* Bison 3.6 ** doc I feel its ugly to use the GNU style to declare functions in the doc. It generates tons of white space in the page, and may contribute to bad page breaks. Also, we seem to teach YYPRINT very early on, although it should be considered deprecated: %printer is superior. ** improve syntax errors (UTF-8, internationalization) Bison depends on the current locale. For instance: %define parse.error verbose %code top { #include #include void yyerror(const char* msg) { fprintf(stderr, "%s\n", msg); } int yylex() { return 0; } } %% exp: "↦" | "🎅🐃" | '\n' %% int main() { return yyparse(); } gives different results with/without LC_ALL=C. $ LC_ALL=C /opt/local/bin/bison /tmp/mangle.y -o ascii.c $ /opt/local/bin/bison /tmp/mangle.y -o utf8.c $ diff -u ascii.c utf8.c -I#line --- ascii.c 2019-01-12 08:15:35.878010093 +0100 +++ utf8.c 2019-01-12 08:15:38.856495929 +0100 @@ -415,9 +415,8 @@ First, the terminals, then, starting at YYNTOKENS, nonterminals. */ static const char *const yytname[] = { - "$end", "error", "$undefined", "\"\\342\\206\\246\"", - "\"\\360\\237\\216\\205\\360\\237\\220\\203\"", "'\\n'", "$accept", - "exp", YY_NULLPTR + "$end", "error", "$undefined", "\"↦\"", "\"🎅🐃\"", "'\\n'", + "$accept", "exp", YY_NULLPTR }; #endif $ gcc ascii.c -o ascii && ./ascii syntax error, unexpected $end, expecting "\342\206\246" or "\360\237\216\205\360\237\220\203" or '\n' $ gcc utf8.c -o utf8 && ./utf8 syntax error, unexpected $end, expecting ↦ or 🎅🐃 or '\n' While at it, we should stop using "$end" by default, in favor of "end of file", or "end of input", whatever. See how lalr1.java does that. ** consistency token vs terminal, variable vs non terminal. ** Stop indentation in diagnostics Before Bison 2.7, we printed "flatly" the dependencies in long diagnostics: input.y:2.7-12: %type redeclaration for exp input.y:1.7-12: previous declaration In Bison 2.7, we indented them input.y:2.7-12: error: %type redeclaration for exp input.y:1.7-12: previous declaration Later we quoted the source in the diagnostics, and today we have: /tmp/foo.y:1.12-14: warning: symbol FOO redeclared [-Wother] 1 | %token FOO FOO | ^~~ /tmp/foo.y:1.8-10: previous declaration 1 | %token FOO FOO | ^~~ The indentation is no longer helping. We should probably get rid of it, or maybe keep it only when -fno-caret. GCC displays this as a "note": $ g++-mp-9 -Wall /tmp/foo.c -c /tmp/foo.c:1:10: error: redefinition of 'int foo' 1 | int foo, foo; | ^~~ /tmp/foo.c:1:5: note: 'int foo' previously declared here 1 | int foo, foo; | ^~~ Likewise for Clang, contrary to what I believed (because "note:" is written in black, so it doesn't show in my terminal :-) $ clang++-mp-8.0 -Wall /tmp/foo.c -c clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated] /tmp/foo.c:1:10: error: redefinition of 'foo' int foo, foo; ^ /tmp/foo.c:1:5: note: previous definition is here int foo, foo; ^ 1 error generated. See also the item "Complaint submessage indentation" below. ** api.token.raw Maybe we should exhibit the YYUNDEFTOK token. It could also be assigned a semantic value so that yyerror could be used to report invalid lexemes. See also the item "$undefined" below. ** C++ Move to int everywhere instead of unsigned? stack_size, etc. The parser itself uses int (for yylen for instance), yet stack is based on size_t. Maybe locations should also move to ints. Paul Eggert already covered most of this. But before publishing these changes, we need to ask our C++ users if they agree with that change, or if we need some migration path. Could be a %define variable, or simply %require "3.5". * Bison 3.7 ** Unit rules / Injection rules (Akim Demaille) Maybe we could expand unit rules (or "injections", see https://homepages.cwi.nl/~daybuild/daily-books/syntax/2-sdf/sdf.html), i.e., transform exp: arith | bool; arith: exp '+' exp; bool: exp '&' exp; into exp: exp '+' exp | exp '&' exp; when there are no actions. This can significantly speed up some grammars. I can't find the papers. In particular the book 'LR parsing: Theory and Practice' is impossible to find, but according to 'Parsing Techniques: a Practical Guide', it includes information about this issue. Does anybody have it? ** clean up (Akim Demaille) Do not work on these items now, as I (Akim) have branches with a lot of changes in this area (hitting several files), and no desire to have to fix conflicts. Addressing these items will happen after my branches have been merged. *** lalr.c Introduce a goto struct, and use it in place of from_state/to_state. Rename states1 as path, length as pathlen. Introduce inline functions for things such as nullable[*rp - ntokens] where we need to map from symbol number to nterm number. There are probably a significant part of the relations management that should be migrated on top of a bitsetv. *** closure It should probably take a "state*" instead of two arguments. *** traces The "automaton" and "set" categories are not so useful. We should probably introduce lr(0) and lalr, just the way we have ielr categories. The "closure" function is too verbose, it should probably have its own category. "set" can still be used for summariring the important sets. That would make tests easy to maintain. *** complain.* Rename these guys as "diagnostics.*" (or "diagnose.*"), since that's the name they have in gcc, clang, etc. Likewise for the complain_* series of functions. *** ritem states/nstates, rules/nrules, ..., ritem/nritems Fix the latter. * Modernization Fix data/skeletons/yacc.c so that it defines YYPTRDIFF_T properly for modern and older C++ compilers. Currently the code defaults to defining it to 'long' for non-GCC compilers, but it should use the proper C++ magic to define it to the same type as the C ptrdiff_t type. * Completion Several features are not available in all the backends. - push parsers: glr.cc, lalr1.cc - ielr: C++ and Java - glr: Java - token constructors: Java and C * Bugs ** Autotest has quotation issues tests/input.at:1730:AT_SETUP([%define errors]) -> $ ./tests/testsuite -l | grep errors | sed q 38: input.at:1730 errors * Short term ** Get rid of YYPRINT and b4_toknum Besides yytoknum is wrong when api.token.raw is defined. ** Better design for diagnostics The current implementation of diagnostics is adhoc, it grew organically. It works as a series of calls to several functions, with dependency of the latter calls on the former. For instance: complain (&sym->location, sym->content->status == needed ? complaint : Wother, _("symbol %s is used, but is not defined as a token" " and has no rules; did you mean %s?"), quote_n (0, sym->tag), quote_n (1, best->tag)); if (feature_flag & feature_caret) location_caret_suggestion (sym->location, best->tag, stderr); We should rewrite this in a more FP way: 1. build a rich structure that denotes the (complete) diagnostic. "Complete" in the sense that it also contains the suggestions, the list of possible matches, etc. 2. send this to the pretty-printing routine. The diagnostic structure should be sufficient so that we can generate all the 'format' of diagnostics, including the fixits. If properly done, this diagnostic module can be detached from Bison and be put in gnulib. It could be used, for instance, for errors caught by xgettext. There's certainly already something alike in GCC. At least that's the impression I get from reading the "-fdiagnostics-format=FORMAT" part of this page: https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Message-Formatting-Options.html ** Graphviz display code thoughts The code for the --graph option is over two files: print_graph, and graphviz. This is because Bison used to also produce VCG graphs, but since this is no longer true, maybe we could consider these files for fusion. An other consideration worth noting is that print_graph.c (correct me if I am wrong) should contain generic functions, whereas graphviz.c and other potential files should contain just the specific code for that output format. It will probably prove difficult to tell if the implementation is actually generic whilst only having support for a single format, but it would be nice to keep stuff a bit tidier: right now, the construction of the bitset used to show reductions is in the graphviz-specific code, and on the opposite side we have some use of \l, which is graphviz-specific, in what should be generic code. Little effort seems to have been given to factoring these files and their rint{,-xml} counterpart. We would very much like to re-use the pretty format of states from .output for the graphs, etc. Since graphviz dies on medium-to-big grammars, maybe consider an other tool? ** push-parser Check it too when checking the different kinds of parsers. And be sure to check that the initial-action is performed once per parsing. ** m4 names b4_shared_declarations is no longer what it is. Make it b4_parser_declaration for instance. ** yychar in lalr1.cc There is a large difference bw maint and master on the handling of yychar (which was removed in lalr1.cc). See what needs to be back-ported. /* User semantic actions sometimes alter yychar, and that requires that yytoken be updated with the new translation. We take the approach of translating immediately before every use of yytoken. One alternative is translating here after every semantic action, but that translation would be missed if the semantic action invokes YYABORT, YYACCEPT, or YYERROR immediately after altering yychar. In the case of YYABORT or YYACCEPT, an incorrect destructor might then be invoked immediately. In the case of YYERROR, subsequent parser actions might lead to an incorrect destructor call or verbose syntax error message before the lookahead is translated. */ /* Make sure we have latest lookahead translation. See comments at user semantic actions for why this is necessary. */ yytoken = yytranslate_ (yychar); ** Get rid of fake #lines [Bison: ...] Possibly as simple as checking whether the column number is nonnegative. I have seen messages like the following from GCC. :0: fatal error: opening dependency file .deps/libltdl/argz.Tpo: No such file or directory ** Discuss about %printer/%destroy in the case of C++. It would be very nice to provide the symbol classes with an operator<< and a destructor. Unfortunately the syntax we have chosen for %destroy and %printer make them hard to reuse. For instance, the user is invited to write something like %printer { debug_stream() << $$; } ; which is hard to reuse elsewhere since it wants to use "debug_stream()" to find the stream to use. The same applies to %destroy: we told the user she could use the members of the Parser class in the printers/destructors, which is not good for an operator<< since it is no longer bound to a particular parser, it's just a (standalone symbol). * Various ** Rewrite glr.cc in C++ (Valentin Tolmer) As a matter of fact, it would be very interesting to see how much we can share between lalr1.cc and glr.cc. Most of the skeletons should be common. It would be a very nice source of inspiration for the other languages. Valentin Tolmer is working on this. ** YYERRCODE Why don't we output the token name of the error token in the output? It is explicitly skipped: /* Skip error token and tokens without identifier. */ if (sym != errtoken && id) Of course there are issues with name spaces, but if we disable we have something which seems to be more simpler and more consistent instead of the special case YYERRCODE. enum yytokentype { error = 256, // ... }; We could (should?) also treat the case of the undef_token, which is numbered 257 for yylex, and 2 internal. Both appear for instance in toknum: const unsigned short parser::yytoken_number_[] = { 0, 256, 257, 258, 259, 260, 261, 262, 263, 264, while here enum yytokentype { TOK_EOF = 0, TOK_EQ = 258, so both 256 and 257 are "mysterious". const char* const parser::yytname_[] = { "\"end of command\"", "error", "$undefined", "\"=\"", "\"break\"", ** yychar == yyempty_ The code in yyerrlab reads: if (yychar <= YYEOF) { /* Return failure if at end of input. */ if (yychar == YYEOF) YYABORT; } There are only two yychar that can be <= YYEOF: YYEMPTY and YYEOF. But I can't produce the situation where yychar is YYEMPTY here, is it really possible? The test suite does not exercise this case. This shows that it would be interesting to manage to install skeleton coverage analysis to the test suite. * From lalr1.cc to yacc.c ** Single stack Merging the three stacks in lalr1.cc simplified the code, prompted for other improvements and also made it faster (probably because memory management is performed once instead of three times). I suggest that we do the same in yacc.c. (Some time later): it's also very nice to have three stacks: it's more dense as we don't lose bits to padding. For instance the typical stack for states will use 8 bits, while it is likely to consume 32 bits in a struct. We need trustworthy benchmarks for Bison, for all our backends. Akim has a few things scattered around; we need to put them in the repo, and make them more useful. ** yysyntax_error The code bw glr.c and yacc.c is really alike, we can certainly factor some parts. This should be worked on when we also address the expected improvements for error generation (e.g., i18n). * Report ** Figures Some statistics about the grammar and the parser would be useful, especially when asking the user to send some information about the grammars she is working on. We should probably also include some information about the variables (I'm not sure for instance we even specify what LR variant was used). ** GLR How would Paul like to display the conflicted actions? In particular, what when two reductions are possible on a given lookahead token, but one is part of $default. Should we make the two reductions explicit, or just keep $default? See the following point. ** Disabled Reductions See 'tests/conflicts.at (Defaulted Conflicted Reduction)', and decide what we want to do. ** Documentation Extend with error productions. The hard part will probably be finding the right rule so that a single state does not exhibit too many yet undocumented ''features''. Maybe an empty action ought to be presented too. Shall we try to make a single grammar with all these features, or should we have several very small grammars? ** --report=conflict-path Provide better assistance for understanding the conflicts by providing a sample text exhibiting the (LALR) ambiguity. See the paper from DeRemer and Penello: they already provide the algorithm. ** Statically check for potential ambiguities in GLR grammars See for an approach. An Experimental Ambiguity Detection Tool ∗ Sylvain Schmitz LORIA, INRIA Nancy - Grand Est, Nancy, France * Extensions ** Multiple start symbols Would be very useful when parsing closely related languages. The idea is to declare several start symbols, for instance %start stmt expr %% stmt: ... expr: ... and to generate parse(), parse_stmt() and parse_expr(). Technically, the above grammar would be transformed into %start yy_start %token YY_START_STMT YY_START_EXPR %% yy_start: YY_START_STMT stmt | YY_START_EXPR expr so that there are no new conflicts in the grammar (as would undoubtedly happen with yy_start: stmt | expr). Then adjust the skeletons so that this initial token (YY_START_STMT, YY_START_EXPR) be shifted first in the corresponding parse function. ** Better error messages The users are not provided with enough tools to forge their error messages. See for instance "Is there an option to change the message produced by YYERROR_VERBOSE?" by Simon Sobisch, on bison-help. See also https://www.cs.tufts.edu/~nr/cs257/archive/clinton-jefferey/lr-error-messages.pdf and https://research.swtch.com/yyerror. ** %include This is a popular demand. We already made many changes in the parser that should make this reasonably easy to implement. Bruce Mardle https://lists.gnu.org/archive/html/bison-patches/2015-09/msg00000.html However, there are many other things to do before having such a feature, because I don't want a % equivalent to #include (which we all learned to hate). I want something that builds "modules" of grammars, and assembles them together, paying attention to keep separate bits separated, in pseudo name spaces. ** Push parsers There is demand for push parsers in Java and C++. And GLR I guess. ** Generate code instead of tables This is certainly quite a lot of work. See http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.50.4539. ** $-1 We should find a means to provide an access to values deep in the stack. For instance, instead of baz: qux { $$ = $-1 + $0 + $1; } we should be able to have: foo($foo) bar($bar) baz($bar): qux($qux) { $baz = $foo + $bar + $qux; } Or something like this. ** %if and the like It should be possible to have %if/%else/%endif. The implementation is not clear: should it be lexical or syntactic. Vadim Maslow thinks it must be in the scanner: we must not parse what is in a switched off part of %if. Akim Demaille thinks it should be in the parser, so as to avoid falling into another CPP mistake. (Later): I'm sure there's actually good case for this. People who need that feature can use m4/cpp on top of Bison. I don't think it is worth the trouble in Bison itself. ** XML Output There are couple of available extensions of Bison targeting some XML output. Some day we should consider including them. One issue is that they seem to be quite orthogonal to the parsing technique, and seem to depend mostly on the possibility to have some code triggered for each reduction. As a matter of fact, such hooks could also be used to generate the yydebug traces. Some generic scheme probably exists in there. XML output for GNU Bison and gcc http://www.cs.may.ie/~jpower/Research/bisonXML/ XML output for GNU Bison http://yaxx.sourceforge.net/ ** Counterexample generation https://lists.gnu.org/archive/html/bug-bison/2016-06/msg00000.html http://www.cs.cornell.edu/andru/papers/cupex/ Andrew Myers and Vincent Imbimbo are working on this item, see https://github.com/akimd/bison/issues/12 * Coding system independence Paul notes: Currently Bison assumes 8-bit bytes (i.e. that UCHAR_MAX is 255). It also assumes that the 8-bit character encoding is the same for the invocation of 'bison' as it is for the invocation of 'cc', but this is not necessarily true when people run bison on an ASCII host and then use cc on an EBCDIC host. I don't think these topics are worth our time addressing (unless we find a gung-ho volunteer for EBCDIC or PDP-10 ports :-) but they should probably be documented somewhere. More importantly, Bison does not currently allow NUL bytes in tokens, either via escapes (e.g., "x\0y") or via a NUL byte in the source code. This should get fixed. * Broken options? ** %token-table ** Skeleton strategy Must we keep %token-table? * Precedence ** Partial order It is unfortunate that there is a total order for precedence. It makes it impossible to have modular precedence information. We should move to partial orders (sounds like series/parallel orders to me). This is a prerequisite for modules. * $undefined From Hans: - If the Bison generated parser experiences an undefined number in the character range, that character is written out in diagnostic messages, an addition to the $undefined value. Suggest: Change the name $undefined to undefined; looks better in outputs. * Pre and post actions. From: Florian Krohm Subject: YYACT_EPILOGUE To: bug-bison@gnu.org X-Sent: 1 week, 4 days, 14 hours, 38 minutes, 11 seconds ago The other day I had the need for explicitly building the parse tree. I used %locations for that and defined YYLLOC_DEFAULT to call a function that returns the tree node for the production. Easy. But I also needed to assign the S-attribute to the tree node. That cannot be done in YYLLOC_DEFAULT, because it is invoked before the action is executed. The way I solved this was to define a macro YYACT_EPILOGUE that would be invoked after the action. For reasons of symmetry I also added YYACT_PROLOGUE. Although I had no use for that I can envision how it might come in handy for debugging purposes. All is needed is to add #if YYLSP_NEEDED YYACT_EPILOGUE (yyval, (yyvsp - yylen), yylen, yyloc, (yylsp - yylen)); #else YYACT_EPILOGUE (yyval, (yyvsp - yylen), yylen); #endif at the proper place to bison.simple. Ditto for YYACT_PROLOGUE. I was wondering what you think about adding YYACT_PROLOGUE/EPILOGUE to bison. If you're interested, I'll work on a patch. * Better graphics Equip the parser with a means to create the (visual) parse tree. Local Variables: mode: outline coding: utf-8 fill-column: 76 End: ----- Copyright (C) 2001-2004, 2006, 2008-2015, 2018-2020 Free Software Foundation, Inc. This file is part of Bison, the GNU Compiler Compiler. This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see .