exit.  Now we show how to rectify this deficiency.

The Bison language itself includes the reserved word @code{error}, which
may be included in the grammar rules.  In the example below it has
been added to one of the alternatives for @code{line}:

@example
@group
line:
  '\n'
| exp '\n'   @{ printf ("\t%.10g\n", $1); @}
| error '\n' @{ yyerrok;                  @}
;
@end group
@end example

This addition to the grammar allows for simple error recovery in the
event of a syntax error.  If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for @code{line},
and parsing will continue.  (The @code{yyerror} function is still called
upon to print its message as well.)  The action executes the statement
@code{yyerrok}, a macro defined automatically by Bison; its meaning is
that error recovery is complete (@pxref{Error Recovery}).  Note the
difference between @code{yyerrok} and @code{yyerror}; neither one is a
misprint.

This form of error recovery deals with syntax errors.  There are other
kinds of errors; for example, division by zero, which raises an exception
signal that is normally fatal.  A real calculator program must handle this
signal and use @code{longjmp} to return to @code{main} and resume parsing
input lines; it would also have to discard the rest of the current line of
input.  We won't discuss this issue further because it is not specific to
Bison programs.

@node Location Tracking Calc
@section Location Tracking Calculator: @code{ltcalc}
@cindex location tracking calculator
@cindex @code{ltcalc}
@cindex calculator, location tracking

This example extends the infix notation calculator with location
tracking.  This feature will be used to improve the error messages.  For
the sake of clarity, this example is a simple integer calculator, since
most of the work needed to use locations will be done in the lexical
analyzer.

@menu
* Ltcalc Declarations::    Bison and C declarations for ltcalc.
* Ltcalc Rules::           Grammar rules for ltcalc, with explanations.
* Ltcalc Lexer::           The lexical analyzer.
@end menu

@node Ltcalc Declarations
@subsection Declarations for @code{ltcalc}

The C and Bison declarations for the location tracking calculator are
the same as the declarations for the infix notation calculator.

@example
/* Location tracking calculator. */

%@{
  #include <math.h>
  int yylex (void);
  void yyerror (char const *);
%@}

/* Bison declarations. */
%define api.value.type @{int@}
%token NUM

%left '-' '+'
%left '*' '/'
%precedence NEG
%right '^'

%% /* The grammar follows. */
@end example

@noindent
Note there are no declarations specific to locations.  Defining a data type
for storing locations is not needed: we will use the type provided by
default (@pxref{Location Type}), which is a four member structure with the
following integer fields: @code{first_line}, @code{first_column},
@code{last_line} and @code{last_column}.  By conventions, and in accordance
with the GNU Coding Standards and common practice, the line and column count
both start at 1.

@node Ltcalc Rules
@subsection Grammar Rules for @code{ltcalc}

Whether handling locations or not has no effect on the syntax of your
language.  Therefore, grammar rules for this example will be very close
to those of the previous example: we will only modify them to benefit
from the new information.

Here, we will use locations to report divisions by zero, and locate the
wrong expressions or subexpressions.

@example
@group
input:
  %empty
| input line
;
@end group

@group
line:
  '\n'
| exp '\n' @{ printf ("%d\n", $1); @}
;
@end group

@group
exp:
  NUM
| exp '+' exp   @{ $$ = $1 + $3; @}
| exp '-' exp   @{ $$ = $1 - $3; @}
| exp '*' exp   @{ $$ = $1 * $3; @}
@end group
@group
| exp '/' exp
    @{
      if ($3)
        $$ = $1 / $3;
      else
        @{
          $$ = 1;
          fprintf (stderr, "%d.%d-%d.%d: division by zero",
                   @@3.first_line, @@3.first_column,
                   @@3.last_line, @@3.last_column);
        @}
    @}
@end group
@group
| '-' exp %prec NEG     @{ $$ = -$2; @}
| exp '^' exp           @{ $$ = pow ($1, $3); @}
| '(' exp ')'           @{ $$ = $2; @}
@end group
@end example

This code shows how to reach locations inside of semantic actions, by
using the pseudo-variables @code{@@@var{n}} for rule components, and the
pseudo-variable @code{@@$} for groupings.

We don't need to assign a value to @code{@@$}: the output parser does it
automatically.  By default, before executing the C code of each action,
@code{@@$} is set to range from the beginning of @code{@@1} to the end of
@code{@@@var{n}}, for a rule with @var{n} components.  This behavior can be
redefined (@pxref{Location Default Action}), and for very specific rules,
@code{@@$} can be computed by hand.

@node Ltcalc Lexer
@subsection The @code{ltcalc} Lexical Analyzer.

Until now, we relied on Bison's defaults to enable location
tracking.  The next step is to rewrite the lexical analyzer, and make it
able to feed the parser with the token locations, as it already does for
semantic values.

To this end, we must take into account every single character of the
input text, to avoid the computed locations of being fuzzy or wrong:

@example
@group
int
yylex (void)
@{
  int c;
@end group

@group
  /* Skip white space. */
  while ((c = getchar ()) == ' ' || c == '\t')
    ++yylloc.last_column;
@end group

@group
  /* Step. */
  yylloc.first_line = yylloc.last_line;
  yylloc.first_column = yylloc.last_column;
@end group

@group
  /* Process numbers. */
  if (isdigit (c))
    @{
      yylval = c - '0';
      ++yylloc.last_column;
      while (isdigit (c = getchar ()))
        @{
          ++yylloc.last_column;
          yylval = yylval * 10 + c - '0';
        @}
      ungetc (c, stdin);
      return NUM;
    @}
@end group

  /* Return end-of-input. */
  if (c == EOF)
    return YYEOF;

@group
  /* Return a single char, and update location. */
  if (c == '\n')
    @{
      ++yylloc.last_line;
      yylloc.last_column = 0;
    @}
  else
    ++yylloc.last_column;
  return c;
@}
@end group
@end example

Basically, the lexical analyzer performs the same processing as before: it
skips blanks and tabs, and reads numbers or single-character tokens.  In
addition, it updates @code{yylloc}, the global variable (of type
@code{YYLTYPE}) containing the token's location.

Now, each time this function returns a token, the parser has its kind as
well as its semantic value, and its location in the text.  The last needed
change is to initialize @code{yylloc}, for example in the controlling
function:

@example
@group
int
main (void)
@{
  yylloc.first_line = yylloc.last_line = 1;
  yylloc.first_column = yylloc.last_column = 0;
  return yyparse ();
@}
@end group
@end example

Remember that computing locations is not a matter of syntax.  Every
character must be associated to a location update, whether it is in
valid input, in comments, in literal strings, and so on.

@node Multi-function Calc
@section Multi-Function Calculator: @code{mfcalc}
@cindex multi-function calculator
@cindex @code{mfcalc}
@cindex calculator, multi-function

Now that the basics of Bison have been discussed, it is time to move on to
a more advanced problem.  The above calculators provided only five
functions, @samp{+}, @samp{-}, @samp{*}, @samp{/} and @samp{^}.  It would
be nice to have a calculator that provides other mathematical functions such
as @code{sin}, @code{cos}, etc.

It is easy to add new operators to the infix calculator as long as they are
only single-character literals.  The lexical analyzer @code{yylex} passes
back all nonnumeric characters as tokens, so new grammar rules suffice for
adding a new operator.  But we want something more flexible: built-in
functions whose syntax has this form:

@example
@var{function_name} (@var{argument})
@end example

@noindent
At the same time, we will add memory to the calculator, by allowing you
to create named variables, store values in them, and use them later.
Here is a sample session with the multi-function calculator:

@example
@group
$ @kbd{mfcalc}
@kbd{pi = 3.141592653589}
@result{} 3.1415926536
@end group
@group
@kbd{sin(pi)}
@result{} 0.0000000000
@end group
@kbd{alpha = beta1 = 2.3}
@result{} 2.3000000000
@kbd{alpha}
@result{} 2.3000000000
@kbd{ln(alpha)}
@result{} 0.8329091229
@kbd{exp(ln(beta1))}
@result{} 2.3000000000
$
@end example

Note that multiple assignment and nested function calls are permitted.

@menu
* Mfcalc Declarations::    Bison declarations for multi-function calculator.
* Mfcalc Rules::           Grammar rules for the calculator.
* Mfcalc Symbol Table::    Symbol table management subroutines.
* Mfcalc Lexer::           The lexical analyzer.
* Mfcalc Main::            The controlling function.
@end menu

@node Mfcalc Declarations
@subsection Declarations for @code{mfcalc}

Here are the C and Bison declarations for the multi-function calculator.

@comment file: mfcalc.y: 1
@example
@group
%@{
  #include <stdio.h>  /* For printf, etc. */
  #include <math.h>   /* For pow, used in the grammar. */
  #include "calc.h"   /* Contains definition of 'symrec'. */
  int yylex (void);
  void yyerror (char const *);
%@}
@end group

%define api.value.type union /* Generate YYSTYPE from these types: */
%token <double>  NUM     /* Double precision number. */
%token <symrec*> VAR FUN /* Symbol table pointer: variable/function. */
%nterm <double>  exp

@group
%precedence '='
%left '-' '+'
%left '*' '/'
%precedence NEG /* negation--unary minus */
%right '^'      /* exponentiation */
@end group
@end example

The above grammar introduces only two new features of the Bison language.
These features allow semantic values to have various data types
(@pxref{Multiple Types}).

The special @code{union} value assigned to the @code{%define} variable
@code{api.value.type} specifies that the symbols are defined with their data
types.  Bison will generate an appropriate definition of @code{YYSTYPE} to
store these values.

Since values can now have various types, it is necessary to associate a type
with each grammar symbol whose semantic value is used.  These symbols are
@code{NUM}, @code{VAR}, @code{FUN}, and @code{exp}.  Their declarations are
augmented with their data type (placed between angle brackets).  For
instance, values of @code{NUM} are stored in @code{double}.

The Bison construct @code{%nterm} is used for declaring nonterminal symbols,
just as @code{%token} is used for declaring token kinds.  Previously we did
not use @code{%nterm} before because nonterminal symbols are normally
declared implicitly by the rules that define them.  But @code{exp} must be
declared explicitly so we can specify its value type.  @xref{Type Decl}.

@node Mfcalc Rules
@subsection Grammar Rules for @code{mfcalc}

Here are the grammar rules for the multi-function calculator.
Most of them are copied directly from @code{calc}; three rules,
those which mention @code{VAR} or @code{FUN}, are new.

@comment file: mfcalc.y: 3
@example
%% /* The grammar follows. */
@group
input:
  %empty
| input line
;
@end group

@group
line:
  '\n'
| exp '\n'   @{ printf ("%.10g\n", $1); @}
| error '\n' @{ yyerrok;                @}
;
@end group

@group
exp:
  NUM
| VAR                @{ $$ = $1->value.var;              @}
| VAR '=' exp        @{ $$ = $3; $1->value.var = $3;     @}
| FUN '(' exp ')'    @{ $$ = $1->value.fun ($3);         @}
| exp '+' exp        @{ $$ = $1 + $3;                    @}
| exp '-' exp        @{ $$ = $1 - $3;                    @}
| exp '*' exp        @{ $$ = $1 * $3;                    @}
| exp '/' exp        @{ $$ = $1 / $3;                    @}
| '-' exp  %prec NEG @{ $$ = -$2;                        @}
| exp '^' exp        @{ $$ = pow ($1, $3);               @}
| '(' exp ')'        @{ $$ = $2;                         @}
;
@end group
/* End of grammar. */
%%
@end example

@node Mfcalc Symbol Table
@subsection The @code{mfcalc} Symbol Table
@cindex symbol table example

The multi-function calculator requires a symbol table to keep track of the
names and meanings of variables and functions.  This doesn't affect the
grammar rules (except for the actions) or the Bison declarations, but it
requires some additional C functions for support.

The symbol table itself consists of a linked list of records.  Its
definition, which is kept in the header @file{calc.h}, is as follows.  It
provides for either functions or variables to be placed in the table.

@comment file: calc.h
@example
@group
/* Function type. */
typedef double (func_t) (double);
@end group

@group
/* Data type for links in the chain of symbols. */
struct symrec
@{
  char *name;  /* name of symbol */
  int type;    /* type of symbol: either VAR or FUN */
  union
  @{
    double var;    /* value of a VAR */
    func_t *fun;   /* value of a FUN */
  @} value;
  struct symrec *next;  /* link field */
@};
@end group

@group
typedef struct symrec symrec;

/* The symbol table: a chain of 'struct symrec'. */
extern symrec *sym_table;

symrec *putsym (char const *name, int sym_type);
symrec *getsym (char const *name);
@end group
@end example

The new version of @code{main} will call @code{init_table} to initialize
the symbol table:

@comment file: mfcalc.y: 3
@example
@group
struct init
@{
  char const *name;
  func_t *fun;
@};
@end group

@group
struct init const funs[] =
@{
  @{ "atan", atan @},
  @{ "cos",  cos  @},
  @{ "exp",  exp  @},
  @{ "ln",   log  @},
  @{ "sin",  sin  @},
  @{ "sqrt", sqrt @},
  @{ 0, 0 @},
@};
@end group

@group
/* The symbol table: a chain of 'struct symrec'. */
symrec *sym_table;
@end group

@group
/* Put functions in table. */
static void
init_table (void)
@end group
@group
@{
  for (int i = 0; funs[i].name; i++)
    @{
      symrec *ptr = putsym (funs[i].name, FUN);
      ptr->value.fun = funs[i].fun;
    @}
@}
@end group
@end example

By simply editing the initialization list and adding the necessary include
files, you can add additional functions to the calculator.

Two important functions allow look-up and installation of symbols in the
symbol table.  The function @code{putsym} is passed a name and the kind
(@code{VAR} or @code{FUN}) of the object to be installed.  The object is
linked to the front of the list, and a pointer to the object is returned.
The function @code{getsym} is passed the name of the symbol to look up.  If
found, a pointer to that symbol is returned; otherwise zero is returned.

@comment file: mfcalc.y: 3
@example
@group
/* The mfcalc code assumes that malloc and realloc
   always succeed, and that integer calculations
   never overflow.  Production-quality code should
   not make these assumptions.  */
#include <stdlib.h> /* malloc, realloc. */
#include <string.h> /* strlen. */
@end group

@group
symrec *
putsym (char const *name, int sym_type)
@{
  symrec *res = (symrec *) malloc (sizeof (symrec));
  res->name = strdup (name);
  res->type = sym_type;
  res->value.var = 0; /* Set value to 0 even if fun. */
  res->next = sym_table;
  sym_table = res;
  return res;
@}
@end group

@group
symrec *
getsym (char const *name)
@{
  for (symrec *p = sym_table; p; p = p->next)
    if (strcmp (p->name, name) == 0)
      return p;
  return NULL;
@}
@end group
@end example

@node Mfcalc Lexer
@subsection The @code{mfcalc} Lexer

The function @code{yylex} must now recognize variables, numeric values, and
the single-character arithmetic operators.  Strings of alphanumeric
characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.

The string is passed to @code{getsym} for look up in the symbol table.  If
the name appears in the table, a pointer to its location and its type
(@code{VAR} or @code{FUN}) is returned to @code{yyparse}.  If it is not
already in the table, then it is installed as a @code{VAR} using
@code{putsym}.  Again, a pointer and its type (which must be @code{VAR}) is
returned to @code{yyparse}.

No change is needed in the handling of numeric values and arithmetic
operators in @code{yylex}.

@comment file: mfcalc.y: 3
@example
#include <ctype.h>
#include <stddef.h>

@group
int
yylex (void)
@{
  int c = getchar ();

  /* Ignore white space, get first nonwhite character. */
  while (c == ' ' || c == '\t')
    c = getchar ();

  if (c == EOF)
    return YYEOF;
@end group

@group
  /* Char starts a number => parse the number. */
  if (c == '.' || isdigit (c))
    @{
      ungetc (c, stdin);
      scanf ("%lf", &yylval.NUM);
      return NUM;
    @}
@end group
@end example

@noindent
Bison generated a definition of @code{YYSTYPE} with a member named
@code{NUM} to store value of @code{NUM} symbols.

@comment file: mfcalc.y: 3
@example
@group
  /* Char starts an identifier => read the name. */
  if (isalpha (c))
    @{
      static ptrdiff_t bufsize = 0;
      static char *symbuf = 0;
@end group
      ptrdiff_t i = 0;
      do
@group
        @{
          /* If buffer is full, make it bigger. */
          if (bufsize <= i)
            @{
              bufsize = 2 * bufsize + 40;
              symbuf = realloc (symbuf, bufsize);
            @}
          /* Add this character to the buffer. */
          symbuf[i++] = c;
          /* Get another character. */
          c = getchar ();
        @}
@end group
@group
      while (isalnum (c));

      ungetc (c, stdin);
      symbuf[i] = '\0';
@end group

@group
      symrec *s = getsym (symbuf);
      if (!s)
        s = putsym (symbuf, VAR);
      yylval.VAR = s; /* or yylval.FUN = s. */
      return s->type;
    @}

  /* Any other character is a token by itself. */
  return c;
@}
@end group
@end example

@node Mfcalc Main
@subsection The @code{mfcalc} Main

The error reporting function is unchanged, and the new version of
@code{main} includes a call to @code{init_table} and sets the @code{yydebug}
on user demand (@xref{Tracing}, for details):

@comment file: mfcalc.y: 3
@example
@group
/* Called by yyparse on error. */
void yyerror (char const *s)
@{
  fprintf (stderr, "%s\n", s);
@}
@end group

@group
int main (int argc, char const* argv[])
@end group
@group
@{
  /* Enable parse traces on option -p. */
  if (argc == 2 && strcmp(argv[1], "-p") == 0)
    yydebug = 1;
@end group
@group
  init_table ();
  return yyparse ();
@}
@end group
@end example

This program is both powerful and flexible.  You may easily add new
functions, and it is a simple job to modify this code to install
predefined variables such as @code{pi} or @code{e} as well.

@node Exercises
@section Exercises
@cindex exercises

@enumerate
@item
Add some new functions from @file{math.h} to the initialization list.

@item
Add another array that contains constants and their values.  Then
modify @code{init_table} to add these constants to the symbol table.
It will be easiest to give the constants type @code{VAR}.

@item
Make the program report an error if the user refers to an
uninitialized variable in any way except to store a value in it.
@end enumerate

@node Grammar File
@chapter Bison Grammar Files

Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar.

The Bison grammar file conventionally has a name ending in @samp{.y}.
@xref{Invocation}.

@menu
* Grammar Outline::    Overall layout of the grammar file.
* Symbols::            Terminal and nonterminal symbols.
* Rules::              How to write grammar rules.
* Semantics::          Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References::   Using named references in actions.
* Declarations::       All kinds of Bison declarations are described here.
* Multiple Parsers::   Putting more than one Bison parser in one program.
@end menu

@node Grammar Outline
@section Outline of a Bison Grammar
@cindex comment
@findex // @dots{}
@findex /* @dots{} */

A Bison grammar file has four main sections, shown here with the
appropriate delimiters:

@example
%@{
  @var{Prologue}
%@}

@var{Bison declarations}

%%
@var{Grammar rules}
%%

@var{Epilogue}
@end example

Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections.
As a GNU extension, @samp{//} introduces a comment that continues until end
of line.

@menu
* Prologue::              Syntax and usage of the prologue.
* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations::    Syntax and usage of the Bison declarations section.
* Grammar Rules::         Syntax and usage of the grammar rules section.
* Epilogue::              Syntax and usage of the epilogue.
@end menu

@node Prologue
@subsection The prologue
@cindex declarations section
@cindex Prologue
@cindex declarations

The @var{Prologue} section contains macro definitions and declarations of
functions and variables that are used in the actions in the grammar rules.
These are copied to the beginning of the parser implementation file so that
they precede the definition of @code{yyparse}.  You can use @samp{#include}
to get the declarations from a header file.  If you don't need any C
declarations, you may omit the @samp{%@{} and @samp{%@}} delimiters that
bracket this section.

The @var{Prologue} section is terminated by the first occurrence of
@samp{%@}} that is outside a comment, a string literal, or a character
constant.

You may have more than one @var{Prologue} section, intermixed with the
@var{Bison declarations}.  This allows you to have C and Bison declarations
that refer to each other.  For example, the @code{%union} declaration may
use types defined in a header file, and you may wish to prototype functions
that take arguments of type @code{YYSTYPE}.  This can be done with two
@var{Prologue} blocks, one before and one after the @code{%union}
declaration.

@example
@group
%@{
  #define _GNU_SOURCE
  #include <stdio.h>
  #include "ptypes.h"
%@}
@end group

@group
%union @{
  long n;
  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
@end group

@group
%@{
  static void print_token (yytoken_kind_t token, YYSTYPE val);
%@}
@end group

@dots{}
@end example

When in doubt, it is usually safer to put prologue code before all Bison
declarations, rather than after.  For example, any definitions of feature
test macros like @code{_GNU_SOURCE} or @code{_POSIX_C_SOURCE} should appear
before all Bison declarations, as feature test macros can affect the
behavior of Bison-generated @code{#include} directives.

@node Prologue Alternatives
@subsection Prologue Alternatives
@cindex Prologue Alternatives

@findex %code
@findex %code requires
@findex %code provides
@findex %code top

The functionality of @var{Prologue} sections can often be subtle and
inflexible.  As an alternative, Bison provides a @code{%code} directive with
an explicit qualifier field, which identifies the purpose of the code and
thus the location(s) where Bison should generate it.  For C/C++, the
qualifier can be omitted for the default location, or it can be one of
@code{requires}, @code{provides}, @code{top}.  @xref{%code Summary}.

Look again at the example of the previous section:

@example
@group
%@{
  #define _GNU_SOURCE
  #include <stdio.h>
  #include "ptypes.h"
%@}
@end group

@group
%union @{
  long n;
  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
@end group

@group
%@{
  static void print_token (yytoken_kind_t token, YYSTYPE val);
%@}
@end group

@dots{}
@end example

@noindent
Notice that there are two @var{Prologue} sections here, but there's a subtle
distinction between their functionality.  For example, if you decide to
override Bison's default definition for @code{YYLTYPE}, in which
@var{Prologue} section should you write your new definition?  You should
write it in the first since Bison will insert that code into the parser
implementation file @emph{before} the default @code{YYLTYPE} definition.  In
which @var{Prologue} section should you prototype an internal function,
@code{trace_token}, that accepts @code{YYLTYPE} and @code{yytoken_kind_t} as
arguments?  You should prototype it in the second since Bison will insert
that code @emph{after} the @code{YYLTYPE} and @code{yytoken_kind_t}
definitions.

This distinction in functionality between the two @var{Prologue} sections is
established by the appearance of the @code{%union} between them.  This
behavior raises a few questions.  First, why should the position of a
@code{%union} affect definitions related to @code{YYLTYPE} and
@code{yytoken_kind_t}?  Second, what if there is no @code{%union}?  In that
case, the second kind of @var{Prologue} section is not available.  This
behavior is not intuitive.

To avoid this subtle @code{%union} dependency, rewrite the example using a
@code{%code top} and an unqualified @code{%code}.  Let's go ahead and add
the new @code{YYLTYPE} definition and the @code{trace_token} prototype at
the same time:

@example
%code top @{
  #define _GNU_SOURCE
  #include <stdio.h>

  /* WARNING: The following code really belongs
   * in a '%code requires'; see below. */

  #include "ptypes.h"
  #define YYLTYPE YYLTYPE
  typedef struct YYLTYPE
  @{
    int first_line;
    int first_column;
    int last_line;
    int last_column;
    char *filename;
  @} YYLTYPE;
@}

@group
%union @{
  long n;
  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
@end group

@group
%code @{
  static void print_token (yytoken_kind_t token, YYSTYPE val);
  static void trace_token (yytoken_kind_t token, YYLTYPE loc);
@}
@end group

@dots{}
@end example

@noindent
In this way, @code{%code top} and the unqualified @code{%code} achieve the
same functionality as the two kinds of @var{Prologue} sections, but it's
always explicit which kind you intend.  Moreover, both kinds are always
available even in the absence of @code{%union}.

The @code{%code top} block above logically contains two parts.  The first
two lines before the warning need to appear near the top of the parser
implementation file.  The first line after the warning is required by
@code{YYSTYPE} and thus also needs to appear in the parser implementation
file.  However, if you've instructed Bison to generate a parser header file
(@pxref{Decl Summary}), you probably want that line to appear
before the @code{YYSTYPE} definition in that header file as well.  The
@code{YYLTYPE} definition should also appear in the parser header file to
override the default @code{YYLTYPE} definition there.

In other words, in the @code{%code top} block above, all but the first two
lines are dependency code required by the @code{YYSTYPE} and @code{YYLTYPE}
definitions.
Thus, they belong in one or more @code{%code requires}:

@example
@group
%code top @{
  #define _GNU_SOURCE
  #include <stdio.h>
@}
@end group

@group
%code requires @{
  #include "ptypes.h"
@}
@end group
@group
%union @{
  long n;
  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
@end group

@group
%code requires @{
  #define YYLTYPE YYLTYPE
  typedef struct YYLTYPE
  @{
    int first_line;
    int first_column;
    int last_line;
    int last_column;
    char *filename;
  @} YYLTYPE;
@}
@end group

@group
%code @{
  static void print_token (yytoken_kind_t token, YYSTYPE val);
  static void trace_token (yytoken_kind_t token, YYLTYPE loc);
@}
@end group

@dots{}
@end example

@noindent
Now Bison will insert @code{#include "ptypes.h"} and the new @code{YYLTYPE}
definition before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE}
definitions in both the parser implementation file and the parser header
file.  (By the same reasoning, @code{%code requires} would also be the
appropriate place to write your own definition for @code{YYSTYPE}.)

When you are writing dependency code for @code{YYSTYPE} and @code{YYLTYPE},
you should prefer @code{%code requires} over @code{%code top} regardless of
whether you instruct Bison to generate a parser header file.  When you are
writing code that you need Bison to insert only into the parser
implementation file and that has no special need to appear at the top of
that file, you should prefer the unqualified @code{%code} over @code{%code
top}.  These practices will make the purpose of each block of your code
explicit to Bison and to other developers reading your grammar file.
Following these practices, we expect the unqualified @code{%code} and
@code{%code requires} to be the most important of the four @var{Prologue}
alternatives.

At some point while developing your parser, you might decide to provide
@code{trace_token} to modules that are external to your parser.  Thus, you
might wish for Bison to insert the prototype into both the parser header
file and the parser implementation file.  Since this function is not a
dependency required by @code{YYSTYPE} or @code{YYLTYPE}, it doesn't make
sense to move its prototype to a @code{%code requires}.  More importantly,
since it depends upon @code{YYLTYPE} and @code{yytoken_kind_t}, @code{%code
requires} is not sufficient.  Instead, move its prototype from the
unqualified @code{%code} to a @code{%code provides}:

@example
@group
%code top @{
  #define _GNU_SOURCE
  #include <stdio.h>
@}
@end group

@group
%code requires @{
  #include "ptypes.h"
@}
@end group
@group
%union @{
  long n;
  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
@end group

@group
%code requires @{
  #define YYLTYPE YYLTYPE
  typedef struct YYLTYPE
  @{
    int first_line;
    int first_column;
    int last_line;
    int last_column;
    char *filename;
  @} YYLTYPE;
@}
@end group

@group
%code provides @{
  void trace_token (yytoken_kind_t token, YYLTYPE loc);
@}
@end group

@group
%code @{
  static void print_token (FILE *file, int token, YYSTYPE val);
@}
@end group

@dots{}
@end example

@noindent
Bison will insert the @code{trace_token} prototype into both the parser
header file and the parser implementation file after the definitions for
@code{yytoken_kind_t}, @code{YYLTYPE}, and @code{YYSTYPE}.

The above examples are careful to write directives in an order that reflects
the layout of the generated parser implementation and header files:
@code{%code top}, @code{%code requires}, @code{%code provides}, and then
@code{%code}.  While your grammar files may generally be easier to read if
you also follow this order, Bison does not require it.  Instead, Bison lets
you choose an organization that makes sense to you.

You may declare any of these directives multiple times in the grammar file.
In that case, Bison concatenates the contained code in declaration order.
This is the only way in which the position of one of these directives within
the grammar file affects its functionality.

The result of the previous two properties is greater flexibility in how you may
organize your grammar file.
For example, you may organize semantic-type-related directives by semantic
type:

@example
@group
%code requires @{ #include "type1.h" @}
%union @{ type1 field1; @}
%destructor @{ type1_free ($$); @} <field1>
%printer @{ type1_print (yyo, $$); @} <field1>
@end group

@group
%code requires @{ #include "type2.h" @}
%union @{ type2 field2; @}
%destructor @{ type2_free ($$); @} <field2>
%printer @{ type2_print (yyo, $$); @} <field2>
@end group
@end example

@noindent
You could even place each of the above directive groups in the rules section of
the grammar file next to the set of rules that uses the associated semantic
type.
(In the rules section, you must terminate each of those directives with a
semicolon.)
And you don't have to worry that some directive (like a @code{%union}) in the
definitions section is going to adversely affect their functionality in some
counter-intuitive manner just because it comes first.
Such an organization is not possible using @var{Prologue} sections.

This section has been concerned with explaining the advantages of the four
@var{Prologue} alternatives over the original Yacc @var{Prologue}.
However, in most cases when using these directives, you shouldn't need to
think about all the low-level ordering issues discussed here.
Instead, you should simply use these directives to label each block of your
code according to its purpose and let Bison handle the ordering.
@code{%code} is the most generic label.
Move code to @code{%code requires}, @code{%code provides}, or @code{%code top}
as needed.

@node Bison Declarations
@subsection The Bison Declarations Section
@cindex Bison declarations (introduction)
@cindex declarations, Bison (introduction)

The @var{Bison declarations} section contains declarations that define
terminal and nonterminal symbols, specify precedence, and so on.
In some simple grammars you may not need any declarations.
@xref{Declarations}.

@node Grammar Rules
@subsection The Grammar Rules Section
@cindex grammar rules section
@cindex rules section for grammar

The @dfn{grammar rules} section contains one or more Bison grammar
rules, and nothing else.  @xref{Rules}.

There must always be at least one grammar rule, and the first
@samp{%%} (which precedes the grammar rules) may never be omitted even
if it is the first thing in the file.

@node Epilogue
@subsection The epilogue
@cindex additional C code section
@cindex epilogue
@cindex C code, section for additional

The @var{Epilogue} is copied verbatim to the end of the parser
implementation file, just as the @var{Prologue} is copied to the
beginning.  This is the most convenient place to put anything that you
want to have in the parser implementation file but which need not come
before the definition of @code{yyparse}.  For example, the definitions
of @code{yylex} and @code{yyerror} often go here.  Because C requires
functions to be declared before being used, you often need to declare
functions like @code{yylex} and @code{yyerror} in the Prologue, even
if you define them in the Epilogue.  @xref{Interface}.

If the last section is empty, you may omit the @samp{%%} that separates it
from the grammar rules.

The Bison parser itself contains many macros and identifiers whose names
start with @samp{yy} or @samp{YY}, so it is a good idea to avoid using
any such names (except those documented in this manual) in the epilogue
of the grammar file.

@node Symbols
@section Symbols, Terminal and Nonterminal
@cindex nonterminal symbol
@cindex terminal symbol
@cindex token kind
@cindex symbol

@dfn{Symbols} in Bison grammars represent the grammatical classifications
of the language.

A @dfn{terminal symbol} (also known as a @dfn{token kind}) represents a
class of syntactically equivalent tokens.  You use the symbol in grammar
rules to mean that a token in that class is allowed.  The symbol is
represented in the Bison parser by a numeric code, and the @code{yylex}
function returns a token kind code to indicate what kind of token has been
read.  You don't need to know what the code value is; you can use the symbol
to stand for it.

A @dfn{nonterminal symbol} stands for a class of syntactically
equivalent groupings.  The symbol name is used in writing grammar rules.
By convention, it should be all lower case.

Symbol names can contain letters, underscores, periods, and non-initial
digits and dashes.  Dashes in symbol names are a GNU extension, incompatible
with POSIX Yacc.  Periods and dashes make symbol names less convenient to
use with named references, which require brackets around such names
(@pxref{Named References}).  Terminal symbols that contain periods or dashes
make little sense: since they are not valid symbols (in most programming
languages) they are not exported as token names.

There are three ways of writing terminal symbols in the grammar:

@itemize @bullet
@item
A @dfn{named token kind} is written with an identifier, like an identifier
in C@.  By convention, it should be all upper case.  Each such name must be
defined with a Bison declaration such as @code{%token}.  @xref{Token Decl}.

@item
@cindex character token
@cindex literal token
@cindex single-character literal
A @dfn{character token kind} (or @dfn{literal character token}) is written
in the grammar using the same syntax used in C for character constants; for
example, @code{'+'} is a character token kind.  A character token kind
doesn't need to be declared unless you need to specify its semantic value
data type (@pxref{Value Type}), associativity, or precedence
(@pxref{Precedence}).

By convention, a character token kind is used only to represent a token that
consists of that particular character.  Thus, the token kind @code{'+'} is
used to represent the character @samp{+} as a token.  Nothing enforces this
convention, but if you depart from it, your program will confuse other
readers.

All the usual escape sequences used in character literals in C can be used
in Bison as well, but you must not use the null character as a character
literal because its numeric code, zero, signifies end-of-input
(@pxref{Calling Convention}).  Also, unlike standard C, trigraphs have no
special meaning in Bison character literals, nor is backslash-newline
allowed.

@item
@cindex string token
@cindex literal string token
@cindex multicharacter literal
A @dfn{literal string token} is written like a C string constant; for
example, @code{"<="} is a literal string token.  A literal string token
doesn't need to be declared unless you need to specify its semantic
value data type (@pxref{Value Type}), associativity, or precedence
(@pxref{Precedence}).

You can associate the literal string token with a symbolic name as an alias,
using the @code{%token} declaration (@pxref{Token Decl}).  If you don't do
that, the lexical analyzer has to retrieve the token code for the literal
string token from the @code{yytname} table (@pxref{Calling Convention}).

@strong{Warning}: literal string tokens do not work in Yacc.

By convention, a literal string token is used only to represent a token
that consists of that particular string.  Thus, you should use the token
kind @code{"<="} to represent the string @samp{<=} as a token.  Bison
does not enforce this convention, but if you depart from it, people who
read your program will be confused.

All the escape sequences used in string literals in C can be used in
Bison as well, except that you must not use a null character within a
string literal.  Also, unlike Standard C, trigraphs have no special
meaning in Bison string literals, nor is backslash-newline allowed.  A
literal string token must contain two or more characters; for a token
containing just one character, use a character token (see above).
@end itemize

How you choose to write a terminal symbol has no effect on its
grammatical meaning.  That depends only on where it appears in rules and
on when the parser function returns that symbol.

The value returned by @code{yylex} is always one of the terminal
symbols, except that a zero or negative value signifies end-of-input.
Whichever way you write the token kind in the grammar rules, you write
it the same way in the definition of @code{yylex}.  The numeric code
for a character token kind is simply the positive numeric code of the
character, so @code{yylex} can use the identical value to generate the
requisite code, though you may need to convert it to @code{unsigned
char} to avoid sign-extension on hosts where @code{char} is signed.
Each named token kind becomes a C macro in the parser implementation
file, so @code{yylex} can use the name to stand for the code.  (This
is why periods don't make sense in terminal symbols.)  @xref{Calling
Convention}.

If @code{yylex} is defined in a separate file, you need to arrange for the
token-kind definitions to be available there.  Use the @samp{-d} option when
you run Bison, so that it will write these definitions into a separate
header file @file{@var{name}.tab.h} which you can include in the other
source files that need it.  @xref{Invocation}.

If you want to write a grammar that is portable to any Standard C
host, you must use only nonnull character tokens taken from the basic
execution character set of Standard C@.  This set consists of the ten
digits, the 52 lower- and upper-case English letters, and the
characters in the following C-language string:

@example
"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~"
@end example

The @code{yylex} function and Bison must use a consistent character set
and encoding for character tokens.  For example, if you run Bison in an
ASCII environment, but then compile and run the resulting
program in an environment that uses an incompatible character set like
EBCDIC, the resulting program may not work because the tables
generated by Bison will assume ASCII numeric values for
character tokens.  It is standard practice for software distributions to
contain C source files that were generated by Bison in an
ASCII environment, so installers on platforms that are
incompatible with ASCII must rebuild those files before
compiling them.

The symbol @code{error} is a terminal symbol reserved for error recovery
(@pxref{Error Recovery}); you shouldn't use it for any other purpose.
In particular, @code{yylex} should never return this value.  The default
value of the error token is 256, unless you explicitly assigned 256 to
one of your tokens with a @code{%token} declaration.

@node Rules
@section Grammar Rules

A Bison grammar is a list of rules.

@menu
* Rules Syntax::   Syntax of the rules.
* Empty Rules::    Symbols that can match the empty string.
* Recursion::      Writing recursive rules.
@end menu

@node Rules Syntax
@subsection Syntax of Grammar Rules
@cindex rule syntax
@cindex grammar rule syntax
@cindex syntax of grammar rules

A Bison grammar rule has the following general form:

@example
@var{result}: @var{components}@dots{};
@end example

@noindent
where @var{result} is the nonterminal symbol that this rule describes,
and @var{components} are various terminal and nonterminal symbols that
are put together by this rule (@pxref{Symbols}).

For example,

@example
exp: exp '+' exp;
@end example

@noindent
says that two groupings of type @code{exp}, with a @samp{+} token in between,
can be combined into a larger grouping of type @code{exp}.

White space in rules is significant only to separate symbols.  You can add
extra white space as you wish.

Scattered among the components can be @var{actions} that determine
the semantics of the rule.  An action looks like this:

@example
@{@var{C statements}@}
@end example

@noindent
@cindex braced code
This is an example of @dfn{braced code}, that is, C code surrounded by
braces, much like a compound statement in C@.  Braced code can contain
any sequence of C tokens, so long as its braces are balanced.  Bison
does not check the braced code for correctness directly; it merely
copies the code to the parser implementation file, where the C
compiler can check it.

Within braced code, the balanced-brace count is not affected by braces
within comments, string literals, or character constants, but it is
affected by the C digraphs @samp{<%} and @samp{%>} that represent
braces.  At the top level braced code must be terminated by @samp{@}}
and not by a digraph.  Bison does not look for trigraphs, so if braced
code uses trigraphs you should ensure that they do not affect the
nesting of braces or the boundaries of comments, string literals, or
character constants.

Usually there is only one action and it follows the components.
@xref{Actions}.

@findex |
Multiple rules for the same @var{result} can be written separately or can
be joined with the vertical-bar character @samp{|} as follows:

@example
@group
@var{result}:
  @var{rule1-components}@dots{}
| @var{rule2-components}@dots{}
@dots{}
;
@end group
@end example

@noindent
They are still considered distinct rules even when joined in this way.

@node Empty Rules
@subsection Empty Rules
@cindex empty rule
@cindex rule, empty
@findex %empty

A rule is said to be @dfn{empty} if its right-hand side (@var{components})
is empty.  It means that @var{result} in the previous example can match the
empty string.  As another example, here is how to define an optional
semicolon:

@example
semicolon.opt: | ";";
@end example

@noindent
It is easy not to see an empty rule, especially when @code{|} is used.  The
@code{%empty} directive allows to make explicit that a rule is empty on
purpose:

@example
@group
semicolon.opt:
  %empty
| ";"
;
@end group
@end example

Flagging a non-empty rule with @code{%empty} is an error.  If run with
@option{-Wempty-rule}, @command{bison} will report empty rules without
@code{%empty}.  Using @code{%empty} enables this warning, unless
@option{-Wno-empty-rule} was specified.

The @code{%empty} directive is a Bison extension, it does not work with
Yacc.  To remain compatible with POSIX Yacc, it is customary to write a
comment @samp{/* empty */} in each rule with no components:

@example
@group
semicolon.opt:
  /* empty */
| ";"
;
@end group
@end example


@node Recursion
@subsection Recursive Rules
@cindex recursive rule
@cindex rule, recursive

A rule is called @dfn{recursive} when its @var{result} nonterminal
appears also on its right hand side.  Nearly all Bison grammars need to
use recursion, because that is the only way to define a sequence of any
number of a particular thing.  Consider this recursive definition of a
comma-separated sequence of one or more expressions:

@example
@group
expseq1:
  exp
| expseq1 ',' exp
;
@end group
@end example

@cindex left recursion
@cindex right recursion
@noindent
Since the recursive use of @code{expseq1} is the leftmost symbol in the
right hand side, we call this @dfn{left recursion}.  By contrast, here
the same construct is defined using @dfn{right recursion}:

@example
@group
expseq1:
  exp
| exp ',' expseq1
;
@end group
@end example

@noindent
Any kind of sequence can be defined using either left recursion or right
recursion, but you should always use left recursion, because it can
parse a sequence of any number of elements with bounded stack space.
Right recursion uses up space on the Bison stack in proportion to the
number of elements in the sequence, because all the elements must be
shifted onto the stack before the rule can be applied even once.
@xref{Algorithm}, for further explanation
of this.

@cindex mutual recursion
@dfn{Indirect} or @dfn{mutual} recursion occurs when the result of the
rule does not appear directly on its right hand side, but does appear
in rules for other nonterminals which do appear on its right hand
side.

For example:

@example
@group
expr:
  primary
| primary '+' primary
;
@end group

@group
primary:
  constant
| '(' expr ')'
;
@end group
@end example

@noindent
defines two mutually-recursive nonterminals, since each refers to the
other.

@node Semantics
@section Defining Language Semantics
@cindex defining language semantics
@cindex language semantics, defining

The grammar rules for a language determine only the syntax.  The semantics
are determined by the semantic values associated with various tokens and
groupings, and by the actions taken when various groupings are recognized.

For example, the calculator calculates properly because the value
associated with each expression is the proper number; it adds properly
because the action for the grouping @w{@samp{@var{x} + @var{y}}} is to add
the numbers associated with @var{x} and @var{y}.

@menu
* Value Type::        Specifying one data type for all semantic values.
* Multiple Types::    Specifying several alternative data types.
* Type Generation::   Generating the semantic value type.
* Union Decl::        Declaring the set of all semantic value types.
* Structured Value Type::  Providing a structured semantic value type.
* Actions::           An action is the semantic definition of a grammar rule.
* Action Types::      Specifying data types for actions to operate on.
* Midrule Actions::   Most actions go at the end of a rule.
                      This says when, why and how to use the exceptional
                        action in the middle of a rule.
@end menu

@node Value Type
@subsection Data Types of Semantic Values
@cindex semantic value type
@cindex value type, semantic
@cindex data types of semantic values
@cindex default data type

In a simple program it may be sufficient to use the same data type for
the semantic values of all language constructs.  This was true in the
RPN and infix calculator examples (@pxref{RPN Calc}).

Bison normally uses the type @code{int} for semantic values if your program
uses the same data type for all language constructs.  To specify some other
type, define the @code{%define} variable @code{api.value.type} like this:

@example
%define api.value.type @{double@}
@end example

@noindent
or

@example
%define api.value.type @{struct semantic_type@}
@end example

The value of @code{api.value.type} should be a type name that does not
contain parentheses or square brackets.

Alternatively, instead of relying of Bison's @code{%define} support, you may
rely on the C/C++ preprocessor and define @code{YYSTYPE} as a macro, like
this:

@example
#define YYSTYPE double
@end example

@noindent
This macro definition must go in the prologue of the grammar file
(@pxref{Grammar Outline}).  If compatibility with POSIX Yacc matters to you,
use this.  Note however that Bison cannot know @code{YYSTYPE}'s value, not
even whether it is defined, so there are services it cannot provide.
Besides this works only for languages that have a preprocessor.

@node Multiple Types
@subsection More Than One Value Type

In most programs, you will need different data types for different kinds
of tokens and groupings.  For example, a numeric constant may need type
@code{int} or @code{long}, while a string constant needs type
@code{char *}, and an identifier might need a pointer to an entry in the
symbol table.

To use more than one data type for semantic values in one parser, Bison
requires you to do two things:

@itemize @bullet
@item
Specify the entire collection of possible data types.  There are several
options:
@itemize @bullet
@item
let Bison compute the union type from the tags you assign to symbols;

@item
use the @code{%union} Bison declaration (@pxref{Union Decl});

@item
define the @code{%define} variable @code{api.value.type} to be a union type
whose members are the type tags (@pxref{Structured Value Type});

@item
use a @code{typedef} or a @code{#define} to define @code{YYSTYPE} to be a
union type whose member names are the type tags.
@end itemize

@item
Choose one of those types for each symbol (terminal or nonterminal) for
which semantic values are used.  This is done for tokens with the
@code{%token} Bison declaration (@pxref{Token Decl}) and
for groupings with the @code{%nterm}/@code{%type} Bison declarations
(@pxref{Type Decl}).
@end itemize

@node Type Generation
@subsection Generating the Semantic Value Type
@cindex declaring value types
@cindex value types, declaring
@findex %define api.value.type union

The special value @code{union} of the @code{%define} variable
@code{api.value.type} instructs Bison that the type tags (used with the
@code{%token}, @code{%nterm} and @code{%type} directives) are genuine types,
not names of members of @code{YYSTYPE}.

For example:

@example
%define api.value.type union
%token <int> INT "integer"
%token <int> 'n'
%nterm <int> expr
%token <char const *> ID "identifier"
@end example

@noindent
generates an appropriate value of @code{YYSTYPE} to support each symbol
type.  The name of the member of @code{YYSTYPE} for tokens than have a
declared identifier @var{id} (such as @code{INT} and @code{ID} above, but
not @code{'n'}) is @code{@var{id}}.  The other symbols have unspecified
names on which you should not depend; instead, relying on C casts to access
the semantic value with the appropriate type:

@example
/* For an "integer". */
yylval.INT = 42;
return INT;

/* For an 'n', also declared as int. */
*((int*)&yylval) = 42;
return 'n';

/* For an "identifier". */
yylval.ID = "42";
return ID;
@end example

If the @code{%define} variable @code{api.token.prefix} is defined
(@pxref{%define Summary}), then it is also used to prefix
the union member names.  For instance, with @samp{%define api.token.prefix
@{TOK_@}}:

@example
/* For an "integer". */
yylval.TOK_INT = 42;
return TOK_INT;
@end example

This Bison extension cannot work if @code{%yacc} (or
@option{-y}/@option{--yacc}) is enabled, as POSIX mandates that Yacc
generate tokens as macros (e.g., @samp{#define INT 258}, or @samp{#define
TOK_INT 258}).

A similar feature is provided for C++ that in addition overcomes C++
limitations (that forbid non-trivial objects to be part of a @code{union}):
@samp{%define api.value.type variant}, see @ref{C++ Variants}.

@node Union Decl
@subsection The Union Declaration
@cindex declaring value types
@cindex value types, declaring
@findex %union

The @code{%union} declaration specifies the entire collection of possible
data types for semantic values.  The keyword @code{%union} is followed by
braced code containing the same thing that goes inside a @code{union} in C@.

For example:

@example
@group
%union @{
  double val;
  symrec *tptr;
@}
@end group
@end example

@noindent
This says that the two alternative types are @code{double} and @code{symrec
*}.  They are given names @code{val} and @code{tptr}; these names are used
in the @code{%token}, @code{%nterm} and @code{%type} declarations to pick
one of the types for a terminal or nonterminal symbol (@pxref{Type Decl}).

As an extension to POSIX, a tag is allowed after the @code{%union}.  For
example:

@example
@group
%union value @{
  double val;
  symrec *tptr;
@}
@end group
@end example

@noindent
specifies the union tag @code{value}, so the corresponding C type is
@code{union value}.  If you do not specify a tag, it defaults to
@code{YYSTYPE} (@pxref{%define Summary}).

As another extension to POSIX, you may specify multiple @code{%union}
declarations; their contents are concatenated.  However, only the first
@code{%union} declaration can specify a tag.

Note that, unlike making a @code{union} declaration in C, you need not write
a semicolon after the closing brace.

@node Structured Value Type
@subsection Providing a Structured Semantic Value Type
@cindex declaring value types
@cindex value types, declaring
@findex %union

Instead of @code{%union}, you can define and use your own union type
@code{YYSTYPE} if your grammar contains at least one @samp{<@var{type}>}
tag.  For example, you can put the following into a header file
@file{parser.h}:

@example
@group
union YYSTYPE @{
  double val;
  symrec *tptr;
@};
@end group
@end example

@noindent
and then your grammar can use the following instead of @code{%union}:

@example
@group
%@{
#include "parser.h"
%@}
%define api.value.type @{union YYSTYPE@}
%nterm <val> expr
%token <tptr> ID
@end group
@end example

Actually, you may also provide a @code{struct} rather that a @code{union},
which may be handy if you want to track information for every symbol (such
as preceding comments).

The type you provide may even be structured and include pointers, in which
case the type tags you provide may be composite, with @samp{.} and @samp{->}
operators.

@node Actions
@subsection Actions
@cindex action
@vindex $$
@vindex $@var{n}
@vindex $@var{name}
@vindex $[@var{name}]

An action accompanies a syntactic rule and contains C code to be executed
each time an instance of that rule is recognized.  The task of most actions
is to compute a semantic value for the grouping built by the rule from the
semantic values associated with tokens or smaller groupings.

An action consists of braced code containing C statements, and can be
placed at any position in the rule;
it is executed at that position.  Most rules have just one action at the
end of the rule, following all the components.  Actions in the middle of
a rule are tricky and used only for special purposes (@pxref{Midrule
Actions}).

The C code in an action can refer to the semantic values of the
components matched by the rule with the construct @code{$@var{n}},
which stands for the value of the @var{n}th component.  The semantic
value for the grouping being constructed is @code{$$}.  In addition,
the semantic values of symbols can be accessed with the named
references construct @code{$@var{name}} or @code{$[@var{name}]}.
Bison translates both of these constructs into expressions of the
appropriate type when it copies the actions into the parser
implementation file.  @code{$$} (or @code{$@var{name}}, when it stands
for the current grouping) is translated to a modifiable lvalue, so it
can be assigned to.

Here is a typical example:

@example
@group
exp:
@dots{}
| exp '+' exp     @{ $$ = $1 + $3; @}
@end group
@end example

Or, in terms of named references:

@example
@group
exp[result]:
@dots{}
| exp[left] '+' exp[right]  @{ $result = $left + $right; @}
@end group
@end example

@noindent
This rule constructs an @code{exp} from two smaller @code{exp} groupings
connected by a plus-sign token.  In the action, @code{$1} and @code{$3}
(@code{$left} and @code{$right})
refer to the semantic values of the two component @code{exp} groupings,
which are the first and third symbols on the right hand side of the rule.
The sum is stored into @code{$$} (@code{$result}) so that it becomes the
semantic value of
the addition-expression just recognized by the rule.  If there were a
useful semantic value associated with the @samp{+} token, it could be
referred to as @code{$2}.

@xref{Named References}, for more information about using the named
references construct.

Note that the vertical-bar character @samp{|} is really a rule
separator, and actions are attached to a single rule.  This is a
difference with tools like Flex, for which @samp{|} stands for either
``or'', or ``the same action as that of the next rule''.  In the
following example, the action is triggered only when @samp{b} is found:

@example
a-or-b: 'a'|'b'   @{ a_or_b_found = 1; @};
@end example

@cindex default action
If you don't specify an action for a rule, Bison supplies a default:
@w{@code{$$ = $1}.}  Thus, the value of the first symbol in the rule
becomes the value of the whole rule.  Of course, the default action is
valid only if the two data types match.  There is no meaningful default
action for an empty rule; every empty rule must have an explicit action
unless the rule's value does not matter.

@code{$@var{n}} with @var{n} zero or negative is allowed for reference
to tokens and groupings on the stack @emph{before} those that match the
current rule.  This is a very risky practice, and to use it reliably
you must be certain of the context in which the rule is applied.  Here
is a case in which you can use this reliably:

@example
@group
foo:
  expr bar '+' expr  @{ @dots{} @}
| expr bar '-' expr  @{ @dots{} @}
;
@end group

@group
bar:
  %empty    @{ previous_expr = $0; @}
;
@end group
@end example

As long as @code{bar} is used only in the fashion shown here, @code{$0}
always refers to the @code{expr} which precedes @code{bar} in the
definition of @code{foo}.

@vindex yylval
It is also possible to access the semantic value of the lookahead token, if
any, from a semantic action.
This semantic value is stored in @code{yylval}.
@xref{Action Features}.

@node Action Types
@subsection Data Types of Values in Actions
@cindex action data types
@cindex data types in actions

If you have chosen a single data type for semantic values, the @code{$$}
and @code{$@var{n}} constructs always have that data type.

If you have used @code{%union} to specify a variety of data types, then you
must declare a choice among these types for each terminal or nonterminal
symbol that can have a semantic value.  Then each time you use @code{$$} or
@code{$@var{n}}, its data type is determined by which symbol it refers to
in the rule.  In this example,

@example
@group
exp:
  @dots{}
| exp '+' exp    @{ $$ = $1 + $3; @}
@end group
@end example

@noindent
@code{$1} and @code{$3} refer to instances of @code{exp}, so they all
have the data type declared for the nonterminal symbol @code{exp}.  If
@code{$2} were used, it would have the data type declared for the
terminal symbol @code{'+'}, whatever that might be.

Alternatively, you can specify the data type when you refer to the value,
by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the
reference.  For example, if you have defined types as shown here:

@example
@group
%union @{
  int itype;
  double dtype;
@}
@end group
@end example

@noindent
then you can write @code{$<itype>1} to refer to the first subunit of the
rule as an integer, or @code{$<dtype>1} to refer to it as a double.

@node Midrule Actions
@subsection Actions in Midrule
@cindex actions in midrule
@cindex midrule actions

Occasionally it is useful to put an action in the middle of a rule.
These actions are written just like usual end-of-rule actions, but they
are executed before the parser even recognizes the following components.

@menu
* Using Midrule Actions::       Putting an action in the middle of a rule.
* Typed Midrule Actions::       Specifying the semantic type of their values.
* Midrule Action Translation::  How midrule actions are actually processed.
* Midrule Conflicts::           Midrule actions can cause conflicts.
@end menu

@node Using Midrule Actions
@subsubsection Using Midrule Actions

A midrule action may refer to the components preceding it using
@code{$@var{n}}, but it may not refer to subsequent components because
it is run before they are parsed.

The midrule action itself counts as one of the components of the rule.
This makes a difference when there is another action later in the same rule
(and usually there is another at the end): you have to count the actions
along with the symbols when working out which number @var{n} to use in
@code{$@var{n}}.

The midrule action can also have a semantic value.  The action can set
its value with an assignment to @code{$$}, and actions later in the rule
can refer to the value using @code{$@var{n}}.  Since there is no symbol
to name the action, there is no way to declare a data type for the value
in advance, so you must use the @samp{$<@dots{}>@var{n}} construct to
specify a data type each time you refer to this value.

There is no way to set the value of the entire rule with a midrule
action, because assignments to @code{$$} do not have that effect.  The
only way to set the value for the entire rule is with an ordinary action
at the end of the rule.

Here is an example from a hypothetical compiler, handling a @code{let}
statement that looks like @samp{let (@var{variable}) @var{statement}} and
serves to create a variable named @var{variable} temporarily for the
duration of @var{statement}.  To parse this construct, we must put
@var{variable} into the symbol table while @var{statement} is parsed, then
remove it afterward.  Here is how it is done:

@example
@group
stmt:
  "let" '(' var ')'
    @{
      $<context>$ = push_context ();
      declare_variable ($3);
    @}
  stmt
    @{
      $$ = $6;
      pop_context ($<context>5);
    @}
@end group
@end example

@noindent
As soon as @samp{let (@var{variable})} has been recognized, the first
action is run.  It saves a copy of the current semantic context (the
list of accessible variables) as its semantic value, using alternative
@code{context} in the data-type union.  Then it calls
@code{declare_variable} to add the new variable to that list.  Once the
first action is finished, the embedded statement @code{stmt} can be
parsed.

Note that the midrule action is component number 5, so the @samp{stmt} is
component number 6.  Named references can be used to improve the readability
and maintainability (@pxref{Named References}):

@example
@group
stmt:
  "let" '(' var ')'
    @{
      $<context>let = push_context ();
      declare_variable ($3);
    @}[let]
  stmt
    @{
      $$ = $6;
      pop_context ($<context>let);
    @}
@end group
@end example

After the embedded statement is parsed, its semantic value becomes the
value of the entire @code{let}-statement.  Then the semantic value from the
earlier action is used to restore the prior list of variables.  This
removes the temporary @code{let}-variable from the list so that it won't
appear to exist while the rest of the program is parsed.

Because the types of the semantic values of midrule actions are unknown to
Bison, type-based features (e.g., @samp{%printer}, @samp{%destructor}) do
not work, which could result in memory leaks.  They also forbid the use of
the @code{variant} implementation of the @code{api.value.type} in C++
(@pxref{C++ Variants}).

@xref{Typed Midrule Actions}, for one way to address this issue, and
@ref{Midrule Action Translation}, for another: turning mid-action actions
into regular actions.


@node Typed Midrule Actions
@subsubsection Typed Midrule Actions

@findex %destructor
@cindex discarded symbols, midrule actions
@cindex error recovery, midrule actions
In the above example, if the parser initiates error recovery (@pxref{Error
Recovery}) while parsing the tokens in the embedded statement @code{stmt},
it might discard the previous semantic context @code{$<context>5} without
restoring it.  Thus, @code{$<context>5} needs a destructor
(@pxref{Destructor Decl}), and Bison needs the
type of the semantic value (@code{context}) to select the right destructor.

As an extension to Yacc's midrule actions, Bison offers a means to type
their semantic value: specify its type tag (@samp{<...>} before the midrule
action.

Consider the previous example, with an untyped midrule action:

@example
@group
stmt:
  "let" '(' var ')'
    @{
      $<context>$ = push_context (); // ***
      declare_variable ($3);
    @}
  stmt
    @{
      $$ = $6;
      pop_context ($<context>5);     // ***
    @}
@end group
@end example

@noindent
If instead you write:

@example
@group
stmt:
  "let" '(' var ')'
    <context>@{                       // ***
      $$ = push_context ();          // ***
      declare_variable ($3);
    @}
  stmt
    @{
      $$ = $6;
      pop_context ($5);              // ***
    @}
@end group
@end example

@noindent
then @code{%printer} and @code{%destructor} work properly (no more leaks!),
C++ @code{variant}s can be used, and redundancy is reduced (@code{<context>}
is specified once).


@node Midrule Action Translation
@subsubsection Midrule Action Translation
@vindex $@@@var{n}
@vindex @@@var{n}

Midrule actions are actually transformed into regular rules and actions.
The various reports generated by Bison (textual, graphical, etc., see
@ref{Understanding}) reveal this translation,
best explained by means of an example.  The following rule:

@example
exp: @{ a(); @} "b" @{ c(); @} @{ d(); @} "e" @{ f(); @};
@end example

@noindent
is translated into:

@example
$@@1: %empty @{ a(); @};
$@@2: %empty @{ c(); @};
$@@3: %empty @{ d(); @};
exp: $@@1 "b" $@@2 $@@3 "e" @{ f(); @};
@end example

@noindent
with new nonterminal symbols @code{$@@@var{n}}, where @var{n} is a number.

A midrule action is expected to generate a value if it uses @code{$$}, or
the (final) action uses @code{$@var{n}} where @var{n} denote the midrule
action.  In that case its nonterminal is rather named @code{@@@var{n}}:

@example
exp: @{ a(); @} "b" @{ $$ = c(); @} @{ d(); @} "e" @{ f = $1; @};
@end example

@noindent
is translated into

@example
@@1: %empty @{ a(); @};
@@2: %empty @{ $$ = c(); @};
$@@3: %empty @{ d(); @};
exp: @@1 "b" @@2 $@@3 "e" @{ f = $1; @}
@end example

There are probably two errors in the above example: the first midrule action
does not generate a value (it does not use @code{$$} although the final
action uses it), and the value of the second one is not used (the final
action does not use @code{$3}).  Bison reports these errors when the
@code{midrule-value} warnings are enabled (@pxref{Invocation}):

@example
$ @kbd{bison -Wmidrule-value mid.y}
@group
mid.y:2.6-13: @dwarning{warning}: unset value: $$
    2 | exp: @dwarning{@{ a(); @}} "b" @{ $$ = c(); @} @{ d(); @} "e" @{ f = $1; @};
      |      @dwarning{^~~~~~~~}
@end group
@group
mid.y:2.19-31: @dwarning{warning}: unused value: $3
    2 | exp: @{ a(); @} "b" @dwarning{@{ $$ = c(); @}} @{ d(); @} "e" @{ f = $1; @};
      |                   @dwarning{^~~~~~~~~~~~~}
@end group
@end example

@sp 1

It is sometimes useful to turn midrule actions into regular actions, e.g.,
to factor them, or to escape from their limitations.  For instance, as an
alternative to @emph{typed} midrule action, you may bury the midrule action
inside a nonterminal symbol and to declare a printer and a destructor for
that symbol:

@example
@group
%nterm <context> let
%destructor @{ pop_context ($$); @} let
%printer @{ print_context (yyo, $$); @} let
@end group

%%

@group
stmt:
  let stmt
    @{
      $$ = $2;
      pop_context ($let);
    @};
@end group

@group
let:
  "let" '(' var ')'
    @{
      $let = push_context ();
      declare_variable ($var);
    @};

@end group
@end example




@node Midrule Conflicts
@subsubsection Conflicts due to Midrule Actions
Taking action before a rule is completely recognized often leads to
conflicts since the parser must commit to a parse in order to execute the
action.  For example, the following two rules, without midrule actions,
can coexist in a working parser because the parser can shift the open-brace
token and look at what follows before deciding whether there is a
declaration or not:

@example
@group
compound:
  '@{' declarations statements '@}'
| '@{' statements '@}'
;
@end group
@end example

@noindent
But when we add a midrule action as follows, the rules become nonfunctional:

@example
@group
compound:
  @{ prepare_for_local_variables (); @}
     '@{' declarations statements '@}'
@end group
@group
|    '@{' statements '@}'
;
@end group
@end example

@noindent
Now the parser is forced to decide whether to run the midrule action
when it has read no farther than the open-brace.  In other words, it
must commit to using one rule or the other, without sufficient
information to do it correctly.  (The open-brace token is what is called
the @dfn{lookahead} token at this time, since the parser is still
deciding what to do about it.  @xref{Lookahead}.)

You might think that you could correct the problem by putting identical
actions into the two rules, like this:

@example
@group
compound:
  @{ prepare_for_local_variables (); @}
    '@{' declarations statements '@}'
| @{ prepare_for_local_variables (); @}
    '@{' statements '@}'
;
@end group
@end example

@noindent
But this does not help, because Bison does not realize that the two actions
are identical.  (Bison never tries to understand the C code in an action.)

If the grammar is such that a declaration can be distinguished from a
statement by the first token (which is true in C), then one solution which
does work is to put the action after the open-brace, like this:

@example
@group
compound:
  '@{' @{ prepare_for_local_variables (); @}
    declarations statements '@}'
| '@{' statements '@}'
;
@end group
@end example

@noindent
Now the first token of the following declaration or statement,
which would in any case tell Bison which rule to use, can still do so.

Another solution is to bury the action inside a nonterminal symbol which
serves as a subroutine:

@example
@group
subroutine:
  %empty  @{ prepare_for_local_variables (); @}
;
@end group

@group
compound:
  subroutine '@{' declarations statements '@}'
| subroutine '@{' statements '@}'
;
@end group
@end example

@noindent
Now Bison can execute the action in the rule for @code{subroutine} without
deciding which rule for @code{compound} it will eventually use.


@node Tracking Locations
@section Tracking Locations
@cindex location
@cindex textual location
@cindex location, textual

Though grammar rules and semantic actions are enough to write a fully
functional parser, it can be useful to process some additional information,
especially symbol locations.

The way locations are handled is defined by providing a data type, and
actions to take when rules are matched.

@menu
* Location Type::               Specifying a data type for locations.
* Actions and Locations::       Using locations in actions.
* Location Default Action::     Defining a general way to compute locations.
@end menu

@node Location Type
@subsection Data Type of Locations
@cindex data type of locations
@cindex default location type

Defining a data type for locations is much simpler than for semantic values,
since all tokens and groupings always use the same type.

You can specify the type of locations by defining a macro called
@code{YYLTYPE}, just as you can specify the semantic value type by defining
a @code{YYSTYPE} macro (@pxref{Value Type}).  When @code{YYLTYPE} is not
defined, Bison uses a default structure type with four members:

@example
typedef struct YYLTYPE
@{
  int first_line;
  int first_column;
  int last_line;
  int last_column;
@} YYLTYPE;
@end example

When @code{YYLTYPE} is not defined, at the beginning of the parsing, Bison
initializes all these fields to 1 for @code{yylloc}.  To initialize
@code{yylloc} with a custom location type (or to chose a different
initialization), use the @code{%initial-action} directive.  @xref{Initial
Action Decl}.

@node Actions and Locations
@subsection Actions and Locations
@cindex location actions
@cindex actions, location
@vindex @@$
@vindex @@@var{n}
@vindex @@@var{name}
@vindex @@[@var{name}]

Actions are not only useful for defining language semantics, but also for
describing the behavior of the output parser with locations.

The most obvious way for building locations of syntactic groupings is very
similar to the way semantic values are computed.  In a given rule, several
constructs can be used to access the locations of the elements being matched.
The location of the @var{n}th component of the right hand side is
@code{@@@var{n}}, while the location of the left hand side grouping is
@code{@@$}.

In addition, the named references construct @code{@@@var{name}} and
@code{@@[@var{name}]} may also be used to address the symbol locations.
@xref{Named References}, for more information about using the named
references construct.

Here is a basic example using the default data type for locations:

@example
@group
exp:
  @dots{}
| exp '/' exp
    @{
      @@$.first_column = @@1.first_column;
      @@$.first_line = @@1.first_line;
      @@$.last_column = @@3.last_column;
      @@$.last_line = @@3.last_line;
      if ($3)
        $$ = $1 / $3;
      else
        @{
          $$ = 1;
          fprintf (stderr, "%d.%d-%d.%d: division by zero",
                   @@3.first_line, @@3.first_column,
                   @@3.last_line, @@3.last_column);
        @}
    @}
@end group
@end example

As for semantic values, there is a default action for locations that is
run each time a rule is matched.  It sets the beginning of @code{@@$} to the
beginning of the first symbol, and the end of @code{@@$} to the end of the
last symbol.

With this default action, the location tracking can be fully automatic.  The
example above simply rewrites this way:

@example
@group
exp:
  @dots{}
| exp '/' exp
    @{
      if ($3)
        $$ = $1 / $3;
      else
        @{
          $$ = 1;
          fprintf (stderr, "%d.%d-%d.%d: division by zero",
                   @@3.first_line, @@3.first_column,
                   @@3.last_line, @@3.last_column);
        @}
    @}
@end group
@end example

@vindex yylloc
It is also possible to access the location of the lookahead token, if any,
from a semantic action.
This location is stored in @code{yylloc}.
@xref{Action Features}.

@node Location Default Action
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT
@cindex GLR parsers and @code{YYLLOC_DEFAULT}

Actually, actions are not the best place to compute locations.  Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each
rule.  The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
matched, before the associated action is run.  It is also invoked
while processing a syntax error, to compute the error's location.
Before reporting an unresolvable syntactic ambiguity, a GLR
parser invokes @code{YYLLOC_DEFAULT} recursively to compute the location
of that ambiguity.

Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.

The @code{YYLLOC_DEFAULT} macro takes three parameters.  The first one is
the location of the grouping (the result of the computation).  When a
rule is matched, the second parameter identifies locations of
all right hand side elements of the rule being matched, and the third
parameter is the size of the rule's right hand side.
When a GLR parser reports an ambiguity, which of multiple candidate
right hand sides it passes to @code{YYLLOC_DEFAULT} is undefined.
When processing a syntax error, the second parameter identifies locations
of the symbols that were discarded during error processing, and the third
parameter is the number of discarded symbols.

By default, @code{YYLLOC_DEFAULT} is defined this way:

@example
@group
# define YYLLOC_DEFAULT(Cur, Rhs, N)                      \
do                                                        \
  if (N)                                                  \
    @{                                                     \
      (Cur).first_line   = YYRHSLOC(Rhs, 1).first_line;   \
      (Cur).first_column = YYRHSLOC(Rhs, 1).first_column; \
      (Cur).last_line    = YYRHSLOC(Rhs, N).last_line;    \
      (Cur).last_column  = YYRHSLOC(Rhs, N).last_column;  \
    @}                                                     \
  else                                                    \
    @{                                                     \
      (Cur).first_line   = (Cur).last_line   =            \
        YYRHSLOC(Rhs, 0).last_line;                       \
      (Cur).first_column = (Cur).last_column =            \
        YYRHSLOC(Rhs, 0).last_column;                     \
    @}                                                     \
while (0)
@end group
@end example

@noindent
where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol
in @var{rhs} when @var{k} is positive, and the location of the symbol
just before the reduction when @var{k} and @var{n} are both zero.

When defining @code{YYLLOC_DEFAULT}, you should consider that:

@itemize @bullet
@item
All arguments are free of side-effects.  However, only the first one (the
result) should be modified by @code{YYLLOC_DEFAULT}.

@item
For consistency with semantic actions, valid indexes within the
right hand side range from 1 to @var{n}.  When @var{n} is zero, only 0 is a
valid index, and it refers to the symbol just before the reduction.
During error processing @var{n} is always positive.

@item
Your macro should parenthesize its arguments, if need be, since the
actual arguments may not be surrounded by parentheses.  Also, your
macro should expand to something that can be used as a single
statement when it is followed by a semicolon.
@end itemize

@node Named References
@section Named References
@cindex named references

As described in the preceding sections, the traditional way to refer to any
semantic value or location is a @dfn{positional reference}, which takes the
form @code{$@var{n}}, @code{$$}, @code{@@@var{n}}, and @code{@@$}.  However,
such a reference is not very descriptive.  Moreover, if you later decide to
insert or remove symbols in the right-hand side of a grammar rule, the need
to renumber such references can be tedious and error-prone.

To avoid these issues, you can also refer to a semantic value or location
using a @dfn{named reference}.  First of all, original symbol names may be
used as named references.  For example:

@example
@group
invocation: op '(' args ')'
  @{ $invocation = new_invocation ($op, $args, @@invocation); @}
@end group
@end example

@noindent
Positional and named references can be mixed arbitrarily.  For example:

@example
@group
invocation: op '(' args ')'
  @{ $$ = new_invocation ($op, $args, @@$); @}
@end group
@end example

@noindent
However, sometimes regular symbol names are not sufficient due to
ambiguities:

@example
@group
exp: exp '/' exp
  @{ $exp = $exp / $exp; @} // $exp is ambiguous.

exp: exp '/' exp
  @{ $$ = $1 / $exp; @} // One usage is ambiguous.

exp: exp '/' exp
  @{ $$ = $1 / $3; @} // No error.
@end group
@end example

@noindent
When ambiguity occurs, explicitly declared names may be used for values and
locations.  Explicit names are declared as a bracketed name after a symbol
appearance in rule definitions.  For example:
@example
@group
exp[result]: exp[left] '/' exp[right]
  @{ $result = $left / $right; @}
@end group
@end example

@noindent
In order to access a semantic value generated by a midrule action, an
explicit name may also be declared by putting a bracketed name after the
closing brace of the midrule action code:
@example
@group
exp[res]: exp[x] '+' @{$left = $x;@}[left] exp[right]
  @{ $res = $left + $right; @}
@end group
@end example

@noindent

In references, in order to specify names containing dots and dashes, an explicit
bracketed syntax @code{$[name]} and @code{@@[name]} must be used:
@example
@group
if-stmt: "if" '(' expr ')' "then" then.stmt ';'
  @{ $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); @}
@end group
@end example

It often happens that named references are followed by a dot, dash or other
C punctuation marks and operators.  By default, Bison will read
@samp{$name.suffix} as a reference to symbol value @code{$name} followed by
@samp{.suffix}, i.e., an access to the @code{suffix} field of the semantic
value.  In order to force Bison to recognize @samp{name.suffix} in its
entirety as the name of a semantic value, the bracketed syntax
@samp{$[name.suffix]} must be used.

@node Declarations
@section Bison Declarations
@cindex declarations, Bison
@cindex Bison declarations

The @dfn{Bison declarations} section of a Bison grammar defines the symbols
used in formulating the grammar and the data types of semantic values.
@xref{Symbols}.

All token kind names (but not single-character literal tokens such as
@code{'+'} and @code{'*'}) must be declared.  Nonterminal symbols must be
declared if you need to specify which data type to use for the semantic
value (@pxref{Multiple Types}).

The first rule in the grammar file also specifies the start symbol, by
default.  If you want some other symbol to be the start symbol, you
must declare it explicitly (@pxref{Language and Grammar}).

@menu
* Require Decl::      Requiring a Bison version.
* Token Decl::        Declaring terminal symbols.
* Precedence Decl::   Declaring terminals with precedence and associativity.
* Type Decl::         Declaring the choice of type for a nonterminal symbol.
* Symbol Decls::      Summary of the Syntax of Symbol Declarations.
* Initial Action Decl::  Code run before parsing starts.
* Destructor Decl::   Declaring how symbols are freed.
* Printer Decl::      Declaring how symbol values are displayed.
* Expect Decl::       Suppressing warnings about parsing conflicts.
* Start Decl::        Specifying the start symbol.
* Pure Decl::         Requesting a reentrant parser.
* Push Decl::         Requesting a push parser.
* Decl Summary::      Table of all Bison declarations.
* %define Summary::   Defining variables to adjust Bison's behavior.
* %code Summary::     Inserting code into the parser source.
@end menu

@node Require Decl
@subsection Require a Version of Bison
@cindex version requirement
@cindex requiring a version of Bison
@findex %require

You may require the minimum version of Bison to process the grammar.  If
the requirement is not met, @command{bison} exits with an error (exit
status 63).

@example
%require "@var{version}"
@end example

Some deprecated behaviors are disabled for some required @var{version}:
@table @asis
@item @code{"3.2"} (or better)
The C++ deprecated files @file{position.hh} and @file{stack.hh} are no
longer generated.

@item @code{"3.4"} (or better)
To comply with the
@uref{https://marc.info/?l=graphviz-devel&m=129418103126092, recommendations
of the Graphviz team}, use the @code{.gv} extension instead of @code{.dot}
for the name of the generated DOT file.  @xref{Graphviz}.
@end table


@node Token Decl
@subsection Token Kind Names
@cindex declaring token kind names
@cindex token kind names, declaring
@cindex declaring literal string tokens
@findex %token

The basic way to declare a token kind name (terminal symbol) is as follows:

@example
%token @var{name}
@end example

Bison will convert this into a definition in the parser, so that the
function @code{yylex} (if it is in this file) can use the name @var{name} to
stand for this token kind's code.

Alternatively, you can use @code{%left}, @code{%right}, @code{%precedence},
or @code{%nonassoc} instead of @code{%token}, if you wish to specify
associativity and precedence.  @xref{Precedence Decl}.  However, for
clarity, we recommend to use these directives only to declare associativity
and precedence, and not to add string aliases, semantic types, etc.

You can explicitly specify the numeric code for a token kind by appending a
nonnegative decimal or hexadecimal integer value in the field immediately
following the token name:

@example
%token NUM 300
%token XNUM 0x12d // a GNU extension
@end example

@noindent
It is generally best, however, to let Bison choose the numeric codes for all
token kinds.  Bison will automatically select codes that don't conflict with
each other or with normal characters.

In the event that the stack type is a union, you must augment the
@code{%token} or other token declaration to include the data type
alternative delimited by angle-brackets (@pxref{Multiple Types}).

For example:

@example
@group
%union @{              /* define stack type */
  double val;
  symrec *tptr;
@}
%token <val> NUM      /* define token NUM and its type */
@end group
@end example

You can associate a literal string token with a token kind name by writing
the literal string at the end of a @code{%token} declaration which declares
the name.  For example:

@example
%token ARROW "=>"
@end example

@noindent
For example, a grammar for the C language might specify these names with
equivalent literal string tokens:

@example
%token  <operator>  OR      "||"
%token  <operator>  LE 134  "<="
%left  OR  "<="
@end example

@noindent
Once you equate the literal string and the token kind name, you can use them
interchangeably in further declarations or the grammar rules.  The
@code{yylex} function can use the token name or the literal string to obtain
the token kind code (@pxref{Calling Convention}).

String aliases allow for better error messages using the literal strings
instead of the token names, such as @samp{syntax error, unexpected ||,
expecting number or (} rather than @samp{syntax error, unexpected OR,
expecting NUM or LPAREN}.

String aliases may also be marked for internationalization (@pxref{Token
I18n}):

@example
%token
    OR     "||"
    LPAREN "("
    RPAREN ")"
    '\n'   _("end of line")
  <double>
    NUM    _("number")
@end example

@noindent
would produce in French @samp{erreur de syntaxe, || inattendu, attendait
nombre ou (} rather than @samp{erreur de syntaxe, || inattendu, attendait
number ou (}.

@node Precedence Decl
@subsection Operator Precedence
@cindex precedence declarations
@cindex declaring operator precedence
@cindex operator precedence, declaring

Use the @code{%left}, @code{%right}, @code{%nonassoc}, or @code{%precedence}
declaration to declare a token and specify its precedence and associativity,
all at once.  These are called @dfn{precedence declarations}.
@xref{Precedence}, for general information on operator
precedence.

The syntax of a precedence declaration is nearly the same as that of
@code{%token}: either

@example
%left @var{symbols}@dots{}
@end example

@noindent
or

@example
%left <@var{type}> @var{symbols}@dots{}
@end example

And indeed any of these declarations serves the purposes of @code{%token}.
But in addition, they specify the associativity and relative precedence for
all the @var{symbols}:

@itemize @bullet
@item
The associativity of an operator @var{op} determines how repeated uses of
the operator nest: whether @samp{@var{x} @var{op} @var{y} @var{op} @var{z}}
is parsed by grouping @var{x} with @var{y} first or by grouping @var{y} with
@var{z} first.  @code{%left} specifies left-associativity (grouping @var{x}
with @var{y} first) and @code{%right} specifies right-associativity
(grouping @var{y} with @var{z} first).  @code{%nonassoc} specifies no
associativity, which means that @samp{@var{x} @var{op} @var{y} @var{op}
@var{z}} is considered a syntax error.

@code{%precedence} gives only precedence to the @var{symbols}, and defines
no associativity at all.  Use this to define precedence only, and leave any
potential conflict due to associativity enabled.

@item
The precedence of an operator determines how it nests with other operators.
All the tokens declared in a single precedence declaration have equal
precedence and nest together according to their associativity.  When two
tokens declared in different precedence declarations associate, the one
declared later has the higher precedence and is grouped first.
@end itemize

For backward compatibility, there is a confusing difference between the
argument lists of @code{%token} and precedence declarations.  Only a
@code{%token} can associate a literal string with a token kind name.  A
precedence declaration always interprets a literal string as a reference to
a separate token.  For example:

@example
%left  OR "<="         // Does not declare an alias.
%left  OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".
@end example

@node Type Decl
@subsection Nonterminal Symbols
@cindex declaring value types, nonterminals
@cindex value types, nonterminals, declaring
@findex %nterm
@findex %type

@noindent
When you use @code{%union} to specify multiple value types, you must
declare the value type of each nonterminal symbol for which values are
used.  This is done with a @code{%type} declaration, like this:

@example
%type <@var{type}> @var{nonterminal}@dots{}
@end example

@noindent
Here @var{nonterminal} is the name of a nonterminal symbol, and @var{type}
is the name given in the @code{%union} to the alternative that you want
(@pxref{Union Decl}).  You can give any number of nonterminal symbols in the
same @code{%type} declaration, if they have the same value type.  Use spaces
to separate the symbol names.

While POSIX Yacc allows @code{%type} only for nonterminals, Bison accepts
that this directive be also applied to terminal symbols.  To declare
exclusively nonterminal symbols, use the safer @code{%nterm}:

@example
%nterm <@var{type}> @var{nonterminal}@dots{}
@end example


@node Symbol Decls
@subsection Syntax of Symbol Declarations
@findex %left
@findex %nterm
@findex %token
@findex %type

The syntax of the various directives to declare symbols is as follows.

@example
%token @var{tag}? ( @var{id} @var{number}? @var{string}? )+ ( @var{tag} ( @var{id} @var{number}? @var{string}? )+ )*
%left  @var{tag}? ( @var{id} @var{number}?)+ ( @var{tag} ( @var{id} @var{number}? )+ )*
%type  @var{tag}? ( @var{id} | @var{char} | @var{string} )+ ( @var{tag} ( @var{id} | @var{char} | @var{string} )+ )*
%nterm @var{tag}? @var{id}+ ( @var{tag} @var{id}+ )*
@end example

@noindent
where @var{tag} denotes a type tag such as @samp{<ival>}, @var{id} denotes
an identifier such as @samp{NUM}, @var{number} a decimal or hexadecimal
integer such as @samp{300} or @samp{0x12d}, @var{char} a character literal
such as @samp{'+'}, and @var{string} a string literal such as
@samp{"number"}.  The postfix quantifiers are @samp{?} (zero or one),
@samp{*} (zero or more) and @samp{+} (one or more).

The directives @code{%precedence}, @code{%right} and @code{%nonassoc} behave
like @code{%left}.

@node Initial Action Decl
@subsection Performing Actions before Parsing
@findex %initial-action

Sometimes your parser needs to perform some initializations before parsing.
The @code{%initial-action} directive allows for such arbitrary code.

@deffn {Directive} %initial-action @{ @var{code} @}
@findex %initial-action
Declare that the braced @var{code} must be invoked before parsing each time
@code{yyparse} is called.  The @var{code} may use @code{$$} (or
@code{$<@var{tag}>$}) and @code{@@$} --- initial value and location of the
lookahead --- and the @code{%parse-param}.
@end deffn

For instance, if your locations use a file name, you may use

@example
%parse-param @{ char const *file_name @};
%initial-action
@{
  @@$.initialize (file_name);
@};
@end example


@node Destructor Decl
@subsection Freeing Discarded Symbols
@cindex freeing discarded symbols
@findex %destructor
@findex <*>
@findex <>
During error recovery (@pxref{Error Recovery}), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
until the parser falls on its feet.  If the parser runs out of memory,
or if it returns via @code{YYABORT} or @code{YYACCEPT}, all the
symbols on the stack must be discarded.  Even if the parser succeeds, it
must discard the start symbol.

When discarded symbols convey heap based information, this memory is
lost.  While this behavior can be tolerable for batch parsers, such as
in traditional compilers, it is unacceptable for programs like shells or
protocol implementations that may parse and execute indefinitely.

The @code{%destructor} directive defines code that is called when a
symbol is automatically discarded.

@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
@findex %destructor
Invoke the braced @var{code} whenever the parser discards one of the
@var{symbols}.  Within @var{code}, @code{$$} (or @code{$<@var{tag}>$})
designates the semantic value associated with the discarded symbol, and
@code{@@$} designates its location.  The additional parser parameters are
also available (@pxref{Parser Function}).

When a symbol is listed among @var{symbols}, its @code{%destructor} is called a
per-symbol @code{%destructor}.
You may also define a per-type @code{%destructor} by listing a semantic type
tag among @var{symbols}.
In that case, the parser will invoke this @var{code} whenever it discards any
grammar symbol that has that semantic type tag unless that symbol has its own
per-symbol @code{%destructor}.

Finally, you can define two different kinds of default @code{%destructor}s.
You can place each of @code{<*>} and @code{<>} in the @var{symbols} list of
exactly one @code{%destructor} declaration in your grammar file.
The parser will invoke the @var{code} associated with one of these whenever it
discards any user-defined grammar symbol that has no per-symbol and no per-type
@code{%destructor}.
The parser uses the @var{code} for @code{<*>} in the case of such a grammar
symbol for which you have formally declared a semantic type tag (@code{%token},
@code{%nterm}, and @code{%type}
count as such a declaration, but @code{$<tag>$} does not).
The parser uses the @var{code} for @code{<>} in the case of such a grammar
symbol that has no declared semantic type tag.
@end deffn

@noindent
For example:

@example
%union @{ char *string; @}
%token <string> STRING1 STRING2
%nterm <string> string1 string2
%union @{ char character; @}
%token <character> CHR
%nterm <character> chr
%token TAGLESS

%destructor @{ @} <character>
%destructor @{ free ($$); @} <*>
%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
%destructor @{ printf ("Discarding tagless symbol.\n"); @} <>
@end example

@noindent
guarantees that, when the parser discards any user-defined symbol that has a
semantic type tag other than @code{<character>}, it passes its semantic value
to @code{free} by default.
However, when the parser discards a @code{STRING1} or a @code{string1},
it uses the third @code{%destructor}, which frees it and
prints its line number to @code{stdout} (@code{free} is invoked only once).
Finally, the parser merely prints a message whenever it discards any symbol,
such as @code{TAGLESS}, that has no semantic type tag.

A Bison-generated parser invokes the default @code{%destructor}s only for
user-defined as opposed to Bison-defined symbols.
For example, the parser will not invoke either kind of default
@code{%destructor} for the special Bison-defined symbols @code{$accept},
@code{$undefined}, or @code{$end} (@pxref{Table of Symbols}),
none of which you can reference in your grammar.
It also will not invoke either for the @code{error} token (@pxref{Table of
Symbols}), which is always defined by Bison regardless of whether you
reference it in your grammar.
However, it may invoke one of them for the end token (token 0) if you
redefine it from @code{$end} to, for example, @code{END}:

@example
%token END 0
@end example

@cindex actions in midrule
@cindex midrule actions
Finally, Bison will never invoke a @code{%destructor} for an unreferenced
midrule semantic value (@pxref{Midrule Actions}).
That is, Bison does not consider a midrule to have a semantic value if you
do not reference @code{$$} in the midrule's action or @code{$@var{n}}
(where @var{n} is the right-hand side symbol position of the midrule) in
any later action in that rule.  However, if you do reference either, the
Bison-generated parser will invoke the @code{<>} @code{%destructor} whenever
it discards the midrule symbol.

@ignore
@noindent
In the future, it may be possible to redefine the @code{error} token as a
nonterminal that captures the discarded symbols.
In that case, the parser will invoke the default destructor for it as well.
@end ignore

@sp 1

@cindex discarded symbols
@dfn{Discarded symbols} are the following:

@itemize
@item
stacked symbols popped during the first phase of error recovery,
@item
incoming terminals during the second phase of error recovery,
@item
the current lookahead and the entire stack (except the current
right-hand side symbols) when the parser returns immediately, and
@item
the current lookahead and the entire stack (including the current right-hand
side symbols) when the C++ parser (@file{lalr1.cc}) catches an exception in
@code{parse},
@item
the start symbol, when the parser succeeds.
@end itemize

The parser can @dfn{return immediately} because of an explicit call to
@code{YYABORT} or @code{YYACCEPT}, or failed error recovery, or memory
exhaustion.

Right-hand side symbols of a rule that explicitly triggers a syntax
error via @code{YYERROR} are not discarded automatically.  As a rule
of thumb, destructors are invoked only when user actions cannot manage
the memory.

@node Printer Decl
@subsection Printing Semantic Values
@cindex printing semantic values
@findex %printer
@findex <*>
@findex <>
When run-time traces are enabled (@pxref{Tracing}),
the parser reports its actions, such as reductions.  When a symbol involved
in an action is reported, only its kind is displayed, as the parser cannot
know how semantic values should be formatted.

The @code{%printer} directive defines code that is called when a symbol is
reported.  Its syntax is the same as @code{%destructor} (@pxref{Destructor
Decl}).

@deffn {Directive} %printer @{ @var{code} @} @var{symbols}
@findex %printer
@vindex yyo
@c This is the same text as for %destructor.
Invoke the braced @var{code} whenever the parser displays one of the
@var{symbols}.  Within @var{code}, @code{yyo} denotes the output stream (a
@code{FILE*} in C, and an @code{std::ostream&} in C++), @code{$$} (or
@code{$<@var{tag}>$}) designates the semantic value associated with the
symbol, and @code{@@$} its location.  The additional parser parameters are
also available (@pxref{Parser Function}).

The @var{symbols} are defined as for @code{%destructor} (@pxref{Destructor
Decl}.): they can be per-type (e.g.,
@samp{<ival>}), per-symbol (e.g., @samp{exp}, @samp{NUM}, @samp{"float"}),
typed per-default (i.e., @samp{<*>}, or untyped per-default (i.e.,
@samp{<>}).
@end deffn

@noindent
For example:

@example
%union @{ char *string; @}
%token <string> STRING1 STRING2
%nterm <string> string1 string2
%union @{ char character; @}
%token <character> CHR
%nterm <character> chr
%token TAGLESS

%printer @{ fprintf (yyo, "'%c'", $$); @} <character>
%printer @{ fprintf (yyo, "&%p", $$); @} <*>
%printer @{ fprintf (yyo, "\"%s\"", $$); @} STRING1 string1
%printer @{ fprintf (yyo, "<>"); @} <>
@end example

@noindent
guarantees that, when the parser print any symbol that has a semantic type
tag other than @code{<character>}, it display the address of the semantic
value by default.  However, when the parser displays a @code{STRING1} or a
@code{string1}, it formats it as a string in double quotes.  It performs
only the second @code{%printer} in this case, so it prints only once.
Finally, the parser print @samp{<>} for any symbol, such as @code{TAGLESS},
that has no semantic type tag.  @xref{Mfcalc Traces}, for a complete example.



@node Expect Decl
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings
@cindex preventing warnings about conflicts
@cindex warnings, preventing
@cindex conflicts, suppressing warnings of
@findex %expect
@findex %expect-rr

Bison normally warns if there are any conflicts in the grammar
(@pxref{Shift/Reduce}), but most real grammars
have harmless shift/reduce conflicts which are resolved in a predictable
way and would be difficult to eliminate.  It is desirable to suppress
the warning about these conflicts unless the number of conflicts
changes.  You can do this with the @code{%expect} declaration.

The declaration looks like this:

@example
%expect @var{n}
@end example

Here @var{n} is a decimal integer.  The declaration says there should
be @var{n} shift/reduce conflicts and no reduce/reduce conflicts.
Bison reports an error if the number of shift/reduce conflicts differs
from @var{n}, or if there are any reduce/reduce conflicts.

For deterministic parsers, reduce/reduce conflicts are more
serious, and should be eliminated entirely.  Bison will always report
reduce/reduce conflicts for these parsers.  With GLR
parsers, however, both kinds of conflicts are routine; otherwise,
there would be no need to use GLR parsing.  Therefore, it is
also possible to specify an expected number of reduce/reduce conflicts
in GLR parsers, using the declaration:

@example
%expect-rr @var{n}
@end example

You may wish to be more specific in your
specification of expected conflicts.  To this end, you can also attach
@code{%expect} and @code{%expect-rr} modifiers to individual rules.
The interpretation of these modifiers differs from their use as
declarations.  When attached to rules, they indicate the number of states
in which the rule is involved in a conflict.  You will need to consult the
output resulting from @samp{-v} to determine appropriate numbers to use.
For example, for the following grammar fragment, the first rule for
@code{empty_dims} appears in two states in which the @samp{[} token is a
lookahead.  Having determined that, you can document this fact with an
@code{%expect} modifier as follows:

@example
dims:
  empty_dims
| '[' expr ']' dims
;

empty_dims:
  %empty   %expect 2
| empty_dims '[' ']'
;
@end example

Mid-rule actions generate implicit rules that are also subject to conflicts
(@pxref{Midrule Conflicts}). To attach
an @code{%expect} or @code{%expect-rr} annotation to an implicit
mid-rule action's rule, put it before the action.  For example,

@example
%glr-parser
%expect-rr 1

%%

clause:
  "condition" %expect-rr 1 @{ value_mode(); @} '(' exprs ')'
| "condition" %expect-rr 1 @{ class_mode(); @} '(' types ')'
;
@end example

@noindent
Here, the appropriate mid-rule action will not be determined until after
the @samp{(} token is shifted.  Thus,
the two actions will clash with each other, and we should expect one
reduce/reduce conflict for each.

In general, using @code{%expect} involves these steps:

@itemize @bullet
@item
Compile your grammar without @code{%expect}.  Use the @samp{-v} option
to get a verbose list of where the conflicts occur.  Bison will also
print the number of conflicts.

@item
Check each of the conflicts to make sure that Bison's default
resolution is what you really want.  If not, rewrite the grammar and
go back to the beginning.

@item
Add an @code{%expect} declaration, copying the number @var{n} from the
number that Bison printed.  With GLR parsers, add an
@code{%expect-rr} declaration as well.

@item
Optionally, count up the number of states in which one or more
conflicted reductions for particular rules appear and add these numbers
to the affected rules as @code{%expect-rr} or @code{%expect} modifiers
as appropriate.  Rules that are in conflict appear in the output listing
surrounded by square brackets or, in the case of reduce/reduce conflicts,
as reductions having the same lookahead symbol as a square-bracketed
reduction in the same state.
@end itemize

Now Bison will report an error if you introduce an unexpected conflict,
but will keep silent otherwise.

@node Start Decl
@subsection The Start-Symbol
@cindex declaring the start symbol
@cindex start symbol, declaring
@cindex default start symbol
@findex %start

Bison assumes by default that the start symbol for the grammar is the first
nonterminal specified in the grammar specification section.  The programmer
may override this restriction with the @code{%start} declaration as follows:

@example
%start @var{symbol}
@end example

@node Pure Decl
@subsection A Pure (Reentrant) Parser
@cindex reentrant parser
@cindex pure parser
@findex %define api.pure

A @dfn{reentrant} program is one which does not alter in the course of
execution; in other words, it consists entirely of @dfn{pure} (read-only)
code.  Reentrancy is important whenever asynchronous execution is possible;
for example, a nonreentrant program may not be safe to call from a signal
handler.  In systems with multiple threads of control, a nonreentrant
program must be called only within interlocks.

Normally, Bison generates a parser which is not reentrant.  This is
suitable for most uses, and it permits compatibility with Yacc.  (The
standard Yacc interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with @code{yylex},
including @code{yylval} and @code{yylloc}.)

Alternatively, you can generate a pure, reentrant parser.  The Bison
declaration @samp{%define api.pure} says that you want the parser to be
reentrant.  It looks like this:

@example
%define api.pure full
@end example

The result is that the communication variables @code{yylval} and
@code{yylloc} become local variables in @code{yyparse}, and a different
calling convention is used for the lexical analyzer function @code{yylex}.
@xref{Pure Calling}, for the details of this.  The variable @code{yynerrs}
becomes local in @code{yyparse} in pull mode but it becomes a member of
@code{yypstate} in push mode.  (@pxref{Error Reporting Function}).  The
convention for calling @code{yyparse} itself is unchanged.

Whether the parser is pure has nothing to do with the grammar rules.
You can generate either a pure parser or a nonreentrant parser from any
valid grammar.

@node Push Decl
@subsection A Push Parser
@cindex push parser
@cindex push parser
@findex %define api.push-pull

A pull parser is called once and it takes control until all its input
is completely parsed.  A push parser, on the other hand, is called
each time a new token is made available.

A push parser is typically useful when the parser is part of a
main event loop in the client's application.  This is typically
a requirement of a GUI, when the main event loop needs to be triggered
within a certain time period.

Normally, Bison generates a pull parser.
The following Bison declaration says that you want the parser to be a push
parser (@pxref{%define Summary}):

@example
%define api.push-pull push
@end example

In almost all cases, you want to ensure that your push parser is also
a pure parser (@pxref{Pure Decl}).  The only
time you should create an impure push parser is to have backwards
compatibility with the impure Yacc pull mode interface.  Unless you know
what you are doing, your declarations should look like this:

@example
%define api.pure full
%define api.push-pull push
@end example

There is a major notable functional difference between the pure push parser
and the impure push parser.  It is acceptable for a pure push parser to have
many parser instances, of the same type of parser, in memory at the same time.
An impure push parser should only use one parser at a time.

When a push parser is selected, Bison will generate some new symbols in
the generated parser.  @code{yypstate} is a structure that the generated
parser uses to store the parser's state.  @code{yypstate_new} is the
function that will create a new parser instance.  @code{yypstate_delete}
will free the resources associated with the corresponding parser instance.
Finally, @code{yypush_parse} is the function that should be called whenever a
token is available to provide the parser.  A trivial example
of using a pure push parser would look like this:

@example
int status;
yypstate *ps = yypstate_new ();
do @{
  status = yypush_parse (ps, yylex (), NULL);
@} while (status == YYPUSH_MORE);
yypstate_delete (ps);
@end example

If the user decided to use an impure push parser, a few things about
the generated parser will change.  The @code{yychar} variable becomes
a global variable instead of a variable in the @code{yypush_parse} function.
For this reason, the signature of the @code{yypush_parse} function is
changed to remove the token as a parameter.  A nonreentrant push parser
example would thus look like this:

@example
extern int yychar;
int status;
yypstate *ps = yypstate_new ();
do @{
  yychar = yylex ();
  status = yypush_parse (ps);
@} while (status == YYPUSH_MORE);
yypstate_delete (ps);
@end example

That's it. Notice the next token is put into the global variable @code{yychar}
for use by the next invocation of the @code{yypush_parse} function.

Bison also supports both the push parser interface along with the pull parser
interface in the same generated parser.  In order to get this functionality,
you should replace the @samp{%define api.push-pull push} declaration with the
@samp{%define api.push-pull both} declaration.  Doing this will create all of
the symbols mentioned earlier along with the two extra symbols, @code{yyparse}
and @code{yypull_parse}.  @code{yyparse} can be used exactly as it normally
would be used.  However, the user should note that it is implemented in the
generated parser by calling @code{yypull_parse}.
This makes the @code{yyparse} function that is generated with the
@samp{%define api.push-pull both} declaration slower than the normal
@code{yyparse} function.  If the user
calls the @code{yypull_parse} function it will parse the rest of the input
stream.  It is possible to @code{yypush_parse} tokens to select a subgrammar
and then @code{yypull_parse} the rest of the input stream.  If you would like
to switch back and forth between between parsing styles, you would have to
write your own @code{yypull_parse} function that knows when to quit looking
for input.  An example of using the @code{yypull_parse} function would look
like this:

@example
yypstate *ps = yypstate_new ();
yypull_parse (ps); /* Will call the lexer */
yypstate_delete (ps);
@end example

Adding the @samp{%define api.pure} declaration does exactly the same thing to
the generated parser with @samp{%define api.push-pull both} as it did for
@samp{%define api.push-pull push}.

@node Decl Summary
@subsection Bison Declaration Summary
@cindex Bison declaration summary
@cindex declaration summary
@cindex summary, Bison declaration

Here is a summary of the declarations used to define a grammar:

@deffn {Directive} %union
Declare the collection of data types that semantic values may have
(@pxref{Union Decl}).
@end deffn

@deffn {Directive} %token
Declare a terminal symbol (token kind name) with no precedence
or associativity specified (@pxref{Token Decl}).
@end deffn

@deffn {Directive} %right
Declare a terminal symbol (token kind name) that is right-associative
(@pxref{Precedence Decl}).
@end deffn

@deffn {Directive} %left
Declare a terminal symbol (token kind name) that is left-associative
(@pxref{Precedence Decl}).
@end deffn

@deffn {Directive} %nonassoc
Declare a terminal symbol (token kind name) that is nonassociative
(@pxref{Precedence Decl}).
Using it in a way that would be associative is a syntax error.
@end deffn

@ifset defaultprec
@deffn {Directive} %default-prec
Assign a precedence to rules lacking an explicit @code{%prec} modifier
(@pxref{Contextual Precedence}).
@end deffn
@end ifset

@deffn {Directive} %nterm
Declare the type of semantic values for a nonterminal symbol (@pxref{Type
Decl}).
@end deffn

@deffn {Directive} %type
Declare the type of semantic values for a symbol (@pxref{Type Decl}).
@end deffn

@deffn {Directive} %start
Specify the grammar's start symbol (@pxref{Start Decl}).
@end deffn

@deffn {Directive} %expect
Declare the expected number of shift-reduce conflicts, either overall or
for a given rule
(@pxref{Expect Decl}).
@end deffn

@deffn {Directive} %expect-rr
Declare the expected number of reduce-reduce conflicts, either overall or
for a given rule
(@pxref{Expect Decl}).
@end deffn


@sp 1
@noindent
In order to change the behavior of @command{bison}, use the following
directives:

@deffn {Directive} %code @{@var{code}@}
@deffnx {Directive} %code @var{qualifier} @{@var{code}@}
@findex %code
Insert @var{code} verbatim into the output parser source at the
default location or at the location specified by @var{qualifier}.
@xref{%code Summary}.
@end deffn

@deffn {Directive} %debug
Instrument the parser for traces.  Obsoleted by @samp{%define
parse.trace}.
@xref{Tracing}.
@end deffn

@deffn {Directive} %define @var{variable}
@deffnx {Directive} %define @var{variable} @var{value}
@deffnx {Directive} %define @var{variable} @{@var{value}@}
@deffnx {Directive} %define @var{variable} "@var{value}"
Define a variable to adjust Bison's behavior.  @xref{%define Summary}.
@end deffn

@deffn {Directive} %defines
Write a parser header file containing definitions for the token kind names
defined in the grammar as well as a few other declarations.  If the parser
implementation file is named @file{@var{name}.c} then the parser header file
is named @file{@var{name}.h}.

For C parsers, the parser header file declares @code{YYSTYPE} unless
@code{YYSTYPE} is already defined as a macro or you have used a
@code{<@var{type}>} tag without using @code{%union}.  Therefore, if you are
using a @code{%union} (@pxref{Multiple Types}) with components that require
other definitions, or if you have defined a @code{YYSTYPE} macro or type
definition (@pxref{Value Type}), you need to arrange for these definitions
to be propagated to all modules, e.g., by putting them in a prerequisite
header that is included both by your parser and by any other module that
needs @code{YYSTYPE}.

Unless your parser is pure, the parser header file declares
@code{yylval} as an external variable.  @xref{Pure Decl}.

If you have also used locations, the parser header file declares
@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of the
@code{YYSTYPE} macro and @code{yylval}.  @xref{Tracking Locations}.

This parser header file is normally essential if you wish to put the
definition of @code{yylex} in a separate source file, because
@code{yylex} typically needs to be able to refer to the
above-mentioned declarations and to the token kind codes.  @xref{Token
Values}.

@findex %code requires
@findex %code provides
If you have declared @code{%code requires} or @code{%code provides}, the output
header also contains their code.
@xref{%code Summary}.

@cindex Header guard
The generated header is protected against multiple inclusions with a C
preprocessor guard: @samp{YY_@var{PREFIX}_@var{FILE}_INCLUDED}, where
@var{PREFIX} and @var{FILE} are the prefix (@pxref{Multiple Parsers}) and
generated file name turned uppercase, with each series of non alphanumerical
characters converted to a single underscore.

For instance with @samp{%define api.prefix @{calc@}} and @samp{%defines
"lib/parse.h"}, the header will be guarded as follows.
@example
#ifndef YY_CALC_LIB_PARSE_H_INCLUDED
# define YY_CALC_LIB_PARSE_H_INCLUDED
...
#endif /* ! YY_CALC_LIB_PARSE_H_INCLUDED */
@end example
@end deffn

@deffn {Directive} %defines @var{defines-file}
Same as above, but save in the file @file{@var{defines-file}}.
@end deffn

@deffn {Directive} %destructor
Specify how the parser should reclaim the memory associated to
discarded symbols.  @xref{Destructor Decl}.
@end deffn

@deffn {Directive} %file-prefix "@var{prefix}"
Specify a prefix to use for all Bison output file names.  The names
are chosen as if the grammar file were named @file{@var{prefix}.y}.
@end deffn

@deffn {Directive} %language "@var{language}"
Specify the programming language for the generated parser.  Currently
supported languages include C, C++, and Java.  @var{language} is
case-insensitive.
@end deffn

@deffn {Directive} %locations
Generate the code processing the locations (@pxref{Action Features}).  This
mode is enabled as soon as the grammar uses the special @samp{@@@var{n}}
tokens, but if your grammar does not use it, using @samp{%locations} allows
for more accurate syntax error messages.
@end deffn

@deffn {Directive} %name-prefix "@var{prefix}"
Obsoleted by @samp{%define api.prefix @{@var{prefix}@}}.  @xref{Multiple
Parsers}.  For C++ parsers, see the
@samp{%define api.namespace} documentation in this section.

Rename the external symbols used in the parser so that they start with
@var{prefix} instead of @samp{yy}.  The precise list of symbols renamed in C
parsers is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
@code{yylval}, @code{yychar}, @code{yydebug}, and (if locations are used)
@code{yylloc}.  If you use a push parser, @code{yypush_parse},
@code{yypull_parse}, @code{yypstate}, @code{yypstate_new} and
@code{yypstate_delete} will also be renamed.  For example, if you use
@samp{%name-prefix "c_"}, the names become @code{c_parse}, @code{c_lex}, and
so on.

Contrary to defining @code{api.prefix}, some symbols are @emph{not} renamed
by @code{%name-prefix}, for instance @code{YYDEBUG}, @code{YYTOKENTYPE},
@code{yytoken_kind_t}, @code{YYSTYPE}, @code{YYLTYPE}.
@end deffn

@ifset defaultprec
@deffn {Directive} %no-default-prec
Do not assign a precedence to rules lacking an explicit @code{%prec}
modifier (@pxref{Contextual Precedence}).
@end deffn
@end ifset

@deffn {Directive} %no-lines
Don't generate any @code{#line} preprocessor commands in the parser
implementation file.  Ordinarily Bison writes these commands in the parser
implementation file so that the C compiler and debuggers will associate
errors and object code with your source file (the grammar file).  This
directive causes them to associate errors with the parser implementation
file, treating it as an independent source file in its own right.
@end deffn

@deffn {Directive} %output "@var{file}"
Generate the parser implementation in @file{@var{file}}.
@end deffn

@deffn {Directive} %pure-parser
Deprecated version of @samp{%define api.pure} (@pxref{%define
Summary}), for which Bison is more careful to warn about
unreasonable usage.
@end deffn

@deffn {Directive} %require "@var{version}"
Require version @var{version} or higher of Bison.  @xref{Require Decl}.
@end deffn

@deffn {Directive} %skeleton "@var{file}"
Specify the skeleton to use.

@c You probably don't need this option unless you are developing Bison.
@c You should use @code{%language} if you want to specify the skeleton for a
@c different language, because it is clearer and because it will always choose the
@c correct skeleton for non-deterministic or push parsers.

If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton
file in the Bison installation directory.
If it does, @var{file} is an absolute file name or a file name relative to the
directory of the grammar file.
This is similar to how most shells resolve commands.
@end deffn

@deffn {Directive} %token-table
This feature is obsolescent, avoid it in new projects.

Generate an array of token names in the parser implementation file.  The
name of the array is @code{yytname}; @code{yytname[@var{i}]} is the name of
the token whose internal Bison token code is @var{i}.  The first three
elements of @code{yytname} correspond to the predefined tokens
@code{"$end"}, @code{"error"}, and @code{"$undefined"}; after these come the
symbols defined in the grammar file.

The name in the table includes all the characters needed to represent the
token in Bison.  For single-character literals and literal strings, this
includes the surrounding quoting characters and any escape sequences.  For
example, the Bison single-character literal @code{'+'} corresponds to a
three-character name, represented in C as @code{"'+'"}; and the Bison
two-character literal string @code{"\\/"} corresponds to a five-character
name, represented in C as @code{"\"\\\\/\""}.

When you specify @code{%token-table}, Bison also generates macro definitions
for macros @code{YYNTOKENS}, @code{YYNNTS}, and @code{YYNRULES}, and
@code{YYNSTATES}:

@table @code
@item YYNTOKENS
The number of terminal symbols, i.e., the highest token code, plus one.
@item YYNNTS
The number of nonterminal symbols.
@item YYNRULES
The number of grammar rules,
@item YYNSTATES
The number of parser states (@pxref{Parser States}).
@end table

Here's code for looking up a multicharacter token in @code{yytname},
assuming that the characters of the token are stored in @code{token_buffer},
and assuming that the token does not contain any characters like @samp{"}
that require escaping.

@example
for (int i = 0; i < YYNTOKENS; i++)
  if (yytname[i]
      && yytname[i][0] == '"'
      && ! strncmp (yytname[i] + 1, token_buffer,
                    strlen (token_buffer))
      && yytname[i][strlen (token_buffer) + 1] == '"'
      && yytname[i][strlen (token_buffer) + 2] == 0)
    break;
@end example

This method is discouraged: the primary purpose of string aliases is forging
good error messages, not describing the spelling of keywords.  In addition,
looking for the token kind at runtime incurs a (small but noticeable) cost.

Finally, @code{%token-table} is incompatible with the @code{custom} and
@code{detailed} values of the @code{parse.error} @code{%define} variable.
@end deffn

@deffn {Directive} %verbose
Write an extra output file containing verbose descriptions of the parser
states and what is done for each type of lookahead token in that state.
@xref{Understanding}, for more information.
@end deffn

@deffn {Directive} %yacc
Pretend the option @option{--yacc} was given, i.e., imitate Yacc, including
its naming conventions.  Only makes sense with the @file{yacc.c}
skeleton. @xref{Tuning the Parser}, for more.

Of course @code{%yacc} is a Bison extension@dots{}
@end deffn


@node %define Summary
@subsection %define Summary

There are many features of Bison's behavior that can be controlled by
assigning the feature a single value.  For historical reasons, some such
features are assigned values by dedicated directives, such as @code{%start},
which assigns the start symbol.  However, newer such features are associated
with variables, which are assigned by the @code{%define} directive:

@deffn {Directive} %define @var{variable}
@deffnx {Directive} %define @var{variable} @var{value}
@deffnx {Directive} %define @var{variable} @{@var{value}@}
@deffnx {Directive} %define @var{variable} "@var{value}"
Define @var{variable} to @var{value}.

The type of the values depend on the syntax.  Braces denote value in the
target language (e.g., a namespace, a type, etc.).  Keyword values (no
delimiters) denote finite choice (e.g., a variation of a feature).  String
values denote remaining cases (e.g., a file name).

It is an error if a @var{variable} is defined by @code{%define} multiple
times, but see @ref{Tuning the Parser,,@option{-D @var{name}[=@var{value}]}}.
@end deffn

The rest of this section summarizes variables and values that @code{%define}
accepts.

Some @var{variable}s take Boolean values.  In this case, Bison will complain
if the variable definition does not meet one of the following four
conditions:

@enumerate
@item @code{@var{value}} is @code{true}

@item @code{@var{value}} is omitted (or @code{""} is specified).
This is equivalent to @code{true}.

@item @code{@var{value}} is @code{false}.

@item @var{variable} is never defined.
In this case, Bison selects a default value.
@end enumerate

What @var{variable}s are accepted, as well as their meanings and default
values, depend on the selected target language and/or the parser skeleton
(@pxref{Decl Summary}, @pxref{Decl Summary}).
Unaccepted @var{variable}s produce an error.  Some of the accepted
@var{variable}s are described below.


@c ================================================== api.namespace
@deffn Directive {%define api.namespace} @{@var{namespace}@}
@itemize
@item Languages(s): C++

@item Purpose: Specify the namespace for the parser class.
For example, if you specify:

@example
%define api.namespace @{foo::bar@}
@end example

Bison uses @code{foo::bar} verbatim in references such as:

@example
foo::bar::parser::semantic_type
@end example

However, to open a namespace, Bison removes any leading @code{::} and then
splits on any remaining occurrences:

@example
namespace foo @{ namespace bar @{
  class position;
  class location;
@} @}
@end example

@item Accepted Values:
Any absolute or relative C++ namespace reference without a trailing
@code{"::"}.  For example, @code{"foo"} or @code{"::foo::bar"}.

@item Default Value:
@code{yy}, unless you used the obsolete @samp{%name-prefix "@var{prefix}"}
directive.
@end itemize
@end deffn
@c api.namespace


@c ================================================== api.location.file
@deffn {Directive} {%define api.location.file} "@var{file}"
@deffnx {Directive} {%define api.location.file} @code{none}

@itemize @bullet
@item Language(s): C++

@item Purpose:
Define the name of the file in which Bison's default location and position
types are generated. @xref{Exposing the Location Classes}.

@item Accepted Values:
@table @asis
@item @code{none}
If locations are enabled, generate the definition of the @code{position} and
@code{location} classes in the header file if @code{%defines}, otherwise in
the parser implementation.

@item "@var{file}"
Generate the definition of the @code{position} and @code{location} classes
in @var{file}.  This file name can be relative (to where the parser file is
output) or absolute.
@end table

@item Default Value:
Not applicable if locations are not enabled, or if a user location type is
specified (see @code{api.location.type}).  Otherwise, Bison's
@code{location} is generated in @file{location.hh} (@pxref{C++ location}).

@item History:
Introduced in Bison 3.2.
@end itemize
@end deffn


@c ================================================== api.location.file
@deffn {Directive} {%define api.location.include} @{"@var{file}"@}
@deffnx {Directive} {%define api.location.include} @{<@var{file}>@}

@itemize @bullet
@item Language(s): C++

@item Purpose:
Specify how the generated file that defines the @code{position} and
@code{location} classes is included.  This makes sense when the
@code{location} class is exposed to the rest of your application/library in
another directory.  @xref{Exposing the Location Classes}.

@item Accepted Values: Argument for @code{#include}.

@item Default Value:
@samp{"@var{dir}/location.hh"} where @var{dir} is the directory part of the
output.  For instance @file{src/parse} if
@option{--output=src/parse/parser.cc} was given.

@item History:
Introduced in Bison 3.2.
@end itemize

@end deffn


@c ================================================== api.location.type
@deffn {Directive} {%define api.location.type} @{@var{type}@}

@itemize @bullet
@item Language(s): C, C++, Java

@item Purpose: Define the location type.
@xref{User Defined Location Type}.

@item Accepted Values: String

@item Default Value: none

@item History:
Introduced in Bison 2.7 for C++ and Java, in Bison 3.4 for C.
@end itemize
@end deffn


@c ================================================== api.parser.class
@deffn Directive {%define api.parser.class} @{@var{name}@}
@itemize @bullet
@item Language(s):
C++, Java

@item Purpose:
The name of the parser class.

@item Accepted Values:
Any valid identifier.

@item Default Value:
In C++, @code{parser}.  In Java, @code{YYParser} or
@code{@var{api.prefix}Parser} (@pxref{Java Bison Interface}).

@item History:
Introduced in Bison 3.3 to replace @code{parser_class_name}.
@end itemize
@end deffn
@c api.parser.class


@c ================================================== api.prefix
@deffn {Directive} {%define api.prefix} @{@var{prefix}@}

@itemize @bullet
@item Language(s): All

@item Purpose: Rename exported symbols.
@xref{Multiple Parsers}.

@item Accepted Values: String

@item Default Value: @code{YY} for Java, @code{yy} otherwise.

@item History: introduced in Bison 2.6
@end itemize
@end deffn


@c ================================================== api.pure
@deffn Directive {%define api.pure} @var{purity}

@itemize @bullet
@item Language(s): C

@item Purpose: Request a pure (reentrant) parser program.
@xref{Pure Decl}.

@item Accepted Values: @code{true}, @code{false}, @code{full}

The value may be omitted: this is equivalent to specifying @code{true}, as is
the case for Boolean values.

When @code{%define api.pure full} is used, the parser is made reentrant. This
changes the signature for @code{yylex} (@pxref{Pure Calling}), and also that of
@code{yyerror} when the tracking of locations has been activated, as shown
below.

The @code{true} value is very similar to the @code{full} value, the only
difference is in the signature of @code{yyerror} on Yacc parsers without
@code{%parse-param}, for historical reasons.

I.e., if @samp{%locations %define api.pure} is passed then the prototypes for
@code{yyerror} are:

@example
void yyerror (char const *msg);                 // Yacc parsers.
void yyerror (YYLTYPE *locp, char const *msg);  // GLR parsers.
@end example

But if @samp{%locations %define api.pure %parse-param @{int *nastiness@}} is
used, then both parsers have the same signature:

@example
void yyerror (YYLTYPE *llocp, int *nastiness, char const *msg);
@end example

(@pxref{Error Reporting Function})

@item Default Value: @code{false}

@item History:
the @code{full} value was introduced in Bison 2.7
@end itemize
@end deffn
@c api.pure



@c ================================================== api.push-pull
@deffn Directive {%define api.push-pull} @var{kind}

@itemize @bullet
@item Language(s): C (deterministic parsers only), Java

@item Purpose: Request a pull parser, a push parser, or both.
@xref{Push Decl}.

@item Accepted Values: @code{pull}, @code{push}, @code{both}

@item Default Value: @code{pull}
@end itemize
@end deffn
@c api.push-pull



@c ================================================== api.symbol.prefix
@deffn Directive {%define api.symbol.prefix} @{@var{prefix}@}

@itemize
@item Languages(s): all

@item Purpose:
Add a prefix to the name of the symbol kinds.  For instance

@example
%define api.symbol.prefix @{S_@}
%token FILE for ERROR
%%
start: FILE for ERROR;
@end example

@noindent
generates this definition in C:

@example
/* Symbol kind.  */
enum yysymbol_kind_t
@{
  S_YYEMPTY = -2,   /* No symbol.  */
  S_YYEOF = 0,      /* $end  */
  S_YYERROR = 1,    /* error  */
  S_YYUNDEF = 2,    /* $undefined  */
  S_FILE = 3,       /* FILE  */
  S_for = 4,        /* for  */
  S_ERROR = 5,      /* ERROR  */
  S_YYACCEPT = 6,   /* $accept  */
  S_start = 7       /* start  */
@};
@end example

@item Accepted Values:
Any non empty string.  Must be a valid identifier in the target language
(typically a non empty sequence of letters, underscores, and ---not at the
beginning--- digits).

The empty prefix is invalid:
@itemize
@item
in C it would create collision with the @code{YYERROR} macro, and
potentially token kind definitions and symbol kind definitions would
collide;
@item
unnamed symbols (such as @samp{'+'}) have a name which starts with a digit;
@item
even in languages with scoped enumerations such as Java, an empty prefix is
dangerous: symbol names may collide with the target language keywords, or
with other members of the @code{SymbolKind} class.
@end itemize


@item Default Value:
@code{YYSYMBOL_} in C.  @code{S_} in C++, D and Java.
@item History:
introduced in Bison 3.6.
@end itemize
@end deffn
@c api.symbol.prefix


@c ================================================== api.token.constructor
@deffn Directive {%define api.token.constructor}

@itemize @bullet
@item Language(s):
C++

@item Purpose:
When variant-based semantic values are enabled (@pxref{C++ Variants}),
request that symbols be handled as a whole (type, value, and possibly
location) in the scanner.  @xref{Complete Symbols}, for details.

@item Accepted Values:
Boolean.

@item Default Value:
@code{false}
@item History:
introduced in Bison 3.0.
@end itemize
@end deffn
@c api.token.constructor


@c ================================================== api.token.prefix
@anchor{api-token-prefix}
@deffn Directive {%define api.token.prefix} @{@var{prefix}@}
@itemize
@item Languages(s): all

@item Purpose:
Add a prefix to the token names when generating their definition in the
target language.  For instance

@example
%define api.token.prefix @{TOK_@}
%token FILE for ERROR
%%
start: FILE for ERROR;
@end example

@noindent
generates the definition of the symbols @code{TOK_FILE}, @code{TOK_for}, and
@code{TOK_ERROR} in the generated source files.  In particular, the scanner
must use these prefixed token names, while the grammar itself may still use
the short names (as in the sample rule given above).  The generated
informational files (@file{*.output}, @file{*.xml}, @file{*.gv}) are not
modified by this prefix.

Bison also prefixes the generated member names of the semantic value union.
@xref{Type Generation}, for more
details.

See @ref{Calc++ Parser} and @ref{Calc++ Scanner}, for a complete example.

@item Accepted Values:
Any string.  Must be a valid identifier prefix in the target language
(typically, a possibly empty sequence of letters, underscores, and ---not at
the beginning--- digits).

@item Default Value:
empty
@item History:
introduced in Bison 3.0.
@end itemize
@end deffn
@c api.token.prefix


@c ================================================== api.token.raw
@deffn Directive {%define api.token.raw}

@itemize @bullet
@item Language(s):
all

@item Purpose:
The output files normally define the enumeration of the @emph{token kinds}
with Yacc-compatible token codes: sequential numbers starting at 257 except
for single character tokens which stand for themselves (e.g., in ASCII,
@samp{'a'} is numbered 65).  The parser however uses @emph{symbol kinds}
which are assigned numbers sequentially starting at 0.  Therefore each time
the scanner returns an (external) token kind, it must be mapped to the
(internal) symbol kind.

When @code{api.token.raw} is set, the code of the token kinds are forced to
coincide with the symbol kind.  This saves one table lookup per token to map
them from the token kind to the symbol kind, and also saves the generation
of the mapping table.  The gain is typically moderate, but in extreme cases
(very simple user actions), a 10% improvement can be observed.

When @code{api.token.raw} is set, the grammar cannot use character literals
(such as @samp{'a'}).

@item Accepted Values: Boolean.

@item Default Value:
@code{false}
@item History:
introduced in Bison 3.5.  Was initially introduced in Bison 1.25 as
@samp{%raw}, but never worked and was removed in Bison 1.29.
@end itemize
@end deffn
@c api.token.raw


@c ================================================== api.value.automove
@deffn Directive {%define api.value.automove}

@itemize @bullet
@item Language(s):
C++

@item Purpose:
Let occurrences of semantic values of the right-hand sides of a rule be
implicitly turned in rvalues.  When enabled, a grammar such as:

@example
exp:
  "number"     @{ $$ = make_number ($1); @}
| exp "+" exp  @{ $$ = make_binary (add, $1, $3); @}
| "(" exp ")"  @{ $$ = $2; @}
@end example

@noindent
is actually compiled as if you had written:

@example
exp:
  "number"     @{ $$ = make_number (std::move ($1)); @}
| exp "+" exp  @{ $$ = make_binary (add,
                                   std::move ($1),
                                   std::move ($3)); @}
| "(" exp ")"  @{ $$ = std::move ($2); @}
@end example

Using a value several times with automove enabled is typically an error.
For instance, instead of:

@example
exp: "twice" exp  @{ $$ = make_binary (add, $2, $2); @}
@end example

@noindent
write:

@example
exp: "twice" exp @{ auto v = $2; $$ = make_binary (add, v, v); @}
@end example

@noindent
It is tempting to use @code{std::move} on one of the @code{v}, but the
argument evaluation order in C++ is unspecified.

@item Accepted Values:
Boolean.

@item Default Value:
@code{false}
@item History:
introduced in Bison 3.2
@end itemize
@end deffn
@c api.value.automove


@c ================================================== api.value.type
@deffn Directive {%define api.value.type} @var{support}
@deffnx Directive {%define api.value.type} @{@var{type}@}
@itemize @bullet
@item Language(s):
all

@item Purpose:
The type for semantic values.

@item Accepted Values:
@table @asis
@item @samp{@{@}}
This grammar has no semantic value at all.  This is not properly supported
yet.
@item @samp{union-directive} (C, C++)
The type is defined thanks to the @code{%union} directive.  You don't have
to define @code{api.value.type} in that case, using @code{%union} suffices.
@xref{Union Decl}.
For instance:
@example
%define api.value.type union-directive
%union
@{
  int ival;
  char *sval;
@}
%token <ival> INT "integer"
%token <sval> STR "string"
@end example

@item @samp{union} (C, C++)
The symbols are defined with type names, from which Bison will generate a
@code{union}.  For instance:
@example
%define api.value.type union
%token <int> INT "integer"
%token <char *> STR "string"
@end example
Most C++ objects cannot be stored in a @code{union}, use @samp{variant}
instead.

@item @samp{variant} (C++)
This is similar to @code{union}, but special storage techniques are used to
allow any kind of C++ object to be used. For instance:
@example
%define api.value.type variant
%token <int> INT "integer"
%token <std::string> STR "string"
@end example
@xref{C++ Variants}.

@item @samp{@{@var{type}@}}
Use this @var{type} as semantic value.
@example
%code requires
@{
  struct my_value
  @{
    enum
    @{
      is_int, is_str
    @} kind;
    union
    @{
      int ival;
      char *sval;
    @} u;
  @};
@}
%define api.value.type @{struct my_value@}
%token <u.ival> INT "integer"
%token <u.sval> STR "string"
@end example
@end table

@item Default Value:
@itemize @minus
@item
@code{union-directive} if @code{%union} is used, otherwise @dots{}
@item
@code{int} if type tags are used (i.e., @samp{%token <@var{type}>@dots{}} or
@samp{%nterm <@var{type}>@dots{}} is used), otherwise @dots{}
@item
undefined.
@end itemize

@item History:
introduced in Bison 3.0.  Was introduced for Java only in 2.3b as
@code{stype}.
@end itemize
@end deffn
@c api.value.type


@c ================================================== api.value.union.name
@deffn Directive {%define api.value.union.name} @var{name}
@itemize @bullet
@item Language(s):
C

@item Purpose:
The tag of the generated @code{union} (@emph{not} the name of the
@code{typedef}).  This variable is set to @code{@var{id}} when @samp{%union
@var{id}} is used.  There is no clear reason to give this union a name.

@item Accepted Values:
Any valid identifier.

@item Default Value:
@code{YYSTYPE}.

@item History:
Introduced in Bison 3.0.3.
@end itemize
@end deffn
@c api.value.type


@c ================================================== location_type
@deffn Directive {%define location_type}
Obsoleted by @code{api.location.type} since Bison 2.7.
@end deffn


@c ================================================== lr.default-reduction

@deffn Directive {%define lr.default-reduction} @var{when}

@itemize @bullet
@item Language(s): all

@item Purpose: Specify the kind of states that are permitted to
contain default reductions.  @xref{Default Reductions}.

@item Accepted Values: @code{most}, @code{consistent}, @code{accepting}
@item Default Value:
@itemize
@item @code{accepting} if @code{lr.type} is @code{canonical-lr}.
@item @code{most} otherwise.
@end itemize
@item History:
introduced as @code{lr.default-reductions} in 2.5, renamed as
@code{lr.default-reduction} in 3.0.
@end itemize
@end deffn


@c ============================================ lr.keep-unreachable-state

@deffn Directive {%define lr.keep-unreachable-state}

@itemize @bullet
@item Language(s): all
@item Purpose: Request that Bison allow unreachable parser states to
remain in the parser tables.  @xref{Unreachable States}.
@item Accepted Values: Boolean
@item Default Value: @code{false}
@item History:
introduced as @code{lr.keep_unreachable_states} in 2.3b, renamed as
@code{lr.keep-unreachable-states} in 2.5, and as
@code{lr.keep-unreachable-state} in 3.0.
@end itemize
@end deffn
@c lr.keep-unreachable-state


@c ================================================== lr.type

@deffn Directive {%define lr.type} @var{type}

@itemize @bullet
@item Language(s): all

@item Purpose: Specify the type of parser tables within the
LR(1) family.  @xref{LR Table Construction}.

@item Accepted Values: @code{lalr}, @code{ielr}, @code{canonical-lr}

@item Default Value: @code{lalr}
@end itemize
@end deffn


@c ================================================== namespace
@deffn Directive %define namespace @{@var{namespace}@}
Obsoleted by @code{api.namespace}
@end deffn
@c namespace


@c ================================================== parse.assert
@deffn Directive {%define parse.assert}

@itemize
@item Languages(s): C, C++

@item Purpose: Issue runtime assertions to catch invalid uses.
In C, some important invariants in the implementation of the parser are
checked when this option is enabled.

In C++, when variants are used (@pxref{C++ Variants}), symbols must be
constructed and destroyed properly.  This option checks these constraints.

@item Accepted Values: Boolean

@item Default Value: @code{false}
@end itemize
@end deffn
@c parse.assert


@c ================================================== parse.error
@deffn Directive {%define parse.error} @var{verbosity}
@itemize
@item Languages(s):
all
@item Purpose:
Control the generation of syntax error messages.  @xref{Error Reporting}.
@item Accepted Values:
@itemize
@item @code{simple}
Error messages passed to @code{yyerror} are simply @w{@code{"syntax
error"}}.

@item @code{detailed}
Error messages report the unexpected token, and possibly the expected ones.
However, this report can often be incorrect when LAC is not enabled
(@pxref{LAC}).  Token name internationalization is supported.

@item @code{verbose}
Similar (but inferior) to @code{detailed}.

Error messages report the unexpected token, and possibly the expected ones.
However, this report can often be incorrect when LAC is not enabled
(@pxref{LAC}).

Does not support token internationalization.  Using non-ASCII characters in
token aliases is not portable.

@item @code{custom}
The user is in charge of generating the syntax error message by defining the
@code{yyreport_syntax_error} function.  @xref{Syntax Error Reporting
Function}.
@end itemize

@item Default Value:
@code{simple}

@item History:
introduced in 3.0 with support for @code{simple} and @code{verbose}.  Values
@code{custom} and @code{detailed} were introduced in 3.6.
@end itemize
@end deffn
@c parse.error


@c ================================================== parse.lac
@deffn Directive {%define parse.lac} @var{when}

@itemize
@item Languages(s): C (deterministic parsers only)

@item Purpose: Enable LAC (lookahead correction) to improve
syntax error handling.  @xref{LAC}.
@item Accepted Values: @code{none}, @code{full}
@item Default Value: @code{none}
@end itemize
@end deffn
@c parse.lac


@c ================================================== parse.trace
@deffn Directive {%define parse.trace}

@itemize
@item Languages(s): C, C++, Java

@item Purpose: Require parser instrumentation for tracing.
@xref{Tracing}.

In C/C++, define the macro @code{YYDEBUG} (or @code{@var{prefix}DEBUG} with
@samp{%define api.prefix @{@var{prefix}@}}), see @ref{Multiple Parsers}) to
1 in the parser implementation file if it is not already defined, so that
the debugging facilities are compiled.

@item Accepted Values: Boolean

@item Default Value: @code{false}
@end itemize
@end deffn
@c parse.trace


@c ================================================== parser_class_name
@deffn Directive %define parser_class_name @{@var{name}@}
Obsoleted by @code{api.parser.class}
@end deffn
@c parser_class_name






@node %code Summary
@subsection %code Summary
@findex %code
@cindex Prologue

The @code{%code} directive inserts code verbatim into the output
parser source at any of a predefined set of locations.  It thus serves
as a flexible and user-friendly alternative to the traditional Yacc
prologue, @code{%@{@var{code}%@}}.  This section summarizes the
functionality of @code{%code} for the various target languages
supported by Bison.  For a detailed discussion of how to use
@code{%code} in place of @code{%@{@var{code}%@}} for C/C++ and why it
is advantageous to do so, @pxref{Prologue Alternatives}.

@deffn {Directive} %code @{@var{code}@}
This is the unqualified form of the @code{%code} directive.  It
inserts @var{code} verbatim at a language-dependent default location
in the parser implementation.

For C/C++, the default location is the parser implementation file
after the usual contents of the parser header file.  Thus, the
unqualified form replaces @code{%@{@var{code}%@}} for most purposes.

For Java, the default location is inside the parser class.
@end deffn

@deffn {Directive} %code @var{qualifier} @{@var{code}@}
This is the qualified form of the @code{%code} directive.
@var{qualifier} identifies the purpose of @var{code} and thus the
location(s) where Bison should insert it.  That is, if you need to
specify location-sensitive @var{code} that does not belong at the
default location selected by the unqualified @code{%code} form, use
this form instead.
@end deffn

For any particular qualifier or for the unqualified form, if there are
multiple occurrences of the @code{%code} directive, Bison concatenates
the specified code in the order in which it appears in the grammar
file.

Not all qualifiers are accepted for all target languages.  Unaccepted
qualifiers produce an error.  Some of the accepted qualifiers are:

@table @code
@item requires
@findex %code requires

@itemize @bullet
@item Language(s): C, C++

@item Purpose: This is the best place to write dependency code required for
@code{YYSTYPE} and @code{YYLTYPE}.  In other words, it's the best place to
define types referenced in @code{%union} directives.  If you use
@code{#define} to override Bison's default @code{YYSTYPE} and @code{YYLTYPE}
definitions, then it is also the best place.  However you should rather
@code{%define} @code{api.value.type} and @code{api.location.type}.

@item Location(s): The parser header file and the parser implementation file
before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE}
definitions.
@end itemize

@item provides
@findex %code provides

@itemize @bullet
@item Language(s): C, C++

@item Purpose: This is the best place to write additional definitions and
declarations that should be provided to other modules.

@item Location(s): The parser header file and the parser implementation
file after the Bison-generated @code{YYSTYPE}, @code{YYLTYPE}, and
token definitions.
@end itemize

@item top
@findex %code top

@itemize @bullet
@item Language(s): C, C++

@item Purpose: The unqualified @code{%code} or @code{%code requires}
should usually be more appropriate than @code{%code top}.  However,
occasionally it is necessary to insert code much nearer the top of the
parser implementation file.  For example:

@example
%code top @{
  #define _GNU_SOURCE
  #include <stdio.h>
@}
@end example

@item Location(s): Near the top of the parser implementation file.
@end itemize

@item imports
@findex %code imports

@itemize @bullet
@item Language(s): Java

@item Purpose: This is the best place to write Java import directives.

@item Location(s): The parser Java file after any Java package directive and
before any class definitions.
@end itemize
@end table

Though we say the insertion locations are language-dependent, they are
technically skeleton-dependent.  Writers of non-standard skeletons
however should choose their locations consistently with the behavior
of the standard Bison skeletons.


@node Multiple Parsers
@section Multiple Parsers in the Same Program

Most programs that use Bison parse only one language and therefore contain
only one Bison parser.  But what if you want to parse more than one language
with the same program?  Then you need to avoid name conflicts between
different definitions of functions and variables such as @code{yyparse},
@code{yylval}.  To use different parsers from the same compilation unit, you
also need to avoid conflicts on types and macros (e.g., @code{YYSTYPE})
exported in the generated header.

The easy way to do this is to define the @code{%define} variable
@code{api.prefix}.  With different @code{api.prefix}s it is guaranteed that
headers do not conflict when included together, and that compiled objects
can be linked together too.  Specifying @samp{%define api.prefix
@{@var{prefix}@}} (or passing the option @samp{-Dapi.prefix=@{@var{prefix}@}}, see
@ref{Invocation}) renames the interface functions and
variables of the Bison parser to start with @var{prefix} instead of
@samp{yy}, and all the macros to start by @var{PREFIX} (i.e., @var{prefix}
upper-cased) instead of @samp{YY}.

The renamed symbols include @code{yyparse}, @code{yylex}, @code{yyerror},
@code{yynerrs}, @code{yylval}, @code{yylloc}, @code{yychar} and
@code{yydebug}.  If you use a push parser, @code{yypush_parse},
@code{yypull_parse}, @code{yypstate}, @code{yypstate_new} and
@code{yypstate_delete} will also be renamed.  The renamed macros include
@code{YYSTYPE}, @code{YYLTYPE}, and @code{YYDEBUG}, which is treated
specifically --- more about this below.

For example, if you use @samp{%define api.prefix @{c@}}, the names become
@code{cparse}, @code{clex}, @dots{}, @code{CSTYPE}, @code{CLTYPE}, and so
on.

Users of Flex must update the signature of the generated @code{yylex}
function.  Since the Flex scanner usually includes the generated header of
the parser (to get the definitions of the tokens, etc.), the most convenient
way is to insert the declaration of @code{yylex} in the @code{provides}
section:

@example
%define api.prefix @{c@}
// Emitted in the header file, after the definition of YYSTYPE.
%code provides
@{
  // Tell Flex the expected prototype of yylex.
  #define YY_DECL                             \
    int clex (CSTYPE *yylval, CLTYPE *yylloc)

  // Declare the scanner.
  YY_DECL;
@}
@end example

@sp 1

The @code{%define} variable @code{api.prefix} works in two different ways.
In the implementation file, it works by adding macro definitions to the
beginning of the parser implementation file, defining @code{yyparse} as
@code{@var{prefix}parse}, and so on:

@example
#define YYSTYPE CTYPE
#define yyparse cparse
#define yylval  clval
...
YYSTYPE yylval;
int yyparse (void);
@end example

This effectively substitutes one name for the other in the entire parser
implementation file, thus the ``original'' names (@code{yylex},
@code{YYSTYPE}, @dots{}) are also usable in the parser implementation file.

However, in the parser header file, the symbols are defined renamed, for
instance:

@example
extern CSTYPE clval;
int cparse (void);
@end example

The macro @code{YYDEBUG} is commonly used to enable the tracing support in
parsers.  To comply with this tradition, when @code{api.prefix} is used,
@code{YYDEBUG} (not renamed) is used as a default value:

@example
/* Debug traces. */
#ifndef CDEBUG
# if defined YYDEBUG
#  if YYDEBUG
#   define CDEBUG 1
#  else
#   define CDEBUG 0
#  endif
# else
#  define CDEBUG 0
# endif
#endif
#if CDEBUG
extern int cdebug;
#endif
@end example

@sp 2

Prior to Bison 2.6, a feature similar to @code{api.prefix} was provided by
the obsolete directive @code{%name-prefix} (@pxref{Table of Symbols}) and
the option @option{--name-prefix} (@pxref{Output Files}).

@node Interface
@chapter Parser C-Language Interface
@cindex C-language interface
@cindex interface

The Bison parser is actually a C function named @code{yyparse}.  Here we
describe the interface conventions of @code{yyparse} and the other
functions that it needs to use.

Keep in mind that the parser uses many C identifiers starting with
@samp{yy} and @samp{YY} for internal purposes.  If you use such an
identifier (aside from those in this manual) in an action or in epilogue
in the grammar file, you are likely to run into trouble.

@menu
* Parser Function::         How to call @code{yyparse} and what it returns.
* Push Parser Function::    How to call @code{yypush_parse} and what it returns.
* Pull Parser Function::    How to call @code{yypull_parse} and what it returns.
* Parser Create Function::  How to call @code{yypstate_new} and what it returns.
* Parser Delete Function::  How to call @code{yypstate_delete} and what it returns.
* Lexical::                 You must supply a function @code{yylex}
                              which reads tokens.
* Error Reporting::         Passing error messages to the user.
* Action Features::         Special features for use in actions.
* Internationalization::    How to let the parser speak in the user's
                              native language.
@end menu

@node Parser Function
@section The Parser Function @code{yyparse}
@findex yyparse

You call the function @code{yyparse} to cause parsing to occur.  This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error.  You can also
write an action which directs @code{yyparse} to return immediately
without reading further.


@deftypefun int yyparse (@code{void})
The value returned by @code{yyparse} is 0 if parsing was successful (return
is due to end-of-input).

The value is 1 if parsing failed because of invalid input, i.e., input
that contains a syntax error or that causes @code{YYABORT} to be
invoked.

The value is 2 if parsing failed due to memory exhaustion.
@end deftypefun

In an action, you can cause immediate return from @code{yyparse} by using
these macros:

@defmac YYACCEPT
@findex YYACCEPT
Return immediately with value 0 (to report success).
@end defmac

@defmac YYABORT
@findex YYABORT
Return immediately with value 1 (to report failure).
@end defmac

If you use a reentrant parser, you can optionally pass additional
parameter information to it in a reentrant way.  To do so, use the
declaration @code{%parse-param}:

@deffn {Directive} %parse-param @{@var{argument-declaration}@} @dots{}
@findex %parse-param
Declare that one or more
@var{argument-declaration} are additional @code{yyparse} arguments.
The @var{argument-declaration} is used when declaring
functions or prototypes.  The last identifier in
@var{argument-declaration} must be the argument name.
@end deffn

Here's an example.  Write this in the parser:

@example
%parse-param @{int *nastiness@} @{int *randomness@}
@end example

@noindent
Then call the parser like this:

@example
@{
  int nastiness, randomness;
  @dots{}  /* @r{Store proper data in @code{nastiness} and @code{randomness}.} */
  value = yyparse (&nastiness, &randomness);
  @dots{}
@}
@end example

@noindent
In the grammar actions, use expressions like this to refer to the data:

@example
exp: @dots{}    @{ @dots{}; *randomness += 1; @dots{} @}
@end example

@noindent
Using the following:
@example
%parse-param @{int *randomness@}
@end example

Results in these signatures:
@example
void yyerror (int *randomness, const char *msg);
int  yyparse (int *randomness);
@end example

@noindent
Or, if both @code{%define api.pure full} (or just @code{%define api.pure})
and @code{%locations} are used:

@example
void yyerror (YYLTYPE *llocp, int *randomness, const char *msg);
int  yyparse (int *randomness);
@end example

@node Push Parser Function
@section The Push Parser Function @code{yypush_parse}
@findex yypush_parse

You call the function @code{yypush_parse} to parse a single token.  This
function is available if either the @samp{%define api.push-pull push} or
@samp{%define api.push-pull both} declaration is used.
@xref{Push Decl}.

@deftypefun int yypush_parse (@code{yypstate *}@var{yyps})
The value returned by @code{yypush_parse} is the same as for @code{yyparse}
with the following exception: it returns @code{YYPUSH_MORE} if more input is
required to finish parsing the grammar.

After @code{yypush_parse} returns a status other than @code{YYPUSH_MORE},
the parser instance @code{yyps} may be reused for a new parse.
@end deftypefun

The fact that the parser state is reusable even after an error simplifies
reuse.  For example, a calculator application which parses each input line
as an expression can just keep reusing the same @code{yyps} even if an input
was invalid.

@node Pull Parser Function
@section The Pull Parser Function @code{yypull_parse}
@findex yypull_parse

You call the function @code{yypull_parse} to parse the rest of the input
stream.  This function is available if the @samp{%define api.push-pull both}
declaration is used.
@xref{Push Decl}.

@deftypefun int yypull_parse (@code{yypstate *}@var{yyps})
The value returned by @code{yypull_parse} is the same as for @code{yyparse}.

The parser instance @code{yyps} may be reused for new parses.
@end deftypefun

@node Parser Create Function
@section The Parser Create Function @code{yystate_new}
@findex yypstate_new

You call the function @code{yypstate_new} to create a new parser instance.
This function is available if either the @samp{%define api.push-pull push} or
@samp{%define api.push-pull both} declaration is used.
@xref{Push Decl}.

@deftypefun {yypstate*} yypstate_new (@code{void})
The function will return a valid parser instance if there was memory available
or 0 if no memory was available.
In impure mode, it will also return 0 if a parser instance is currently
allocated.
@end deftypefun

@node Parser Delete Function
@section The Parser Delete Function @code{yystate_delete}
@findex yypstate_delete

You call the function @code{yypstate_delete} to delete a parser instance.
function is available if either the @samp{%define api.push-pull push} or
@samp{%define api.push-pull both} declaration is used.
@xref{Push Decl}.

@deftypefun void yypstate_delete (@code{yypstate *}@var{yyps})
This function will reclaim the memory associated with a parser instance.
After this call, you should no longer attempt to use the parser instance.
@end deftypefun

@node Lexical
@section The Lexical Analyzer Function @code{yylex}
@findex yylex
@cindex lexical analyzer

The @dfn{lexical analyzer} function, @code{yylex}, recognizes tokens from
the input stream and returns them to the parser.  Bison does not create
this function automatically; you must write it so that @code{yyparse} can
call it.  The function is sometimes referred to as a lexical scanner.

In simple programs, @code{yylex} is often defined at the end of the Bison
grammar file.  If @code{yylex} is defined in a separate source file, you
need to arrange for the token-kind definitions to be available there.  To do
this, use the @samp{-d} option when you run Bison, so that it will write
these definitions into the separate parser header file,
@file{@var{name}.tab.h}, which you can include in the other source files
that need it.  @xref{Invocation}.

@menu
* Calling Convention::  How @code{yyparse} calls @code{yylex}.
* Special Tokens::      Signaling end-of-file and errors to the parser.
* Tokens from Literals:: Finding token kinds from string aliases.
* Token Values::        How @code{yylex} must return the semantic value
                          of the token it has read.
* Token Locations::     How @code{yylex} must return the text location
                          (line number, etc.) of the token, if the
                          actions want that.
* Pure Calling::        How the calling convention differs in a pure parser
                          (@pxref{Pure Decl}).
@end menu

@node Calling Convention
@subsection Calling Convention for @code{yylex}

The value that @code{yylex} returns must be the positive numeric code for
the kind of token it has just found; a zero or negative value signifies
end-of-input.

When a token kind is referred to in the grammar rules by a name, that name
in the parser implementation file becomes an enumerator of the enum
@code{yytoken_kind_t} whose definition is the proper numeric code for that
token kind.  So @code{yylex} should use the name to indicate that type.
@xref{Symbols}.

When a token is referred to in the grammar rules by a character literal, the
numeric code for that character is also the code for the token kind.  So
@code{yylex} can simply return that character code, possibly converted to
@code{unsigned char} to avoid sign-extension.  The null character must not
be used this way, because its code is zero and that signifies end-of-input.

Here is an example showing these things:

@example
int
yylex (void)
@{
  @dots{}
  if (c == EOF)    /* Detect end-of-input. */
    return YYEOF;
  @dots{}
  else if (c == '+' || c == '-')
    return c;      /* Assume token kind for '+' is '+'. */
  @dots{}
  else
    return INT;    /* Return the kind of the token. */
  @dots{}
@}
@end example

@noindent
This interface has been designed so that the output from the @code{lex}
utility can be used without change as the definition of @code{yylex}.


@node Special Tokens
@subsection Special Tokens

In addition to the user defined tokens, Bison generates a few special tokens
that @code{yylex} may return.

The @code{YYEOF} token denotes the end of file, and signals to the parser
that there is nothing left afterwards.  @xref{Calling Convention}, for an
example.

Returning @code{YYUNDEF} tells the parser that some lexical error was found.
It will emit an error message about an ``invalid token'', and enter
error-recovery (@pxref{Error Recovery}).  Returning an unknown token kind
results in the exact same behavior.

Returning @code{YYerror} requires the parser to enter error-recovery
@emph{without} emitting an error message.  This way the lexical analyzer can
produce an accurate error messages about the invalid input (something the
parser cannot do), and yet benefit from the error-recovery features of the
parser.

@example
int
yylex (void)
@{
  @dots{}
  switch (c)
    @{
      @dots{}
      case '0': case '1': case '2': case '3': case '4':
      case '5': case '6': case '7': case '8': case '9':
        @dots{}
        return TOK_NUM;
      @dots{}
      case EOF:
        return YYEOF;
      default:
        yyerror ("syntax error: invalid character: %c", c);
        return YYerror;
    @}
@}
@end example

@node Tokens from Literals
@subsection Finding Tokens by String Literals

If the grammar uses literal string tokens, there are two ways that
@code{yylex} can determine the token kind codes for them:

@itemize @bullet
@item
If the grammar defines symbolic token names as aliases for the literal
string tokens, @code{yylex} can use these symbolic names like all others.
In this case, the use of the literal string tokens in the grammar file has
no effect on @code{yylex}.

This is the preferred approach.

@item
@code{yylex} can search for the multicharacter token in the @code{yytname}
table.  This method is discouraged: the primary purpose of string aliases is
forging good error messages, not describing the spelling of keywords.  In
addition, looking for the token kind at runtime incurs a (small but
noticeable) cost.

The @code{yytname} table is generated only if you use the
@code{%token-table} declaration.  @xref{Decl Summary}.
@end itemize


@node Token Values
@subsection Semantic Values of Tokens

@vindex yylval
In an ordinary (nonreentrant) parser, the semantic value of the token must
be stored into the global variable @code{yylval}.  When you are using just
one data type for semantic values, @code{yylval} has that type.  Thus, if
the type is @code{int} (the default), you might write this in @code{yylex}:

@example
@group
  @dots{}
  yylval = value;  /* Put value onto Bison stack. */
  return INT;      /* Return the kind of the token. */
  @dots{}
@end group
@end example

When you are using multiple data types, @code{yylval}'s type is a union made
from the @code{%union} declaration (@pxref{Union Decl}).  So when you store
a token's value, you must use the proper member of the union.  If the
@code{%union} declaration looks like this:

@example
@group
%union @{
  int intval;
  double val;
  symrec *tptr;
@}
@end group
@end example

@noindent
then the code in @code{yylex} might look like this:

@example
@group
  @dots{}
  yylval.intval = value; /* Put value onto Bison stack. */
  return INT;            /* Return the kind of the token. */
  @dots{}
@end group
@end example

@node Token Locations
@subsection Textual Locations of Tokens

@vindex yylloc
If you are using the @samp{@@@var{n}}-feature (@pxref{Tracking Locations})
in actions to keep track of the textual locations of tokens and groupings,
then you must provide this information in @code{yylex}.  The function
@code{yyparse} expects to find the textual location of a token just parsed
in the global variable @code{yylloc}.  So @code{yylex} must store the proper
data in that variable.

By default, the value of @code{yylloc} is a structure and you need only
initialize the members that are going to be used by the actions.  The
four members are called @code{first_line}, @code{first_column},
@code{last_line} and @code{last_column}.  Note that the use of this
feature makes the parser noticeably slower.

@tindex YYLTYPE
The data type of @code{yylloc} has the name @code{YYLTYPE}.

@node Pure Calling
@subsection Calling Conventions for Pure Parsers

When you use the Bison declaration @code{%define api.pure full} to request a
pure, reentrant parser, the global communication variables @code{yylval} and
@code{yylloc} cannot be used.  (@xref{Pure Decl}.)  In such parsers the two
global variables are replaced by pointers passed as arguments to
@code{yylex}.  You must declare them as shown here, and pass the information
back by storing it through those pointers.

@example
int
yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
@{
  @dots{}
  *lvalp = value;  /* Put value onto Bison stack. */
  return INT;      /* Return the kind of the token. */
  @dots{}
@}
@end example

If the grammar file does not use the @samp{@@} constructs to refer to
textual locations, then the type @code{YYLTYPE} will not be defined.  In
this case, omit the second argument; @code{yylex} will be called with
only one argument.

If you wish to pass additional arguments to @code{yylex}, use
@code{%lex-param} just like @code{%parse-param} (@pxref{Parser
Function}).  To pass additional arguments to both @code{yylex} and
@code{yyparse}, use @code{%param}.

@deffn {Directive} %lex-param @{@var{argument-declaration}@} @dots{}
@findex %lex-param
Specify that @var{argument-declaration} are additional @code{yylex} argument
declarations.  You may pass one or more such declarations, which is
equivalent to repeating @code{%lex-param}.
@end deffn

@deffn {Directive} %param @{@var{argument-declaration}@} @dots{}
@findex %param
Specify that @var{argument-declaration} are additional
@code{yylex}/@code{yyparse} argument declaration.  This is equivalent to
@samp{%lex-param @{@var{argument-declaration}@} @dots{} %parse-param
@{@var{argument-declaration}@} @dots{}}.  You may pass one or more
declarations, which is equivalent to repeating @code{%param}.
@end deffn

@noindent
For instance:

@example
%lex-param   @{scanner_mode *mode@}
%parse-param @{parser_mode *mode@}
%param       @{environment_type *env@}
@end example

@noindent
results in the following signatures:

@example
int yylex   (scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);
@end example

If @samp{%define api.pure full} is added:

@example
int yylex   (YYSTYPE *lvalp, scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);
@end example

@noindent
and finally, if both @samp{%define api.pure full} and @code{%locations} are
used:

@example
int yylex   (YYSTYPE *lvalp, YYLTYPE *llocp,
             scanner_mode *mode, environment_type *env);
int yyparse (parser_mode *mode, environment_type *env);
@end example


@node Error Reporting
@section Error Reporting

During its execution the parser may have error messages to pass to the user,
such as syntax error, or memory exhaustion.  How this message is delivered
to the user must be specified by the developer.

@menu
* Error Reporting Function::         You must supply a @code{yyerror} function.
* Syntax Error Reporting Function::  You can supply a @code{yyreport_syntax_error} function.
@end menu

@node Error Reporting Function
@subsection The Error Reporting Function @code{yyerror}
@cindex error reporting function
@findex yyerror
@cindex parse error
@cindex syntax error

The Bison parser detects a @dfn{syntax error} (or @dfn{parse error})
whenever it reads a token which cannot satisfy any syntax rule.  An
action in the grammar can also explicitly proclaim an error, using the
macro @code{YYERROR} (@pxref{Action Features}).

The Bison parser expects to report the error by calling an error
reporting function named @code{yyerror}, which you must supply.  It is
called by @code{yyparse} whenever a syntax error is found, and it
receives one argument.  For a syntax error, the string is normally
@w{@code{"syntax error"}}.

@findex %define parse.error detailed
@findex %define parse.error verbose
If you invoke @samp{%define parse.error detailed} (or @samp{custom}) in the
Bison declarations section (@pxref{Bison Declarations}), then Bison provides
a more verbose and specific error message string instead of just plain
@w{@code{"syntax error"}}.  However, that message sometimes contains
incorrect information if LAC is not enabled (@pxref{LAC}).

The parser can detect one other kind of error: memory exhaustion.  This
can happen when the input contains constructions that are very deeply
nested.  It isn't likely you will encounter this, since the Bison
parser normally extends its stack automatically up to a very large limit.  But
if memory is exhausted, @code{yyparse} calls @code{yyerror} in the usual
fashion, except that the argument string is @w{@code{"memory exhausted"}}.

In some cases diagnostics like @w{@code{"syntax error"}} are
translated automatically from English to some other language before
they are passed to @code{yyerror}.  @xref{Internationalization}.

The following definition suffices in simple programs:

@example
@group
void
yyerror (char const *s)
@{
@end group
@group
  fprintf (stderr, "%s\n", s);
@}
@end group
@end example

After @code{yyerror} returns to @code{yyparse}, the latter will attempt
error recovery if you have written suitable error recovery grammar rules
(@pxref{Error Recovery}).  If recovery is impossible, @code{yyparse} will
immediately return 1.

Obviously, in location tracking pure parsers, @code{yyerror} should have
an access to the current location. With @code{%define api.pure}, this is
indeed the case for the GLR parsers, but not for the Yacc parser, for
historical reasons, and this is the why @code{%define api.pure full} should be
preferred over @code{%define api.pure}.

When @code{%locations %define api.pure full} is used, @code{yyerror} has the
following signature:

@example
void yyerror (YYLTYPE *locp, char const *msg);
@end example

@noindent
The prototypes are only indications of how the code produced by Bison
uses @code{yyerror}.  Bison-generated code always ignores the returned
value, so @code{yyerror} can return any type, including @code{void}.
Also, @code{yyerror} can be a variadic function; that is why the
message is always passed last.

Traditionally @code{yyerror} returns an @code{int} that is always
ignored, but this is purely for historical reasons, and @code{void} is
preferable since it more accurately describes the return type for
@code{yyerror}.

@vindex yynerrs
The variable @code{yynerrs} contains the number of syntax errors
reported so far.  Normally this variable is global; but if you
request a pure parser (@pxref{Pure Decl})
then it is a local variable which only the actions can access.


@node Syntax Error Reporting Function
@subsection The Syntax Error Reporting Function @code{yyreport_syntax_error}

@findex %define parse.error custom
If you invoke @samp{%define parse.error custom} (@pxref{Bison
Declarations}), then the parser no longer passes syntax error messages to
@code{yyerror}, rather it delegates that task to the user by calling the
@code{yyreport_syntax_error} function.

@deftypefun int yyreport_syntax_error (@code{const yypcontext_t *}@var{ctx})
Report a syntax error to the user.  Return 0 on success, @code{YYENOMEM} on
memory exhaustion.  Whether it uses @code{yyerror} is up to the user.
@end deftypefun

Use the following types and functions to build the error message.

@deffn {Type} yypcontext_t
An opaque type that captures the circumstances of the syntax error.
@end deffn

@deffn {Type} yysymbol_kind_t
An enum of all the grammar symbols, tokens and nonterminals.  Its
enumerators are forged from the symbol names:

@example
enum yysymbol_kind_t
@{
  YYSYMBOL_YYEMPTY = -2,      /* No symbol.  */
  YYSYMBOL_YYEOF = 0,         /* "end of file"  */
  YYSYMBOL_YYerror = 1,       /* error  */
  YYSYMBOL_YYUNDEF = 2,       /* "invalid token"  */
  YYSYMBOL_PLUS = 3,          /* "+"  */
  YYSYMBOL_MINUS = 4,         /* "-"  */
  [...]
  YYSYMBOL_VAR = 14,          /* "variable"  */
  YYSYMBOL_NEG = 15,          /* NEG  */
  YYSYMBOL_YYACCEPT = 16,     /* $accept  */
  YYSYMBOL_exp = 17,          /* exp  */
  YYSYMBOL_input = 18         /* input  */
@};
typedef enum yysymbol_kind_t yysymbol_kind_t;
@end example
@end deffn

@deftypefun {yysymbol_kind_t} yypcontext_token (@code{const yypcontext_t *}@var{ctx})
The ``unexpected'' token: the symbol kind of the lookahead token that caused
the syntax error.  Return @code{YYSYMBOL_YYEMPTY} if there is no lookahead.
@end deftypefun

@deftypefun {YYLTYPE *} yypcontext_location (@code{const yypcontext_t *}@var{ctx})
The location of the syntax error (that of the unexpected token).
@end deftypefun

@deftypefun int yypcontext_expected_tokens (@code{const yypcontext_t *}ctx, @code{yysymbol_kind_t} @var{argv}@code{[]}, @code{int} @var{argc})
Fill @var{argv} with the expected tokens, which never includes
@code{YYSYMBOL_YYEMPTY}, @code{YYSYMBOL_YYerror}, or
@code{YYSYMBOL_YYUNDEF}.

Never put more than @var{argc} elements into @var{argv}, and on success
return the effective number of tokens stored in @var{argv}.  Return 0 if
there are more than @var{argc} expected tokens, yet fill @var{argv} up to
@var{argc}.  When LAC is enabled, may return a negative number on errors,
such as @code{YYENOMEM} on memory exhaustion.

If @var{argv} is null, return the size needed to store all the possible
values, which is always less than @code{YYNTOKENS}.
@end deftypefun

@deftypefun {@r{string type}} yysymbol_name (@code{symbol_kind_t} @var{symbol})
The name of the symbol whose kind is @var{symbol}, possibly translated.
Depending on the options, may return a @code{const char*} or a
@code{std::string}.
@end deftypefun

A custom syntax error function looks as follows.  This implementation is
inappropriate for internationalization, see the @file{c/bistromathic}
example for a better alternative.

@example
int
yyreport_syntax_error (const yypcontext_t *ctx)
@{
  int res = 0;
  YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
  fprintf (stderr, ": syntax error");
  // Report the tokens expected at this point.
  @{
    enum @{ TOKENMAX = 5 @};
    yysymbol_kind_t expected[TOKENMAX];
    int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
    if (n < 0)
      // Forward errors to yyparse.
      res = n;
    else
      for (int i = 0; i < n; ++i)
        fprintf (stderr, "%s %s",
                 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
  @}
  // Report the unexpected token.
  @{
    yysymbol_kind_t lookahead = yypcontext_token (ctx);
    if (lookahead != YYSYMBOL_YYEMPTY)
      fprintf (stderr, " before %s", yysymbol_name (lookahead));
  @}
  fprintf (stderr, "\n");
  return res;
@}
@end example

You still must provide a @code{yyerror} function, used for instance to
report memory exhaustion.

@node Action Features
@section Special Features for Use in Actions
@cindex summary, action features
@cindex action features summary

Here is a table of Bison constructs, variables and macros that are useful in
actions.

@deffn {Variable} $$
Acts like a variable that contains the semantic value for the
grouping made by the current rule.  @xref{Actions}.
@end deffn

@deffn {Variable} $@var{n}
Acts like a variable that contains the semantic value for the
@var{n}th component of the current rule.  @xref{Actions}.
@end deffn

@deffn {Variable} $<@var{typealt}>$
Like @code{$$} but specifies alternative @var{typealt} in the union
specified by the @code{%union} declaration.  @xref{Action Types}.
@end deffn

@deffn {Variable} $<@var{typealt}>@var{n}
Like @code{$@var{n}} but specifies alternative @var{typealt} in the
union specified by the @code{%union} declaration.
@xref{Action Types}.
@end deffn

@deffn {Macro} YYABORT @code{;}
Return immediately from @code{yyparse}, indicating failure.
@xref{Parser Function}.
@end deffn

@deffn {Macro} YYACCEPT @code{;}
Return immediately from @code{yyparse}, indicating success.
@xref{Parser Function}.
@end deffn

@deffn {Macro} YYBACKUP (@var{token}, @var{value})@code{;}
@findex YYBACKUP
Unshift a token.  This macro is allowed only for rules that reduce
a single value, and only when there is no lookahead token.
It is also disallowed in GLR parsers.
It installs a lookahead token with token kind @var{token} and
semantic value @var{value}; then it discards the value that was
going to be reduced by this rule.

If the macro is used when it is not valid, such as when there is
a lookahead token already, then it reports a syntax error with
a message @samp{cannot back up} and performs ordinary error
recovery.

In either case, the rest of the action is not executed.
@end deffn

@deffn {Macro} YYEMPTY
Value stored in @code{yychar} when there is no lookahead token.
@end deffn

@deffn {Macro} YYEOF
Value stored in @code{yychar} when the lookahead is the end of the input
stream.
@end deffn

@deffn {Macro} YYERROR @code{;}
Cause an immediate syntax error.  This statement initiates error
recovery just as if the parser itself had detected an error; however, it
does not call @code{yyerror}, and does not print any message.  If you
want to print an error message, call @code{yyerror} explicitly before
the @samp{YYERROR;} statement.  @xref{Error Recovery}.
@end deffn

@deffn {Macro} YYRECOVERING
@findex YYRECOVERING
The expression @code{YYRECOVERING ()} yields 1 when the parser
is recovering from a syntax error, and 0 otherwise.
@xref{Error Recovery}.
@end deffn

@deffn {Variable} yychar
Variable containing either the lookahead token, or @code{YYEOF} when the
lookahead is the end of the input stream, or @code{YYEMPTY} when no lookahead
has been performed so the next token is not yet known.
Do not modify @code{yychar} in a deferred semantic action (@pxref{GLR Semantic
Actions}).
@xref{Lookahead}.
@end deffn

@deffn {Macro} yyclearin @code{;}
Discard the current lookahead token.  This is useful primarily in
error rules.
Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR
Semantic Actions}).
@xref{Error Recovery}.
@end deffn

@deffn {Macro} yyerrok @code{;}
Resume generating error messages immediately for subsequent syntax
errors.  This is useful primarily in error rules.
@xref{Error Recovery}.
@end deffn

@deffn {Variable} yylloc
Variable containing the lookahead token location when @code{yychar} is not set
to @code{YYEMPTY} or @code{YYEOF}.
Do not modify @code{yylloc} in a deferred semantic action (@pxref{GLR Semantic
Actions}).
@xref{Actions and Locations}.
@end deffn

@deffn {Variable} yylval
Variable containing the lookahead token semantic value when @code{yychar} is
not set to @code{YYEMPTY} or @code{YYEOF}.
Do not modify @code{yylval} in a deferred semantic action (@pxref{GLR Semantic
Actions}).
@xref{Actions}.
@end deffn

@deffn {Value} @@$
Acts like a structure variable containing information on the textual
location of the grouping made by the current rule.  @xref{Tracking
Locations}.

@c Check if those paragraphs are still useful or not.

@c @example
@c struct @{
@c   int first_line, last_line;
@c   int first_column, last_column;
@c @};
@c @end example

@c Thus, to get the starting line number of the third component, you would
@c use @samp{@@3.first_line}.

@c In order for the members of this structure to contain valid information,
@c you must make @code{yylex} supply this information about each token.
@c If you need only certain members, then @code{yylex} need only fill in
@c those members.

@c The use of this feature makes the parser noticeably slower.
@end deffn

@deffn {Value} @@@var{n}
@findex @@@var{n}
Acts like a structure variable containing information on the textual
location of the @var{n}th component of the current rule.  @xref{Tracking
Locations}.
@end deffn

@node Internationalization
@section Parser Internationalization
@cindex internationalization
@cindex i18n
@cindex NLS
@cindex gettext
@cindex bison-po

A Bison-generated parser can print diagnostics, including error and
tracing messages.  By default, they appear in English.  However, Bison
also supports outputting diagnostics in the user's native language.  To
make this work, the user should set the usual environment variables.
@xref{Users, , The User's View, gettext, GNU @code{gettext} utilities}.
For example, the shell command @samp{export LC_ALL=fr_CA.UTF-8} might
set the user's locale to French Canadian using the UTF-8
encoding.  The exact set of available locales depends on the user's
installation.

@menu
* Enabling I18n::    Preparing your project to support internationalization.
* Token I18n::       Preparing tokens for internationalization in error messages.
@end menu

@node Enabling I18n
@subsection Enabling Internationalization

The maintainer of a package that uses a Bison-generated parser enables
the internationalization of the parser's output through the following
steps.  Here we assume a package that uses GNU Autoconf and
GNU Automake.

@enumerate
@item
@cindex bison-i18n.m4
Into the directory containing the GNU Autoconf macros used
by the package ---often called @file{m4}--- copy the
@file{bison-i18n.m4} file installed by Bison under
@samp{share/aclocal/bison-i18n.m4} in Bison's installation directory.
For example:

@example
cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
@end example

@item
@findex BISON_I18N
@vindex BISON_LOCALEDIR
@vindex YYENABLE_NLS
In the top-level @file{configure.ac}, after the @code{AM_GNU_GETTEXT}
invocation, add an invocation of @code{BISON_I18N}.  This macro is
defined in the file @file{bison-i18n.m4} that you copied earlier.  It
causes @code{configure} to find the value of the
@code{BISON_LOCALEDIR} variable, and it defines the source-language
symbol @code{YYENABLE_NLS} to enable translations in the
Bison-generated parser.

@item
In the @code{main} function of your program, designate the directory
containing Bison's runtime message catalog, through a call to
@samp{bindtextdomain} with domain name @samp{bison-runtime}.
For example:

@example
bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
@end example

Typically this appears after any other call @code{bindtextdomain
(PACKAGE, LOCALEDIR)} that your package already has.  Here we rely on
@samp{BISON_LOCALEDIR} to be defined as a string through the
@file{Makefile}.

@item
In the @file{Makefile.am} that controls the compilation of the @code{main}
function, make @samp{BISON_LOCALEDIR} available as a C preprocessor macro,
either in @samp{DEFS} or in @samp{AM_CPPFLAGS}.  For example:

@example
DEFS = @@DEFS@@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
@end example

or:

@example
AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
@end example

@item
Finally, invoke the command @command{autoreconf} to generate the build
infrastructure.
@end enumerate

@node Token I18n
@subsection Token Internationalization

When the @code{%define} variable @code{parse.error} is set to @code{custom}
or @code{detailed}, token aliases can be internationalized:

@example
%token
    '\n'   _("end of line")
  <double>
    NUM    _("number")
  <symrec*>
    FUN    _("function")
    VAR    _("variable")
@end example

The remainder of the grammar may freely use either the token symbol
(@code{FUN}) or its alias (@code{"function"}), but not with the
internationalization marker (@code{_("function")}).

If at least one token alias is internationalized, then the generated parser
will use both @code{N_} and @code{_}, that must be defined
(@pxref{Programmers, , The Programmer’s View, gettext, GNU @code{gettext}
utilities}).  They are used only on string aliases marked for translation.
In other words, even if your catalog features a translation for
``function'', then with

@example
%token
  <symrec*>
    FUN      "function"
    VAR    _("variable")
@end example

@noindent
``function'' will appear untranslated in debug traces and error messages.

Unless defined by the user, the end-of-file token, @code{YYEOF}, is provided
``end of file'' as an alias.  It is also internationalized if the user
internationalized tokens.  To map it to another string, use:

@example
%token END 0 _("end of input")
@end example


@node Algorithm
@chapter The Bison Parser Algorithm
@cindex Bison parser algorithm
@cindex algorithm of parser
@cindex shifting
@cindex reduction
@cindex parser stack
@cindex stack, parser

As Bison reads tokens, it pushes them onto a stack along with their
semantic values.  The stack is called the @dfn{parser stack}.  Pushing a
token is traditionally called @dfn{shifting}.

For example, suppose the infix calculator has read @samp{1 + 5 *}, with a
@samp{3} to come.  The stack will have four elements, one for each token
that was shifted.

But the stack does not always have an element for each token read.  When
the last @var{n} tokens and groupings shifted match the components of a
grammar rule, they can be combined according to that rule.  This is called
@dfn{reduction}.  Those tokens and groupings are replaced on the stack by a
single grouping whose symbol is the result (left hand side) of that rule.
Running the rule's action is part of the process of reduction, because this
is what computes the semantic value of the resulting grouping.

For example, if the infix calculator's parser stack contains this:

@example
1 + 5 * 3
@end example

@noindent
and the next input token is a newline character, then the last three
elements can be reduced to 15 via the rule:

@example
expr: expr '*' expr;
@end example

@noindent
Then the stack contains just these three elements:

@example
1 + 15
@end example

@noindent
At this point, another reduction can be made, resulting in the single value
16.  Then the newline token can be shifted.

The parser tries, by shifts and reductions, to reduce the entire input down
to a single grouping whose symbol is the grammar's start-symbol
(@pxref{Language and Grammar}).

This kind of parser is known in the literature as a bottom-up parser.

@menu
* Lookahead::         Parser looks one token ahead when deciding what to do.
* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
* Precedence::        Operator precedence works by resolving conflicts.
* Contextual Precedence::  When an operator's precedence depends on context.
* Parser States::     The parser is a finite-state-machine with stack.
* Reduce/Reduce::     When two rules are applicable in the same situation.
* Mysterious Conflicts:: Conflicts that look unjustified.
* Tuning LR::         How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted.  How to avoid it.
@end menu

@node Lookahead
@section Lookahead Tokens
@cindex lookahead token

The Bison parser does @emph{not} always reduce immediately as soon as the
last @var{n} tokens and groupings match a rule.  This is because such a
simple strategy is inadequate to handle most languages.  Instead, when a
reduction is possible, the parser sometimes ``looks ahead'' at the next
token in order to decide what to do.

When a token is read, it is not immediately shifted; first it becomes the
@dfn{lookahead token}, which is not on the stack.  Now the parser can
perform one or more reductions of tokens and groupings on the stack, while
the lookahead token remains off to the side.  When no more reductions
should take place, the lookahead token is shifted onto the stack.  This
does not mean that all possible reductions have been done; depending on the
token kind of the lookahead token, some rules may choose to delay their
application.

Here is a simple case where lookahead is needed.  These three rules define
expressions which contain binary addition operators and postfix unary
factorial operators (@samp{!}), and allow parentheses for grouping.

@example
@group
expr:
  term '+' expr
| term
;
@end group

@group
term:
  '(' expr ')'
| term '!'
| "number"
;
@end group
@end example

Suppose that the tokens @w{@samp{1 + 2}} have been read and shifted; what
should be done?  If the following token is @samp{)}, then the first three
tokens must be reduced to form an @code{expr}.  This is the only valid
course, because shifting the @samp{)} would produce a sequence of symbols
@w{@code{term ')'}}, and no rule allows this.

If the following token is @samp{!}, then it must be shifted immediately so
that @w{@samp{2 !}} can be reduced to make a @code{term}.  If instead the
parser were to reduce before shifting, @w{@samp{1 + 2}} would become an
@code{expr}.  It would then be impossible to shift the @samp{!} because
doing so would produce on the stack the sequence of symbols @code{expr
'!'}.  No rule allows that sequence.

@vindex yychar
@vindex yylval
@vindex yylloc
The lookahead token is stored in the variable @code{yychar}.
Its semantic value and location, if any, are stored in the variables
@code{yylval} and @code{yylloc}.
@xref{Action Features}.

@node Shift/Reduce
@section Shift/Reduce Conflicts
@cindex conflicts
@cindex shift/reduce conflicts
@cindex dangling @code{else}
@cindex @code{else}, dangling

Suppose we are parsing a language which has if-then and if-then-else
statements, with a pair of rules like this:

@example
@group
if_stmt:
  "if" expr "then" stmt
| "if" expr "then" stmt "else" stmt
;
@end group
@end example

@noindent
Here @code{"if"}, @code{"then"} and @code{"else"} are terminal symbols for
specific keyword tokens.

When the @code{"else"} token is read and becomes the lookahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule.  But it is also legitimate to shift the
@code{"else"}, because that would lead to eventual reduction by the second
rule.

This situation, where either a shift or a reduction would be valid, is
called a @dfn{shift/reduce conflict}.  Bison is designed to resolve
these conflicts by choosing to shift, unless otherwise directed by
operator precedence declarations.  To see the reason for this, let's
contrast it with the other alternative.

Since the parser prefers to shift the @code{"else"}, the result is to attach
the else-clause to the innermost if-statement, making these two inputs
equivalent:

@example
if x then if y then win; else lose;

if x then do; if y then win; else lose; end;
@end example

But if the parser chose to reduce when possible rather than shift, the
result would be to attach the else-clause to the outermost if-statement,
making these two inputs equivalent:

@example
if x then if y then win; else lose;

if x then do; if y then win; end; else lose;
@end example

The conflict exists because the grammar as written is ambiguous: either
parsing of the simple nested if-statement is legitimate.  The established
convention is that these ambiguities are resolved by attaching the
else-clause to the innermost if-statement; this is what Bison accomplishes
by choosing to shift rather than reduce.  (It would ideally be cleaner to
write an unambiguous grammar, but that is very hard to do in this case.)
This particular ambiguity was first encountered in the specifications of
Algol 60 and is called the ``dangling @code{else}'' ambiguity.

To avoid warnings from Bison about predictable, legitimate shift/reduce
conflicts, you can use the @code{%expect @var{n}} declaration.
There will be no warning as long as the number of shift/reduce conflicts
is exactly @var{n}, and Bison will report an error if there is a
different number.
@xref{Expect Decl}.  However, we don't
recommend the use of @code{%expect} (except @samp{%expect 0}!), as an equal
number of conflicts does not mean that they are the @emph{same}.  When
possible, you should rather use precedence directives to @emph{fix} the
conflicts explicitly (@pxref{Non Operators}).

The definition of @code{if_stmt} above is solely to blame for the
conflict, but the conflict does not actually appear without additional
rules.  Here is a complete Bison grammar file that actually manifests
the conflict:

@example
%%
@group
stmt:
  expr
| if_stmt
;
@end group

@group
if_stmt:
  "if" expr "then" stmt
| "if" expr "then" stmt "else" stmt
;
@end group

expr:
  "identifier"
;
@end example

@node Precedence
@section Operator Precedence
@cindex operator precedence
@cindex precedence of operators

Another situation where shift/reduce conflicts appear is in arithmetic
expressions.  Here shifting is not always the preferred resolution; the
Bison declarations for operator precedence allow you to specify when to
shift and when to reduce.

@menu
* Why Precedence::    An example showing why precedence is needed.
* Using Precedence::  How to specify precedence and associativity.
* Precedence Only::   How to specify precedence only.
* Precedence Examples::  How these features are used in the previous example.
* How Precedence::    How they work.
* Non Operators::     Using precedence for general conflicts.
@end menu

@node Why Precedence
@subsection When Precedence is Needed

Consider the following ambiguous grammar fragment (ambiguous because the
input @w{@samp{1 - 2 * 3}} can be parsed in two different ways):

@example
@group
expr:
  expr '-' expr
| expr '*' expr
| expr '<' expr
| '(' expr ')'
@dots{}
;
@end group
@end example

@noindent
Suppose the parser has seen the tokens @samp{1}, @samp{-} and @samp{2};
should it reduce them via the rule for the subtraction operator?  It
depends on the next token.  Of course, if the next token is @samp{)}, we
must reduce; shifting is invalid because no single rule can reduce the
token sequence @w{@samp{- 2 )}} or anything starting with that.  But if
the next token is @samp{*} or @samp{<}, we have a choice: either
shifting or reduction would allow the parse to complete, but with
different results.

To decide which one Bison should do, we must consider the results.  If
the next operator token @var{op} is shifted, then it must be reduced
first in order to permit another opportunity to reduce the difference.
The result is (in effect) @w{@samp{1 - (2 @var{op} 3)}}.  On the other
hand, if the subtraction is reduced before shifting @var{op}, the result
is @w{@samp{(1 - 2) @var{op} 3}}.  Clearly, then, the choice of shift or
reduce should depend on the relative precedence of the operators
@samp{-} and @var{op}: @samp{*} should be shifted first, but not
@samp{<}.

@cindex associativity
What about input such as @w{@samp{1 - 2 - 5}}; should this be
@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}?  For most
operators we prefer the former, which is called @dfn{left association}.
The latter alternative, @dfn{right association}, is desirable for
assignment operators.  The choice of left or right association is a
matter of whether the parser chooses to shift or reduce when the stack
contains @w{@samp{1 - 2}} and the lookahead token is @samp{-}: shifting
makes right-associativity.

@node Using Precedence
@subsection Specifying Operator Precedence
@findex %left
@findex %nonassoc
@findex %precedence
@findex %right

Bison allows you to specify these choices with the operator precedence
declarations @code{%left} and @code{%right}.  Each such declaration
contains a list of tokens, which are operators whose precedence and
associativity is being declared.  The @code{%left} declaration makes all
those operators left-associative and the @code{%right} declaration makes
them right-associative.  A third alternative is @code{%nonassoc}, which
declares that it is a syntax error to find the same operator twice ``in a
row''.
The last alternative, @code{%precedence}, allows to define only
precedence and no associativity at all.  As a result, any
associativity-related conflict that remains will be reported as an
compile-time error.  The directive @code{%nonassoc} creates run-time
error: using the operator in a associative way is a syntax error.  The
directive @code{%precedence} creates compile-time errors: an operator
@emph{can} be involved in an associativity-related conflict, contrary to
what expected the grammar author.

The relative precedence of different operators is controlled by the
order in which they are declared.  The first precedence/associativity
declaration in the file declares the operators whose
precedence is lowest, the next such declaration declares the operators
whose precedence is a little higher, and so on.

@node Precedence Only
@subsection Specifying Precedence Only
@findex %precedence

Since POSIX Yacc defines only @code{%left}, @code{%right}, and
@code{%nonassoc}, which all defines precedence and associativity, little
attention is paid to the fact that precedence cannot be defined without
defining associativity.  Yet, sometimes, when trying to solve a
conflict, precedence suffices.  In such a case, using @code{%left},
@code{%right}, or @code{%nonassoc} might hide future (associativity
related) conflicts that would remain hidden.

The dangling @code{else} ambiguity (@pxref{Shift/Reduce}) can be solved
explicitly.  This shift/reduce conflicts occurs in the following situation,
where the period denotes the current parsing state:

@example
if @var{e1} then if  @var{e2} then @var{s1} . else @var{s2}
@end example

The conflict involves the reduction of the rule @samp{IF expr THEN
stmt}, which precedence is by default that of its last token
(@code{THEN}), and the shifting of the token @code{ELSE}.  The usual
disambiguation (attach the @code{else} to the closest @code{if}),
shifting must be preferred, i.e., the precedence of @code{ELSE} must be
higher than that of @code{THEN}.  But neither is expected to be involved
in an associativity related conflict, which can be specified as follows.

@example
%precedence THEN
%precedence ELSE
@end example

The unary-minus is another typical example where associativity is usually
over-specified, see @ref{Infix Calc}.  The @code{%left} directive is
traditionally used to declare the precedence of @code{NEG}, which is more
than needed since it also defines its associativity.  While this is harmless
in the traditional example, who knows how @code{NEG} might be used in future
evolutions of the grammar@dots{}

@node Precedence Examples
@subsection Precedence Examples

In our example, we would want the following declarations:

@example
%left '<'
%left '-'
%left '*'
@end example

In a more complete example, which supports other operators as well, we
would declare them in groups of equal precedence.  For example, @code{'+'} is
declared with @code{'-'}:

@example
%left '<' '>' '=' "!=" "<=" ">="
%left '+' '-'
%left '*' '/'
@end example

@node How Precedence
@subsection How Precedence Works

The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared.  The second effect is to assign
precedence levels to certain rules: each rule gets its precedence from
the last terminal symbol mentioned in the components.  (You can also
specify explicitly the precedence of a rule.  @xref{Contextual
Precedence}.)

Finally, the resolution of conflicts works by comparing the precedence
of the rule being considered with that of the lookahead token.  If the
token's precedence is higher, the choice is to shift.  If the rule's
precedence is higher, the choice is to reduce.  If they have equal
precedence, the choice is made based on the associativity of that
precedence level.  The verbose output file made by @samp{-v}
(@pxref{Invocation}) says how each conflict was
resolved.

Not all rules and not all tokens have precedence.  If either the rule or
the lookahead token has no precedence, then the default is to shift.

@node Non Operators
@subsection Using Precedence For Non Operators

Using properly precedence and associativity directives can help fixing
shift/reduce conflicts that do not involve arithmetic-like operators.  For
instance, the ``dangling @code{else}'' problem (@pxref{Shift/Reduce}) can be
solved elegantly in two different ways.

In the present case, the conflict is between the token @code{"else"} willing
to be shifted, and the rule @samp{if_stmt: "if" expr "then" stmt}, asking
for reduction.  By default, the precedence of a rule is that of its last
token, here @code{"then"}, so the conflict will be solved appropriately
by giving @code{"else"} a precedence higher than that of @code{"then"}, for
instance as follows:

@example
@group
%precedence "then"
%precedence "else"
@end group
@end example

Alternatively, you may give both tokens the same precedence, in which case
associativity is used to solve the conflict.  To preserve the shift action,
use right associativity:

@example
%right "then" "else"
@end example

Neither solution is perfect however.  Since Bison does not provide, so far,
``scoped'' precedence, both force you to declare the precedence
of these keywords with respect to the other operators your grammar.
Therefore, instead of being warned about new conflicts you would be unaware
of (e.g., a shift/reduce conflict due to @samp{if test then 1 else 2 + 3}
being ambiguous: @samp{if test then 1 else (2 + 3)} or @samp{(if test then 1
else 2) + 3}?), the conflict will be already ``fixed''.

@node Contextual Precedence
@section Context-Dependent Precedence
@cindex context-dependent precedence
@cindex unary operator precedence
@cindex precedence, context-dependent
@cindex precedence, unary operator
@findex %prec

Often the precedence of an operator depends on the context.  This sounds
outlandish at first, but it is really very common.  For example, a minus
sign typically has a very high precedence as a unary operator, and a
somewhat lower precedence (lower than multiplication) as a binary operator.

The Bison precedence declarations
can only be used once for a given token; so a token has
only one precedence declared in this way.  For context-dependent
precedence, you need to use an additional mechanism: the @code{%prec}
modifier for rules.

The @code{%prec} modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that rule.
It's not necessary for that symbol to appear otherwise in the rule.  The
modifier's syntax is:

@example
%prec @var{terminal-symbol}
@end example

@noindent
and it is written after the components of the rule.  Its effect is to
assign the rule the precedence of @var{terminal-symbol}, overriding
the precedence that would be deduced for it in the ordinary way.  The
altered rule precedence then affects how conflicts involving that rule
are resolved (@pxref{Precedence}).

Here is how @code{%prec} solves the problem of unary minus.  First, declare
a precedence for a fictitious terminal symbol named @code{UMINUS}.  There
are no tokens of this type, but the symbol serves to stand for its
precedence:

@example
@dots{}
%left '+' '-'
%left '*'
%left UMINUS
@end example

Now the precedence of @code{UMINUS} can be used in specific rules:

@example
@group
exp:
  @dots{}
| exp '-' exp
  @dots{}
| '-' exp %prec UMINUS
@end group
@end example

@ifset defaultprec
If you forget to append @code{%prec UMINUS} to the rule for unary
minus, Bison silently assumes that minus has its usual precedence.
This kind of problem can be tricky to debug, since one typically
discovers the mistake only by testing the code.

The @code{%no-default-prec;} declaration makes it easier to discover
this kind of problem systematically.  It causes rules that lack a
@code{%prec} modifier to have no precedence, even if the last terminal
symbol mentioned in their components has a declared precedence.

If @code{%no-default-prec;} is in effect, you must specify @code{%prec}
for all rules that participate in precedence conflict resolution.
Then you will see any shift/reduce conflict until you tell Bison how
to resolve it, either by changing your grammar or by adding an
explicit precedence.  This will probably add declarations to the
grammar, but it helps to protect against incorrect rule precedences.

The effect of @code{%no-default-prec;} can be reversed by giving
@code{%default-prec;}, which is the default.
@end ifset

@node Parser States
@section Parser States
@cindex finite-state machine
@cindex parser state
@cindex state (of parser)

The function @code{yyparse} is implemented using a finite-state machine.
The values pushed on the parser stack are not simply token kind codes; they
represent the entire sequence of terminal and nonterminal symbols at or
near the top of the stack.  The current state collects all the information
about previous input which is relevant to deciding what to do next.

Each time a lookahead token is read, the current parser state together with
the kind of lookahead token are looked up in a table.  This table entry can
say, ``Shift the lookahead token.''  In this case, it also specifies the new
parser state, which is pushed onto the top of the parser stack.  Or it can
say, ``Reduce using rule number @var{n}.''  This means that a certain number
of tokens or groupings are taken off the top of the stack, and replaced by
one grouping.  In other words, that number of states are popped from the
stack, and one new state is pushed.

There is one other alternative: the table can say that the lookahead token
is erroneous in the current state.  This causes error processing to begin
(@pxref{Error Recovery}).

@node Reduce/Reduce
@section Reduce/Reduce Conflicts
@cindex reduce/reduce conflict
@cindex conflicts, reduce/reduce

A reduce/reduce conflict occurs if there are two or more rules that apply
to the same sequence of input.  This usually indicates a serious error
in the grammar.

For example, here is an erroneous attempt to define a sequence
of zero or more @code{word} groupings.

@example
@group
sequence:
  %empty         @{ printf ("empty sequence\n"); @}
| maybeword
| sequence word  @{ printf ("added word %s\n", $2); @}
;
@end group

@group
maybeword:
  %empty    @{ printf ("empty maybeword\n"); @}
| word      @{ printf ("single word %s\n", $1); @}
;
@end group
@end example

@noindent
The error is an ambiguity: there is more than one way to parse a single
@code{word} into a @code{sequence}.  It could be reduced to a
@code{maybeword} and then into a @code{sequence} via the second rule.
Alternatively, nothing-at-all could be reduced into a @code{sequence}
via the first rule, and this could be combined with the @code{word}
using the third rule for @code{sequence}.

There is also more than one way to reduce nothing-at-all into a
@code{sequence}.  This can be done directly via the first rule,
or indirectly via @code{maybeword} and then the second rule.

You might think that this is a distinction without a difference, because it
does not change whether any particular input is valid or not.  But it does
affect which actions are run.  One parsing order runs the second rule's
action; the other runs the first rule's action and the third rule's action.
In this example, the output of the program changes.

Bison resolves a reduce/reduce conflict by choosing to use the rule that
appears first in the grammar, but it is very risky to rely on this.  Every
reduce/reduce conflict must be studied and usually eliminated.  Here is the
proper way to define @code{sequence}:

@example
@group
sequence:
  %empty         @{ printf ("empty sequence\n"); @}
| sequence word  @{ printf ("added word %s\n", $2); @}
;
@end group
@end example

Here is another common error that yields a reduce/reduce conflict:

@example
@group
sequence:
  %empty
| sequence words
| sequence redirects
;
@end group

@group
words:
  %empty
| words word
;
@end group

@group
redirects:
  %empty
| redirects redirect
;
@end group
@end example

@noindent
The intention here is to define a sequence which can contain either
@code{word} or @code{redirect} groupings.  The individual definitions of
@code{sequence}, @code{words} and @code{redirects} are error-free, but the
three together make a subtle ambiguity: even an empty input can be parsed
in infinitely many ways!

Consider: nothing-at-all could be a @code{words}.  Or it could be two
@code{words} in a row, or three, or any number.  It could equally well be a
@code{redirects}, or two, or any number.  Or it could be a @code{words}
followed by three @code{redirects} and another @code{words}.  And so on.

Here are two ways to correct these rules.  First, to make it a single level
of sequence:

@example
sequence:
  %empty
| sequence word
| sequence redirect
;
@end example

Second, to prevent either a @code{words} or a @code{redirects}
from being empty:

@example
@group
sequence:
  %empty
| sequence words
| sequence redirects
;
@end group

@group
words:
  word
| words word
;
@end group

@group
redirects:
  redirect
| redirects redirect
;
@end group
@end example

Yet this proposal introduces another kind of ambiguity!  The input
@samp{word word} can be parsed as a single @code{words} composed of two
@samp{word}s, or as two one-@code{word} @code{words} (and likewise for
@code{redirect}/@code{redirects}).  However this ambiguity is now a
shift/reduce conflict, and therefore it can now be addressed with precedence
directives.

To simplify the matter, we will proceed with @code{word} and @code{redirect}
being tokens: @code{"word"} and @code{"redirect"}.

To prefer the longest @code{words}, the conflict between the token
@code{"word"} and the rule @samp{sequence: sequence words} must be resolved
as a shift.  To this end, we use the same techniques as exposed above, see
@ref{Non Operators}.  One solution
relies on precedences: use @code{%prec} to give a lower precedence to the
rule:

@example
%precedence "word"
%precedence "sequence"
%%
@group
sequence:
  %empty
| sequence word      %prec "sequence"
| sequence redirect  %prec "sequence"
;
@end group

@group
words:
  word
| words "word"
;
@end group
@end example

Another solution relies on associativity: provide both the token and the
rule with the same precedence, but make them right-associative:

@example
%right "word" "redirect"
%%
@group
sequence:
  %empty
| sequence word      %prec "word"
| sequence redirect  %prec "redirect"
;
@end group
@end example

@node Mysterious Conflicts
@section Mysterious Conflicts
@cindex Mysterious Conflicts

Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:

@example
@group
%%
def: param_spec return_spec ',';
param_spec:
  type
| name_list ':' type
;
@end group

@group
return_spec:
  type
| name ':' type
;
@end group

type: "id";

@group
name: "id";
name_list:
  name
| name ',' name_list
;
@end group
@end example

It would seem that this grammar can be parsed with only a single token of
lookahead: when a @code{param_spec} is being read, an @code{"id"} is a
@code{name} if a comma or colon follows, or a @code{type} if another
@code{"id"} follows.  In other words, this grammar is LR(1).

@cindex LR
@cindex LALR
However, for historical reasons, Bison cannot by default handle all
LR(1) grammars.
In this grammar, two contexts, that after an @code{"id"} at the beginning
of a @code{param_spec} and likewise at the beginning of a
@code{return_spec}, are similar enough that Bison assumes they are the
same.
They appear similar because the same set of rules would be
active---the rule for reducing to a @code{name} and that for reducing to
a @code{type}.  Bison is unable to determine at that stage of processing
that the rules would require different lookahead tokens in the two
contexts, so it makes a single parser state for them both.  Combining
the two contexts causes a conflict later.  In parser terminology, this
occurrence means that the grammar is not LALR(1).

@cindex IELR
@cindex canonical LR
For many practical grammars (specifically those that fall into the non-LR(1)
class), the limitations of LALR(1) result in difficulties beyond just
mysterious reduce/reduce conflicts.  The best way to fix all these problems
is to select a different parser table construction algorithm.  Either
IELR(1) or canonical LR(1) would suffice, but the former is more efficient
and easier to debug during development.  @xref{LR Table Construction}, for
details.

If you instead wish to work around LALR(1)'s limitations, you
can often fix a mysterious conflict by identifying the two parser states
that are being confused, and adding something to make them look
distinct.  In the above example, adding one rule to
@code{return_spec} as follows makes the problem go away:

@example
@group
@dots{}
return_spec:
  type
| name ':' type
| "id" "bogus"       /* This rule is never used. */
;
@end group
@end example

This corrects the problem because it introduces the possibility of an
additional active rule in the context after the @code{"id"} at the beginning of
@code{return_spec}.  This rule is not active in the corresponding context
in a @code{param_spec}, so the two contexts receive distinct parser states.
As long as the token @code{"bogus"} is never generated by @code{yylex},
the added rule cannot alter the way actual input is parsed.

In this particular example, there is another way to solve the problem:
rewrite the rule for @code{return_spec} to use @code{"id"} directly
instead of via @code{name}.  This also causes the two confusing
contexts to have different sets of active rules, because the one for
@code{return_spec} activates the altered rule for @code{return_spec}
rather than the one for @code{name}.

@example
@group
param_spec:
  type
| name_list ':' type
;
@end group

@group
return_spec:
  type
| "id" ':' type
;
@end group
@end example

For a more detailed exposition of LALR(1) parsers and parser
generators, @pxref{Bibliography}.

@node Tuning LR
@section Tuning LR

The default behavior of Bison's LR-based parsers is chosen mostly for
historical reasons, but that behavior is often not robust.  For example, in
the previous section, we discussed the mysterious conflicts that can be
produced by LALR(1), Bison's default parser table construction algorithm.
Another example is Bison's @code{%define parse.error verbose} directive,
which instructs the generated parser to produce verbose syntax error
messages, which can sometimes contain incorrect information.

In this section, we explore several modern features of Bison that allow you
to tune fundamental aspects of the generated LR-based parsers.  Some of
these features easily eliminate shortcomings like those mentioned above.
Others can be helpful purely for understanding your parser.

@menu
* LR Table Construction:: Choose a different construction algorithm.
* Default Reductions::    Disable default reductions.
* LAC::                   Correct lookahead sets in the parser states.
* Unreachable States::    Keep unreachable parser states for debugging.
@end menu

@node LR Table Construction
@subsection LR Table Construction
@cindex Mysterious Conflict
@cindex LALR
@cindex IELR
@cindex canonical LR
@findex %define lr.type

For historical reasons, Bison constructs LALR(1) parser tables by default.
However, LALR does not possess the full language-recognition power of LR.
As a result, the behavior of parsers employing LALR parser tables is often
mysterious.  We presented a simple example of this effect in @ref{Mysterious
Conflicts}.

As we also demonstrated in that example, the traditional approach to
eliminating such mysterious behavior is to restructure the grammar.
Unfortunately, doing so correctly is often difficult.  Moreover, merely
discovering that LALR causes mysterious behavior in your parser can be
difficult as well.

Fortunately, Bison provides an easy way to eliminate the possibility of such
mysterious behavior altogether.  You simply need to activate a more powerful
parser table construction algorithm by using the @code{%define lr.type}
directive.

@deffn {Directive} {%define lr.type} @var{type}
Specify the type of parser tables within the LR(1) family.  The accepted
values for @var{type} are:

@itemize
@item @code{lalr} (default)
@item @code{ielr}
@item @code{canonical-lr}
@end itemize
@end deffn

For example, to activate IELR, you might add the following directive to you
grammar file:

@example
%define lr.type ielr
@end example

@noindent For the example in @ref{Mysterious Conflicts}, the mysterious
conflict is then eliminated, so there is no need to invest time in
comprehending the conflict or restructuring the grammar to fix it.  If,
during future development, the grammar evolves such that all mysterious
behavior would have disappeared using just LALR, you need not fear that
continuing to use IELR will result in unnecessarily large parser tables.
That is, IELR generates LALR tables when LALR (using a deterministic parsing
algorithm) is sufficient to support the full language-recognition power of
LR.  Thus, by enabling IELR at the start of grammar development, you can
safely and completely eliminate the need to consider LALR's shortcomings.

While IELR is almost always preferable, there are circumstances where LALR
or the canonical LR parser tables described by Knuth
(@pxref{Bibliography}) can be useful.  Here we summarize the
relative advantages of each parser table construction algorithm within
Bison:

@itemize
@item LALR

There are at least two scenarios where LALR can be worthwhile:

@itemize
@item GLR without static conflict resolution.

@cindex GLR with LALR
When employing GLR parsers (@pxref{GLR Parsers}), if you do not resolve any
conflicts statically (for example, with @code{%left} or @code{%precedence}),
then
the parser explores all potential parses of any given input.  In this case,
the choice of parser table construction algorithm is guaranteed not to alter
the language accepted by the parser.  LALR parser tables are the smallest
parser tables Bison can currently construct, so they may then be preferable.
Nevertheless, once you begin to resolve conflicts statically, GLR behaves
more like a deterministic parser in the syntactic contexts where those
conflicts appear, and so either IELR or canonical LR can then be helpful to
avoid LALR's mysterious behavior.

@item Malformed grammars.

Occasionally during development, an especially malformed grammar with a
major recurring flaw may severely impede the IELR or canonical LR parser
table construction algorithm.  LALR can be a quick way to construct parser
tables in order to investigate such problems while ignoring the more subtle
differences from IELR and canonical LR.
@end itemize

@item IELR

IELR (Inadequacy Elimination LR) is a minimal LR algorithm.  That is, given
any grammar (LR or non-LR), parsers using IELR or canonical LR parser tables
always accept exactly the same set of sentences.  However, like LALR, IELR
merges parser states during parser table construction so that the number of
parser states is often an order of magnitude less than for canonical LR.
More importantly, because canonical LR's extra parser states may contain
duplicate conflicts in the case of non-LR grammars, the number of conflicts
for IELR is often an order of magnitude less as well.  This effect can
significantly reduce the complexity of developing a grammar.

@item Canonical LR

@cindex delayed syntax error detection
@cindex LAC
@findex %nonassoc
While inefficient, canonical LR parser tables can be an interesting means to
explore a grammar because they possess a property that IELR and LALR tables
do not.  That is, if @code{%nonassoc} is not used and default reductions are
left disabled (@pxref{Default Reductions}), then, for every left context of
every canonical LR state, the set of tokens accepted by that state is
guaranteed to be the exact set of tokens that is syntactically acceptable in
that left context.  It might then seem that an advantage of canonical LR
parsers in production is that, under the above constraints, they are
guaranteed to detect a syntax error as soon as possible without performing
any unnecessary reductions.  However, IELR parsers that use LAC are also
able to achieve this behavior without sacrificing @code{%nonassoc} or
default reductions.  For details and a few caveats of LAC, @pxref{LAC}.
@end itemize

For a more detailed exposition of the mysterious behavior in LALR parsers
and the benefits of IELR, @pxref{Bibliography}, and
@ref{Bibliography}.

@node Default Reductions
@subsection Default Reductions
@cindex default reductions
@findex %define lr.default-reduction
@findex %nonassoc

After parser table construction, Bison identifies the reduction with the
largest lookahead set in each parser state.  To reduce the size of the
parser state, traditional Bison behavior is to remove that lookahead set and
to assign that reduction to be the default parser action.  Such a reduction
is known as a @dfn{default reduction}.

Default reductions affect more than the size of the parser tables.  They
also affect the behavior of the parser:

@itemize
@item Delayed @code{yylex} invocations.

@cindex delayed yylex invocations
@cindex consistent states
@cindex defaulted states
A @dfn{consistent state} is a state that has only one possible parser
action.  If that action is a reduction and is encoded as a default
reduction, then that consistent state is called a @dfn{defaulted state}.
Upon reaching a defaulted state, a Bison-generated parser does not bother to
invoke @code{yylex} to fetch the next token before performing the reduction.
In other words, whether default reductions are enabled in consistent states
determines how soon a Bison-generated parser invokes @code{yylex} for a
token: immediately when it @emph{reaches} that token in the input or when it
eventually @emph{needs} that token as a lookahead to determine the next
parser action.  Traditionally, default reductions are enabled, and so the
parser exhibits the latter behavior.

The presence of defaulted states is an important consideration when
designing @code{yylex} and the grammar file.  That is, if the behavior of
@code{yylex} can influence or be influenced by the semantic actions
associated with the reductions in defaulted states, then the delay of the
next @code{yylex} invocation until after those reductions is significant.
For example, the semantic actions might pop a scope stack that @code{yylex}
uses to determine what token to return.  Thus, the delay might be necessary
to ensure that @code{yylex} does not look up the next token in a scope that
should already be considered closed.

@item Delayed syntax error detection.

@cindex delayed syntax error detection
When the parser fetches a new token by invoking @code{yylex}, it checks
whether there is an action for that token in the current parser state.  The
parser detects a syntax error if and only if either (1) there is no action
for that token or (2) the action for that token is the error action (due to
the use of @code{%nonassoc}).  However, if there is a default reduction in
that state (which might or might not be a defaulted state), then it is
impossible for condition 1 to exist.  That is, all tokens have an action.
Thus, the parser sometimes fails to detect the syntax error until it reaches
a later state.

@cindex LAC
@c If there's an infinite loop, default reductions can prevent an incorrect
@c sentence from being rejected.
While default reductions never cause the parser to accept syntactically
incorrect sentences, the delay of syntax error detection can have unexpected
effects on the behavior of the parser.  However, the delay can be caused
anyway by parser state merging and the use of @code{%nonassoc}, and it can
be fixed by another Bison feature, LAC.  We discuss the effects of delayed
syntax error detection and LAC more in the next section (@pxref{LAC}).
@end itemize

For canonical LR, the only default reduction that Bison enables by default
is the accept action, which appears only in the accepting state, which has
no other action and is thus a defaulted state.  However, the default accept
action does not delay any @code{yylex} invocation or syntax error detection
because the accept action ends the parse.

For LALR and IELR, Bison enables default reductions in nearly all states by
default.  There are only two exceptions.  First, states that have a shift
action on the @code{error} token do not have default reductions because
delayed syntax error detection could then prevent the @code{error} token
from ever being shifted in that state.  However, parser state merging can
cause the same effect anyway, and LAC fixes it in both cases, so future
versions of Bison might drop this exception when LAC is activated.  Second,
GLR parsers do not record the default reduction as the action on a lookahead
token for which there is a conflict.  The correct action in this case is to
split the parse instead.

To adjust which states have default reductions enabled, use the
@code{%define lr.default-reduction} directive.

@deffn {Directive} {%define lr.default-reduction} @var{where}
Specify the kind of states that are permitted to contain default reductions.
The accepted values of @var{where} are:
@itemize
@item @code{most} (default for LALR and IELR)
@item @code{consistent}
@item @code{accepting} (default for canonical LR)
@end itemize
@end deffn

@node LAC
@subsection LAC
@findex %define parse.lac
@cindex LAC
@cindex lookahead correction

Canonical LR, IELR, and LALR can suffer from a couple of problems upon
encountering a syntax error.  First, the parser might perform additional
parser stack reductions before discovering the syntax error.  Such
reductions can perform user semantic actions that are unexpected because
they are based on an invalid token, and they cause error recovery to begin
in a different syntactic context than the one in which the invalid token was
encountered.  Second, when verbose error messages are enabled (@pxref{Error
Reporting}), the expected token list in the syntax error message can both
contain invalid tokens and omit valid tokens.

The culprits for the above problems are @code{%nonassoc}, default reductions
in inconsistent states (@pxref{Default Reductions}), and parser state
merging.  Because IELR and LALR merge parser states, they suffer the most.
Canonical LR can suffer only if @code{%nonassoc} is used or if default
reductions are enabled for inconsistent states.

LAC (Lookahead Correction) is a new mechanism within the parsing algorithm
that solves these problems for canonical LR, IELR, and LALR without
sacrificing @code{%nonassoc}, default reductions, or state merging.  You can
enable LAC with the @code{%define parse.lac} directive.

@deffn {Directive} {%define parse.lac} @var{value}
Enable LAC to improve syntax error handling.
@itemize
@item @code{none} (default)
@item @code{full}
@end itemize
This feature is currently only available for deterministic parsers in C and C++.
@end deffn

Conceptually, the LAC mechanism is straight-forward.  Whenever the parser
fetches a new token from the scanner so that it can determine the next
parser action, it immediately suspends normal parsing and performs an
exploratory parse using a temporary copy of the normal parser state stack.
During this exploratory parse, the parser does not perform user semantic
actions.  If the exploratory parse reaches a shift action, normal parsing
then resumes on the normal parser stacks.  If the exploratory parse reaches
an error instead, the parser reports a syntax error.  If verbose syntax
error messages are enabled, the parser must then discover the list of
expected tokens, so it performs a separate exploratory parse for each token
in the grammar.

There is one subtlety about the use of LAC.  That is, when in a consistent
parser state with a default reduction, the parser will not attempt to fetch
a token from the scanner because no lookahead is needed to determine the
next parser action.  Thus, whether default reductions are enabled in
consistent states (@pxref{Default Reductions}) affects how soon the parser
detects a syntax error: immediately when it @emph{reaches} an erroneous
token or when it eventually @emph{needs} that token as a lookahead to
determine the next parser action.  The latter behavior is probably more
intuitive, so Bison currently provides no way to achieve the former behavior
while default reductions are enabled in consistent states.

Thus, when LAC is in use, for some fixed decision of whether to enable
default reductions in consistent states, canonical LR and IELR behave almost
exactly the same for both syntactically acceptable and syntactically
unacceptable input.  While LALR still does not support the full
language-recognition power of canonical LR and IELR, LAC at least enables
LALR's syntax error handling to correctly reflect LALR's
language-recognition power.

There are a few caveats to consider when using LAC:

@itemize
@item Infinite parsing loops.

IELR plus LAC does have one shortcoming relative to canonical LR.  Some
parsers generated by Bison can loop infinitely.  LAC does not fix infinite
parsing loops that occur between encountering a syntax error and detecting
it, but enabling canonical LR or disabling default reductions sometimes
does.

@item Verbose error message limitations.

Because of internationalization considerations, Bison-generated parsers
limit the size of the expected token list they are willing to report in a
verbose syntax error message.  If the number of expected tokens exceeds that
limit, the list is simply dropped from the message.  Enabling LAC can
increase the size of the list and thus cause the parser to drop it.  Of
course, dropping the list is better than reporting an incorrect list.

@item Performance.

Because LAC requires many parse actions to be performed twice, it can have a
performance penalty.  However, not all parse actions must be performed
twice.  Specifically, during a series of default reductions in consistent
states and shift actions, the parser never has to initiate an exploratory
parse.  Moreover, the most time-consuming tasks in a parse are often the
file I/O, the lexical analysis performed by the scanner, and the user's
semantic actions, but none of these are performed during the exploratory
parse.  Finally, the base of the temporary stack used during an exploratory
parse is a pointer into the normal parser state stack so that the stack is
never physically copied.  In our experience, the performance penalty of LAC
has proved insignificant for practical grammars.
@end itemize

While the LAC algorithm shares techniques that have been recognized in the
parser community for years, for the publication that introduces LAC,
@pxref{Bibliography}.

@node Unreachable States
@subsection Unreachable States
@findex %define lr.keep-unreachable-state
@cindex unreachable states

If there exists no sequence of transitions from the parser's start state to
some state @var{s}, then Bison considers @var{s} to be an @dfn{unreachable
state}.  A state can become unreachable during conflict resolution if Bison
disables a shift action leading to it from a predecessor state.

By default, Bison removes unreachable states from the parser after conflict
resolution because they are useless in the generated parser.  However,
keeping unreachable states is sometimes useful when trying to understand the
relationship between the parser and the grammar.

@deffn {Directive} {%define lr.keep-unreachable-state} @var{value}
Request that Bison allow unreachable states to remain in the parser tables.
@var{value} must be a Boolean.  The default is @code{false}.
@end deffn

There are a few caveats to consider:

@itemize @bullet
@item Missing or extraneous warnings.

Unreachable states may contain conflicts and may use rules not used in any
other state.  Thus, keeping unreachable states may induce warnings that are
irrelevant to your parser's behavior, and it may eliminate warnings that are
relevant.  Of course, the change in warnings may actually be relevant to a
parser table analysis that wants to keep unreachable states, so this
behavior will likely remain in future Bison releases.

@item Other useless states.

While Bison is able to remove unreachable states, it is not guaranteed to
remove other kinds of useless states.  Specifically, when Bison disables
reduce actions during conflict resolution, some goto actions may become
useless, and thus some additional states may become useless.  If Bison were
to compute which goto actions were useless and then disable those actions,
it could identify such states as unreachable and then remove those states.
However, Bison does not compute which goto actions are useless.
@end itemize

@node Generalized LR Parsing
@section Generalized LR (GLR) Parsing
@cindex GLR parsing
@cindex generalized LR (GLR) parsing
@cindex ambiguous grammars
@cindex nondeterministic parsing

Bison produces @emph{deterministic} parsers that choose uniquely
when to reduce and which reduction to apply
based on a summary of the preceding input and on one extra token of lookahead.
As a result, normal Bison handles a proper subset of the family of
context-free languages.
Ambiguous grammars, since they have strings with more than one possible
sequence of reductions cannot have deterministic parsers in this sense.
The same is true of languages that require more than one symbol of
lookahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (@pxref{Mysterious Conflicts}),
there are languages where Bison's default choice of how to
summarize the input seen so far loses necessary information.

When you use the @samp{%glr-parser} declaration in your grammar file,
Bison generates a parser that uses a different algorithm, called
Generalized LR (or GLR).  A Bison GLR
parser uses the same basic
algorithm for parsing as an ordinary Bison parser, but behaves
differently in cases where there is a shift-reduce conflict that has not
been resolved by precedence rules (@pxref{Precedence}) or a
reduce-reduce conflict.  When a GLR parser encounters such a
situation, it
effectively @emph{splits} into a several parsers, one for each possible
shift or reduction.  These parsers then proceed as usual, consuming
tokens in lock-step.  Some of the stacks may encounter other conflicts
and split further, with the result that instead of a sequence of states,
a Bison GLR parsing stack is what is in effect a tree of states.

In effect, each stack represents a guess as to what the proper parse
is.  Additional input may indicate that a guess was wrong, in which case
the appropriate stack silently disappears.  Otherwise, the semantics
actions generated in each stack are saved, rather than being executed
immediately.  When a stack disappears, its saved semantic actions never
get executed.  When a reduction causes two stacks to become equivalent,
their sets of semantic actions are both saved with the state that
results from the reduction.  We say that two stacks are equivalent
when they both represent the same sequence of states,
and each pair of corresponding states represents a
grammar symbol that produces the same segment of the input token
stream.

Whenever the parser makes a transition from having multiple
states to having one, it reverts to the normal deterministic parsing
algorithm, after resolving and executing the saved-up actions.
At this transition, some of the states on the stack will have semantic
values that are sets (actually multisets) of possible actions.  The
parser tries to pick one of the actions by first finding one whose rule
has the highest dynamic precedence, as set by the @samp{%dprec}
declaration.  Otherwise, if the alternative actions are not ordered by
precedence, but there the same merging function is declared for both
rules by the @samp{%merge} declaration,
Bison resolves and evaluates both and then calls the merge function on
the result.  Otherwise, it reports an ambiguity.

It is possible to use a data structure for the GLR parsing tree that
permits the processing of any LR(1) grammar in linear time (in the
size of the input), any unambiguous (not necessarily
LR(1)) grammar in
quadratic worst-case time, and any general (possibly ambiguous)
context-free grammar in cubic worst-case time.  However, Bison currently
uses a simpler data structure that requires time proportional to the
length of the input times the maximum number of stacks required for any
prefix of the input.  Thus, really ambiguous or nondeterministic
grammars can require exponential time and space to process.  Such badly
behaving examples, however, are not generally of practical interest.
Usually, nondeterminism in a grammar is local---the parser is ``in
doubt'' only for a few tokens at a time.  Therefore, the current data
structure should generally be adequate.  On LR(1) portions of a
grammar, in particular, it is only slightly slower than with the
deterministic LR(1) Bison parser.

For a more detailed exposition of GLR parsers, @pxref{Bibliography}.

@node Memory Management
@section Memory Management, and How to Avoid Memory Exhaustion
@cindex memory exhaustion
@cindex memory management
@cindex stack overflow
@cindex parser stack overflow
@cindex overflow of parser stack

The Bison parser stack can run out of memory if too many tokens are shifted and
not reduced.  When this happens, the parser function @code{yyparse}
calls @code{yyerror} and then returns 2.

Because Bison parsers have growing stacks, hitting the upper limit
usually results from using a right recursion instead of a left
recursion, see @ref{Recursion}.

@vindex YYMAXDEPTH
By defining the macro @code{YYMAXDEPTH}, you can control how deep the
parser stack can become before memory is exhausted.  Define the
macro with a value that is an integer.  This value is the maximum number
of tokens that can be shifted (and not reduced) before overflow.

The stack space allowed is not necessarily allocated.  If you specify a
large value for @code{YYMAXDEPTH}, the parser normally allocates a small
stack at first, and then makes it bigger by stages as needed.  This
increasing allocation happens automatically and silently.  Therefore,
you do not need to make @code{YYMAXDEPTH} painfully small merely to save
space for ordinary inputs that do not need much stack.

However, do not allow @code{YYMAXDEPTH} to be a value so large that
arithmetic overflow could occur when calculating the size of the stack
space.  Also, do not allow @code{YYMAXDEPTH} to be less than
@code{YYINITDEPTH}.

@cindex default stack limit
The default value of @code{YYMAXDEPTH}, if you do not define it, is
10000.

@vindex YYINITDEPTH
You can control how much stack is allocated initially by defining the
macro @code{YYINITDEPTH} to a positive integer.  For the deterministic
parser in C, this value must be a compile-time constant
unless you are assuming C99 or some other target language or compiler
that allows variable-length arrays.  The default is 200.

Do not allow @code{YYINITDEPTH} to be greater than @code{YYMAXDEPTH}.

You can generate a deterministic parser containing C++ user code from the
default (C) skeleton, as well as from the C++ skeleton (@pxref{C++
Parsers}).  However, if you do use the default skeleton and want to allow
the parsing stack to grow, be careful not to use semantic types or location
types that require non-trivial copy constructors.  The C skeleton bypasses
these constructors when copying data to new, larger stacks.

@node Error Recovery
@chapter Error Recovery
@cindex error recovery
@cindex recovery from errors

It is not usually acceptable to have a program terminate on a syntax
error.  For example, a compiler should recover sufficiently to parse the
rest of the input file and check it for errors; a calculator should accept
another expression.

In a simple interactive command parser where each input is one line, it may
be sufficient to allow @code{yyparse} to return 1 on error and have the
caller ignore the rest of the input line when that happens (and then call
@code{yyparse} again).  But this is inadequate for a compiler, because it
forgets all the syntactic context leading up to the error.  A syntax error
deep within a function in the compiler input should not cause the compiler
to treat the following line like the beginning of a source file.

@findex error
You can define how to recover from a syntax error by writing rules to
recognize the special token @code{error}.  This is a terminal symbol that
is always defined (you need not declare it) and reserved for error
handling.  The Bison parser generates an @code{error} token whenever a
syntax error happens; if you have provided a rule to recognize this token
in the current context, the parse can continue.

For example:

@example
stmts:
  %empty
| stmts '\n'
| stmts exp '\n'
| stmts error '\n'
@end example

The fourth rule in this example says that an error followed by a newline
makes a valid addition to any @code{stmts}.

What happens if a syntax error occurs in the middle of an @code{exp}?  The
error recovery rule, interpreted strictly, applies to the precise sequence
of a @code{stmts}, an @code{error} and a newline.  If an error occurs in
the middle of an @code{exp}, there will probably be some additional tokens
and subexpressions on the stack after the last @code{stmts}, and there
will be tokens to read before the next newline.  So the rule is not
applicable in the ordinary way.

But Bison can force the situation to fit the rule, by discarding part of the
semantic context and part of the input.  First it discards states and
objects from the stack until it gets back to a state in which the
@code{error} token is acceptable.  (This means that the subexpressions
already parsed are discarded, back to the last complete @code{stmts}.)  At
this point the @code{error} token can be shifted.  Then, if the old
lookahead token is not acceptable to be shifted next, the parser reads
tokens and discards them until it finds a token which is acceptable.  In
this example, Bison reads and discards input until the next newline so that
the fourth rule can apply.  Note that discarded symbols are possible sources
of memory leaks, see @ref{Destructor Decl}, for a means to reclaim this
memory.

The choice of error rules in the grammar is a choice of strategies for
error recovery.  A simple and useful strategy is simply to skip the rest of
the current input line or current statement if an error is detected:

@example
stmt: error ';'  /* On error, skip until ';' is read. */
@end example

It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed.  Otherwise the
close-delimiter will probably appear to be unmatched, and generate another,
spurious error message:

@example
primary:
  '(' expr ')'
| '(' error ')'
@dots{}
;
@end example

Error recovery strategies are necessarily guesses.  When they guess wrong,
one syntax error often leads to another.  In the above example, the error
recovery rule guesses that an error is due to bad input within one
@code{stmt}.  Suppose that instead a spurious semicolon is inserted in the
middle of a valid @code{stmt}.  After the error recovery rule recovers
from the first error, another syntax error will be found straightaway,
since the text following the spurious semicolon is also an invalid
@code{stmt}.

To prevent an outpouring of error messages, the parser will output no error
message for another syntax error that happens shortly after the first; only
after three consecutive input tokens have been successfully shifted will
error messages resume.

Note that rules which accept the @code{error} token may have actions, just
as any other rules can.

@findex yyerrok
You can make error messages resume immediately by using the macro
@code{yyerrok} in an action.  If you do this in the error rule's action, no
error messages will be suppressed.  This macro requires no arguments;
@samp{yyerrok;} is a valid C statement.

@findex yyclearin
The previous lookahead token is reanalyzed immediately after an error.  If
this is unacceptable, then the macro @code{yyclearin} may be used to clear
this token.  Write the statement @samp{yyclearin;} in the error rule's
action.
@xref{Action Features}.

For example, suppose that on a syntax error, an error handling routine is
called that advances the input stream to some point where parsing should
once again commence.  The next symbol returned by the lexical scanner is
probably correct.  The previous lookahead token ought to be discarded
with @samp{yyclearin;}.

@vindex YYRECOVERING
The expression @code{YYRECOVERING ()} yields 1 when the parser
is recovering from a syntax error, and 0 otherwise.
Syntax error diagnostics are suppressed while recovering from a syntax
error.

@node Context Dependency
@chapter Handling Context Dependencies

The Bison paradigm is to parse tokens first, then group them into larger
syntactic units.  In many languages, the meaning of a token is affected by
its context.  Although this violates the Bison paradigm, certain techniques
(known as @dfn{kludges}) may enable you to write Bison parsers for such
languages.

@menu
* Semantic Tokens::   Token parsing can depend on the semantic context.
* Lexical Tie-ins::   Token parsing can depend on the syntactic context.
* Tie-in Recovery::   Lexical tie-ins have implications for how
                        error recovery rules must be written.
@end menu

(Actually, ``kludge'' means any technique that gets its job done but is
neither clean nor robust.)

@node Semantic Tokens
@section Semantic Info in Token Kinds

The C language has a context dependency: the way an identifier is used
depends on what its current meaning is.  For example, consider this:

@example
foo (x);
@end example

This looks like a function call statement, but if @code{foo} is a typedef
name, then this is actually a declaration of @code{x}.  How can a Bison
parser for C decide how to parse this input?

The method used in GNU C is to have two different token kinds,
@code{IDENTIFIER} and @code{TYPENAME}.  When @code{yylex} finds an
identifier, it looks up the current declaration of the identifier in order
to decide which token kind to return: @code{TYPENAME} if the identifier is
declared as a typedef, @code{IDENTIFIER} otherwise.

The grammar rules can then express the context dependency by the choice of
token kind to recognize.  @code{IDENTIFIER} is accepted as an expression,
but @code{TYPENAME} is not.  @code{TYPENAME} can start a declaration, but
@code{IDENTIFIER} cannot.  In contexts where the meaning of the identifier
is @emph{not} significant, such as in declarations that can shadow a
typedef name, either @code{TYPENAME} or @code{IDENTIFIER} is
accepted---there is one rule for each of the two token kinds.

This technique is simple to use if the decision of which kinds of
identifiers to allow is made at a place close to where the identifier is
parsed.  But in C this is not always so: C allows a declaration to
redeclare a typedef name provided an explicit type has been specified
earlier:

@example
typedef int foo, bar;
int baz (void)
@group
@{
  static bar (bar);      /* @r{redeclare @code{bar} as static variable} */
  extern foo foo (foo);  /* @r{redeclare @code{foo} as function} */
  return foo (bar);
@}
@end group
@end example

Unfortunately, the name being declared is separated from the declaration
construct itself by a complicated syntactic structure---the ``declarator''.

As a result, part of the Bison parser for C needs to be duplicated, with
all the nonterminal names changed: once for parsing a declaration in
which a typedef name can be redefined, and once for parsing a
declaration in which that can't be done.  Here is a part of the
duplication, with actions omitted for brevity:

@example
@group
initdcl:
  declarator maybeasm '=' init
| declarator maybeasm
;
@end group

@group
notype_initdcl:
  notype_declarator maybeasm '=' init
| notype_declarator maybeasm
;
@end group
@end example

@noindent
Here @code{initdcl} can redeclare a typedef name, but @code{notype_initdcl}
cannot.  The distinction between @code{declarator} and
@code{notype_declarator} is the same sort of thing.

There is some similarity between this technique and a lexical tie-in
(described next), in that information which alters the lexical analysis is
changed during parsing by other parts of the program.  The difference is
here the information is global, and is used for other purposes in the
program.  A true lexical tie-in has a special-purpose flag controlled by
the syntactic context.

@node Lexical Tie-ins
@section Lexical Tie-ins
@cindex lexical tie-in

One way to handle context-dependency is the @dfn{lexical tie-in}: a flag
which is set by Bison actions, whose purpose is to alter the way tokens are
parsed.

For example, suppose we have a language vaguely like C, but with a special
construct @samp{hex (@var{hex-expr})}.  After the keyword @code{hex} comes
an expression in parentheses in which all integers are hexadecimal.  In
particular, the token @samp{a1b} must be treated as an integer rather than
as an identifier if it appears in that context.  Here is how you can do it:

@example
@group
%@{
  int hexflag;
  int yylex (void);
  void yyerror (char const *);
%@}
%%
@dots{}
@end group
@group
expr:
  IDENTIFIER
| constant
| HEX '('        @{ hexflag = 1; @}
    expr ')'     @{ hexflag = 0; $$ = $4; @}
| expr '+' expr  @{ $$ = make_sum ($1, $3); @}
@dots{}
;
@end group

@group
constant:
  INTEGER
| STRING
;
@end group
@end example

@noindent
Here we assume that @code{yylex} looks at the value of @code{hexflag}; when
it is nonzero, all integers are parsed in hexadecimal, and tokens starting
with letters are parsed as integers if possible.

The declaration of @code{hexflag} shown in the prologue of the grammar file
is needed to make it accessible to the actions (@pxref{Prologue}).  You must
also write the code in @code{yylex} to obey the flag.

@node Tie-in Recovery
@section Lexical Tie-ins and Error Recovery

Lexical tie-ins make strict demands on any error recovery rules you have.
@xref{Error Recovery}.

The reason for this is that the purpose of an error recovery rule is to
abort the parsing of one construct and resume in some larger construct.
For example, in C-like languages, a typical error recovery rule is to skip
tokens until the next semicolon, and then start a new statement, like this:

@example
stmt:
  expr ';'
| IF '(' expr ')' stmt @{ @dots{} @}
@dots{}
| error ';'  @{ hexflag = 0; @}
;
@end example

If there is a syntax error in the middle of a @samp{hex (@var{expr})}
construct, this error rule will apply, and then the action for the
completed @samp{hex (@var{expr})} will never run.  So @code{hexflag} would
remain set for the entire rest of the input, or until the next @code{hex}
keyword, causing identifiers to be misinterpreted as integers.

To avoid this problem the error recovery rule itself clears @code{hexflag}.

There may also be an error recovery rule that works within expressions.
For example, there could be a rule which applies within parentheses
and skips to the close-parenthesis:

@example
@group
expr:
  @dots{}
| '(' expr ')'   @{ $$ = $2; @}
| '(' error ')'
@dots{}
@end group
@end example

If this rule acts within the @code{hex} construct, it is not going to abort
that construct (since it applies to an inner level of parentheses within
the construct).  Therefore, it should not clear the flag: the rest of
the @code{hex} construct should be parsed with the flag still in effect.

What if there is an error recovery rule which might abort out of the
@code{hex} construct or might not, depending on circumstances?  There is no
way you can write the action to determine whether a @code{hex} construct is
being aborted or not.  So if you are using a lexical tie-in, you had better
make sure your error recovery rules are not of this kind.  Each rule must
be such that you can be sure that it always will, or always won't, have to
clear the flag.

@c ================================================== Debugging Your Parser

@node Debugging
@chapter Debugging Your Parser

Developing a parser can be a challenge, especially if you don't understand
the algorithm (@pxref{Algorithm}).  This
chapter explains how to understand and debug a parser.

The first sections focus on the static part of the parser: its structure.
They explain how to generate and read the detailed description of the
automaton.  There are several formats available:
@itemize @minus
@item
as text, see @ref{Understanding};

@item
as a graph, see @ref{Graphviz};

@item
or as a markup report that can be turned, for instance, into HTML, see
@ref{Xml}.
@end itemize

The last section focuses on the dynamic part of the parser: how to enable
and understand the parser run-time traces (@pxref{Tracing}).

@menu
* Understanding::     Understanding the structure of your parser.
* Graphviz::          Getting a visual representation of the parser.
* Xml::               Getting a markup representation of the parser.
* Tracing::           Tracing the execution of your parser.
@end menu

@node Understanding
@section Understanding Your Parser

As documented elsewhere (@pxref{Algorithm}) Bison parsers are
@dfn{shift/reduce automata}.  In some cases (much more frequent than one
would hope), looking at this automaton is required to tune or simply fix a
parser.

The textual file is generated when the options @option{--report} or
@option{--verbose} are specified, see @ref{Invocation}.  Its name is made by
removing @samp{.tab.c} or @samp{.c} from the parser implementation file
name, and adding @samp{.output} instead.  Therefore, if the grammar file is
@file{foo.y}, then the parser implementation file is called @file{foo.tab.c}
by default.  As a consequence, the verbose output file is called
@file{foo.output}.

The following grammar file, @file{calc.y}, will be used in the sequel:

@example
@group
%union
@{
  int ival;
  const char *sval;
@}
@end group
@group
%token <ival> NUM
%nterm <ival> exp
@end group
@group
%token <sval> STR
%nterm <sval> useless
@end group
@group
%left '+' '-'
%left '*'
@end group
%%
@group
exp:
  exp '+' exp
| exp '-' exp
| exp '*' exp
| exp '/' exp
| NUM
;
@end group
useless: STR;
%%
@end example

@command{bison} reports:

@example
calc.y: @dwarning{warning}: 1 nonterminal useless in grammar [@dwarning{-Wother}]
calc.y: @dwarning{warning}: 1 rule useless in grammar [@dwarning{-Wother}]
calc.y:19.1-7: @dwarning{warning}: nonterminal useless in grammar: useless [@dwarning{-Wother}]
   19 | @dwarning{useless: STR;}
      | @dwarning{^~~~~~~}
calc.y: @dwarning{warning}: 7 shift/reduce conflicts [@dwarning{-Wconflicts-sr}]
@end example

When given @option{--report=state}, in addition to @file{calc.tab.c}, it
creates a file @file{calc.output} with contents detailed below.  The
order of the output and the exact presentation might vary, but the
interpretation is the same.

@noindent
@cindex token, useless
@cindex useless token
@cindex nonterminal, useless
@cindex useless nonterminal
@cindex rule, useless
@cindex useless rule
The first section reports useless tokens, nonterminals and rules.  Useless
nonterminals and rules are removed in order to produce a smaller parser, but
useless tokens are preserved, since they might be used by the scanner (note
the difference between ``useless'' and ``unused'' below):

@example
Nonterminals useless in grammar
   useless

Terminals unused in grammar
   STR

Rules useless in grammar
    6 useless: STR
@end example

@noindent
The next section lists states that still have conflicts.

@example
State 8 conflicts: 1 shift/reduce
State 9 conflicts: 1 shift/reduce
State 10 conflicts: 1 shift/reduce
State 11 conflicts: 4 shift/reduce
@end example

@noindent
Then Bison reproduces the exact grammar it used:

@example
Grammar

    0 $accept: exp $end

    1 exp: exp '+' exp
    2    | exp '-' exp
    3    | exp '*' exp
    4    | exp '/' exp
    5    | NUM
@end example

@noindent
and reports the uses of the symbols:

@example
@group
Terminals, with rules where they appear

$end (0) 0
'*' (42) 3
'+' (43) 1
'-' (45) 2
'/' (47) 4
error (256)
NUM <ival> (258) 5
STR <sval> (259)
@end group

@group
Nonterminals, with rules where they appear

$accept (9)
    on left: 0
exp <ival> (10)
    on left: 1 2 3 4 5, on right: 0 1 2 3 4
@end group
@end example

@noindent
@cindex item
@cindex pointed rule
@cindex rule, pointed
Bison then proceeds onto the automaton itself, describing each state
with its set of @dfn{items}, also known as @dfn{pointed rules}.  Each
item is a production rule together with a point (@samp{.}) marking
the location of the input cursor.

@example
State 0

    0 $accept: . exp $end

    NUM  shift, and go to state 1

    exp  go to state 2
@end example

This reads as follows: ``state 0 corresponds to being at the very
beginning of the parsing, in the initial rule, right before the start
symbol (here, @code{exp}).  When the parser returns to this state right
after having reduced a rule that produced an @code{exp}, the control
flow jumps to state 2.  If there is no such transition on a nonterminal
symbol, and the lookahead is a @code{NUM}, then this token is shifted onto
the parse stack, and the control flow jumps to state 1.  Any other
lookahead triggers a syntax error.''

@cindex core, item set
@cindex item set core
@cindex kernel, item set
@cindex item set core
Even though the only active rule in state 0 seems to be rule 0, the
report lists @code{NUM} as a lookahead token because @code{NUM} can be
at the beginning of any rule deriving an @code{exp}.  By default Bison
reports the so-called @dfn{core} or @dfn{kernel} of the item set, but if
you want to see more detail you can invoke @command{bison} with
@option{--report=itemset} to list the derived items as well:

@example
State 0

    0 $accept: . exp $end
    1 exp: . exp '+' exp
    2    | . exp '-' exp
    3    | . exp '*' exp
    4    | . exp '/' exp
    5    | . NUM

    NUM  shift, and go to state 1

    exp  go to state 2
@end example

@noindent
In the state 1@dots{}

@example
State 1

    5 exp: NUM .

    $default  reduce using rule 5 (exp)
@end example

@noindent
the rule 5, @samp{exp: NUM;}, is completed.  Whatever the lookahead token
(@samp{$default}), the parser will reduce it.  If it was coming from
State 0, then, after this reduction it will return to state 0, and will
jump to state 2 (@samp{exp: go to state 2}).

@example
State 2

    0 $accept: exp . $end
    1 exp: exp . '+' exp
    2    | exp . '-' exp
    3    | exp . '*' exp
    4    | exp . '/' exp

    $end  shift, and go to state 3
    '+'   shift, and go to state 4
    '-'   shift, and go to state 5
    '*'   shift, and go to state 6
    '/'   shift, and go to state 7
@end example

@noindent
In state 2, the automaton can only shift a symbol.  For instance,
because of the item @samp{exp: exp . '+' exp}, if the lookahead is
@samp{+} it is shifted onto the parse stack, and the automaton
jumps to state 4, corresponding to the item @samp{exp: exp '+' . exp}.
Since there is no default action, any lookahead not listed triggers a syntax
error.

@cindex accepting state
The state 3 is named the @dfn{final state}, or the @dfn{accepting
state}:

@example
State 3

    0 $accept: exp $end .

    $default  accept
@end example

@noindent
the initial rule is completed (the start symbol and the end-of-input were
read), the parsing exits successfully.

The interpretation of states 4 to 7 is straightforward, and is left to
the reader.

@example
State 4

    1 exp: exp '+' . exp

    NUM  shift, and go to state 1

    exp  go to state 8


State 5

    2 exp: exp '-' . exp

    NUM  shift, and go to state 1

    exp  go to state 9


State 6

    3 exp: exp '*' . exp

    NUM  shift, and go to state 1

    exp  go to state 10


State 7

    4 exp: exp '/' . exp

    NUM  shift, and go to state 1

    exp  go to state 11
@end example

As was announced in beginning of the report, @samp{State 8 conflicts:
1 shift/reduce}:

@example
State 8

    1 exp: exp . '+' exp
    1    | exp '+' exp .
    2    | exp . '-' exp
    3    | exp . '*' exp
    4    | exp . '/' exp

    '*'  shift, and go to state 6
    '/'  shift, and go to state 7

    '/'       [reduce using rule 1 (exp)]
    $default  reduce using rule 1 (exp)
@end example

Indeed, there are two actions associated to the lookahead @samp{/}:
either shifting (and going to state 7), or reducing rule 1.  The
conflict means that either the grammar is ambiguous, or the parser lacks
information to make the right decision.  Indeed the grammar is
ambiguous, as, since we did not specify the precedence of @samp{/}, the
sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM /
NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) /
NUM}, which corresponds to reducing rule 1.

Because in deterministic parsing a single decision can be made, Bison
arbitrarily chose to disable the reduction, see @ref{Shift/Reduce}.
Discarded actions are reported between square brackets.

Note that all the previous states had a single possible action: either
shifting the next token and going to the corresponding state, or
reducing a single rule.  In the other cases, i.e., when shifting
@emph{and} reducing is possible or when @emph{several} reductions are
possible, the lookahead is required to select the action.  State 8 is
one such state: if the lookahead is @samp{*} or @samp{/} then the action
is shifting, otherwise the action is reducing rule 1.  In other words,
the first two items, corresponding to rule 1, are not eligible when the
lookahead token is @samp{*}, since we specified that @samp{*} has higher
precedence than @samp{+}.  More generally, some items are eligible only
with some set of possible lookahead tokens.  When run with
@option{--report=lookahead}, Bison specifies these lookahead tokens:

@example
State 8

    1 exp: exp . '+' exp
    1    | exp '+' exp .  [$end, '+', '-', '/']
    2    | exp . '-' exp
    3    | exp . '*' exp
    4    | exp . '/' exp

    '*'  shift, and go to state 6
    '/'  shift, and go to state 7

    '/'       [reduce using rule 1 (exp)]
    $default  reduce using rule 1 (exp)
@end example

Note however that while @samp{NUM + NUM / NUM} is ambiguous (which results in
the conflicts on @samp{/}), @samp{NUM + NUM * NUM} is not: the conflict was
solved thanks to associativity and precedence directives.  If invoked with
@option{--report=solved}, Bison includes information about the solved
conflicts in the report:

@example
Conflict between rule 1 and token '+' resolved as reduce (%left '+').
Conflict between rule 1 and token '-' resolved as reduce (%left '-').
Conflict between rule 1 and token '*' resolved as shift ('+' < '*').
@end example


The remaining states are similar:

@example
@group
State 9

    1 exp: exp . '+' exp
    2    | exp . '-' exp
    2    | exp '-' exp .
    3    | exp . '*' exp
    4    | exp . '/' exp

    '*'  shift, and go to state 6
    '/'  shift, and go to state 7

    '/'       [reduce using rule 2 (exp)]
    $default  reduce using rule 2 (exp)
@end group

@group
State 10

    1 exp: exp . '+' exp
    2    | exp . '-' exp
    3    | exp . '*' exp
    3    | exp '*' exp .
    4    | exp . '/' exp

    '/'  shift, and go to state 7

    '/'       [reduce using rule 3 (exp)]
    $default  reduce using rule 3 (exp)
@end group

@group
State 11

    1 exp: exp . '+' exp
    2    | exp . '-' exp
    3    | exp . '*' exp
    4    | exp . '/' exp
    4    | exp '/' exp .

    '+'  shift, and go to state 4
    '-'  shift, and go to state 5
    '*'  shift, and go to state 6
    '/'  shift, and go to state 7

    '+'       [reduce using rule 4 (exp)]
    '-'       [reduce using rule 4 (exp)]
    '*'       [reduce using rule 4 (exp)]
    '/'       [reduce using rule 4 (exp)]
    $default  reduce using rule 4 (exp)
@end group
@end example

@noindent
Observe that state 11 contains conflicts not only due to the lack of
precedence of @samp{/} with respect to @samp{+}, @samp{-}, and @samp{*}, but
also because the associativity of @samp{/} is not specified.

Bison may also produce an HTML version of this output, via an XML file and
XSLT processing (@pxref{Xml}).

@c ================================================= Graphical Representation

@node Graphviz
@section Visualizing Your Parser
@cindex dot

As another means to gain better understanding of the shift/reduce
automaton corresponding to the Bison parser, a DOT file can be generated. Note
that debugging a real grammar with this is tedious at best, and impractical
most of the times, because the generated files are huge (the generation of
a PDF or PNG file from it will take very long, and more often than not it will
fail due to memory exhaustion). This option was rather designed for beginners,
to help them understand LR parsers.

This file is generated when the @option{--graph} option is specified
(@pxref{Invocation}).  Its name is made by removing
@samp{.tab.c} or @samp{.c} from the parser implementation file name, and
adding @samp{.gv} instead.  If the grammar file is @file{foo.y}, the
Graphviz output file is called @file{foo.gv}.  A DOT file may also be
produced via an XML file and XSLT processing (@pxref{Xml}).


The following grammar file, @file{rr.y}, will be used in the sequel:

@example
%%
@group
exp: a ";" | b ".";
a: "0";
b: "0";
@end group
@end example

The graphical output
@ifnotinfo
(see @ref{fig:graph})
@end ifnotinfo
is very similar to the textual one, and as such it is easier understood by
making direct comparisons between them.  @xref{Debugging}, for a detailed
analysis of the textual report.

@ifnotinfo
@float Figure,fig:graph
@center @image{figs/example, 430pt,,,.svg}
@caption{A graphical rendering of the parser.}
@end float
@end ifnotinfo

@subheading Graphical Representation of States

The items (pointed rules) for each state are grouped together in graph nodes.
Their numbering is the same as in the verbose file. See the following points,
about transitions, for examples

When invoked with @option{--report=lookaheads}, the lookahead tokens, when
needed, are shown next to the relevant rule between square brackets as a
comma separated list. This is the case in the figure for the representation of
reductions, below.

@sp 1

The transitions are represented as directed edges between the current and
the target states.

@subheading Graphical Representation of Shifts

Shifts are shown as solid arrows, labeled with the lookahead token for that
shift. The following describes a reduction in the @file{rr.output} file:

@example
@group
State 3

    1 exp: a . ";"

    ";"  shift, and go to state 6
@end group
@end example

A Graphviz rendering of this portion of the graph could be:

@center @image{figs/example-shift, 100pt,,,.svg}

@subheading Graphical Representation of Reductions

Reductions are shown as solid arrows, leading to a diamond-shaped node
bearing the number of the reduction rule. The arrow is labeled with the
appropriate comma separated lookahead tokens. If the reduction is the default
action for the given state, there is no such label.

This is how reductions are represented in the verbose file @file{rr.output}:
@example
State 1

    3 a: "0" .  [";"]
    4 b: "0" .  ["."]

    "."       reduce using rule 4 (b)
    $default  reduce using rule 3 (a)
@end example

A Graphviz rendering of this portion of the graph could be:

@center @image{figs/example-reduce, 120pt,,,.svg}

When unresolved conflicts are present, because in deterministic parsing
a single decision can be made, Bison can arbitrarily choose to disable a
reduction, see @ref{Shift/Reduce}.  Discarded actions
are distinguished by a red filling color on these nodes, just like how they are
reported between square brackets in the verbose file.

The reduction corresponding to the rule number 0 is the acceptation
state. It is shown as a blue diamond, labeled ``Acc''.

@subheading Graphical Representation of Gotos

The @samp{go to} jump transitions are represented as dotted lines bearing
the name of the rule being jumped to.

@c ================================================= XML

@node Xml
@section Visualizing your parser in multiple formats
@cindex xml

Bison supports two major report formats: textual output
(@pxref{Understanding}) when invoked
with option @option{--verbose}, and DOT
(@pxref{Graphviz}) when invoked with
option @option{--graph}. However,
another alternative is to output an XML file that may then be, with
@command{xsltproc}, rendered as either a raw text format equivalent to the
verbose file, or as an HTML version of the same file, with clickable
transitions, or even as a DOT. The @file{.output} and DOT files obtained via
XSLT have no difference whatsoever with those obtained by invoking
@command{bison} with options @option{--verbose} or @option{--graph}.

The XML file is generated when the options @option{-x} or
@option{--xml[=FILE]} are specified, see @ref{Invocation}.
If not specified, its name is made by removing @samp{.tab.c} or @samp{.c}
from the parser implementation file name, and adding @samp{.xml} instead.
For instance, if the grammar file is @file{foo.y}, the default XML output
file is @file{foo.xml}.

Bison ships with a @file{data/xslt} directory, containing XSL Transformation
files to apply to the XML file. Their names are non-ambiguous:

@table @file
@item xml2dot.xsl
Used to output a copy of the DOT visualization of the automaton.
@item xml2text.xsl
Used to output a copy of the @samp{.output} file.
@item xml2xhtml.xsl
Used to output an xhtml enhancement of the @samp{.output} file.
@end table

Sample usage (requires @command{xsltproc}):
@example
$ @kbd{bison -x gr.y}
@group
$ @kbd{bison --print-datadir}
/usr/local/share/bison
@end group
$ @kbd{xsltproc /usr/local/share/bison/xslt/xml2xhtml.xsl gr.xml >gr.html}
@end example

@c ================================================= Tracing

@node Tracing
@section Tracing Your Parser
@findex yydebug
@cindex debugging
@cindex tracing the parser

When a Bison grammar compiles properly but parses ``incorrectly'', the
@code{yydebug} parser-trace feature helps figuring out why.

@menu
* Enabling Traces::    Activating run-time trace support
* Mfcalc Traces::      Extending @code{mfcalc} to support traces
* The YYPRINT Macro::  Obsolete interface for semantic value reports
@end menu

@node Enabling Traces
@subsection  Enabling Traces
There are several means to enable compilation of trace facilities, in
decreasing order of preference:

@table @asis
@item the variable @samp{parse.trace}
@findex %define parse.trace
Add the @samp{%define parse.trace} directive (@pxref{%define
Summary}), or pass the @option{-Dparse.trace} option
(@pxref{Tuning the Parser}).  This is a Bison extension.  Unless POSIX and
Yacc portability matter to you, this is the preferred solution.

@item the option @option{-t} (POSIX Yacc compliant)
@itemx the option @option{--debug} (Bison extension)
Use the @samp{-t} option when you run Bison (@pxref{Invocation}).  With
@samp{%define api.prefix @{c@}}, it defines @code{CDEBUG} to 1, otherwise it
defines @code{YYDEBUG} to 1.

@item the directive @samp{%debug} (deprecated)
@findex %debug
Add the @code{%debug} directive (@pxref{Decl Summary}).  This Bison
extension is maintained for backward compatibility with previous versions of
Bison; use @code{%define parse.trace} instead.

@item the macro @code{YYDEBUG} (C/C++ only)
@findex YYDEBUG
Define the macro @code{YYDEBUG} to a nonzero value when you compile the
parser.  This is compliant with POSIX Yacc.  You could use
@samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define
YYDEBUG 1} in the prologue of the grammar file (@pxref{Prologue}).

If the @code{%define} variable @code{api.prefix} is used (@pxref{Multiple
Parsers}), for instance @samp{%define
api.prefix @{c@}}, then if @code{CDEBUG} is defined, its value controls the
tracing feature (enabled if and only if nonzero); otherwise tracing is
enabled if and only if @code{YYDEBUG} is nonzero.
@end table

We suggest that you always enable the trace option so that debugging is
always possible.

@findex YYFPRINTF
In C the trace facility outputs messages with macro calls of the form
@code{YYFPRINTF (stderr, @var{format}, @var{args})} where @var{format} and
@var{args} are the usual @code{printf} format and variadic arguments.  If
you define @code{YYDEBUG} to a nonzero value but do not define
@code{YYFPRINTF}, @code{<stdio.h>} is automatically included and
@code{YYFPRINTF} is defined to @code{fprintf}.

Once you have compiled the program with trace facilities, the way to request
a trace is to store a nonzero value in the variable @code{yydebug}.  You can
do this by making the C code do it (in @code{main}, perhaps), or you can
alter the value with a C debugger.

Each step taken by the parser when @code{yydebug} is nonzero produces a line
or two of trace information, written on @code{stderr}.  The trace messages
tell you these things:

@itemize @bullet
@item
Each time the parser calls @code{yylex}, what kind of token was read.

@item
Each time a token is shifted, the depth and complete contents of the state
stack (@pxref{Parser States}).

@item
Each time a rule is reduced, which rule it is, and the complete contents of
the state stack afterward.
@end itemize

To make sense of this information, it helps to refer to the automaton
description file (@pxref{Understanding}).  This
file shows the meaning of each state in terms of positions in various rules,
and also what each state will do with each possible input token.  As you
read the successive trace messages, you can see that the parser is
functioning according to its specification in the listing file.  Eventually
you will arrive at the place where something undesirable happens, and you
will see which parts of the grammar are to blame.

The parser implementation file is a C/C++/Java program and you can use
debuggers on it, but it's not easy to interpret what it is doing.  The
parser function is a finite-state machine interpreter, and aside from the
actions it executes the same code over and over.  Only the values of
variables show where in the grammar it is working.

@node Mfcalc Traces
@subsection Enabling Debug Traces for @code{mfcalc}

The debugging information normally gives the token kind of each token read,
but not its semantic value.  The @code{%printer} directive allows specify
how semantic values are reported, see @ref{Printer Decl}.

As a demonstration of @code{%printer}, consider the multi-function
calculator, @code{mfcalc} (@pxref{Multi-function Calc}).  To enable run-time
traces, and semantic value reports, insert the following directives in its
prologue:

@comment file: mfcalc.y: 2
@example
/* Generate the parser description file. */
%verbose
/* Enable run-time traces (yydebug). */
%define parse.trace

/* Formatting semantic values. */
%printer @{ fprintf (yyo, "%s", $$->name); @} VAR;
%printer @{ fprintf (yyo, "%s()", $$->name); @} FUN;
%printer @{ fprintf (yyo, "%g", $$); @} <double>;
@end example

The @code{%define} directive instructs Bison to generate run-time trace
support.  Then, activation of these traces is controlled at run-time by the
@code{yydebug} variable, which is disabled by default.  Because these traces
will refer to the ``states'' of the parser, it is helpful to ask for the
creation of a description of that parser; this is the purpose of (admittedly
ill-named) @code{%verbose} directive.

The set of @code{%printer} directives demonstrates how to format the
semantic value in the traces.  Note that the specification can be done
either on the symbol type (e.g., @code{VAR} or @code{FUN}), or on the type
tag: since @code{<double>} is the type for both @code{NUM} and @code{exp},
this printer will be used for them.

Here is a sample of the information provided by run-time traces.  The traces
are sent onto standard error.

@example
$ @kbd{echo 'sin(1-1)' | ./mfcalc -p}
Starting parse
Entering state 0
Reducing stack by rule 1 (line 34):
-> $$ = nterm input ()
Stack now 0
Entering state 1
@end example

@noindent
This first batch shows a specific feature of this grammar: the first rule
(which is in line 34 of @file{mfcalc.y} can be reduced without even having
to look for the first token.  The resulting left-hand symbol (@code{$$}) is
a valueless (@samp{()}) @code{input} nonterminal (@code{nterm}).

Then the parser calls the scanner.
@example
Reading a token
Next token is token FUN (sin())
Shifting token FUN (sin())
Entering state 6
@end example

@noindent
That token (@code{token}) is a function (@code{FUN}) whose value is
@samp{sin} as formatted per our @code{%printer} specification: @samp{sin()}.
The parser stores (@code{Shifting}) that token, and others, until it can do
something about it.

@example
Reading a token
Next token is token '(' ()
Shifting token '(' ()
Entering state 14
Reading a token
Next token is token NUM (1.000000)
Shifting token NUM (1.000000)
Entering state 4
Reducing stack by rule 6 (line 44):
   $1 = token NUM (1.000000)
-> $$ = nterm exp (1.000000)
Stack now 0 1 6 14
Entering state 24
@end example

@noindent
The previous reduction demonstrates the @code{%printer} directive for
@code{<double>}: both the token @code{NUM} and the resulting nonterminal
@code{exp} have @samp{1} as value.

@example
Reading a token
Next token is token '-' ()
Shifting token '-' ()
Entering state 17
Reading a token
Next token is token NUM (1.000000)
Shifting token NUM (1.000000)
Entering state 4
Reducing stack by rule 6 (line 44):
   $1 = token NUM (1.000000)
-> $$ = nterm exp (1.000000)
Stack now 0 1 6 14 24 17
Entering state 26
Reading a token
Next token is token ')' ()
Reducing stack by rule 11 (line 49):
   $1 = nterm exp (1.000000)
   $2 = token '-' ()
   $3 = nterm exp (1.000000)
-> $$ = nterm exp (0.000000)
Stack now 0 1 6 14
Entering state 24
@end example

@noindent
The rule for the subtraction was just reduced.  The parser is about to
discover the end of the call to @code{sin}.

@example
Next token is token ')' ()
Shifting token ')' ()
Entering state 31
Reducing stack by rule 9 (line 47):
   $1 = token FUN (sin())
   $2 = token '(' ()
   $3 = nterm exp (0.000000)
   $4 = token ')' ()
-> $$ = nterm exp (0.000000)
Stack now 0 1
Entering state 11
@end example

@noindent
Finally, the end-of-line allow the parser to complete the computation, and
display its result.

@example
Reading a token
Next token is token '\n' ()
Shifting token '\n' ()
Entering state 22
Reducing stack by rule 4 (line 40):
   $1 = nterm exp (0.000000)
   $2 = token '\n' ()
@result{} 0
-> $$ = nterm line ()
Stack now 0 1
Entering state 10
Reducing stack by rule 2 (line 35):
   $1 = nterm input ()
   $2 = nterm line ()
-> $$ = nterm input ()
Stack now 0
Entering state 1
@end example

The parser has returned into state 1, in which it is waiting for the next
expression to evaluate, or for the end-of-file token, which causes the
completion of the parsing.

@example
Reading a token
Now at end of input.
Shifting token $end ()
Entering state 2
Stack now 0 1 2
Cleanup: popping token $end ()
Cleanup: popping nterm input ()
@end example


@node The YYPRINT Macro
@subsection The @code{YYPRINT} Macro
@findex YYPRINT

The @code{%printer} directive was introduced in Bison 1.50 (November 2002).
Before then, @code{YYPRINT} provided a similar feature, but only for
terminal symbols and only with the @file{yacc.c} skeleton.

@deffn {Macro} YYPRINT (@var{stream}, @var{token}, @var{value});
@findex YYPRINT
Deprecated, will be removed eventually.

If you define @code{YYPRINT}, it should take three arguments.  The parser
will pass a standard I/O stream, the numeric code for the token kind, and
the token value (from @code{yylval}).

For @file{yacc.c} only.  Obsoleted by @code{%printer}.
@end deffn

Here is an example of @code{YYPRINT} suitable for the multi-function
calculator (@pxref{Mfcalc Declarations}):

@example
%@{
  static void print_token_value (FILE *file, int type, YYSTYPE value);
  #define YYPRINT(File, Type, Value)            \
    print_token_value (File, Type, Value)
%@}

@dots{} %% @dots{} %% @dots{}

static void
print_token_value (FILE *file, yytoken_kind_t kind, YYSTYPE value)
@{
  if (kind == VAR)
    fprintf (file, "%s", value.tptr->name);
  else if (kind == NUM)
    fprintf (file, "%d", value.val);
@}
@end example

@xref{Mfcalc Traces}, for the proper use of @code{%printer}.

@c ================================================= Invoking Bison

@node Invocation
@chapter Invoking Bison
@cindex invoking Bison
@cindex Bison invocation
@cindex options for invoking Bison

The usual way to invoke Bison is as follows:

@example
$ @kbd{bison @var{file}}
@end example

Here @var{file} is the grammar file name, which usually ends in @samp{.y}.
The parser implementation file's name is made by replacing the @samp{.y}
with @samp{.tab.c} and removing any leading directory.  Thus, the
@samp{bison foo.y} file name yields @file{foo.tab.c}, and the @samp{bison
hack/foo.y} file name yields @file{foo.tab.c}.  It's also possible, in case
you are writing C++ code instead of C in your grammar file, to name it
@file{foo.ypp} or @file{foo.y++}.  Then, the output files will take an
extension like the given one as input (respectively @file{foo.tab.cpp} and
@file{foo.tab.c++}).  This feature takes effect with all options that
manipulate file names like @samp{-o} or @samp{-d}.

For example:

@example
$ @kbd{bison -d @var{file.yxx}}
@end example
@noindent
will produce @file{file.tab.cxx} and @file{file.tab.hxx}, and

@example
$ @kbd{bison -d -o @var{output.c++} @var{file.y}}
@end example
@noindent
will produce @file{output.c++} and @file{output.h++}.

For compatibility with POSIX, the standard Bison distribution also contains
a shell script called @command{yacc} that invokes Bison with the @option{-y}
option.

@sp 1

The exit status of @command{bison} is:
@table @asis
@item 0 (success)
when there were no errors.  Warnings, which are diagnostics about dubious
constructs, do not change the exit status, unless they are turned into
errors (@pxref{-Werror,,@option{-Werror}}).

@item 1 (failure)
when there were errors.  No file was generated (except the reports generated
by @option{--verbose}, etc.).  In particular, the output files that possibly
existed were not changed.

@item 63 (mistmatch)
when @command{bison} does not meet the version requirements of the grammar
file. @xref{Require Decl}.  No file was generated or changed.
@end table


@menu
* Bison Options::     All the options described in detail,
                        in alphabetical order by short options.
* Option Cross Key::  Alphabetical list of long options.
* Yacc Library::      Yacc-compatible @code{yylex} and @code{main}.
@end menu

@node Bison Options
@section Bison Options

Bison supports both traditional single-letter options and mnemonic long
option names.  Long option names are indicated with @samp{--} instead of
@samp{-}.  Abbreviations for option names are allowed as long as they
are unique.  When a long option takes an argument, like
@samp{--file-prefix}, connect the option name and the argument with
@samp{=}.

Here is a list of options that can be used with Bison.  It is followed by a
cross key alphabetized by long option.

@menu
* Operation Modes::    Options controlling the global behavior of @command{bison}
* Diagnostics::        Options controlling the diagnostics
* Tuning the Parser::  Options changing the generated parsers
* Output Files::       Options controlling the output
@end menu

@node Operation Modes
@subsection Operation Modes

Options controlling the global behavior of @command{bison}.

@c Please, keep this ordered as in 'bison --help'.
@table @option
@item -h
@itemx --help
Print a summary of the command-line options to Bison and exit.

@item -V
@itemx --version
Print the version number of Bison and exit.

@item --print-localedir
Print the name of the directory containing locale-dependent data.

@item --print-datadir
Print the name of the directory containing skeletons, CSS and XSLT.

@item -u
@item --update
Update the grammar file (remove duplicates, update deprecated directives,
etc.) and exit (i.e., do not generate any of the output files).  Leaves a
backup of the original file with a @code{~} appended.  For instance:

@example
@group
$ @kbd{cat foo.y}
%error-verbose
%define parse.error verbose
%%
exp:;
@end group
@group
$ @kbd{bison -u foo.y}
foo.y:1.1-14: @dwarning{warning}: deprecated directive, use '%define parse.error verbose' [@dwarning{-Wdeprecated}]
    1 | @dwarning{%error-verbose}
      | @dwarning{^~~~~~~~~~~~~~}
foo.y:2.1-27: @dwarning{warning}: %define variable 'parse.error' redefined [@dwarning{-Wother}]
    2 | @dwarning{%define parse.error verbose}
      | @dwarning{^~~~~~~~~~~~~~~~~~~~~~~~~~~}
foo.y:1.1-14:     previous definition
    1 | @dnotice{%error-verbose}
      | @dnotice{^~~~~~~~~~~~~~}
bison: file 'foo.y' was updated (backup: 'foo.y~')
@end group
@group
$ @kbd{cat foo.y}
%define parse.error verbose
%%
exp:;
@end group
@end example

See the documentation of @option{--feature=fixit} below for more details.

@item -f [@var{feature}]
@itemx --feature[=@var{feature}]
Activate miscellaneous @var{feature}s. @var{Feature} can be one of:
@table @code
@item caret
@itemx diagnostics-show-caret
Show caret errors, in a manner similar to GCC's
@option{-fdiagnostics-show-caret}, or Clang's
@option{-fcaret-diagnostics}. The location provided with the message is used
to quote the corresponding line of the source file, underlining the
important part of it with carets (@samp{^}). Here is an example, using the
following file @file{in.y}:

@example
%nterm <ival> exp
%%
exp: exp '+' exp @{ $exp = $1 + $2; @};
@end example

When invoked with @option{-fcaret} (or nothing), Bison will report:

@example
@group
in.y:3.20-23: @derror{error}: ambiguous reference: '$exp'
    3 | exp: exp '+' exp @{ @derror{$exp} = $1 + $2; @};
      |                    @derror{^~~~}
@end group
@group
in.y:3.1-3:       refers to: $exp at $$
    3 | @dnotice{exp}: exp '+' exp @{ $exp = $1 + $2; @};
      | @dnotice{^~~}
@end group
@group
in.y:3.6-8:       refers to: $exp at $1
    3 | exp: @dnotice{exp} '+' exp @{ $exp = $1 + $2; @};
      |      @dnotice{^~~}
@end group
@group
in.y:3.14-16:     refers to: $exp at $3
    3 | exp: exp '+' @dnotice{exp} @{ $exp = $1 + $2; @};
      |              @dnotice{^~~}
@end group
@group
in.y:3.32-33: @derror{error}: $2 of 'exp' has no declared type
    3 | exp: exp '+' exp @{ $exp = $1 + @derror{$2}; @};
      |                                @derror{^~}
@end group
@end example

Whereas, when invoked with @option{-fno-caret}, Bison will only report:

@example
@group
in.y:3.20-23: @derror{error}: ambiguous reference: '$exp'
in.y:3.1-3:       refers to: $exp at $$
in.y:3.6-8:       refers to: $exp at $1
in.y:3.14-16:     refers to: $exp at $3
in.y:3.32-33: @derror{error}: $2 of 'exp' has no declared type
@end group
@end example

This option is activated by default.

@item fixit
@itemx diagnostics-parseable-fixits
Show machine-readable fixes, in a manner similar to GCC's and Clang's
@option{-fdiagnostics-parseable-fixits}.

Fix-its are generated for duplicate directives:

@example
@group
$ @kbd{cat foo.y}
%define api.prefix @{foo@}
%define api.prefix @{bar@}
%%
exp:;
@end group

@group
$ @kbd{bison -ffixit foo.y}
foo.y:2.1-24: @derror{error}: %define variable 'api.prefix' redefined
    2 | @derror{%define api.prefix @{bar@}}
      | @derror{^~~~~~~~~~~~~~~~~~~~~~~~}
foo.y:1.1-24:     previous definition
    1 | @dnotice{%define api.prefix @{foo@}}
      | @dnotice{^~~~~~~~~~~~~~~~~~~~~~~~}
fix-it:"foo.y":@{2:1-2:25@}:""
foo.y: @dwarning{warning}: fix-its can be applied.  Rerun with option '--update'. [@dwarning{-Wother}]
@end group
@end example

They are also generated to update deprecated directives, unless
@option{-Wno-deprecated} was given:

@example
@group
$ @kbd{cat /tmp/foo.yy}
%error-verbose
%name-prefix "foo"
%%
exp:;
@end group
@group
$ @kbd{bison foo.y}
foo.y:1.1-14: @dwarning{warning}: deprecated directive, use '%define parse.error verbose' [@dwarning{-Wdeprecated}]
    1 | @dwarning{%error-verbose}
      | @dwarning{^~~~~~~~~~~~~~}
foo.y:2.1-18: @dwarning{warning}: deprecated directive, use '%define api.prefix @{foo@}' [@dwarning{-Wdeprecated}]
    2 | @dwarning{%name-prefix "foo"}
      | @dwarning{^~~~~~~~~~~~~~~~~~}
foo.y: @dwarning{warning}: fix-its can be applied.  Rerun with option '--update'. [@dwarning{-Wother}]
@end group
@end example

The fix-its are applied by @command{bison} itself when given the option
@option{-u}/@option{--update}.  See its documentation above.

@item syntax-only
Do not generate the output files.  The name of this feature is somewhat
misleading as more than just checking the syntax is done: every stage is run
(including checking for conflicts for instance), except the generation of
the output files.

@end table
@end table

@node Diagnostics
@subsection Diagnostics

Options controlling the diagnostics.

@c Please, keep this ordered as in 'bison --help'.
@table @code
@item -W [@var{category}]
@itemx --warnings[=@var{category}]
Output warnings falling in @var{category}.  @var{category} can be one
of:
@table @code
@item conflicts-sr
@itemx conflicts-rr
S/R and R/R conflicts.  These warnings are enabled by default.  However, if
the @code{%expect} or @code{%expect-rr} directive is specified, an
unexpected number of conflicts is an error, and an expected number of
conflicts is not reported, so @option{-W} and @option{--warning} then have
no effect on the conflict report.

@item dangling-alias
Report string literals that are not bound to a token symbol.

String literals, which allow for better error messages, are (too) liberally
accepted by Bison, which might result in silent errors.  For instance

@example
%type <exVal> cond "condition"
@end example

@noindent
does not define ``condition'' as a string alias to @code{cond}---nonterminal
symbols do not have string aliases.  It is rather equivalent to

@example
%nterm <exVal> cond
%token <exVal> "condition"
@end example

@noindent
i.e., it gives the @samp{"condition"} token the type @code{exVal}.

Also, because string aliases do not need to be defined, typos such as
@samp{"baz"} instead of @samp{"bar"} will be not reported.

The option @option{-Wdangling-alias} catches these situations.  On

@example
%token BAR "bar"
%type <ival> foo "foo"
%%
foo: "baz" @{@}
@end example

@noindent
@command{bison -Wdangling-alias} reports

@example
@dwarning{warning}: string literal not attached to a symbol
      | %type <ival> foo @dwarning{"foo"}
      |                  @dwarning{^~~~~}
@dwarning{warning}: string literal not attached to a symbol
      | foo: @dwarning{"baz"} @{@}
      |      @dwarning{^~~~~}
@end example

@item deprecated
Deprecated constructs whose support will be removed in future versions of
Bison.

@item empty-rule
Empty rules without @code{%empty}.  @xref{Empty Rules}.  Disabled by
default, but enabled by uses of @code{%empty}, unless
@option{-Wno-empty-rule} was specified.

@item midrule-values
Warn about midrule values that are set but not used within any of the actions
of the parent rule.
For example, warn about unused @code{$2} in:

@example
exp: '1' @{ $$ = 1; @} '+' exp @{ $$ = $1 + $4; @};
@end example

Also warn about midrule values that are used but not set.
For example, warn about unset @code{$$} in the midrule action in:

@example
exp: '1' @{ $1 = 1; @} '+' exp @{ $$ = $2 + $4; @};
@end example

These warnings are not enabled by default since they sometimes prove to
be false alarms in existing grammars employing the Yacc constructs
@code{$0} or @code{$-@var{n}} (where @var{n} is some positive integer).

@item precedence
Useless precedence and associativity directives.  Disabled by default.

Consider for instance the following grammar:

@example
@group
%nonassoc "="
%left "+"
%left "*"
%precedence "("
@end group
%%
@group
stmt:
  exp
| "var" "=" exp
;
@end group

@group
exp:
  exp "+" exp
| exp "*" "number"
| "(" exp ")"
| "number"
;
@end group
@end example

Bison reports:

@c cannot leave the location and the [-Wprecedence] for lack of
@c width in PDF.
@example
@group
@dwarning{warning}: useless precedence and associativity for "="
      | %nonassoc @dwarning{"="}
      |           @dwarning{^~~}
@end group
@group
@dwarning{warning}: useless associativity for "*", use %precedence
      | %left @dwarning{"*"}
      |       @dwarning{^~~}
@end group
@group
@dwarning{warning}: useless precedence for "("
      | %precedence @dwarning{"("}
      |             @dwarning{^~~}
@end group
@end example

One would get the exact same parser with the following directives instead:

@example
@group
%left "+"
%precedence "*"
@end group
@end example

@item yacc
Incompatibilities with POSIX Yacc.

@item other
All warnings not categorized above.  These warnings are enabled by default.

This category is provided merely for the sake of completeness.  Future
releases of Bison may move warnings from this category to new, more specific
categories.

@item all
All the warnings except @code{dangling-alias} and @code{yacc}.

@item none
Turn off all the warnings.

@item error
See @option{-Werror}, below.
@end table

A category can be turned off by prefixing its name with @samp{no-}.  For
instance, @option{-Wno-yacc} will hide the warnings about
POSIX Yacc incompatibilities.

@item -Werror
@anchor{-Werror}
Turn enabled warnings for every @var{category} into errors, unless they are
explicitly disabled by @option{-Wno-error=@var{category}}.

@item -Werror=@var{category}
Enable warnings falling in @var{category}, and treat them as errors.

@var{category} is the same as for @option{--warnings}, with the exception that
it may not be prefixed with @samp{no-} (see above).

Note that the precedence of the @samp{=} and @samp{,} operators is such that
the following commands are @emph{not} equivalent, as the first will not treat
S/R conflicts as errors.

@example
$ @kbd{bison -Werror=yacc,conflicts-sr input.y}
$ @kbd{bison -Werror=yacc,error=conflicts-sr input.y}
@end example

@item -Wno-error
Do not turn enabled warnings for every @var{category} into errors, unless
they are explicitly enabled by @option{-Werror=@var{category}}.

@item -Wno-error=@var{category}
Deactivate the error treatment for this @var{category}. However, the warning
itself won't be disabled, or enabled, by this option.

@item --color
Equivalent to @option{--color=always}.

@item --color=@var{when}
Control whether diagnostics are colorized, depending on @var{when}:
@table @code
@item always
@itemx yes
Enable colorized diagnostics.

@item never
@itemx no
Disable colorized diagnostics.

@item auto @r{(default)}
@itemx tty
Diagnostics will be colorized if the output device is a tty, i.e. when the
output goes directly to a text screen or terminal emulator window.
@end table

@item --style=@var{file}
Specifies the CSS style @var{file} to use when colorizing. It has an effect
only when the @option{--color} option is effective.  The
@file{bison-default.css} file provide a good example from which to define
your own style file.  See the documentation of libtextstyle for more
details.
@end table

@node Tuning the Parser
@subsection Tuning the Parser

Options changing the generated parsers.

@c Please, keep this ordered as in 'bison --help'.
@table @option
@item -t
@itemx --debug
In the parser implementation file, define the macro @code{YYDEBUG} to 1 if
it is not already defined, so that the debugging facilities are compiled.
@xref{Tracing}.

@item -D @var{name}[=@var{value}]
@itemx --define=@var{name}[=@var{value}]
@itemx -F @var{name}[=@var{value}]
@itemx --force-define=@var{name}[=@var{value}]
Each of these is equivalent to @samp{%define @var{name} @var{value}}
(@pxref{%define Summary}).  Note that the delimiters are part of
@var{value}: @option{-Dapi.value.type=union},
@option{-Dapi.value.type=@{union@}} and @option{-Dapi.value.type="union"}
correspond to @samp{%define api.value.type union}, @samp{%define
api.value.type @{union@}} and @samp{%define api.value.type "union"}.

Bison processes multiple definitions for the same @var{name} as follows:

@itemize
@item
Bison quietly ignores all command-line definitions for @var{name} except
the last.
@item
If that command-line definition is specified by a @option{-D} or
@option{--define}, Bison reports an error for any @code{%define} definition
for @var{name}.
@item
If that command-line definition is specified by a @option{-F} or
@option{--force-define} instead, Bison quietly ignores all @code{%define}
definitions for @var{name}.
@item
Otherwise, Bison reports an error if there are multiple @code{%define}
definitions for @var{name}.
@end itemize

You should avoid using @option{-F} and @option{--force-define} in your
make files unless you are confident that it is safe to quietly ignore
any conflicting @code{%define} that may be added to the grammar file.

@item -L @var{language}
@itemx --language=@var{language}
Specify the programming language for the generated parser, as if
@code{%language} was specified (@pxref{Decl Summary}).  Currently supported
languages include C, C++, and Java.  @var{language} is case-insensitive.

@item --locations
Pretend that @code{%locations} was specified.  @xref{Decl Summary}.

@item -p @var{prefix}
@itemx --name-prefix=@var{prefix}
Pretend that @code{%name-prefix "@var{prefix}"} was specified (@pxref{Decl
Summary}).  Obsoleted by @option{-Dapi.prefix=@var{prefix}}.  @xref{Multiple
Parsers}.

@item -l
@itemx --no-lines
Don't put any @code{#line} preprocessor commands in the parser
implementation file.  Ordinarily Bison puts them in the parser
implementation file so that the C compiler and debuggers will
associate errors with your source file, the grammar file.  This option
causes them to associate errors with the parser implementation file,
treating it as an independent source file in its own right.

@item -S @var{file}
@itemx --skeleton=@var{file}
Specify the skeleton to use, similar to @code{%skeleton}
(@pxref{Decl Summary}).

@c You probably don't need this option unless you are developing Bison.
@c You should use @option{--language} if you want to specify the skeleton for a
@c different language, because it is clearer and because it will always
@c choose the correct skeleton for non-deterministic or push parsers.

If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton
file in the Bison installation directory.
If it does, @var{file} is an absolute file name or a file name relative to the
current working directory.
This is similar to how most shells resolve commands.

@item -k
@itemx --token-table
Pretend that @code{%token-table} was specified.  @xref{Decl Summary}.

@item -y
@itemx --yacc
Act more like the traditional @command{yacc} command.  This can cause
different diagnostics to be generated (it implies @option{-Wyacc}), and may
change behavior in other minor ways.  Most importantly, imitate Yacc's
output file name conventions, so that the parser implementation file is
called @file{y.tab.c}, and the other outputs are called @file{y.output} and
@file{y.tab.h}.  Also, generate @code{#define} statements in addition to an
@code{enum} to associate token codes with token kind names.  Thus, the
following shell script can substitute for Yacc, and the Bison distribution
contains such a script for compatibility with POSIX:

@example
#! /bin/sh
bison -y "$@@"
@end example

The @option{-y}/@option{--yacc} option is intended for use with traditional
Yacc grammars.  This option only makes sense for the default C skeleton,
@file{yacc.c}.  If your grammar uses Bison extensions Bison cannot be
Yacc-compatible, even if this option is specified.
@end table

@node Output Files
@subsection Output Files

Options controlling the output.

@c Please, keep this ordered as in 'bison --help'.
@table @option
@item --defines[=@var{file}]
Pretend that @code{%defines} was specified, i.e., write an extra output file
containing definitions for the token kind names defined in the grammar, as
well as a few other declarations.  @xref{Decl Summary}.

@item -d
This is the same as @option{--defines} except @option{-d} does not accept a
@var{file} argument since POSIX Yacc requires that @option{-d} can be bundled
with other short options.

@item -b @var{file-prefix}
@itemx --file-prefix=@var{prefix}
Pretend that @code{%file-prefix} was specified, i.e., specify prefix to use
for all Bison output file names.  @xref{Decl Summary}.

@item -r @var{things}
@itemx --report=@var{things}
Write an extra output file containing verbose description of the comma
separated list of @var{things} among:

@table @code
@item state
Description of the grammar, conflicts (resolved and unresolved), and
parser's automaton.

@item itemset
Implies @code{state} and augments the description of the automaton with
the full set of items for each state, instead of its core only.

@item lookahead
Implies @code{state} and augments the description of the automaton with
each rule's lookahead set.

@item solved
Implies @code{state}.  Explain how conflicts were solved thanks to
precedence and associativity directives.

@item all
Enable all the items.

@item none
Do not generate the report.
@end table

@item --report-file=@var{file}
Specify the @var{file} for the verbose description.

@item -v
@itemx --verbose
Pretend that @code{%verbose} was specified, i.e., write an extra output
file containing verbose descriptions of the grammar and
parser.  @xref{Decl Summary}.

@item -o @var{file}
@itemx --output=@var{file}
Specify the @var{file} for the parser implementation file.

The other output files' names are constructed from @var{file} as
described under the @samp{-v} and @samp{-d} options.

@item -g [@var{file}]
@itemx --graph[=@var{file}]
Output a graphical representation of the parser's automaton computed by
Bison, in @uref{http://www.graphviz.org/, Graphviz}
@uref{http://www.graphviz.org/doc/info/lang.html, DOT} format.
@code{@var{file}} is optional.  If omitted and the grammar file is
@file{foo.y}, the output file will be @file{foo.gv} if the @code{%required}
version is 3.4 or better, @file{foo.dot} otherwise.

@item -x [@var{file}]
@itemx --xml[=@var{file}]
Output an XML report of the parser's automaton computed by Bison.
@code{@var{file}} is optional.
If omitted and the grammar file is @file{foo.y}, the output file will be
@file{foo.xml}.
@end table

@node Option Cross Key
@section Option Cross Key

Here is a list of options, alphabetized by long option, to help you find
the corresponding short option and directive.

@multitable {@option{--force-define=@var{name}[=@var{value}]}} {@option{-F @var{name}[=@var{value}]}} {@code{%nondeterministic-parser}}
@headitem Long Option @tab Short Option @tab Bison Directive
@include cross-options.texi
@end multitable

@node Yacc Library
@section Yacc Library

The Yacc library contains default implementations of the @code{yyerror} and
@code{main} functions.  These default implementations are normally not
useful, but POSIX requires them.  To use the Yacc library, link your program
with the @option{-ly} option.  Note that Bison's implementation of the Yacc
library is distributed under the terms of the GNU General Public License
(@pxref{Copying}).

If you use the Yacc library's @code{yyerror} function, you should declare
@code{yyerror} as follows:

@example
int yyerror (char const *);
@end example

@noindent
The @code{int} value returned by this @code{yyerror} is ignored.

The implementation of Yacc library's @code{main} function is:

@example
int main (void)
@{
  setlocale (LC_ALL, "");
  return yyparse ();
@}
@end example

@noindent
so if you use it, the internationalization support is enabled (e.g., error
messages are translated), and your @code{yyparse} function should have the
following type signature:

@example
int yyparse (void);
@end example

@c ================================================= C++ Bison

@node Other Languages
@chapter Parsers Written In Other Languages

In addition to C, Bison can generate parsers in C++ and Java.  This chapter
is devoted to these languages.  The reader is expected to understand how
Bison works; read the introductory chapters first if you don't.

@menu
* C++ Parsers::                 The interface to generate C++ parser classes
* Java Parsers::                The interface to generate Java parser classes
@end menu

@node C++ Parsers
@section C++ Parsers

The Bison parser in C++ is an object, an instance of the class
@code{yy::parser}.

@menu
* A Simple C++ Example::        A short introduction to C++ parsers
* C++ Bison Interface::         Asking for C++ parser generation
* C++ Parser Interface::        Instantiating and running the parser
* C++ Semantic Values::         %union vs. C++
* C++ Location Values::         The position and location classes
* C++ Parser Context::          You can supply a @code{report_syntax_error} function.
* C++ Scanner Interface::       Exchanges between yylex and parse
* A Complete C++ Example::      Demonstrating their use
@end menu

@node A Simple C++ Example
@subsection A Simple C++ Example

This tutorial about C++ parsers is based on a simple, self contained
example.  The following sections are the reference manual for Bison with
C++, the last one showing a fully blown example (@pxref{A Complete C++
Example}).

To look nicer, our example will be in C++14.  It is not required: Bison
supports the original C++98 standard.

A Bison file has three parts.  In the first part, the prologue, we start by
making sure we run a version of Bison which is recent enough, and that we
generate C++.

@comment file: c++/simple.yy: 1
@example
%require "3.2"
%language "c++"
@end example

Let's dive directly into the middle part: the grammar.  Our input is a
simple list of strings, that we display once the parsing is done.

@comment file: c++/simple.yy: 2
@example
%%
@group
result:
  list  @{ std::cout << $1 << '\n'; @}
;
@end group

%nterm <std::vector<std::string>> list;
@group
list:
  %empty     @{ /* Generates an empty string list */ @}
| list item  @{ $$ = $1; $$.push_back ($2); @}
;
@end group
@end example

We used a vector of strings as a semantic value!  To use genuine C++ objects
as semantic values---not just PODs---we cannot rely on the union that Bison
uses by default to store them, we need @emph{variants} (@pxref{C++
Variants}):

@comment file: c++/simple.yy: 1
@example
%define api.value.type variant
@end example

Obviously, the rule for @code{result} needs to print a vector of strings.
In the prologue, we add:

@comment file: c++/simple.yy: 1
@example
%code
@{
  // Print a list of strings.
  auto
  operator<< (std::ostream& o, const std::vector<std::string>& ss)
    -> std::ostream&
  @{
    o << '@{';
    const char *sep = "";
@group
    for (const auto& s: ss)
      @{
        o << sep << s;
        sep = ", ";
      @}
@end group
    return o << '@}';
  @}
@}
@end example

@noindent
You may want to move it into the @code{yy} namespace to avoid leaking it in
your default namespace.  We recommend that you keep the actions simple, and
move details into auxiliary functions, as we did with @code{operator<<}.

Our list of strings will be built from two types of items: numbers and
strings:

@comment file: c++/simple.yy: 2
@example
%nterm <std::string> item;
%token <std::string> TEXT;
%token <int> NUMBER;
@group
item:
  TEXT
| NUMBER  @{ $$ = std::to_string ($1); @}
;
@end group
@end example

In the case of @code{TEXT}, the implicit default action applies: @w{@code{$$
= $1}.}

@sp 1

Our scanner deserves some attention.  The traditional interface of
@code{yylex} is not type safe: since the token kind and the token value are
not correlated, you may return a @code{NUMBER} with a string as semantic
value.  To avoid this, we use @emph{token constructors} (@pxref{Complete
Symbols}).  This directive:

@comment file: c++/simple.yy: 1
@example
%define api.token.constructor
@end example

@noindent
requests that Bison generates the functions @code{make_TEXT} and
@code{make_NUMBER}, but also @code{make_YYEOF}, for the end of input.

Everything is in place for our scanner:

@comment file: c++/simple.yy: 1
@example
%code
@{
  namespace yy
  @{
    // Return the next token.
    auto yylex () -> parser::symbol_type
    @{
      static int count = 0;
      switch (int stage = count++)
        @{
@group
        case 0:
          return parser::make_TEXT ("I have three numbers for you.");
@end group
@group
        case 1: case 2: case 3:
          return parser::make_NUMBER (stage);
@end group
@group
        case 4:
          return parser::make_TEXT ("And that's all!");
@end group
@group
        default:
          return parser::make_YYEOF ();
@end group
        @}
    @}
  @}
@}
@end example

In the epilogue, the third part of a Bison grammar file, we leave simple
details: the error reporting function, and the main function.

@comment file: c++/simple.yy: 3
@example
%%
namespace yy
@{
  // Report an error to the user.
  auto parser::error (const std::string& msg) -> void
  @{
    std::cerr << msg << '\n';
  @}
@}

int main ()
@{
  yy::parser parse;
  return parse ();
@}
@end example

Compile, and run!

@example
$ @kbd{bison simple.yy -o simple.cc}
$ @kbd{g++ -std=c++14 simple.cc -o simple}
@group
$ @kbd{./simple}
@{I have three numbers for you., 1, 2, 3, And that's all!@}
@end group
@end example

@node C++ Bison Interface
@subsection C++ Bison Interface
@c - %skeleton "lalr1.cc"
@c - Always pure
@c - initial action

The C++ deterministic parser is selected using the skeleton directive,
@samp{%skeleton "lalr1.cc"}.  @xref{Decl Summary}.

When run, @command{bison} will create several entities in the @samp{yy}
namespace.
@findex %define api.namespace
Use the @samp{%define api.namespace} directive to change the namespace name,
see @ref{%define Summary}.  The various classes are generated
in the following files:

@table @file
@item @var{file}.hh
(Assuming the extension of the grammar file was @samp{.yy}.)  The
declaration of the C++ parser class and auxiliary types.  By default, this
file is not generated (@pxref{Decl Summary}).

@item @var{file}.cc
The implementation of the C++ parser class.  The basename and extension of
these two files (@file{@var{file}.hh} and @file{@var{file}.cc}) follow the
same rules as with regular C parsers (@pxref{Invocation}).

@item location.hh
Generated when both @code{%defines} and @code{%locations} are enabled, this
file contains the definition of the classes @code{position} and
@code{location}, used for location tracking.  It is not generated if
@samp{%define api.location.file none} is specified, or if user defined
locations are used.  @xref{C++ Location Values}.

@item position.hh
@itemx stack.hh
Useless legacy files.  To get rid of then, use @samp{%require "3.2"} or
newer.
@end table

All these files are documented using Doxygen; run @command{doxygen} for a
complete and accurate documentation.

@node C++ Parser Interface
@subsection C++ Parser Interface

The output files @file{@var{file}.hh} and @file{@var{file}.cc} declare and
define the parser class in the namespace @code{yy}.  The class name defaults
to @code{parser}, but may be changed using @samp{%define api.parser.class
@{@var{name}@}}.  The interface of this class is detailed below.  It can be
extended using the @code{%parse-param} feature: its semantics is slightly
changed since it describes an additional member of the parser class, and an
additional argument for its constructor.


@defcv {Type} {parser} {token}
A structure that contains (only) the @code{token_kind_type} enumeration,
which defines the tokens.  To refer to the token @code{FOO}, use
@code{yy::parser::token::FOO}.  The scanner can use @samp{typedef
yy::parser::token token;} to ``import'' the token enumeration (@pxref{Calc++
Scanner}).
@end defcv

@defcv {Type} {parser} {token_kind_type}
An enumeration of the token kinds.  Its enumerators are forged from the
token names, with a possible token prefix
(@pxref{api-token-prefix,,@code{api.token.prefix}}):

@example
/// Token kinds.
struct token
@{
  enum token_kind_type
  @{
    YYEMPTY = -2,              // No token.
    YYEOF = 0,                 // "end of file"
    YYerror = 256,             // error
    YYUNDEF = 257,             // "invalid token"
    PLUS = 258,                // "+"
    MINUS = 259,               // "-"
    [...]
    VAR = 271,                 // "variable"
    NEG = 272                  // NEG
  @};
@};

/// Token kind, as returned by yylex.
typedef token::token_kind_type token_kind_type;
@end example
@end defcv

@defcv {Type} {parser} {semantic_type}
The types for semantic values. @xref{C++ Semantic Values}.
@end defcv

@defcv {Type} {parser} {location_type}
The type of locations, if location tracking is enabled.  @xref{C++ Location
Values}.
@end defcv

@defcv {Type} {parser} {syntax_error}
This class derives from @code{std::runtime_error}.  Throw instances of it
from the scanner or from the actions to raise parse errors.  This is
equivalent with first invoking @code{error} to report the location and
message of the syntax error, and then to invoke @code{YYERROR} to enter the
error-recovery mode.  But contrary to @code{YYERROR} which can only be
invoked from user actions (i.e., written in the action itself), the
exception can be thrown from functions invoked from the user action.
@end defcv

@deftypeop {Constructor} {parser} {} parser ()
@deftypeopx {Constructor} {parser} {} parser (@var{type1} @var{arg1}, ...)
Build a new parser object.  There are no arguments, unless
@samp{%parse-param @{@var{type1} @var{arg1}@}} was used.
@end deftypeop

@deftypeop {Constructor} {syntax_error} {} syntax_error (@code{const location_type&} @var{l}, @code{const std::string&} @var{m})
@deftypeopx {Constructor}  {syntax_error} {} syntax_error (@code{const std::string&} @var{m})
Instantiate a syntax-error exception.
@end deftypeop

@deftypemethod {parser} {int} operator() ()
@deftypemethodx {parser} {int} parse ()
Run the syntactic analysis, and return 0 on success, 1 otherwise.  Both
routines are equivalent, @code{operator()} being more C++ish.

@cindex exceptions
The whole function is wrapped in a @code{try}/@code{catch} block, so that
when an exception is thrown, the @code{%destructor}s are called to release
the lookahead symbol, and the symbols pushed on the stack.

Exception related code in the generated parser is protected by CPP guards
(@code{#if}) and disabled when exceptions are not supported (i.e., passing
@option{-fno-exceptions} to the C++ compiler).
@end deftypemethod

@deftypemethod {parser} {std::ostream&} debug_stream ()
@deftypemethodx {parser} {void} set_debug_stream (@code{std::ostream&} @var{o})
Get or set the stream used for tracing the parsing.  It defaults to
@code{std::cerr}.
@end deftypemethod

@deftypemethod {parser} {debug_level_type} debug_level ()
@deftypemethodx {parser} {void} set_debug_level (debug_level_type @var{l})
Get or set the tracing level (an integral).  Currently its value is either
0, no trace, or nonzero, full tracing.
@end deftypemethod

@deftypemethod {parser} {void} error (@code{const location_type&} @var{l}, @code{const std::string&} @var{m})
@deftypemethodx {parser} {void} error (@code{const std::string&} @var{m})
The definition for this member function must be supplied by the user: the
parser uses it to report a parser error occurring at @var{l}, described by
@var{m}.  If location tracking is not enabled, the second signature is used.
@end deftypemethod


@node C++ Semantic Values
@subsection C++ Semantic Values
@c - No objects in unions
@c - YYSTYPE
@c - Printer and destructor

Bison supports two different means to handle semantic values in C++.  One is
alike the C interface, and relies on unions.  As C++ practitioners know,
unions are inconvenient in C++, therefore another approach is provided,
based on variants.

@menu
* C++ Unions::             Semantic values cannot be objects
* C++ Variants::           Using objects as semantic values
@end menu

@node C++ Unions
@subsubsection C++ Unions

The @code{%union} directive works as for C, see @ref{Union Decl}.  In
particular it produces a genuine @code{union}, which have a few specific
features in C++.
@itemize @minus
@item
The type @code{YYSTYPE} is defined but its use is discouraged: rather you
should refer to the parser's encapsulated type
@code{yy::parser::semantic_type}.
@item
Non POD (Plain Old Data) types cannot be used.  C++98 forbids any instance
of classes with constructors in unions: only @emph{pointers} to such objects
are allowed.  C++11 relaxed this constraints, but at the cost of safety.
@end itemize

Because objects have to be stored via pointers, memory is not
reclaimed automatically: using the @code{%destructor} directive is the
only means to avoid leaks.  @xref{Destructor Decl}.

@node C++ Variants
@subsubsection C++ Variants

Bison provides a @emph{variant} based implementation of semantic values for
C++.  This alleviates all the limitations reported in the previous section,
and in particular, object types can be used without pointers.

To enable variant-based semantic values, set the @code{%define} variable
@code{api.value.type} to @code{variant} (@pxref{%define Summary}).  Then
@code{%union} is ignored; instead of using the name of the fields of the
@code{%union} to ``type'' the symbols, use genuine types.

For instance, instead of:

@example
%union
@{
  int ival;
  std::string* sval;
@}
%token <ival> NUMBER;
%token <sval> STRING;
@end example

@noindent
write:

@example
%token <int> NUMBER;
%token <std::string> STRING;
@end example

@code{STRING} is no longer a pointer, which should fairly simplify the user
actions in the grammar and in the scanner (in particular the memory
management).

Since C++ features destructors, and since it is customary to specialize
@code{operator<<} to support uniform printing of values, variants also
typically simplify Bison printers and destructors.

Variants are stricter than unions.  When based on unions, you may play any
dirty game with @code{yylval}, say storing an @code{int}, reading a
@code{char*}, and then storing a @code{double} in it.  This is no longer
possible with variants: they must be initialized, then assigned to, and
eventually, destroyed.  As a matter of fact, Bison variants forbid the use
of alternative types such as @samp{$<int>2} or @samp{$<std::string>$}, even
in midrule actions.  It is mandatory to use typed midrule actions
(@pxref{Typed Midrule Actions}).

@deftypemethod {semantic_type} {T&} {emplace<T>} ()
@deftypemethodx {semantic_type} {T&} {emplace<T>} (@code{const T&} @var{t})
Available in C++98/C++03 only.  Default construct/copy-construct from
@var{t}.  Return a reference to where the actual value may be stored.
Requires that the variant was not initialized yet.
@end deftypemethod

@deftypemethod {semantic_type} {T&} {emplace<T, U>} (@code{U&&...} @var{u})
Available in C++11 and later only.  Build a variant of type @code{T} from
the variadic forwarding references @var{u...}.
@end deftypemethod

@strong{Warning}: We do not use Boost.Variant, for two reasons.  First, it
appeared unacceptable to require Boost on the user's machine (i.e., the
machine on which the generated parser will be compiled, not the machine on
which @command{bison} was run).  Second, for each possible semantic value,
Boost.Variant not only stores the value, but also a tag specifying its
type.  But the parser already ``knows'' the type of the semantic value, so
that would be duplicating the information.

We do not use C++17's @code{std::variant} either: we want to support all the
C++ standards, and of course @code{std::variant} also stores a tag to record
the current type.

Therefore we developed light-weight variants whose type tag is external (so
they are really like @code{unions} for C++ actually).  There is a number of
limitations in (the current implementation of) variants:
@itemize
@item
Alignment must be enforced: values should be aligned in memory according to
the most demanding type.  Computing the smallest alignment possible requires
meta-programming techniques that are not currently implemented in Bison, and
therefore, since, as far as we know, @code{double} is the most demanding
type on all platforms, alignments are enforced for @code{double} whatever
types are actually used.  This may waste space in some cases.

@item
There might be portability issues we are not aware of.
@end itemize

As far as we know, these limitations @emph{can} be alleviated.  All it takes
is some time and/or some talented C++ hacker willing to contribute to Bison.

@node C++ Location Values
@subsection C++ Location Values
@c - %locations
@c - class position
@c - class location
@c - %define filename_type "const symbol::Symbol"

When the directive @code{%locations} is used, the C++ parser supports
location tracking, see @ref{Tracking Locations}.

By default, two auxiliary classes define a @code{position}, a single point
in a file, and a @code{location}, a range composed of a pair of
@code{position}s (possibly spanning several files).  If the @code{%define}
variable @code{api.location.type} is defined, then these classes will not be
generated, and the user defined type will be used.

@menu
* C++ position::         One point in the source file
* C++ location::         Two points in the source file
* Exposing the Location Classes:: Using the Bison location class in your
                                  project
* User Defined Location Type::    Required interface for locations
@end menu

@node C++ position
@subsubsection C++ @code{position}

@defcv {Type} {position} {counter_type}
The type used to store line and column numbers.  Defined as @code{int}.
@end defcv

@deftypeop {Constructor} {position} {} position (@code{std::string*} @var{file} = nullptr, @code{counter_type} @var{line} = 1, @code{counter_type} @var{col} = 1)
Create a @code{position} denoting a given point.  Note that @code{file} is
not reclaimed when the @code{position} is destroyed: memory managed must be
handled elsewhere.
@end deftypeop

@deftypemethod {position} {void} initialize (@code{std::string*} @var{file} = nullptr, @code{counter_type} @var{line} = 1, @code{counter_type} @var{col} = 1)
Reset the position to the given values.
@end deftypemethod

@deftypeivar {position} {std::string*} file
The name of the file.  It will always be handled as a pointer, the parser
will never duplicate nor deallocate it.  As an experimental feature you may
change it to @samp{@var{type}*} using @samp{%define filename_type
"@var{type}"}.
@end deftypeivar

@deftypeivar {position} {counter_type} line
The line, starting at 1.
@end deftypeivar

@deftypemethod {position} {void} lines (@code{counter_type} @var{height} = 1)
If @var{height} is not null, advance by @var{height} lines, resetting the
column number.  The resulting line number cannot be less than 1.
@end deftypemethod

@deftypeivar {position} {counter_type} column
The column, starting at 1.
@end deftypeivar

@deftypemethod {position} {void} columns (@code{counter_type} @var{width} = 1)
Advance by @var{width} columns, without changing the line number. The
resulting column number cannot be less than 1.
@end deftypemethod

@deftypemethod {position} {position&} operator+= (@code{counter_type} @var{width})
@deftypemethodx {position} {position} operator+ (@code{counter_type} @var{width})
@deftypemethodx {position} {position&} operator-= (@code{counter_type} @var{width})
@deftypemethodx {position} {position} operator- (@code{counter_type} @var{width})
Various forms of syntactic sugar for @code{columns}.
@end deftypemethod

@deftypemethod {position} {bool} operator== (@code{const position&} @var{that})
@deftypemethodx {position} {bool} operator!= (@code{const position&} @var{that})
Whether @code{*this} and @code{that} denote equal/different positions.
@end deftypemethod

@deftypefun {std::ostream&} operator<< (@code{std::ostream&} @var{o}, @code{const position&} @var{p})
Report @var{p} on @var{o} like this:
@samp{@var{file}:@var{line}.@var{column}}, or
@samp{@var{line}.@var{column}} if @var{file} is null.
@end deftypefun

@node C++ location
@subsubsection C++ @code{location}

@deftypeop {Constructor} {location} {} location (@code{const position&} @var{begin}, @code{const position&} @var{end})
Create a @code{Location} from the endpoints of the range.
@end deftypeop

@deftypeop {Constructor} {location} {} location (@code{const position&} @var{pos} = position())
@deftypeopx {Constructor} {location} {} location (@code{std::string*} @var{file}, @code{counter_type} @var{line}, @code{counter_type} @var{col})
Create a @code{Location} denoting an empty range located at a given point.
@end deftypeop

@deftypemethod {location} {void} initialize (@code{std::string*} @var{file} = nullptr, @code{counter_type} @var{line} = 1, @code{counter_type} @var{col} = 1)
Reset the location to an empty range at the given values.
@end deftypemethod

@deftypeivar {location} {position} begin
@deftypeivarx {location} {position} end
The first, inclusive, position of the range, and the first beyond.
@end deftypeivar

@deftypemethod {location} {void} columns (@code{counter_type} @var{width} = 1)
@deftypemethodx {location} {void} lines (@code{counter_type} @var{height} = 1)
Forwarded to the @code{end} position.
@end deftypemethod

@deftypemethod  {location} {location} operator+  (@code{counter_type} @var{width})
@deftypemethodx {location} {location} operator+= (@code{counter_type} @var{width})
@deftypemethodx {location} {location} operator-  (@code{counter_type} @var{width})
@deftypemethodx {location} {location} operator-= (@code{counter_type} @var{width})
Various forms of syntactic sugar for @code{columns}.
@end deftypemethod

@deftypemethod {location} {location} operator+ (@code{const location&} @var{end})
@deftypemethodx {location} {location} operator+= (@code{const location&} @var{end})
Join two locations: starts at the position of the first one, and ends at the
position of the second.
@end deftypemethod

@deftypemethod {location} {void} step ()
Move @code{begin} onto @code{end}.
@end deftypemethod

@deftypemethod {location} {bool} operator== (@code{const location&} @var{that})
@deftypemethodx {location} {bool} operator!= (@code{const location&} @var{that})
Whether @code{*this} and @code{that} denote equal/different ranges of
positions.
@end deftypemethod

@deftypefun {std::ostream&} operator<< (@code{std::ostream&} @var{o}, @code{const location&} @var{p})
Report @var{p} on @var{o}, taking care of special cases such as: no
@code{filename} defined, or equal filename/line or column.
@end deftypefun

@node Exposing the Location Classes
@subsubsection Exposing the Location Classes

When both @code{%defines} and @code{%locations} are enabled, Bison generates
an additional file: @file{location.hh}.  If you don't use locations outside
of the parser, you may avoid its creation with @samp{%define
api.location.file none}.

However this file is useful if, for instance, your parser builds an abstract
syntax tree decorated with locations: you may use Bison's @code{location}
type independently of Bison's parser.  You may name the file differently,
e.g., @samp{%define api.location.file "include/ast/location.hh"}: this name
can have directory components, or even be absolute.  The way the location
file is included is controlled by @code{api.location.include}.

This way it is possible to have several parsers share the same location
file.

For instance, in @file{src/foo/parser.yy}, generate the
@file{include/ast/loc.hh} file:

@example
// src/foo/parser.yy
%locations
%define api.namespace @{foo@}
%define api.location.file "include/ast/loc.hh"
%define api.location.include @{<ast/loc.hh>@}
@end example

@noindent
and use it in @file{src/bar/parser.yy}:

@example
// src/bar/parser.yy
%locations
%define api.namespace @{bar@}
%code requires @{#include <ast/loc.hh>@}
%define api.location.type @{bar::location@}
@end example

Absolute file names are supported; it is safe in your @file{Makefile} to
pass the flag
@option{-Dapi.location.file='"$(top_srcdir)/include/ast/loc.hh"'} to
@command{bison} for @file{src/foo/parser.yy}.  The generated file will not
have references to this absolute path, thanks to @samp{%define
api.location.include @{<ast/loc.hh>@}}.  Adding @samp{-I
$(top_srcdir)/include} to your @code{CPPFLAGS} will suffice for the compiler
to find @file{ast/loc.hh}.

@node User Defined Location Type
@subsubsection User Defined Location Type
@findex %define api.location.type

Instead of using the built-in types you may use the @code{%define} variable
@code{api.location.type} to specify your own type:

@example
%define api.location.type @{@var{LocationType}@}
@end example

The requirements over your @var{LocationType} are:
@itemize
@item
it must be copyable;

@item
in order to compute the (default) value of @code{@@$} in a reduction, the
parser basically runs
@example
@@$.begin = @@1.begin;
@@$.end   = @@@var{N}.end; // The location of last right-hand side symbol.
@end example
@noindent
so there must be copyable @code{begin} and @code{end} members;

@item
alternatively you may redefine the computation of the default location, in
which case these members are not required (@pxref{Location Default Action});

@item
if traces are enabled, then there must exist an @samp{std::ostream&
  operator<< (std::ostream& o, const @var{LocationType}& s)} function.
@end itemize

@sp 1

In programs with several C++ parsers, you may also use the @code{%define}
variable @code{api.location.type} to share a common set of built-in
definitions for @code{position} and @code{location}.  For instance, one
parser @file{master/parser.yy} might use:

@example
%defines
%locations
%define api.namespace @{master::@}
@end example

@noindent
to generate the @file{master/position.hh} and @file{master/location.hh}
files, reused by other parsers as follows:

@example
%define api.location.type @{master::location@}
%code requires @{ #include <master/location.hh> @}
@end example


@node C++ Parser Context
@subsection C++ Parser Context

When @samp{%define parse.error custom} is used (@pxref{Syntax Error
Reporting Function}), the user must define the following function.

@deftypemethod {parser} {void} report_syntax_error (@code{const context_type&}@var{ctx}) @code{const}
Report a syntax error to the user.  Whether it uses @code{yyerror} is up to
the user.
@end deftypemethod

Use the following types and functions to build the error message.

@defcv {Type} {parser} {context}
A type that captures the circumstances of the syntax error.
@end defcv

@defcv {Type} {parser} {symbol_kind_type}
An enum of all the grammar symbols, tokens and nonterminals.  Its
enumerators are forged from the symbol names:

@example
struct symbol_kind
@{
  enum symbol_kind_type
  @{
    S_YYEMPTY = -2,      // No symbol.
    S_YYEOF = 0,         // "end of file"
    S_YYERROR = 1,       // error
    S_YYUNDEF = 2,       // "invalid token"
    S_PLUS = 3,          // "+"
    S_MINUS = 4,         // "-"
    [...]
    S_VAR = 14,          // "variable"
    S_NEG = 15,          // NEG
    S_YYACCEPT = 16,     // $accept
    S_exp = 17,          // exp
    S_input = 18         // input
  @};
@};
typedef symbol_kind::symbol_kind_t symbol_kind_type;
@end example
@end defcv

@deftypemethod {context} {const symbol_type&} lookahead () @code{const}
The ``unexpected'' token: the lookahead that caused the syntax error.
@end deftypemethod

@deftypemethod {context} {symbol_kind_type} token () @code{const}
The symbol kind of the lookahead token that caused the syntax error.  Return
@code{symbol_kind::S_YYEMPTY} if there is no lookahead.
@end deftypemethod

@deftypemethod {context} {const location&} location () @code{const}
The location of the syntax error (that of the lookahead).
@end deftypemethod

@deftypemethod {context} int expected_tokens (@code{symbol_kind_type} @var{argv}@code{[]}, @code{int} @var{argc}) @code{const}
Fill @var{argv} with the expected tokens, which never includes
@code{symbol_kind::S_YYEMPTY}, @code{symbol_kind::S_YYERROR}, or
@code{symbol_kind::S_YYUNDEF}.

Never put more than @var{argc} elements into @var{argv}, and on success
return the effective number of tokens stored in @var{argv}.  Return 0 if
there are more than @var{argc} expected tokens, yet fill @var{argv} up to
@var{argc}.

If @var{argv} is null, return the size needed to store all the possible
values, which is always less than @code{YYNTOKENS}.
@end deftypemethod

@deftypemethod {parser} {const char *} symbol_name (@code{symbol_kind_t} @var{symbol}) @code{const}
The name of the symbol whose kind is @var{symbol}, possibly translated.
@end deftypemethod

A custom syntax error function looks as follows.  This implementation is
inappropriate for internationalization, see the @file{c/bistromathic}
example for a better alternative.

@example
void
yy::parser::report_syntax_error (const context& ctx)
@{
  int res = 0;
  std::cerr << ctx.location () << ": syntax error";
  // Report the tokens expected at this point.
  @{
    enum @{ TOKENMAX = 5 @};
    symbol_kind_type expected[TOKENMAX];
    int n = ctx.expected_tokens (ctx, expected, TOKENMAX);
    for (int i = 0; i < n; ++i)
      std::cerr << i == 0 ? ": expected " : " or "
                << symbol_name (expected[i]);
  @}
  // Report the unexpected token.
  @{
    symbol_kind_type lookahead = ctx.token ();
    if (lookahead != symbol_kind::S_YYEMPTY)
      std::cerr << " before " << symbol_name (lookahead));
  @}
  std::cerr << '\n';
@}
@end example

You still must provide a @code{yyerror} function, used for instance to
report memory exhaustion.


@node C++ Scanner Interface
@subsection C++ Scanner Interface
@c - prefix for yylex.
@c - Pure interface to yylex
@c - %lex-param

The parser invokes the scanner by calling @code{yylex}.  Contrary to C
parsers, C++ parsers are always pure: there is no point in using the
@samp{%define api.pure} directive.  The actual interface with @code{yylex}
depends whether you use unions, or variants.

@menu
* Split Symbols::         Passing symbols as two/three components
* Complete Symbols::      Making symbols a whole
@end menu

@node Split Symbols
@subsubsection Split Symbols

The generated parser expects @code{yylex} to have the following prototype.

@deftypefun {int} yylex (@code{semantic_type*} @var{yylval}, @code{location_type*} @var{yylloc}, @var{type1} @var{arg1}, @dots{})
@deftypefunx {int} yylex (@code{semantic_type*} @var{yylval}, @var{type1} @var{arg1}, @dots{})
Return the next token.  Its kind is the return value, its semantic value and
location (if enabled) being @var{yylval} and @var{yylloc}.  Invocations of
@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments.
@end deftypefun

Note that when using variants, the interface for @code{yylex} is the same,
but @code{yylval} is handled differently.

Regular union-based code in Lex scanner typically looks like:

@example
[0-9]+   @{
           yylval->ival = text_to_int (yytext);
           return yy::parser::token::INTEGER;
         @}
[a-z]+   @{
           yylval->sval = new std::string (yytext);
           return yy::parser::token::IDENTIFIER;
         @}
@end example

Using variants, @code{yylval} is already constructed, but it is not
initialized.  So the code would look like:

@example
[0-9]+   @{
           yylval->emplace<int> () = text_to_int (yytext);
           return yy::parser::token::INTEGER;
         @}
[a-z]+   @{
           yylval->emplace<std::string> () = yytext;
           return yy::parser::token::IDENTIFIER;
         @}
@end example

@noindent
or

@example
[0-9]+   @{
           yylval->emplace (text_to_int (yytext));
           return yy::parser::token::INTEGER;
         @}
[a-z]+   @{
           yylval->emplace (yytext);
           return yy::parser::token::IDENTIFIER;
         @}
@end example


@node Complete Symbols
@subsubsection Complete Symbols

With both @code{%define api.value.type variant} and @code{%define
api.token.constructor}, the parser defines the type @code{symbol_type}, and
expects @code{yylex} to have the following prototype.

@deftypefun {parser::symbol_type} yylex ()
@deftypefunx {parser::symbol_type} yylex (@var{type1} @var{arg1}, @dots{})
Return a @emph{complete} symbol, aggregating its type (i.e., the traditional
value returned by @code{yylex}), its semantic value, and possibly its
location.  Invocations of @samp{%lex-param @{@var{type1} @var{arg1}@}} yield
additional arguments.
@end deftypefun

@defcv {Type} {parser} {symbol_type}
A ``complete symbol'', that binds together its kind, value and (when
applicable) location.
@end defcv

@deftypemethod {symbol_type} {symbol_kind_type} kind () @code{const}
The kind of this symbol.
@end deftypemethod

@sp 1

For each token kind, Bison generates named constructors as follows.

@deftypeop  {Constructor} {parser::symbol_type} {} {symbol_type} (@code{int} @var{token}, @code{const @var{value_type}&} @var{value}, @code{const location_type&} @var{location})
@deftypeopx {Constructor} {parser::symbol_type} {} {symbol_type} (@code{int} @var{token}, @code{const location_type&} @var{location})
@deftypeopx {Constructor} {parser::symbol_type} {} {symbol_type} (@code{int} @var{token}, @code{const @var{value_type}&} @var{value})
@deftypeopx {Constructor} {parser::symbol_type} {} {symbol_type} (@code{int} @var{token})
Build a complete terminal symbol for the token kind @var{token} (including
the @code{api.token.prefix}), whose semantic value, if it has one, is
@var{value} of adequate @var{value_type}.  Pass the @var{location} iff
location tracking is enabled.

Consistency between @var{token} and @var{value_type} is checked via an
@code{assert}.
@end deftypeop

For instance, given the following declarations:

@example
%define api.token.prefix @{TOK_@}
%token <std::string> IDENTIFIER;
%token <int> INTEGER;
%token ':';
@end example

@noindent
you may use these constructors:

@example
symbol_type (int token, const std::string&, const location_type&);
symbol_type (int token, const int&, const location_type&);
symbol_type (int token, const location_type&);
@end example

Correct matching between token kinds and value types is checked via
@code{assert}; for instance, @samp{symbol_type (ID, 42)} would abort.  Named
constructors are preferable (see below), as they offer better type safety
(for instance @samp{make_ID (42)} would not even compile), but symbol_type
constructors may help when token kinds are discovered at run-time, e.g.,

@example
@group
[a-z]+   @{
           if (auto i = lookup_keyword (yytext))
             return yy::parser::symbol_type (i, loc);
           else
             return yy::parser::make_ID (yytext, loc);
         @}
@end group
@end example

@sp 1

Note that it is possible to generate and compile type incorrect code
(e.g. @samp{symbol_type (':', yytext, loc)}).  It will fail at run time,
provided the assertions are enabled (i.e., @option{-DNDEBUG} was not passed
to the compiler).  Bison supports an alternative that guarantees that type
incorrect code will not even compile.  Indeed, it generates @emph{named
constructors} as follows.

@deftypemethod {parser} {symbol_type} {make_@var{token}} (@code{const @var{value_type}&} @var{value}, @code{const location_type&} @var{location})
@deftypemethodx {parser} {symbol_type} {make_@var{token}} (@code{const location_type&} @var{location})
@deftypemethodx {parser} {symbol_type} {make_@var{token}} (@code{const @var{value_type}&} @var{value})
@deftypemethodx {parser} {symbol_type} {make_@var{token}} ()
Build a complete terminal symbol for the token kind @var{token} (not
including the @code{api.token.prefix}), whose semantic value, if it has one,
is @var{value} of adequate @var{value_type}.  Pass the @var{location} iff
location tracking is enabled.
@end deftypemethod

For instance, given the following declarations:

@example
%define api.token.prefix @{TOK_@}
%token <std::string> IDENTIFIER;
%token <int> INTEGER;
%token COLON;
%token EOF 0;
@end example

@noindent
Bison generates:

@example
symbol_type make_IDENTIFIER (const std::string&, const location_type&);
symbol_type make_INTEGER (const int&, const location_type&);
symbol_type make_COLON (const location_type&);
symbol_type make_EOF (const location_type&);
@end example

@noindent
which should be used in a scanner as follows.

@example
[a-z]+   return yy::parser::make_IDENTIFIER (yytext, loc);
[0-9]+   return yy::parser::make_INTEGER (text_to_int (yytext), loc);
":"      return yy::parser::make_COLON (loc);
<<EOF>>  return yy::parser::make_EOF (loc);
@end example

Tokens that do not have an identifier are not accessible: you cannot simply
use characters such as @code{':'}, they must be declared with @code{%token},
including the end-of-file token.


@node A Complete C++ Example
@subsection A Complete C++ Example

This section demonstrates the use of a C++ parser with a simple but complete
example.  This example should be available on your system, ready to compile,
in the directory @dfn{.../share/doc/bison/examples/calc++}.  It focuses on
the use of Bison, therefore the design of the various C++ classes is very
naive: no accessors, no encapsulation of members etc.  We will use a Lex
scanner, and more precisely, a Flex scanner, to demonstrate the various
interactions.  A hand-written scanner is actually easier to interface with.

@menu
* Calc++ --- C++ Calculator::   The specifications
* Calc++ Parsing Driver::       An active parsing context
* Calc++ Parser::               A parser class
* Calc++ Scanner::              A pure C++ Flex scanner
* Calc++ Top Level::            Conducting the band
@end menu

@node Calc++ --- C++ Calculator
@subsubsection Calc++ --- C++ Calculator

Of course the grammar is dedicated to arithmetic, a single expression,
possibly preceded by variable assignments.  An environment containing
possibly predefined variables such as @code{one} and @code{two}, is
exchanged with the parser.  An example of valid input follows.

@example
three := 3
seven := one + two * three
seven * seven
@end example

@node Calc++ Parsing Driver
@subsubsection Calc++ Parsing Driver
@c - An env
@c - A place to store error messages
@c - A place for the result

To support a pure interface with the parser (and the scanner) the technique
of the ``parsing context'' is convenient: a structure containing all the
data to exchange.  Since, in addition to simply launch the parsing, there
are several auxiliary tasks to execute (open the file for scanning,
instantiate the parser etc.), we recommend transforming the simple parsing
context structure into a fully blown @dfn{parsing driver} class.

The declaration of this driver class, in @file{driver.hh}, is as follows.
The first part includes the CPP guard and imports the required standard
library components, and the declaration of the parser class.

@comment file: calc++/driver.hh
@example
#ifndef DRIVER_HH
# define DRIVER_HH
# include <string>
# include <map>
# include "parser.hh"
@end example


@noindent
Then comes the declaration of the scanning function.  Flex expects the
signature of @code{yylex} to be defined in the macro @code{YY_DECL}, and the
C++ parser expects it to be declared.  We can factor both as follows.

@comment file: calc++/driver.hh
@example
// Give Flex the prototype of yylex we want ...
# define YY_DECL \
  yy::parser::symbol_type yylex (driver& drv)
// ... and declare it for the parser's sake.
YY_DECL;
@end example

@noindent
The @code{driver} class is then declared with its most obvious members.

@comment file: calc++/driver.hh
@example
// Conducting the whole scanning and parsing of Calc++.
class driver
@{
public:
  driver ();

  std::map<std::string, int> variables;

  int result;
@end example

@noindent
The main routine is of course calling the parser.

@comment file: calc++/driver.hh
@example
  // Run the parser on file F.  Return 0 on success.
  int parse (const std::string& f);
  // The name of the file being parsed.
  std::string file;
  // Whether to generate parser debug traces.
  bool trace_parsing;
@end example

@noindent
To encapsulate the coordination with the Flex scanner, it is useful to have
member functions to open and close the scanning phase.

@comment file: calc++/driver.hh
@example
  // Handling the scanner.
  void scan_begin ();
  void scan_end ();
  // Whether to generate scanner debug traces.
  bool trace_scanning;
  // The token's location used by the scanner.
  yy::location location;
@};
#endif // ! DRIVER_HH
@end example

The implementation of the driver (@file{driver.cc}) is straightforward.

@comment file: calc++/driver.cc
@example
#include "driver.hh"
#include "parser.hh"

@group
driver::driver ()
  : trace_parsing (false), trace_scanning (false)
@{
  variables["one"] = 1;
  variables["two"] = 2;
@}
@end group
@end example

The @code{parse} member function deserves some attention.

@comment file: calc++/driver.cc
@example
@group
int
driver::parse (const std::string &f)
@{
  file = f;
  location.initialize (&file);
  scan_begin ();
  yy::parser parse (*this);
  parse.set_debug_level (trace_parsing);
  int res = parse ();
  scan_end ();
  return res;
@}
@end group
@end example

@node Calc++ Parser
@subsubsection Calc++ Parser

The grammar file @file{parser.yy} starts by asking for the C++ deterministic
parser skeleton, the creation of the parser header file.  Because the C++
skeleton changed several times, it is safer to require the version you
designed the grammar for.

@comment file: calc++/parser.yy
@example
%skeleton "lalr1.cc" // -*- C++ -*-
%require "@value{VERSION}"
%defines
@end example

@noindent
@findex %define api.token.raw
Because our scanner returns only genuine tokens and never simple characters
(i.e., it returns @samp{PLUS}, not @samp{'+'}), we can avoid conversions.

@comment file: calc++/parser.yy
@example
%define api.token.raw
@end example

@noindent
@findex %define api.token.constructor
@findex %define api.value.type variant
This example uses genuine C++ objects as semantic values, therefore, we
require the variant-based storage of semantic values.  To make sure we
properly use it, we enable assertions.  To fully benefit from type-safety
and more natural definition of ``symbol'', we enable
@code{api.token.constructor}.

@comment file: calc++/parser.yy
@example
%define api.token.constructor
%define api.value.type variant
%define parse.assert
@end example

@noindent
@findex %code requires
Then come the declarations/inclusions needed by the semantic values.
Because the parser uses the parsing driver and reciprocally, both would like
to include the header of the other, which is, of course, insane.  This
mutual dependency will be broken using forward declarations.  Because the
driver's header needs detailed knowledge about the parser class (in
particular its inner types), it is the parser's header which will use a
forward declaration of the driver.  @xref{%code Summary}.

@comment file: calc++/parser.yy
@example
@group
%code requires @{
  # include <string>
  class driver;
@}
@end group
@end example

@noindent
The driver is passed by reference to the parser and to the scanner.
This provides a simple but effective pure interface, not relying on
global variables.

@comment file: calc++/parser.yy
@example
// The parsing context.
%param @{ driver& drv @}
@end example

@noindent
Then we request location tracking.

@comment file: calc++/parser.yy
@example
%locations
@end example

@noindent
Use the following two directives to enable parser tracing and detailed error
messages.  However, detailed error messages can contain incorrect
information if lookahead correction is not enabled (@pxref{LAC}).

@comment file: calc++/parser.yy
@example
%define parse.trace
%define parse.error detailed
%define parse.lac full
@end example

@noindent
@findex %code
The code between @samp{%code @{} and @samp{@}} is output in the @file{*.cc}
file; it needs detailed knowledge about the driver.

@comment file: calc++/parser.yy
@example
@group
%code @{
# include "driver.hh"
@}
@end group
@end example


@noindent
User friendly names are provided for each symbol.  To avoid name clashes in
the generated files (@pxref{Calc++ Scanner}), prefix tokens with @code{TOK_}
(@pxref{%define Summary}).

@comment file: calc++/parser.yy
@example
%define api.token.prefix @{TOK_@}
%token
  ASSIGN  ":="
  MINUS   "-"
  PLUS    "+"
  STAR    "*"
  SLASH   "/"
  LPAREN  "("
  RPAREN  ")"
;
@end example

@noindent
Since we use variant-based semantic values, @code{%union} is not used, and
@code{%token}, @code{%nterm} and @code{%type} expect genuine types, not type
tags.

@comment file: calc++/parser.yy
@example
%token <std::string> IDENTIFIER "identifier"
%token <int> NUMBER "number"
%nterm <int> exp
@end example

@noindent
No @code{%destructor} is needed to enable memory deallocation during error
recovery; the memory, for strings for instance, will be reclaimed by the
regular destructors.  All the values are printed using their
@code{operator<<} (@pxref{Printer Decl}).

@comment file: calc++/parser.yy
@example
%printer @{ yyo << $$; @} <*>;
@end example

@noindent
The grammar itself is straightforward (@pxref{Location Tracking Calc}).

@comment file: calc++/parser.yy
@example
%%
%start unit;
unit: assignments exp  @{ drv.result = $2; @};

assignments:
  %empty                 @{@}
| assignments assignment @{@};

assignment:
  "identifier" ":=" exp @{ drv.variables[$1] = $3; @};

%left "+" "-";
%left "*" "/";
exp:
  "number"
| "identifier"  @{ $$ = drv.variables[$1]; @}
| exp "+" exp   @{ $$ = $1 + $3; @}
| exp "-" exp   @{ $$ = $1 - $3; @}
| exp "*" exp   @{ $$ = $1 * $3; @}
| exp "/" exp   @{ $$ = $1 / $3; @}
| "(" exp ")"   @{ $$ = $2; @}
%%
@end example

@noindent
Finally the @code{error} member function reports the errors.

@comment file: calc++/parser.yy
@example
void
yy::parser::error (const location_type& l, const std::string& m)
@{
  std::cerr << l << ": " << m << '\n';
@}
@end example

@node Calc++ Scanner
@subsubsection Calc++ Scanner

In addition to standard headers, the Flex scanner includes the driver's,
then the parser's to get the set of defined tokens.

@comment file: calc++/scanner.ll
@example
%@{ /* -*- C++ -*- */
# include <cerrno>
# include <climits>
# include <cstdlib>
# include <cstring> // strerror
# include <string>
# include "driver.hh"
# include "parser.hh"
%@}
@end example

@ignore
@comment file: calc++/scanner.ll
@example
%@{
#if defined __clang__
# define CLANG_VERSION (__clang_major__ * 100 + __clang_minor__)
#endif

// Clang and ICC like to pretend they are GCC.
#if defined __GNUC__ && !defined __clang__ && !defined __ICC
# define GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__)
#endif

// Pacify warnings in yy_init_buffer (observed with Flex 2.6.4)
// and GCC 6.4.0, 7.3.0 with -O3.
#if defined GCC_VERSION && 600 <= GCC_VERSION
# pragma GCC diagnostic ignored "-Wnull-dereference"
#endif

// This example uses Flex's C back end, yet compiles it as C++.
// So expect warnings about C style casts and NULL.
#if defined CLANG_VERSION && 500 <= CLANG_VERSION
# pragma clang diagnostic ignored "-Wold-style-cast"
# pragma clang diagnostic ignored "-Wzero-as-null-pointer-constant"
#elif defined GCC_VERSION && 407 <= GCC_VERSION
# pragma GCC diagnostic ignored "-Wold-style-cast"
# pragma GCC diagnostic ignored "-Wzero-as-null-pointer-constant"
#endif

#define FLEX_VERSION (YY_FLEX_MAJOR_VERSION * 100 + YY_FLEX_MINOR_VERSION)

// Old versions of Flex (2.5.35) generate an incomplete documentation comment.
//
//  In file included from src/scan-code-c.c:3:
//  src/scan-code.c:2198:21: error: empty paragraph passed to '@param' command
//        [-Werror,-Wdocumentation]
//   * @param line_number
//     ~~~~~~~~~~~~~~~~~^
//  1 error generated.
#if FLEX_VERSION < 206 && defined CLANG_VERSION
# pragma clang diagnostic ignored "-Wdocumentation"
#endif

// Old versions of Flex (2.5.35) use 'register'.  Warnings introduced in
// GCC 7 and Clang 6.
#if FLEX_VERSION < 206
# if defined CLANG_VERSION && 600 <= CLANG_VERSION
#  pragma clang diagnostic ignored "-Wdeprecated-register"
# elif defined GCC_VERSION && 700 <= GCC_VERSION
#  pragma GCC diagnostic ignored "-Wregister"
# endif
#endif

#if FLEX_VERSION < 206
# if defined CLANG_VERSION
#  pragma clang diagnostic ignored "-Wconversion"
#  pragma clang diagnostic ignored "-Wdocumentation"
#  pragma clang diagnostic ignored "-Wshorten-64-to-32"
#  pragma clang diagnostic ignored "-Wsign-conversion"
# elif defined GCC_VERSION
#  pragma GCC diagnostic ignored "-Wconversion"
#  pragma GCC diagnostic ignored "-Wsign-conversion"
# endif
#endif
%@}
@end example
@end ignore

@noindent
Since our calculator has no @code{#include}-like feature, we don't need
@code{yywrap}.  We don't need the @code{unput} and @code{input} functions
either, and we parse an actual file, this is not an interactive session with
the user.  Finally, we enable scanner tracing.

@comment file: calc++/scanner.ll
@example
%option noyywrap nounput noinput batch debug
@end example

@noindent
The following function will be handy to convert a string denoting a number
into a @code{NUMBER} token.

@comment file: calc++/scanner.ll
@example
%@{
  // A number symbol corresponding to the value in S.
  yy::parser::symbol_type
  make_NUMBER (const std::string &s, const yy::parser::location_type& loc);
%@}
@end example

@noindent
Abbreviations allow for more readable rules.

@comment file: calc++/scanner.ll
@example
id    [a-zA-Z][a-zA-Z_0-9]*
int   [0-9]+
blank [ \t\r]
@end example

@noindent
The following paragraph suffices to track locations accurately.  Each time
@code{yylex} is invoked, the begin position is moved onto the end position.
Then when a pattern is matched, its width is added to the end column.  When
matching ends of lines, the end cursor is adjusted, and each time blanks are
matched, the begin cursor is moved onto the end cursor to effectively ignore
the blanks preceding tokens.  Comments would be treated equally.

@comment file: calc++/scanner.ll
@example
@group
%@{
  // Code run each time a pattern is matched.
  # define YY_USER_ACTION  loc.columns (yyleng);
%@}
@end group
%%
@group
%@{
  // A handy shortcut to the location held by the driver.
  yy::location& loc = drv.location;
  // Code run each time yylex is called.
  loc.step ();
%@}
@end group
@{blank@}+   loc.step ();
\n+        loc.lines (yyleng); loc.step ();
@end example

@noindent
The rules are simple.  The driver is used to report errors.

@comment file: calc++/scanner.ll
@example
"-"        return yy::parser::make_MINUS  (loc);
"+"        return yy::parser::make_PLUS   (loc);
"*"        return yy::parser::make_STAR   (loc);
"/"        return yy::parser::make_SLASH  (loc);
"("        return yy::parser::make_LPAREN (loc);
")"        return yy::parser::make_RPAREN (loc);
":="       return yy::parser::make_ASSIGN (loc);

@{int@}      return make_NUMBER (yytext, loc);
@{id@}       return yy::parser::make_IDENTIFIER (yytext, loc);
@group
.          @{
             throw yy::parser::syntax_error
               (loc, "invalid character: " + std::string(yytext));
@}
@end group
<<EOF>>    return yy::parser::make_YYEOF (loc);
%%
@end example

@noindent
You should keep your rules simple, both in the parser and in the scanner.
Throwing from the auxiliary functions is then very handy to report errors.

@comment file: scanner.ll
@example
@group
yy::parser::symbol_type
make_NUMBER (const std::string &s, const yy::parser::location_type& loc)
@{
  errno = 0;
  long n = strtol (s.c_str(), NULL, 10);
  if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
    throw yy::parser::syntax_error (loc, "integer is out of range: " + s);
  return yy::parser::make_NUMBER ((int) n, loc);
@}
@end group
@end example

@noindent
Finally, because the scanner-related driver's member-functions depend
on the scanner's data, it is simpler to implement them in this file.

@comment file: calc++/scanner.ll
@example
@group
void
driver::scan_begin ()
@{
  yy_flex_debug = trace_scanning;
  if (file.empty () || file == "-")
    yyin = stdin;
  else if (!(yyin = fopen (file.c_str (), "r")))
    @{
      std::cerr << "cannot open " << file << ": " << strerror (errno) << '\n';
      exit (EXIT_FAILURE);
    @}
@}
@end group

@group
void
driver::scan_end ()
@{
  fclose (yyin);
@}
@end group
@end example

@node Calc++ Top Level
@subsubsection Calc++ Top Level

The top level file, @file{calc++.cc}, poses no problem.

@comment file: calc++.cc
@example
#include <iostream>
#include "driver.hh"

@group
int
main (int argc, char *argv[])
@{
  int res = 0;
  driver drv;
  for (int i = 1; i < argc; ++i)
    if (argv[i] == std::string ("-p"))
      drv.trace_parsing = true;
    else if (argv[i] == std::string ("-s"))
      drv.trace_scanning = true;
    else if (!drv.parse (argv[i]))
      std::cout << drv.result << '\n';
    else
      res = 1;
  return res;
@}
@end group
@end example

@node Java Parsers
@section Java Parsers

@menu
* Java Bison Interface::        Asking for Java parser generation
* Java Semantic Values::        %token and %nterm vs. Java
* Java Location Values::        The position and location classes
* Java Parser Interface::       Instantiating and running the parser
* Java Parser Context Interface:: Circumstances of a syntax error
* Java Scanner Interface::      Specifying the scanner for the parser
* Java Action Features::        Special features for use in actions
* Java Push Parser Interface::  Instantiating and running the a push parser
* Java Differences::            Differences between C/C++ and Java Grammars
* Java Declarations Summary::   List of Bison declarations used with Java
@end menu

@node Java Bison Interface
@subsection Java Bison Interface
@c - %language "Java"

The Java parser skeletons are selected using the @code{%language "Java"}
directive or the @option{-L java}/@option{--language=java} option.

@c FIXME: Documented bug.
When generating a Java parser, @samp{bison @var{basename}.y} will create a
single Java source file named @file{@var{basename}.java} containing the
parser implementation.  Using a grammar file without a @file{.y} suffix is
currently broken.  The basename of the parser implementation file can be
changed by the @code{%file-prefix} directive or the
@option{-b}/@option{--file-prefix} option.  The entire parser implementation
file name can be changed by the @code{%output} directive or the
@option{-o}/@option{--output} option.  The parser implementation file
contains a single class for the parser.

You can create documentation for generated parsers using Javadoc.

Contrary to C parsers, Java parsers do not use global variables; the state
of the parser is always local to an instance of the parser class.
Therefore, all Java parsers are ``pure'', and the @code{%define api.pure}
directive does nothing when used in Java.

GLR parsers are currently unsupported in Java.  Do not use the
@code{glr-parser} directive.

No header file can be generated for Java parsers.  Do not use the
@code{%defines} directive or the @option{-d}/@option{--defines} options.

@c FIXME: Possible code change.
Currently, support for tracing is always compiled in.  Thus the
@samp{%define parse.trace} and @samp{%token-table} directives and the
@option{-t}/@option{--debug} and @option{-k}/@option{--token-table} options
have no effect.  This may change in the future to eliminate unused code in
the generated parser, so use @samp{%define parse.trace} explicitly if
needed.  Also, in the future the @code{%token-table} directive might enable
a public interface to access the token names and codes.

Getting a ``code too large'' error from the Java compiler means the code hit
the 64KB bytecode per method limitation of the Java class file.  Try
reducing the amount of code in actions and static initializers; otherwise,
report a bug so that the parser skeleton will be improved.


@node Java Semantic Values
@subsection Java Semantic Values
@c - No %union, specify type in %nterm/%token.
@c - YYSTYPE
@c - Printer and destructor

There is no @code{%union} directive in Java parsers.  Instead, the semantic
values' types (class names) should be specified in the @code{%nterm} or
@code{%token} directive:

@example
%nterm <Expression> expr assignment_expr term factor
%nterm <Integer> number
@end example

By default, the semantic stack is declared to have @code{Object} members,
which means that the class types you specify can be of any class.
To improve the type safety of the parser, you can declare the common
superclass of all the semantic values using the @samp{%define api.value.type}
directive.  For example, after the following declaration:

@example
%define api.value.type @{ASTNode@}
@end example

@noindent
any @code{%token}, @code{%nterm} or @code{%type} specifying a semantic type
which is not a subclass of @code{ASTNode}, will cause a compile-time error.

@c FIXME: Documented bug.
Types used in the directives may be qualified with a package name.
Primitive data types are accepted for Java version 1.5 or later.  Note
that in this case the autoboxing feature of Java 1.5 will be used.
Generic types may not be used; this is due to a limitation in the
implementation of Bison, and may change in future releases.

Java parsers do not support @code{%destructor}, since the language
adopts garbage collection.  The parser will try to hold references
to semantic values for as little time as needed.

Java parsers do not support @code{%printer}, as @code{toString()}
can be used to print the semantic values.  This however may change
(in a backwards-compatible way) in future versions of Bison.


@node Java Location Values
@subsection Java Location Values
@c - %locations
@c - class Position
@c - class Location

When the directive @code{%locations} is used, the Java parser supports
location tracking, see @ref{Tracking Locations}.  An auxiliary user-defined
class defines a @dfn{position}, a single point in a file; Bison itself
defines a class representing a @dfn{location}, a range composed of a pair of
positions (possibly spanning several files).  The location class is an inner
class of the parser; the name is @code{Location} by default, and may also be
renamed using @code{%define api.location.type @{@var{class-name}@}}.

The location class treats the position as a completely opaque value.
By default, the class name is @code{Position}, but this can be changed
with @code{%define api.position.type @{@var{class-name}@}}.  This class must
be supplied by the user.


@deftypeivar {Location} {Position} begin
@deftypeivarx {Location} {Position} end
The first, inclusive, position of the range, and the first beyond.
@end deftypeivar

@deftypeop {Constructor} {Location} {} Location (@code{Position} @var{loc})
Create a @code{Location} denoting an empty range located at a given point.
@end deftypeop

@deftypeop {Constructor} {Location} {} Location (@code{Position} @var{begin}, @code{Position} @var{end})
Create a @code{Location} from the endpoints of the range.
@end deftypeop

@deftypemethod {Location} {String} toString ()
Prints the range represented by the location.  For this to work
properly, the position class should override the @code{equals} and
@code{toString} methods appropriately.
@end deftypemethod


@node Java Parser Interface
@subsection Java Parser Interface

The name of the generated parser class defaults to @code{YYParser}.  The
@code{YY} prefix may be changed using the @samp{%define api.prefix}.
Alternatively, use @samp{%define api.parser.class @{@var{name}@}} to give a
custom name to the class.  The interface of this class is detailed below.

By default, the parser class has package visibility.  A declaration
@samp{%define api.parser.public} will change to public visibility.  Remember
that, according to the Java language specification, the name of the
@file{.java} file should match the name of the class in this case.
Similarly, you can use @code{api.parser.abstract}, @code{api.parser.final}
and @code{api.parser.strictfp} with the @code{%define} declaration to add
other modifiers to the parser class.  A single @samp{%define
api.parser.annotations @{@var{annotations}@}} directive can be used to add
any number of annotations to the parser class.

The Java package name of the parser class can be specified using the
@samp{%define package} directive.  The superclass and the implemented
interfaces of the parser class can be specified with the @code{%define
api.parser.extends} and @samp{%define api.parser.implements} directives.

The parser class defines an inner class, @code{Location}, that is used
for location tracking (see @ref{Java Location Values}), and a inner
interface, @code{Lexer} (see @ref{Java Scanner Interface}).  Other than
these inner class/interface, and the members described in the interface
below, all the other members and fields are preceded with a @code{yy} or
@code{YY} prefix to avoid clashes with user code.

The parser class can be extended using the @code{%parse-param}
directive. Each occurrence of the directive will add a @code{protected
final} field to the parser class, and an argument to its constructor,
which initialize them automatically.

@deftypeop {Constructor} {YYParser} {} YYParser (@var{lex_param}, @dots{}, @var{parse_param}, @dots{})
Build a new parser object with embedded @code{%code lexer}.  There are
no parameters, unless @code{%param}s and/or @code{%parse-param}s and/or
@code{%lex-param}s are used.

Use @code{%code init} for code added to the start of the constructor
body. This is especially useful to initialize superclasses. Use
@samp{%define init_throws} to specify any uncaught exceptions.
@end deftypeop

@deftypeop {Constructor} {YYParser} {} YYParser (@code{Lexer} @var{lexer}, @var{parse_param}, @dots{})
Build a new parser object using the specified scanner.  There are no
additional parameters unless @code{%param}s and/or @code{%parse-param}s are
used.

If the scanner is defined by @code{%code lexer}, this constructor is
declared @code{protected} and is called automatically with a scanner
created with the correct @code{%param}s and/or @code{%lex-param}s.

Use @code{%code init} for code added to the start of the constructor
body. This is especially useful to initialize superclasses. Use
@samp{%define init_throws} to specify any uncaught exceptions.
@end deftypeop

@deftypemethod {YYParser} {boolean} parse ()
Run the syntactic analysis, and return @code{true} on success,
@code{false} otherwise.
@end deftypemethod

@deftypemethod {YYParser} {boolean} getErrorVerbose ()
@deftypemethodx {YYParser} {void} setErrorVerbose (boolean @var{verbose})
Get or set the option to produce verbose error messages.  These are only
available with @samp{%define parse.error detailed} (or @samp{verbose}),
which also turns on verbose error messages.
@end deftypemethod

@deftypemethod {YYParser} {void} yyerror (@code{String} @var{msg})
@deftypemethodx {YYParser} {void} yyerror (@code{Position} @var{pos}, @code{String} @var{msg})
@deftypemethodx {YYParser} {void} yyerror (@code{Location} @var{loc}, @code{String} @var{msg})
Print an error message using the @code{yyerror} method of the scanner
instance in use. The @code{Location} and @code{Position} parameters are
available only if location tracking is active.
@end deftypemethod

@deftypemethod {YYParser} {boolean} recovering ()
During the syntactic analysis, return @code{true} if recovering
from a syntax error.
@xref{Error Recovery}.
@end deftypemethod

@deftypemethod {YYParser} {java.io.PrintStream} getDebugStream ()
@deftypemethodx {YYParser} {void} setDebugStream (@code{java.io.printStream} @var{o})
Get or set the stream used for tracing the parsing.  It defaults to
@code{System.err}.
@end deftypemethod

@deftypemethod {YYParser} {int} getDebugLevel ()
@deftypemethodx {YYParser} {void} setDebugLevel (@code{int} @var{l})
Get or set the tracing level.  Currently its value is either 0, no trace,
or nonzero, full tracing.
@end deftypemethod

@deftypecv {Constant} {YYParser} {String} {bisonVersion}
@deftypecvx {Constant} {YYParser} {String} {bisonSkeleton}
Identify the Bison version and skeleton used to generate this parser.
@end deftypecv

If you enabled token internationalization (@pxref{Token I18n}), you must
provide the parser with the following function:

@deftypecv {Static Method} {YYParser} {String} {i18n} (@code{string} @var{s})
Return the translation of @var{s} in the user's language.  As an example:

@example
%code @{
  static ResourceBundle myResources
    = ResourceBundle.getBundle("domain-name");
  static final String i18n(String s) @{
    return myResources.getString(s);
  @}
@}
@end example
@end deftypecv

@node Java Parser Context Interface
@subsection Java Parser Context Interface

The parser context provides information to build error reports when you
invoke @samp{%define parse.error custom}.

@defcv {Type} {YYParser} {SymbolKind}
An enum of all the grammar symbols, tokens and nonterminals.  Its
enumerators are forged from the symbol names:

@example
public enum SymbolKind
@{
  S_YYEOF(0),          /* "end of file"  */
  S_YYERROR(1),        /* error  */
  S_YYUNDEF(2),        /* "invalid token"  */
  S_BANG(3),           /* "!"  */
  S_PLUS(4),           /* "+"  */
  S_MINUS(5),          /* "-"  */
  [...]
  S_NUM(13),           /* "number"  */
  S_NEG(14),           /* NEG  */
  S_YYACCEPT(15),      /* $accept  */
  S_input(16),         /* input  */
  S_line(17);          /* line  */
@};
@end example
@end defcv

@deftypemethod {YYParser.SymbolKind} {String} getName ()
The name of this symbol, possibly translated.
@end deftypemethod

@deftypemethod {YYParser.Context} {YYParser.SymbolKind} getToken ()
The kind of the lookahead.  Return @code{null} iff there is no lookahead.
@end deftypemethod

@deftypemethod {YYParser.Context} {YYParser.Location} getLocation ()
The location of the lookahead.
@end deftypemethod

@deftypemethod {YYParser.Context} {int} getExpectedTokens (@code{YYParser.SymbolKind[]} @var{argv}, @code{int} @var{argc})
Fill @var{argv} with the expected tokens, which never includes
@code{SymbolKind.S_YYERROR}, or @code{SymbolKind.S_YYUNDEF}.

Never put more than @var{argc} elements into @var{argv}, and on success
return the effective number of tokens stored in @var{argv}.  Return 0 if
there are more than @var{argc} expected tokens, yet fill @var{argv} up to
@var{argc}.

If @var{argv} is null, return the size needed to store all the possible
values, which is always less than @code{YYNTOKENS}.
@end deftypemethod


@node Java Scanner Interface
@subsection Java Scanner Interface
@c - %code lexer
@c - %lex-param
@c - Lexer interface

There are two possible ways to interface a Bison-generated Java parser
with a scanner: the scanner may be defined by @code{%code lexer}, or
defined elsewhere.  In either case, the scanner has to implement the
@code{Lexer} inner interface of the parser class.  This interface also
contain constants for all user-defined token names and the predefined
@code{YYEOF} token.

In the first case, the body of the scanner class is placed in
@code{%code lexer} blocks.  If you want to pass parameters from the
parser constructor to the scanner constructor, specify them with
@code{%lex-param}; they are passed before @code{%parse-param}s to the
constructor.

In the second case, the scanner has to implement the @code{Lexer} interface,
which is defined within the parser class (e.g., @code{YYParser.Lexer}).
The constructor of the parser object will then accept an object
implementing the interface; @code{%lex-param} is not used in this
case.

In both cases, the scanner has to implement the following methods.

@deftypemethod {Lexer} {void} yyerror (@code{Location} @var{loc}, @code{String} @var{msg})
This method is defined by the user to emit an error message.  The first
parameter is omitted if location tracking is not active.  Its type can be
changed using @code{%define api.location.type @{@var{class-name}@}}.
@end deftypemethod

@deftypemethod {Lexer} {int} yylex ()
Return the next token.  Its type is the return value, its semantic
value and location are saved and returned by the their methods in the
interface.

Use @samp{%define lex_throws} to specify any uncaught exceptions.
Default is @code{java.io.IOException}.
@end deftypemethod

@deftypemethod {Lexer} {Position} getStartPos ()
@deftypemethodx {Lexer} {Position} getEndPos ()
Return respectively the first position of the last token that @code{yylex}
returned, and the first position beyond it.  These methods are not needed
unless location tracking is active.

They should return new objects for each call, to avoid that all the symbol
share the same Position boundaries.

The return type can be changed using @code{%define api.position.type
@{@var{class-name}@}}.
@end deftypemethod

@deftypemethod {Lexer} {Object} getLVal ()
Return the semantic value of the last token that yylex returned.

The return type can be changed using @samp{%define api.value.type
@{@var{class-name}@}}.
@end deftypemethod

@deftypemethod {Lexer} {void} reportSyntaxError (@code{YYParser.Context} @var{ctx})
If you invoke @samp{%define parse.error custom} (@pxref{Bison
Declarations}), then the parser no longer passes syntax error messages to
@code{yyerror}, rather it delegates that task to the user by calling the
@code{reportSyntaxError} function.

Whether it uses @code{yyerror} is up to the user.

Here is an example of a reporting function (@pxref{Java Parser Context
Interface}).

@example
public void reportSyntaxError(YYParser.Context ctx) @{
  System.err.print(ctx.getLocation() + ": syntax error");
  // Report the expected tokens.
  @{
    final int TOKENMAX = 5;
    YYParser.SymbolKind[] arg = new YYParser.SymbolKind[TOKENMAX];
    int n = ctx.getExpectedTokens(arg, TOKENMAX);
    for (int i = 0; i < n; ++i)
      System.err.print((i == 0 ? ": expected " : " or ")
                       + arg[i].getName());
  @}
  // Report the unexpected token which triggered the error.
  @{
    YYParser.SymbolKind lookahead = ctx.getToken();
    if (lookahead != null)
      System.err.print(" before " + lookahead.getName());
  @}
  System.err.println("");
@}
@end example

@noindent
This implementation is inappropriate for internationalization, see the
@file{c/bistromathic} example for a better alternative.
@end deftypemethod

@node Java Action Features
@subsection Special Features for Use in Java Actions

The following special constructs can be uses in Java actions.
Other analogous C action features are currently unavailable for Java.

Use @samp{%define throws} to specify any uncaught exceptions from parser
actions, and initial actions specified by @code{%initial-action}.

@defvar $@var{n}
The semantic value for the @var{n}th component of the current rule.
This may not be assigned to.
@xref{Java Semantic Values}.
@end defvar

@defvar $<@var{typealt}>@var{n}
Like @code{$@var{n}} but specifies a alternative type @var{typealt}.
@xref{Java Semantic Values}.
@end defvar

@defvar $$
The semantic value for the grouping made by the current rule.  As a
value, this is in the base type (@code{Object} or as specified by
@samp{%define api.value.type}) as in not cast to the declared subtype because
casts are not allowed on the left-hand side of Java assignments.
Use an explicit Java cast if the correct subtype is needed.
@xref{Java Semantic Values}.
@end defvar

@defvar $<@var{typealt}>$
Same as @code{$$} since Java always allow assigning to the base type.
Perhaps we should use this and @code{$<>$} for the value and @code{$$}
for setting the value but there is currently no easy way to distinguish
these constructs.
@xref{Java Semantic Values}.
@end defvar

@defvar @@@var{n}
The location information of the @var{n}th component of the current rule.
This may not be assigned to.
@xref{Java Location Values}.
@end defvar

@defvar @@$
The location information of the grouping made by the current rule.
@xref{Java Location Values}.
@end defvar

@deftypefn {Statement} return YYABORT @code{;}
Return immediately from the parser, indicating failure.
@xref{Java Parser Interface}.
@end deftypefn

@deftypefn {Statement} return YYACCEPT @code{;}
Return immediately from the parser, indicating success.
@xref{Java Parser Interface}.
@end deftypefn

@deftypefn {Statement} {return} YYERROR @code{;}
Start error recovery (without printing an error message).
@xref{Error Recovery}.
@end deftypefn

@deftypefn {Function} {boolean} recovering ()
Return whether error recovery is being done. In this state, the parser
reads token until it reaches a known state, and then restarts normal
operation.
@xref{Error Recovery}.
@end deftypefn

@deftypefn  {Function} {void} yyerror (@code{String} @var{msg})
@deftypefnx {Function} {void} yyerror (@code{Position} @var{loc}, @code{String} @var{msg})
@deftypefnx {Function} {void} yyerror (@code{Location} @var{loc}, @code{String} @var{msg})
Print an error message using the @code{yyerror} method of the scanner
instance in use. The @code{Location} and @code{Position} parameters are
available only if location tracking is active.
@end deftypefn

@node Java Push Parser Interface
@subsection Java Push Parser Interface
@c - define push_parse
@findex %define api.push-pull

Normally, Bison generates a pull parser for Java.
The following Bison declaration says that you want the parser to be a push
parser (@pxref{%define Summary}):

@example
%define api.push-pull push
@end example

Most of the discussion about the Java pull Parser Interface, (@pxref{Java
Parser Interface}) applies to the push parser interface as well.

When generating a push parser, the method @code{push_parse} is created with
the following signature (depending on if locations are enabled).

@deftypemethod {YYParser} {void} push_parse (@code{int} @var{token}, @code{Object} @var{yylval})
@deftypemethodx {YYParser} {void} push_parse (@code{int} @var{token}, @code{Object} @var{yylval}, @code{Location} @var{yyloc})
@deftypemethodx {YYParser} {void} push_parse (@code{int} @var{token}, @code{Object} @var{yylval}, @code{Position} @var{yypos})
@end deftypemethod

The primary difference with respect to a pull parser is that the parser
method @code{push_parse} is invoked repeatedly to parse each token.  This
function is available if either the "%define api.push-pull push" or "%define
api.push-pull both" declaration is used (@pxref{%define
Summary}).  The @code{Location} and @code{Position}
parameters are available only if location tracking is active.

The value returned by the @code{push_parse} method is one of the following
four constants: @code{YYABORT}, @code{YYACCEPT}, @code{YYERROR}, or
@code{YYPUSH_MORE}.  This new value, @code{YYPUSH_MORE}, may be returned if
more input is required to finish parsing the grammar.

If api.push-pull is declared as @code{both}, then the generated parser class
will also implement the @code{parse} method. This method's body is a loop
that repeatedly invokes the scanner and then passes the values obtained from
the scanner to the @code{push_parse} method.

There is one additional complication.  Technically, the push parser does not
need to know about the scanner (i.e. an object implementing the
@code{YYParser.Lexer} interface), but it does need access to the
@code{yyerror} method.  Currently, the @code{yyerror} method is defined in
the @code{YYParser.Lexer} interface. Hence, an implementation of that
interface is still required in order to provide an implementation of
@code{yyerror}.  The current approach (and subject to change) is to require
the @code{YYParser} constructor to be given an object implementing the
@code{YYParser.Lexer} interface. This object need only implement the
@code{yyerror} method; the other methods can be stubbed since they will
never be invoked.  The simplest way to do this is to add a trivial scanner
implementation to your grammar file using whatever implementation of
@code{yyerror} is desired. The following code sample shows a simple way to
accomplish this.

@example
%code lexer
@{
  public Object getLVal () @{return null;@}
  public int yylex () @{return 0;@}
  public void yyerror (String s) @{System.err.println(s);@}
@}
@end example

@node Java Differences
@subsection Differences between C/C++ and Java Grammars

The different structure of the Java language forces several differences
between C/C++ grammars, and grammars designed for Java parsers.  This
section summarizes these differences.

@itemize
@item
Java lacks a preprocessor, so the @code{YYERROR}, @code{YYACCEPT},
@code{YYABORT} symbols (@pxref{Table of Symbols}) cannot obviously be
macros.  Instead, they should be preceded by @code{return} when they
appear in an action.  The actual definition of these symbols is
opaque to the Bison grammar, and it might change in the future.  The
only meaningful operation that you can do, is to return them.
@xref{Java Action Features}.

Note that of these three symbols, only @code{YYACCEPT} and
@code{YYABORT} will cause a return from the @code{yyparse}
method@footnote{Java parsers include the actions in a separate
method than @code{yyparse} in order to have an intuitive syntax that
corresponds to these C macros.}.

@item
Java lacks unions, so @code{%union} has no effect.  Instead, semantic
values have a common base type: @code{Object} or as specified by
@samp{%define api.value.type}.  Angle brackets on @code{%token}, @code{type},
@code{$@var{n}} and @code{$$} specify subtypes rather than fields of
an union.  The type of @code{$$}, even with angle brackets, is the base
type since Java casts are not allow on the left-hand side of assignments.
Also, @code{$@var{n}} and @code{@@@var{n}} are not allowed on the
left-hand side of assignments. @xref{Java Semantic Values}, and
@ref{Java Action Features}.

@item
The prologue declarations have a different meaning than in C/C++ code.
@table @asis
@item @code{%code imports}
blocks are placed at the beginning of the Java source code.  They may
include copyright notices.  For a @code{package} declarations, it is
suggested to use @samp{%define package} instead.

@item unqualified @code{%code}
blocks are placed inside the parser class.

@item @code{%code lexer}
blocks, if specified, should include the implementation of the
scanner.  If there is no such block, the scanner can be any class
that implements the appropriate interface (@pxref{Java Scanner
Interface}).
@end table

Other @code{%code} blocks are not supported in Java parsers.
In particular, @code{%@{ @dots{} %@}} blocks should not be used
and may give an error in future versions of Bison.

The epilogue has the same meaning as in C/C++ code and it can
be used to define other classes used by the parser @emph{outside}
the parser class.
@end itemize


@node Java Declarations Summary
@subsection Java Declarations Summary

This summary only include declarations specific to Java or have special
meaning when used in a Java parser.

@deffn {Directive} {%language "Java"}
Generate a Java class for the parser.
@end deffn

@deffn {Directive} %lex-param @{@var{type} @var{name}@}
A parameter for the lexer class defined by @code{%code lexer}
@emph{only}, added as parameters to the lexer constructor and the parser
constructor that @emph{creates} a lexer.  Default is none.
@xref{Java Scanner Interface}.
@end deffn

@deffn {Directive} %parse-param @{@var{type} @var{name}@}
A parameter for the parser class added as parameters to constructor(s)
and as fields initialized by the constructor(s).  Default is none.
@xref{Java Parser Interface}.
@end deffn

@deffn {Directive} %token <@var{type}> @var{token} @dots{}
Declare tokens.  Note that the angle brackets enclose a Java @emph{type}.
@xref{Java Semantic Values}.
@end deffn

@deffn {Directive} %nterm <@var{type}> @var{nonterminal} @dots{}
Declare the type of nonterminals.  Note that the angle brackets enclose
a Java @emph{type}.
@xref{Java Semantic Values}.
@end deffn

@deffn {Directive} %code @{ @var{code} @dots{} @}
Code appended to the inside of the parser class.
@xref{Java Differences}.
@end deffn

@deffn {Directive} {%code imports} @{ @var{code} @dots{} @}
Code inserted just after the @code{package} declaration.
@xref{Java Differences}.
@end deffn

@deffn {Directive} {%code init} @{ @var{code} @dots{} @}
Code inserted at the beginning of the parser constructor body.
@xref{Java Parser Interface}.
@end deffn

@deffn {Directive} {%code lexer} @{ @var{code} @dots{} @}
Code added to the body of a inner lexer class within the parser class.
@xref{Java Scanner Interface}.
@end deffn

@deffn {Directive} %% @var{code} @dots{}
Code (after the second @code{%%}) appended to the end of the file,
@emph{outside} the parser class.
@xref{Java Differences}.
@end deffn

@deffn {Directive} %@{ @var{code} @dots{} %@}
Not supported.  Use @code{%code imports} instead.
@xref{Java Differences}.
@end deffn

@deffn {Directive} {%define api.prefix} @{@var{prefix}@}
The prefix of the parser class name @code{@var{prefix}Parser} if
@samp{%define api.parser.class} is not used.  Default is @code{YY}.
@xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define api.parser.abstract}
Whether the parser class is declared @code{abstract}.  Default is false.
@xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define api.parser.annotations} @{@var{annotations}@}
The Java annotations for the parser class.  Default is none.
@xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define api.parser.class} @{@var{name}@}
The name of the parser class.  Default is @code{YYParser} or
@code{@var{api.prefix}Parser}.  @xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define api.parser.extends} @{@var{superclass}@}
The superclass of the parser class.  Default is none.
@xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define api.parser.final}
Whether the parser class is declared @code{final}.  Default is false.
@xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define api.parser.implements} @{@var{interfaces}@}
The implemented interfaces of the parser class, a comma-separated list.
Default is none.
@xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define api.parser.public}
Whether the parser class is declared @code{public}.  Default is false.
@xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define api.parser.strictfp}
Whether the parser class is declared @code{strictfp}.  Default is false.
@xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define init_throws} @{@var{exceptions}@}
The exceptions thrown by @code{%code init} from the parser class
constructor.  Default is none.
@xref{Java Parser Interface}.
@end deffn

@deffn {Directive} {%define lex_throws} @{@var{exceptions}@}
The exceptions thrown by the @code{yylex} method of the lexer, a
comma-separated list.  Default is @code{java.io.IOException}.
@xref{Java Scanner Interface}.
@end deffn

@deffn {Directive} {%define api.location.type} @{@var{class}@}
The name of the class used for locations (a range between two
positions).  This class is generated as an inner class of the parser
class by @command{bison}.  Default is @code{Location}.
Formerly named @code{location_type}.
@xref{Java Location Values}.
@end deffn

@deffn {Directive} {%define package} @{@var{package}@}
The package to put the parser class in.  Default is none.
@xref{Java Bison Interface}.
@end deffn

@deffn {Directive} {%define api.position.type} @{@var{class}@}
The name of the class used for positions. This class must be supplied by
the user.  Default is @code{Position}.
Formerly named @code{position_type}.
@xref{Java Location Values}.
@end deffn

@deffn {Directive} {%define api.value.type} @{@var{class}@}
The base type of semantic values.  Default is @code{Object}.
@xref{Java Semantic Values}.
@end deffn

@deffn {Directive} {%define throws} @{@var{exceptions}@}
The exceptions thrown by user-supplied parser actions and
@code{%initial-action}, a comma-separated list.  Default is none.
@xref{Java Parser Interface}.
@end deffn

@c ================================================= History

@node History
@chapter A Brief History of the Greater Ungulates
@cindex history
@cindex ungulates

@menu
* Yacc::                        The original Yacc
* yacchack::                    An obscure early implementation of reentrancy
* Byacc::                       Berkeley Yacc
* Bison::                       This program
* Other Ungulates::             Similar programs
@end menu

@node Yacc
@section The ancestral Yacc

Bison originated as a workalike of a program called Yacc --- Yet Another
Compiler Compiler.@footnote{Because of the acronym, the name is sometimes
given as ``YACC'', but Johnson used ``Yacc'' in the descriptive paper
included in the
@url{https://s3.amazonaws.com/plan9-bell-labs/7thEdMan/v7vol2b.pdf, Version
7 Unix Manual}.} Yacc was written at Bell Labs as part of the very early
development of Unix; one of its first uses was to develop the original
Portable C Compiler, pcc. The same person, Steven C. Johnson, wrote Yacc and
the original pcc.

According to the author
@footnote{@url{https://lists.gnu.org/archive/html/bison-patches/2019-02/msg00061.html}},
Yacc was first invented in 1971 and reached a form recognizably similar to
the C version in 1973.  Johnson published @cite{A Portable Compiler: Theory
and Practice} (@pxref{Bibliography}).

Yacc was not itself originally written in C but in its predecessor language,
B.  This goes far to explain its odd interface, which exposes a large number
of global variables rather than bundling them into a C struct.  All other
Yacc-like programs are descended from the C port of Yacc.

Yacc, through both its deployment in pcc and as a standalone tool for
generating other parsers, helped drive the early spread of Unix.  Yacc
itself, however, passed out of use after around 1990 when workalikes
with less restrictive licenses and more features became available.

Original Yacc became generally available when Caldera released the sources
of old versions of Unix up to V7 and 32V in 2002.  By that time it had been
long superseded in practical use by Bison even on Yacc's native Unix
variants.

@node yacchack
@section yacchack
@cindex yacchack

One of the deficiencies of original Yacc was its inability to produce
reentrant parsers.  This was first remedied by a set of drop-in
modifications called ``yacchack'', published by Eric S. Raymond on USENET
around 1983.  This code was quickly forgotten when zoo and Berkeley Yacc
became available a few years later.

@node Byacc
@section Berkeley Yacc
@cindex byacc

Berkeley Yacc was originated in 1985 by Robert Corbett
(@pxref{Bibliography}).  It was originally named ``zoo'', but
by October 1989 it became known as Berkeley Yacc or byacc.

Berkeley Yacc had three advantages over the ancestral Yacc: it generated
faster parsers, it could generate reentrant parsers, and the source code
was released to the public domain rather than being under an AT&T
proprietary license.  The better performance came from implementing
techniques from DeRemer and Penello's seminal paper on LALR parsing
(@pxref{Bibliography}).

Use of byacc spread rapidly due to its public domain license. However, once
Bison became available, byacc itself passed out of general use.

@node Bison
@section Bison
@cindex zoo

Robert Corbett actually wrote two (closely related) LALR parsers in 1985,
both using the DeRemer/Penello techniques. One was ``zoo'', the other was
``Byson''. In 1987 Richard Stallman began working on Byson; the name changed
to Bison and the interface became Yacc-compatible.

The main visible difference between Yacc and Byson/Bison at the time of
Byson's first release is that Byson supported the @code{@@@var{n}} construct
(giving access to the starting and ending line number and character number
associated with any of the symbols in the current rule).

There was also the command @samp{%expect @var{n}} which said not to mention the
conflicts if there are @var{n} shift/reduce conflicts and no reduce/reduce
conflicts.  In more recent versions of Bison, @code{%expect} and its
@code{%expect-rr} variant for reduce-reduce conflicts can be applied to
individual rules.

Later versions of Bison added many more new features.

Bison error reporting has been improved in various ways. Notably. ancestral
Yacc and Byson did not have carets in error messages.

Compared to Yacc Bison uses a faster but less space-efficient encoding for
the parse tables (@pxref{Bibliography}), and more modern techniques for
generating the lookahead sets (@pxref{Bibliography}).  This approach is the
standard one since then.

(It has also been plausibly alleged the differences in the algorithms stem
mainly from the horrible kludges that Johnson had to perpetrate to make
the original Yacc fit in a PDP-11.)

Named references, semantic predicates, @code{%locations},
@code{%glr-parser}, @code{%printer}, %destructor, dumps to DOT,
@code{%parse-param}, @code{%lex-param}, and dumps to XSLT, LAC, and IELR(1)
generation are new in Bison.

Bison also has many features to support C++ that were not present in the
ancestral Yacc or Byson.

Bison obsolesced all previous Yacc variants and workalikes generating C by
1995.

@node Other Ungulates
@section Other Ungulates

The Yacc concept has frequently been ported to other languages. Some of the
early ports are extinct along with the languages that hosted them; others
have been superseded by parser skeletons shipped with Bison.

However, independent implementations persist. One of the best-known
still in use is David Beazley's ``PLY'' (Python Lex-Yacc) for
Python. Another is goyacc, supporting the Go language. An ``ocamlyacc''
is shipped as part of the Ocaml compiler suite.

@c ================================================= FAQ

@node FAQ
@chapter Frequently Asked Questions
@cindex frequently asked questions
@cindex questions

Several questions about Bison come up occasionally.  Here some of them
are addressed.

@menu
* Memory Exhausted::            Breaking the Stack Limits
* How Can I Reset the Parser::  @code{yyparse} Keeps some State
* Strings are Destroyed::       @code{yylval} Loses Track of Strings
* Implementing Gotos/Loops::    Control Flow in the Calculator
* Multiple start-symbols::      Factoring closely related grammars
* Secure?  Conform?::           Is Bison POSIX safe?
* Enabling Relocatability::     Moving Bison/using it through network shares
* I can't build Bison::         Troubleshooting
* Where can I find help?::      Troubleshouting
* Bug Reports::                 Troublereporting
* More Languages::              Parsers in C++, Java, and so on
* Beta Testing::                Experimenting development versions
* Mailing Lists::               Meeting other Bison users
@end menu

@node Memory Exhausted
@section Memory Exhausted

@quotation
My parser returns with error with a @samp{memory exhausted}
message.  What can I do?
@end quotation

This question is already addressed elsewhere, see @ref{Recursion}.

@node How Can I Reset the Parser
@section How Can I Reset the Parser

The following phenomenon has several symptoms, resulting in the
following typical questions:

@quotation
I invoke @code{yyparse} several times, and on correct input it works
properly; but when a parse error is found, all the other calls fail
too.  How can I reset the error flag of @code{yyparse}?
@end quotation

@noindent
or

@quotation
My parser includes support for an @samp{#include}-like feature, in which
case I run @code{yyparse} from @code{yyparse}.  This fails although I did
specify @samp{%define api.pure full}.
@end quotation

These problems typically come not from Bison itself, but from
Lex-generated scanners.  Because these scanners use large buffers for
speed, they might not notice a change of input file.  As a
demonstration, consider the following source file,
@file{first-line.l}:

@example
@group
%@{
#include <stdio.h>
#include <stdlib.h>
%@}
@end group
%%
.*\n    ECHO; return 1;
%%
@group
int
yyparse (char const *file)
@{
  yyin = fopen (file, "r");
  if (!yyin)
    @{
      perror ("fopen");
      exit (EXIT_FAILURE);
    @}
@end group
@group
  /* One token only. */
  yylex ();
  if (fclose (yyin) != 0)
    @{
      perror ("fclose");
      exit (EXIT_FAILURE);
    @}
  return 0;
@}
@end group

@group
int
main (void)
@{
  yyparse ("input");
  yyparse ("input");
  return 0;
@}
@end group
@end example

@noindent
If the file @file{input} contains

@example
input:1: Hello,
input:2: World!
@end example

@noindent
then instead of getting the first line twice, you get:

@example
$ @kbd{flex -ofirst-line.c first-line.l}
$ @kbd{gcc  -ofirst-line   first-line.c -ll}
$ @kbd{./first-line}
input:1: Hello,
input:2: World!
@end example

Therefore, whenever you change @code{yyin}, you must tell the
Lex-generated scanner to discard its current buffer and switch to the
new one.  This depends upon your implementation of Lex; see its
documentation for more.  For Flex, it suffices to call
@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}.  If your
Flex-generated scanner needs to read from several input streams to
handle features like include files, you might consider using Flex
functions like @samp{yy_switch_to_buffer} that manipulate multiple
input buffers.

If your Flex-generated scanner uses start conditions (@pxref{Start
conditions, , Start conditions, flex, The Flex Manual}), you might
also want to reset the scanner's state, i.e., go back to the initial
start condition, through a call to @samp{BEGIN (0)}.

@node Strings are Destroyed
@section Strings are Destroyed

@quotation
My parser seems to destroy old strings, or maybe it loses track of
them.  Instead of reporting @samp{"foo", "bar"}, it reports
@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}.
@end quotation

This error is probably the single most frequent ``bug report'' sent to
Bison lists, but is only concerned with a misunderstanding of the role
of the scanner.  Consider the following Lex code:

@example
@group
%@{
#include <stdio.h>
char *yylval = NULL;
%@}
@end group
@group
%%
.*    yylval = yytext; return 1;
\n    continue;
%%
@end group
@group
int
main ()
@{
  /* Similar to using $1, $2 in a Bison action. */
  char *fst = (yylex (), yylval);
  char *snd = (yylex (), yylval);
  printf ("\"%s\", \"%s\"\n", fst, snd);
  return 0;
@}
@end group
@end example

If you compile and run this code, you get:

@example
$ @kbd{flex -osplit-lines.c split-lines.l}
$ @kbd{gcc  -osplit-lines   split-lines.c -ll}
$ @kbd{printf 'one\ntwo\n' | ./split-lines}
"one
two", "two"
@end example

@noindent
this is because @code{yytext} is a buffer provided for @emph{reading}
in the action, but if you want to keep it, you have to duplicate it
(e.g., using @code{strdup}).  Note that the output may depend on how
your implementation of Lex handles @code{yytext}.  For instance, when
given the Lex compatibility option @option{-l} (which triggers the
option @samp{%array}) Flex generates a different behavior:

@example
$ @kbd{flex -l -osplit-lines.c split-lines.l}
$ @kbd{gcc     -osplit-lines   split-lines.c -ll}
$ @kbd{printf 'one\ntwo\n' | ./split-lines}
"two", "two"
@end example


@node Implementing Gotos/Loops
@section Implementing Gotos/Loops

@quotation
My simple calculator supports variables, assignments, and functions,
but how can I implement gotos, or loops?
@end quotation

Although very pedagogical, the examples included in the document blur
the distinction to make between the parser---whose job is to recover
the structure of a text and to transmit it to subsequent modules of
the program---and the processing (such as the execution) of this
structure.  This works well with so called straight line programs,
i.e., precisely those that have a straightforward execution model:
execute simple instructions one after the others.

@cindex abstract syntax tree
@cindex AST
If you want a richer model, you will probably need to use the parser
to construct a tree that does represent the structure it has
recovered; this tree is usually called the @dfn{abstract syntax tree},
or @dfn{AST} for short.  Then, walking through this tree,
traversing it in various ways, will enable treatments such as its
execution or its translation, which will result in an interpreter or a
compiler.

This topic is way beyond the scope of this manual, and the reader is
invited to consult the dedicated literature.


@node Multiple start-symbols
@section Multiple start-symbols

@quotation
I have several closely related grammars, and I would like to share their
implementations.  In fact, I could use a single grammar but with
multiple entry points.
@end quotation

Bison does not support multiple start-symbols, but there is a very
simple means to simulate them.  If @code{foo} and @code{bar} are the two
pseudo start-symbols, then introduce two new tokens, say
@code{START_FOO} and @code{START_BAR}, and use them as switches from the
real start-symbol:

@example
%token START_FOO START_BAR;
%start start;
start:
  START_FOO foo
| START_BAR bar;
@end example

These tokens prevents the introduction of new conflicts.  As far as the
parser goes, that is all that is needed.

Now the difficult part is ensuring that the scanner will send these
tokens first.  If your scanner is hand-written, that should be
straightforward.  If your scanner is generated by Lex, them there is
simple means to do it: recall that anything between @samp{%@{ ... %@}}
after the first @code{%%} is copied verbatim in the top of the generated
@code{yylex} function.  Make sure a variable @code{start_token} is
available in the scanner (e.g., a global variable or using
@code{%lex-param} etc.), and use the following:

@example
  /* @r{Prologue.} */
%%
%@{
  if (start_token)
    @{
      int t = start_token;
      start_token = 0;
      return t;
    @}
%@}
  /* @r{The rules.} */
@end example


@node Secure?  Conform?
@section Secure?  Conform?

@quotation
Is Bison secure?  Does it conform to POSIX?
@end quotation

If you're looking for a guarantee or certification, we don't provide it.
However, Bison is intended to be a reliable program that conforms to the
POSIX specification for Yacc.  If you run into problems, please send us a
bug report.

@include relocatable.texi

@node I can't build Bison
@section I can't build Bison

@quotation
I can't build Bison because @command{make} complains that
@code{msgfmt} is not found.
What should I do?
@end quotation

Like most GNU packages with internationalization support, that feature
is turned on by default.  If you have problems building in the @file{po}
subdirectory, it indicates that your system's internationalization
support is lacking.  You can re-configure Bison with
@option{--disable-nls} to turn off this support, or you can install GNU
gettext from @url{https://ftp.gnu.org/gnu/gettext/} and re-configure
Bison.  See the file @file{ABOUT-NLS} for more information.

@quotation
I can't build Bison because my C compiler is too old.
@end quotation

Except for GLR parsers (@pxref{Compiler Requirements for GLR}), the C
code that Bison generates requires only C89 or later.  However, Bison
itself requires common C99 features such as declarations after
statements.  Bison's @code{configure} script attempts to enable C99 (or
later) support on compilers that default to pre-C99.  If your compiler
lacks these C99 features entirely, GCC may well be a better choice; or
you can try upgrading to your compiler's latest version.

@node Where can I find help?
@section Where can I find help?

@quotation
I'm having trouble using Bison.  Where can I find help?
@end quotation

First, read this fine manual.  Beyond that, you can send mail to
@email{help-bison@@gnu.org}.  This mailing list is intended to be
populated with people who are willing to answer questions about using
and installing Bison.  Please keep in mind that (most of) the people on
the list have aspects of their lives which are not related to Bison (!),
so you may not receive an answer to your question right away.  This can
be frustrating, but please try not to honk them off; remember that any
help they provide is purely voluntary and out of the kindness of their
hearts.

@node Bug Reports
@section Bug Reports

@quotation
I found a bug.  What should I include in the bug report?
@end quotation

Before sending a bug report, make sure you are using the latest
version.  Check @url{https://ftp.gnu.org/pub/gnu/bison/} or one of its
mirrors.  Be sure to include the version number in your bug report.  If
the bug is present in the latest version but not in a previous version,
try to determine the most recent version which did not contain the bug.

If the bug is parser-related, you should include the smallest grammar
you can which demonstrates the bug.  The grammar file should also be
complete (i.e., I should be able to run it through Bison without having
to edit or add anything).  The smaller and simpler the grammar, the
easier it will be to fix the bug.

Include information about your compilation environment, including your
operating system's name and version and your compiler's name and
version.  If you have trouble compiling, you should also include a
transcript of the build session, starting with the invocation of
@code{configure}.  Depending on the nature of the bug, you may be asked to
send additional files as well (such as @file{config.h} or @file{config.cache}).

Patches are most welcome, but not required.  That is, do not hesitate to
send a bug report just because you cannot provide a fix.

Send bug reports to @email{bug-bison@@gnu.org}.

@node More Languages
@section More Languages

@quotation
Will Bison ever have C++ and Java support?  How about @var{insert your
favorite language here}?
@end quotation

C++ and Java support is there now, and is documented.  We'd love to add other
languages; contributions are welcome.

@node Beta Testing
@section Beta Testing

@quotation
What is involved in being a beta tester?
@end quotation

It's not terribly involved.  Basically, you would download a test
release, compile it, and use it to build and run a parser or two.  After
that, you would submit either a bug report or a message saying that
everything is okay.  It is important to report successes as well as
failures because test releases eventually become mainstream releases,
but only if they are adequately tested.  If no one tests, development is
essentially halted.

Beta testers are particularly needed for operating systems to which the
developers do not have easy access.  They currently have easy access to
recent GNU/Linux and Solaris versions.  Reports about other operating
systems are especially welcome.

@node Mailing Lists
@section Mailing Lists

@quotation
How do I join the help-bison and bug-bison mailing lists?
@end quotation

See @url{http://lists.gnu.org/}.

@c ================================================= Table of Symbols

@node Table of Symbols
@appendix Bison Symbols
@cindex Bison symbols, table of
@cindex symbols in Bison, table of

@deffn {Variable} @@$
In an action, the location of the left-hand side of the rule.
@xref{Tracking Locations}.
@end deffn

@deffn {Variable} @@@var{n}
@deffnx {Symbol} @@@var{n}
In an action, the location of the @var{n}-th symbol of the right-hand side
of the rule.  @xref{Tracking Locations}.

In a grammar, the Bison-generated nonterminal symbol for a midrule action
with a semantic value.  @xref{Midrule Action Translation}.
@end deffn

@deffn {Variable} @@@var{name}
@deffnx {Variable} @@[@var{name}]
In an action, the location of a symbol addressed by @var{name}.
@xref{Tracking Locations}.
@end deffn

@deffn {Symbol} $@@@var{n}
In a grammar, the Bison-generated nonterminal symbol for a midrule action
with no semantics value.  @xref{Midrule Action Translation}.
@end deffn

@deffn {Variable} $$
In an action, the semantic value of the left-hand side of the rule.
@xref{Actions}.
@end deffn

@deffn {Variable} $@var{n}
In an action, the semantic value of the @var{n}-th symbol of the
right-hand side of the rule.  @xref{Actions}.
@end deffn

@deffn {Variable} $@var{name}
@deffnx {Variable} $[@var{name}]
In an action, the semantic value of a symbol addressed by @var{name}.
@xref{Actions}.
@end deffn

@deffn {Delimiter} %%
Delimiter used to separate the grammar rule section from the
Bison declarations section or the epilogue.
@xref{Grammar Layout}.
@end deffn

@c Don't insert spaces, or check the DVI output.
@deffn {Delimiter} %@{@var{code}%@}
All code listed between @samp{%@{} and @samp{%@}} is copied verbatim
to the parser implementation file.  Such code forms the prologue of
the grammar file.  @xref{Grammar Outline}.
@end deffn

@deffn {Directive} %?@{@var{expression}@}
Predicate actions.  This is a type of action clause that may appear in
rules. The expression is evaluated, and if false, causes a syntax error.  In
GLR parsers during nondeterministic operation,
this silently causes an alternative parse to die.  During deterministic
operation, it is the same as the effect of YYERROR.
@xref{Semantic Predicates}.
@end deffn

@deffn {Construct} /* @dots{} */
@deffnx {Construct} // @dots{}
Comments, as in C/C++.
@end deffn

@deffn {Delimiter} :
Separates a rule's result from its components.  @xref{Rules}.
@end deffn

@deffn {Delimiter} ;
Terminates a rule.  @xref{Rules}.
@end deffn

@deffn {Delimiter} |
Separates alternate rules for the same result nonterminal.
@xref{Rules}.
@end deffn

@deffn {Directive} <*>
Used to define a default tagged @code{%destructor} or default tagged
@code{%printer}.

@xref{Destructor Decl}.
@end deffn

@deffn {Directive} <>
Used to define a default tagless @code{%destructor} or default tagless
@code{%printer}.

@xref{Destructor Decl}.
@end deffn

@deffn {Symbol} $accept
The predefined nonterminal whose only rule is @samp{$accept: @var{start}
$end}, where @var{start} is the start symbol.  @xref{Start Decl}.  It cannot
be used in the grammar.
@end deffn

@deffn {Directive} %code @{@var{code}@}
@deffnx {Directive} %code @var{qualifier} @{@var{code}@}
Insert @var{code} verbatim into the output parser source at the
default location or at the location specified by @var{qualifier}.
@xref{%code Summary}.
@end deffn

@deffn {Directive} %debug
Equip the parser for debugging.  @xref{Decl Summary}.
@end deffn

@ifset defaultprec
@deffn {Directive} %default-prec
Assign a precedence to rules that lack an explicit @samp{%prec}
modifier.  @xref{Contextual Precedence}.
@end deffn
@end ifset

@deffn {Directive} %define @var{variable}
@deffnx {Directive} %define @var{variable} @var{value}
@deffnx {Directive} %define @var{variable} @{@var{value}@}
@deffnx {Directive} %define @var{variable} "@var{value}"
Define a variable to adjust Bison's behavior.  @xref{%define Summary}.
@end deffn

@deffn {Directive} %defines
Bison declaration to create a parser header file, which is usually
meant for the scanner.  @xref{Decl Summary}.
@end deffn

@deffn {Directive} %defines @var{defines-file}
Same as above, but save in the file @var{defines-file}.
@xref{Decl Summary}.
@end deffn

@deffn {Directive} %destructor
Specify how the parser should reclaim the memory associated to
discarded symbols.  @xref{Destructor Decl}.
@end deffn

@deffn {Directive} %dprec
Bison declaration to assign a precedence to a rule that is used at parse
time to resolve reduce/reduce conflicts.  @xref{GLR Parsers}.
@end deffn

@deffn {Directive} %empty
Bison declaration to declare make explicit that a rule has an empty
right-hand side.  @xref{Empty Rules}.
@end deffn

@deffn {Symbol} $end
The predefined token marking the end of the token stream.  It cannot be
used in the grammar.
@end deffn

@deffn {Symbol} error
A token name reserved for error recovery.  This token may be used in
grammar rules so as to allow the Bison parser to recognize an error in
the grammar without halting the process.  In effect, a sentence
containing an error may be recognized as valid.  On a syntax error, the
token @code{error} becomes the current lookahead token.  Actions
corresponding to @code{error} are then executed, and the lookahead
token is reset to the token that originally caused the violation.
@xref{Error Recovery}.
@end deffn

@deffn {Directive} %error-verbose
An obsolete directive standing for @samp{%define parse.error verbose}.
@end deffn

@deffn {Directive} %file-prefix "@var{prefix}"
Bison declaration to set the prefix of the output files.  @xref{Decl
Summary}.
@end deffn

@deffn {Directive} %glr-parser
Bison declaration to produce a GLR parser.  @xref{GLR
Parsers}.
@end deffn

@deffn {Directive} %initial-action
Run user code before parsing.  @xref{Initial Action Decl}.
@end deffn

@deffn {Directive} %language
Specify the programming language for the generated parser.
@xref{Decl Summary}.
@end deffn

@deffn {Directive} %left
Bison declaration to assign precedence and left associativity to token(s).
@xref{Precedence Decl}.
@end deffn

@deffn {Directive} %lex-param @{@var{argument-declaration}@} @dots{}
Bison declaration to specifying additional arguments that
@code{yylex} should accept.  @xref{Pure Calling}.
@end deffn

@deffn {Directive} %merge
Bison declaration to assign a merging function to a rule.  If there is a
reduce/reduce conflict with a rule having the same merging function, the
function is applied to the two semantic values to get a single result.
@xref{GLR Parsers}.
@end deffn

@deffn {Directive} %name-prefix "@var{prefix}"
Obsoleted by the @code{%define} variable @code{api.prefix} (@pxref{Multiple
Parsers}).

Rename the external symbols (variables and functions) used in the parser so
that they start with @var{prefix} instead of @samp{yy}.  Contrary to
@code{api.prefix}, do no rename types and macros.

The precise list of symbols renamed in C parsers is @code{yyparse},
@code{yylex}, @code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yychar},
@code{yydebug}, and (if locations are used) @code{yylloc}.  If you use a
push parser, @code{yypush_parse}, @code{yypull_parse}, @code{yypstate},
@code{yypstate_new} and @code{yypstate_delete} will also be renamed.  For
example, if you use @samp{%name-prefix "c_"}, the names become
@code{c_parse}, @code{c_lex}, and so on.  For C++ parsers, see the
@code{%define api.namespace} documentation in this section.
@end deffn


@ifset defaultprec
@deffn {Directive} %no-default-prec
Do not assign a precedence to rules that lack an explicit @samp{%prec}
modifier.  @xref{Contextual Precedence}.
@end deffn
@end ifset

@deffn {Directive} %no-lines
Bison declaration to avoid generating @code{#line} directives in the
parser implementation file.  @xref{Decl Summary}.
@end deffn

@deffn {Directive} %nonassoc
Bison declaration to assign precedence and nonassociativity to token(s).
@xref{Precedence Decl}.
@end deffn

@deffn {Directive} %nterm
Bison declaration to declare nonterminals.  @xref{Type Decl}.
@end deffn

@deffn {Directive} %output "@var{file}"
Bison declaration to set the name of the parser implementation file.
@xref{Decl Summary}.
@end deffn

@deffn {Directive} %param @{@var{argument-declaration}@} @dots{}
Bison declaration to specify additional arguments that both
@code{yylex} and @code{yyparse} should accept.  @xref{Parser Function}.
@end deffn

@deffn {Directive} %parse-param @{@var{argument-declaration}@} @dots{}
Bison declaration to specify additional arguments that @code{yyparse}
should accept.  @xref{Parser Function}.
@end deffn

@deffn {Directive} %prec
Bison declaration to assign a precedence to a specific rule.
@xref{Contextual Precedence}.
@end deffn

@deffn {Directive} %precedence
Bison declaration to assign precedence to token(s), but no associativity
@xref{Precedence Decl}.
@end deffn

@deffn {Directive} %pure-parser
Deprecated version of @samp{%define api.pure} (@pxref{%define
Summary}), for which Bison is more careful to warn about
unreasonable usage.
@end deffn

@deffn {Directive} %require "@var{version}"
Require version @var{version} or higher of Bison.  @xref{Require Decl}.
@end deffn

@deffn {Directive} %right
Bison declaration to assign precedence and right associativity to token(s).
@xref{Precedence Decl}.
@end deffn

@deffn {Directive} %skeleton
Specify the skeleton to use; usually for development.
@xref{Decl Summary}.
@end deffn

@deffn {Directive} %start
Bison declaration to specify the start symbol.  @xref{Start Decl}.
@end deffn

@deffn {Directive} %token
Bison declaration to declare token(s) without specifying precedence.
@xref{Token Decl}.
@end deffn

@deffn {Directive} %token-table
Bison declaration to include a token name table in the parser implementation
file.  @xref{Decl Summary}.
@end deffn

@deffn {Directive} %type
Bison declaration to declare symbol value types.  @xref{Type Decl}.
@end deffn

@deffn {Symbol} $undefined
The predefined token onto which all undefined values returned by
@code{yylex} are mapped.  It cannot be used in the grammar, rather, use
@code{error}.
@end deffn

@deffn {Directive} %union
Bison declaration to specify several possible data types for semantic
values.  @xref{Union Decl}.
@end deffn

@deffn {Macro} YYABORT
Macro to pretend that an unrecoverable syntax error has occurred, by making
@code{yyparse} return 1 immediately.  The error reporting function
@code{yyerror} is not called.  @xref{Parser Function}.

For Java parsers, this functionality is invoked using @code{return YYABORT;}
instead.
@end deffn

@deffn {Macro} YYACCEPT
Macro to pretend that a complete utterance of the language has been
read, by making @code{yyparse} return 0 immediately.
@xref{Parser Function}.

For Java parsers, this functionality is invoked using @code{return YYACCEPT;}
instead.
@end deffn

@deffn {Macro} YYBACKUP
Macro to discard a value from the parser stack and fake a lookahead
token.  @xref{Action Features}.
@end deffn

@deffn {Variable} yychar
External integer variable that contains the integer value of the
lookahead token.  (In a pure parser, it is a local variable within
@code{yyparse}.)  Error-recovery rule actions may examine this variable.
@xref{Action Features}.
@end deffn

@deffn {Variable} yyclearin
Macro used in error-recovery rule actions.  It clears the previous
lookahead token.  @xref{Error Recovery}.
@end deffn

@deffn {Macro} YYDEBUG
Macro to define to equip the parser with tracing code.  @xref{Tracing}.
@end deffn

@deffn {Variable} yydebug
External integer variable set to zero by default.  If @code{yydebug}
is given a nonzero value, the parser will output information on input
symbols and parser action.  @xref{Tracing}.
@end deffn

@deffn {Macro} yyerrok
Macro to cause parser to recover immediately to its normal mode
after a syntax error.  @xref{Error Recovery}.
@end deffn

@deffn {Macro} YYERROR
Cause an immediate syntax error.  This statement initiates error
recovery just as if the parser itself had detected an error; however, it
does not call @code{yyerror}, and does not print any message.  If you
want to print an error message, call @code{yyerror} explicitly before
the @samp{YYERROR;} statement.  @xref{Error Recovery}.

For Java parsers, this functionality is invoked using @code{return YYERROR;}
instead.
@end deffn

@deffn {Function} yyerror
User-supplied function to be called by @code{yyparse} on error.
@xref{Error Reporting Function}.
@end deffn

@deffn {Macro} YYFPRINTF
Macro used to output run-time traces in C.
@xref{Enabling Traces}.
@end deffn

@deffn {Macro} YYINITDEPTH
Macro for specifying the initial size of the parser stack.
@xref{Memory Management}.
@end deffn

@deffn {Function} yylex
User-supplied lexical analyzer function, called with no arguments to get
the next token.  @xref{Lexical}.
@end deffn

@deffn {Variable} yylloc
External variable in which @code{yylex} should place the line and column
numbers associated with a token.  (In a pure parser, it is a local
variable within @code{yyparse}, and its address is passed to
@code{yylex}.)
You can ignore this variable if you don't use the @samp{@@} feature in the
grammar actions.
@xref{Token Locations}.
In semantic actions, it stores the location of the lookahead token.
@xref{Actions and Locations}.
@end deffn

@deffn {Type} YYLTYPE
Data type of @code{yylloc}; by default, a structure with four members.
@xref{Location Type}.
@end deffn

@deffn {Variable} yylval
External variable in which @code{yylex} should place the semantic
value associated with a token.  (In a pure parser, it is a local
variable within @code{yyparse}, and its address is passed to
@code{yylex}.)
@xref{Token Values}.
In semantic actions, it stores the semantic value of the lookahead token.
@xref{Actions}.
@end deffn

@deffn {Macro} YYMAXDEPTH
Macro for specifying the maximum size of the parser stack.  @xref{Memory
Management}.
@end deffn

@deffn {Variable} yynerrs
Global variable which Bison increments each time it reports a syntax error.
(In a pure parser, it is a local variable within @code{yyparse}. In a
pure push parser, it is a member of @code{yypstate}.)
@xref{Error Reporting Function}.
@end deffn

@deffn {Function} yyparse
The parser function produced by Bison; call this function to start
parsing.  @xref{Parser Function}.
@end deffn

@deffn {Macro} YYPRINT
Macro used to output token semantic values.  For @file{yacc.c} only.
Deprecated, use @code{%printer} instead (@pxref{Printer Decl}).
@xref{The YYPRINT Macro}.
@end deffn

@deffn {Function} yypstate_delete
The function to delete a parser instance, produced by Bison in push mode;
call this function to delete the memory associated with a parser.
@xref{Parser Delete Function}.  Does nothing when called with a null pointer.
@end deffn

@deffn {Function} yypstate_new
The function to create a parser instance, produced by Bison in push mode;
call this function to create a new parser.
@xref{Parser Create Function}.
@end deffn

@deffn {Function} yypull_parse
The parser function produced by Bison in push mode; call this function to
parse the rest of the input stream.
@xref{Pull Parser Function}.
@end deffn

@deffn {Function} yypush_parse
The parser function produced by Bison in push mode; call this function to
parse a single token.  @xref{Push Parser Function}.
@end deffn

@deffn {Macro} YYRECOVERING
The expression @code{YYRECOVERING ()} yields 1 when the parser
is recovering from a syntax error, and 0 otherwise.
@xref{Action Features}.
@end deffn

@deffn {Macro} YYSTACK_USE_ALLOCA
Macro used to control the use of @code{alloca} when the
deterministic parser in C needs to extend its stacks.  If defined to 0,
the parser will use @code{malloc} to extend its stacks and memory exhaustion
occurs if @code{malloc} fails (@pxref{Memory Management}).  If defined to
1, the parser will use @code{alloca}.  Values other than 0 and 1 are
reserved for future Bison extensions.  If not defined,
@code{YYSTACK_USE_ALLOCA} defaults to 0.

In the all-too-common case where your code may run on a host with a
limited stack and with unreliable stack-overflow checking, you should
set @code{YYMAXDEPTH} to a value that cannot possibly result in
unchecked stack overflow on any of your target hosts when
@code{alloca} is called.  You can inspect the code that Bison
generates in order to determine the proper numeric values.  This will
require some expertise in low-level implementation details.
@end deffn

@deffn {Type} YYSTYPE
Deprecated in favor of the @code{%define} variable @code{api.value.type}.
Data type of semantic values; @code{int} by default.
@xref{Value Type}.
@end deffn

@deffn {Type} yysymbol_kind_t
An enum of all the symbols, tokens and nonterminals, of the grammar.
@xref{Syntax Error Reporting Function}.  The symbol kinds are used
internally by the parser, and should not be confused with the token kinds:
the symbol kind of a terminal symbol is not equal to its token kind! (Unless
@samp{%define api.token.raw} was used.)
@end deffn

@deffn {Type} yytoken_kind_t
An enum of all the @dfn{token kinds} declared with @code{%token}
(@pxref{Token Decl}).  These are the return values for @code{yylex}.  They
should not be confused with the @emph{symbol kinds}, used internally by the
parser.
@end deffn


@node Glossary
@appendix Glossary
@cindex glossary

@table @asis
@item Accepting state
A state whose only action is the accept action.
The accepting state is thus a consistent state.
@xref{Understanding}.

@item Backus-Naur Form (BNF; also called ``Backus Normal Form'')
Formal method of specifying context-free grammars originally proposed
by John Backus, and slightly improved by Peter Naur in his 1960-01-02
committee document contributing to what became the Algol 60 report.
@xref{Language and Grammar}.

@item Consistent state
A state containing only one possible action.  @xref{Default Reductions}.

@item Context-free grammars
Grammars specified as rules that can be applied regardless of context.
Thus, if there is a rule which says that an integer can be used as an
expression, integers are allowed @emph{anywhere} an expression is
permitted.  @xref{Language and Grammar}.

@item Default reduction
The reduction that a parser should perform if the current parser state
contains no other action for the lookahead token.  In permitted parser
states, Bison declares the reduction with the largest lookahead set to be
the default reduction and removes that lookahead set.  @xref{Default
Reductions}.

@item Defaulted state
A consistent state with a default reduction.  @xref{Default Reductions}.

@item Dynamic allocation
Allocation of memory that occurs during execution, rather than at
compile time or on entry to a function.

@item Empty string
Analogous to the empty set in set theory, the empty string is a
character string of length zero.

@item Finite-state stack machine
A ``machine'' that has discrete states in which it is said to exist at
each instant in time.  As input to the machine is processed, the
machine moves from state to state as specified by the logic of the
machine.  In the case of the parser, the input is the language being
parsed, and the states correspond to various stages in the grammar
rules.  @xref{Algorithm}.

@item Generalized LR (GLR)
A parsing algorithm that can handle all context-free grammars, including those
that are not LR(1).  It resolves situations that Bison's
deterministic parsing
algorithm cannot by effectively splitting off multiple parsers, trying all
possible parsers, and discarding those that fail in the light of additional
right context.  @xref{Generalized LR Parsing}.

@item Grouping
A language construct that is (in general) grammatically divisible;
for example, `expression' or `declaration' in C@.
@xref{Language and Grammar}.

@item IELR(1) (Inadequacy Elimination LR(1))
A minimal LR(1) parser table construction algorithm.  That is, given any
context-free grammar, IELR(1) generates parser tables with the full
language-recognition power of canonical LR(1) but with nearly the same
number of parser states as LALR(1).  This reduction in parser states is
often an order of magnitude.  More importantly, because canonical LR(1)'s
extra parser states may contain duplicate conflicts in the case of non-LR(1)
grammars, the number of conflicts for IELR(1) is often an order of magnitude
less as well.  This can significantly reduce the complexity of developing a
grammar.  @xref{LR Table Construction}.

@item Infix operator
An arithmetic operator that is placed between the operands on which it
performs some operation.

@item Input stream
A continuous flow of data between devices or programs.

@item Kind
``Token'' and ``symbol'' are each overloaded to mean either a grammar symbol
(kind) or all parse info (kind, value, location) associated with occurrences
of that grammar symbol from the input.  To disambiguate, we use ``token
kind'' and ``symbol kind'' to mean both grammar symbols and the types that
represent them in a base programming language (C, C++, etc.). However, we
use ``token'' and ``symbol'' without the word ``kind'' to mean parsed
occurrences, and we append the word ``type'' to refer to the types that
represent them in a base programming language.

In summary: When you see ``kind'', interpret ``symbol'' or ``token'' to mean
a @emph{grammar symbol}.  When you don't see ``kind'' (including when you
see ``type''), interpret ``symbol'' or ``token'' to mean a @emph{parsed
symbol}.

@item LAC (Lookahead Correction)
A parsing mechanism that fixes the problem of delayed syntax error
detection, which is caused by LR state merging, default reductions, and the
use of @code{%nonassoc}.  Delayed syntax error detection results in
unexpected semantic actions, initiation of error recovery in the wrong
syntactic context, and an incorrect list of expected tokens in a verbose
syntax error message.  @xref{LAC}.

@item Language construct
One of the typical usage schemas of the language.  For example, one of
the constructs of the C language is the @code{if} statement.
@xref{Language and Grammar}.

@item Left associativity
Operators having left associativity are analyzed from left to right:
@samp{a+b+c} first computes @samp{a+b} and then combines with
@samp{c}.  @xref{Precedence}.

@item Left recursion
A rule whose result symbol is also its first component symbol; for
example, @samp{expseq1 : expseq1 ',' exp;}.  @xref{Recursion}.

@item Left-to-right parsing
Parsing a sentence of a language by analyzing it token by token from
left to right.  @xref{Algorithm}.

@item Lexical analyzer (scanner)
A function that reads an input stream and returns tokens one by one.
@xref{Lexical}.

@item Lexical tie-in
A flag, set by actions in the grammar rules, which alters the way
tokens are parsed.  @xref{Lexical Tie-ins}.

@item Literal string token
A token which consists of two or more fixed characters.  @xref{Symbols}.

@item Lookahead token
A token already read but not yet shifted.  @xref{Lookahead}.

@item LALR(1)
The class of context-free grammars that Bison (like most other parser
generators) can handle by default; a subset of LR(1).
@xref{Mysterious Conflicts}.

@item LR(1)
The class of context-free grammars in which at most one token of
lookahead is needed to disambiguate the parsing of any piece of input.

@item Nonterminal symbol
A grammar symbol standing for a grammatical construct that can
be expressed through rules in terms of smaller constructs; in other
words, a construct that is not a token.  @xref{Symbols}.

@item Parser
A function that recognizes valid sentences of a language by analyzing
the syntax structure of a set of tokens passed to it from a lexical
analyzer.

@item Postfix operator
An arithmetic operator that is placed after the operands upon which it
performs some operation.

@item Reduction
Replacing a string of nonterminals and/or terminals with a single
nonterminal, according to a grammar rule.  @xref{Algorithm}.

@item Reentrant
A reentrant subprogram is a subprogram which can be in invoked any
number of times in parallel, without interference between the various
invocations.  @xref{Pure Decl}.

@item Reverse Polish Notation
A language in which all operators are postfix operators.

@item Right recursion
A rule whose result symbol is also its last component symbol; for
example, @samp{expseq1: exp ',' expseq1;}.  @xref{Recursion}.

@item Semantics
In computer languages, the semantics are specified by the actions
taken for each instance of the language, i.e., the meaning of
each statement.  @xref{Semantics}.

@item Shift
A parser is said to shift when it makes the choice of analyzing
further input from the stream rather than reducing immediately some
already-recognized rule.  @xref{Algorithm}.

@item Single-character literal
A single character that is recognized and interpreted as is.
@xref{Grammar in Bison}.

@item Start symbol
The nonterminal symbol that stands for a complete valid utterance in
the language being parsed.  The start symbol is usually listed as the
first nonterminal symbol in a language specification.
@xref{Start Decl}.

@item Symbol kind
A (finite) enumeration of the grammar symbols, as processed by the parser.
@xref{Symbols}.

@item Symbol table
A data structure where symbol names and associated data are stored during
parsing to allow for recognition and use of existing information in repeated
uses of a symbol.  @xref{Multi-function Calc}.

@item Syntax error
An error encountered during parsing of an input stream due to invalid
syntax.  @xref{Error Recovery}.

@item Terminal symbol
A grammar symbol that has no rules in the grammar and therefore is
grammatically indivisible.  The piece of text it represents is a token.
@xref{Language and Grammar}.

@item Token
A basic, grammatically indivisible unit of a language.  The symbol that
describes a token in the grammar is a terminal symbol.  The input of the
Bison parser is a stream of tokens which comes from the lexical analyzer.
@xref{Symbols}.

@item Token kind
A (finite) enumeration of the grammar terminals, as discriminated by the
scanner.  @xref{Symbols}.

@item Unreachable state
A parser state to which there does not exist a sequence of transitions from
the parser's start state.  A state can become unreachable during conflict
resolution.  @xref{Unreachable States}.
@end table

@node GNU Free Documentation License
@appendix GNU Free Documentation License

@include fdl.texi

@node Bibliography
@unnumbered Bibliography

@c Please follow the following canvas to add more references.

@table @asis
@item [Corbett 1984]
@c author
Robert Paul Corbett,
@c title
Static Semantics in Compiler Error Recovery
@c in
Ph.D. Dissertation, Report No. UCB/CSD 85/251,
@c where
Department of Electrical Engineering and Computer Science, Compute Science
Division, University of California, Berkeley, California
@c when
(June 1985).
@c url
@uref{http://xtf.lib.berkeley.edu/reports/TRWebData/accessPages/CSD-85-251.html}

@item [Denny 2008]
Joel E. Denny and Brian A. Malloy, IELR(1): Practical LR(1) Parser Tables
for Non-LR(1) Grammars with Conflict Resolution, in @cite{Proceedings of the
2008 ACM Symposium on Applied Computing} (SAC'08), ACM, New York, NY, USA,
pp.@: 240--245.  @uref{http://dx.doi.org/10.1145/1363686.1363747}

@item [Denny 2010 May]
Joel E. Denny, PSLR(1): Pseudo-Scannerless Minimal LR(1) for the
Deterministic Parsing of Composite Languages, Ph.D. Dissertation, Clemson
University, Clemson, SC, USA (May 2010).
@uref{http://proquest.umi.com/pqdlink?did=2041473591&Fmt=7&clientId=79356&RQT=309&VName=PQD}

@item [Denny 2010 November]
Joel E. Denny and Brian A. Malloy, The IELR(1) Algorithm for Generating
Minimal LR(1) Parser Tables for Non-LR(1) Grammars with Conflict Resolution,
in @cite{Science of Computer Programming}, Vol.@: 75, Issue 11 (November
2010), pp.@: 943--979.  @uref{http://dx.doi.org/10.1016/j.scico.2009.08.001}

@item [DeRemer 1982]
Frank DeRemer and Thomas Pennello, Efficient Computation of LALR(1)
Look-Ahead Sets, in @cite{ACM Transactions on Programming Languages and
Systems}, Vol.@: 4, No.@: 4 (October 1982), pp.@:
615--649. @uref{http://dx.doi.org/10.1145/69622.357187}

@item [Johnson 1978]
Steven C. Johnson,
A portable compiler: theory and practice,
in @cite{Proceedings of the 5th ACM SIGACT-SIGPLAN symposium on
Principles of programming languages} (POPL '78),
pp.@: 97--104.
@uref{https://dx.doi.org/10.1145/512760.512771}.

@item [Knuth 1965]
Donald E. Knuth, On the Translation of Languages from Left to Right, in
@cite{Information and Control}, Vol.@: 8, Issue 6 (December 1965), pp.@:
607--639. @uref{http://dx.doi.org/10.1016/S0019-9958(65)90426-2}

@item [Scott 2000]
Elizabeth Scott, Adrian Johnstone, and Shamsa Sadaf Hussain,
@cite{Tomita-Style Generalised LR Parsers}, Royal Holloway, University of
London, Department of Computer Science, TR-00-12 (December 2000).
@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps}
@end table

@node Index of Terms
@unnumbered Index of Terms

@printindex cp

@bye

@c LocalWords: texinfo setfilename settitle setchapternewpage finalout texi FSF
@c LocalWords: ifinfo smallbook shorttitlepage titlepage GPL FIXME iftex FSF's
@c LocalWords: akim fn cp syncodeindex vr tp synindex dircategory direntry Naur
@c LocalWords: ifset vskip pt filll insertcopying sp ISBN Etienne Suvasa Multi
@c LocalWords: ifnottex yyparse detailmenu GLR RPN Calc var Decls Rpcalc multi
@c LocalWords: rpcalc Lexer Expr ltcalc mfcalc yylex defaultprec Donnelly Gotos
@c LocalWords: yyerror pxref LR yylval cindex dfn LALR samp gpl BNF xref yypush
@c LocalWords: const int paren ifnotinfo AC noindent emph expr stmt findex lr
@c LocalWords: glr YYSTYPE TYPENAME prog dprec printf decl init stmtMerge POSIX
@c LocalWords: pre STDC GNUC endif yy YY alloca lf stddef stdlib YYDEBUG yypull
@c LocalWords: NUM exp subsubsection kbd Ctrl ctype EOF getchar isdigit nonfree
@c LocalWords: ungetc stdin scanf sc calc ulator ls lm cc NEG prec yyerrok rr
@c LocalWords: longjmp fprintf stderr yylloc YYLTYPE cos ln Stallman Destructor
@c LocalWords: symrec val tptr FUN func struct sym enum IEC syntaxes Byacc
@c LocalWords: fun putsym getsym arith funs atan ptr malloc sizeof Lex pcc
@c LocalWords: strlen strcpy fctn strcmp isalpha symbuf realloc isalnum DOTDOT
@c LocalWords: ptypes itype YYPRINT trigraphs yytname expseq vindex dtype Unary
@c LocalWords: Rhs YYRHSLOC LE nonassoc op deffn typeless yynerrs nonterminal
@c LocalWords: yychar yydebug msg YYNTOKENS YYNNTS YYNRULES YYNSTATES reentrant
@c LocalWords: cparse clex deftypefun NE defmac YYACCEPT YYABORT param yypstate
@c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP subrange
@c LocalWords: YYEMPTY YYEOF YYRECOVERING yyclearin GE def UMINUS maybeword loc
@c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH inline
@c LocalWords: YYINITDEPTH stmts ref initdcl maybeasm notype Lookahead ctx
@c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args Autoconf
@c LocalWords: ypp yxx itemx tex leaderfill Troubleshouting sqrt Graphviz
@c LocalWords: hbox hss hfill tt ly yyin fopen fclose ofirst gcc ll lookahead
@c LocalWords: nbar yytext fst snd osplit ntwo strdup AST Troublereporting th
@c LocalWords: YYSTACK DVI fdl printindex IELR nondeterministic nonterminals ps
@c LocalWords: subexpressions declarator nondeferred config libintl postfix LAC
@c LocalWords: preprocessor nonpositive unary nonnumeric typedef extern rhs sr
@c LocalWords: yytokentype destructor multicharacter nonnull EBCDIC nterm LR's
@c LocalWords: lvalue nonnegative XNUM CHR chr TAGLESS tagless stdout api TOK
@c LocalWords: destructors Reentrancy nonreentrant subgrammar nonassociative Ph
@c LocalWords: deffnx namespace xml goto lalr ielr runtime lex yacc yyps env
@c LocalWords: yystate variadic Unshift NLS gettext po UTF Automake LOCALEDIR
@c LocalWords: YYENABLE bindtextdomain Makefile DEFS CPPFLAGS DBISON DeRemer
@c LocalWords: autoreconf Pennello multisets nondeterminism Generalised baz ACM
@c LocalWords: redeclare automata Dparse localedir datadir XSLT midrule Wno
@c LocalWords: multitable headitem hh basename Doxygen fno filename gdef de
@c LocalWords: doxygen ival sval deftypemethod deallocate pos deftypemethodx
@c LocalWords: Ctor defcv defcvx arg accessors CPP ifndef CALCXX YYerror
@c LocalWords: lexer's calcxx bool LPAREN RPAREN deallocation cerrno climits
@c LocalWords: cstdlib Debian undef yywrap unput noyywrap nounput zA yyleng
@c LocalWords: errno strtol ERANGE str strerror iostream argc argv Javadoc PSLR
@c LocalWords: bytecode initializers superclass stype ASTNode autoboxing nls
@c LocalWords: toString deftypeivar deftypeivarx deftypeop YYParser strictfp
@c LocalWords: superclasses boolean getErrorVerbose setErrorVerbose deftypecv
@c LocalWords: getDebugStream setDebugStream getDebugLevel setDebugLevel url
@c LocalWords: bisonVersion deftypecvx bisonSkeleton getStartPos getEndPos
@c LocalWords: getLVal defvar deftypefn deftypefnx gotos msgfmt Corbett LALR's
@c LocalWords: subdirectory Solaris nonassociativity perror schemas Malloy ints
@c LocalWords: Scannerless ispell american ChangeLog smallexample CSTYPE CLTYPE
@c LocalWords: clval CDEBUG cdebug deftypeopx yyterminate LocationType yyo
@c LocalWords: parsers parser's documentencoding documentlanguage Wempty ss
@c LocalWords: associativity subclasses precedences unresolvable runnable
@c LocalWords: allocators subunit initializations unreferenced untyped dir
@c LocalWords: errorVerbose subtype subtypes Wmidrule midrule's src rvalues
@c LocalWords: automove evolutions Wother Wconflicts PNG lookaheads Acc sep
@c LocalWords: xsltproc XSL xsl xhtml html num Wprecedence Werror fcaret gv
@c LocalWords: fdiagnostics setlocale nullptr ast srcdir iff drv rgbWarning
@c LocalWords: deftypefunx pragma Wnull dereference Wdocumentation elif ish
@c LocalWords: Wdeprecated Wregister noinput yyloc yypos PODs sstream Wsign
@c LocalWords: typename emplace Wconversion Wshorten yacchack reentrancy ou
@c LocalWords: Relocatability exprs fixit Wyacc parseable fixits ffixit svg
@c LocalWords: DNDEBUG cstring Wzero workalike POPL workalikes byacc UCB
@c LocalWords: Penello's Penello Byson Byson's Corbett's CSD TOPLAS PDP
@c LocalWords: Beazley's goyacc ocamlyacc SIGACT SIGPLAN colorWarning exVal
@c LocalWords: setcolor rgbError colorError rgbNotice colorNotice derror
@c LocalWords: colorOff maincolor inlineraw darkviolet darkcyan dwarning
@c LocalWords: dnotice copyable stdint ptrdiff bufsize yyreport invariants
@c LocalWords: xrefautomaticsectiontitle yysyntax yysymbol ARGMAX cond
@c LocalWords: Wdangling yytoken erreur syntaxe inattendu attendait nombre
@c LocalWords: YYUNDEF SymbolKind yypcontext YYENOMEM TOKENMAX getBundle
@c LocalWords: ResourceBundle myResources getString getName getToken
@c LocalWords: getLocation getExpectedTokens reportSyntaxError bistromathic

@c Local Variables:
@c ispell-dictionary: "american"
@c fill-column: 76
@c End: