summaryrefslogtreecommitdiff
path: root/pod/perlfaq7.pod
blob: 1bfab258546695899b32cde456d5ddd0f05c9096 (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
=head1 NAME

perlfaq7 - General Perl Language Issues

=head1 DESCRIPTION

This section deals with general Perl language issues that don't
clearly fit into any of the other sections.

=head2 Can I get a BNF/yacc/RE for the Perl language?

There is no BNF, but you can paw your way through the yacc grammar in
perly.y in the source distribution if you're particularly brave.  The
grammar relies on very smart tokenizing code, so be prepared to
venture into toke.c as well.

In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF.
The work of parsing perl is distributed between yacc, the lexer, smoke
and mirrors."

=head2 What are all these $@%&* punctuation signs, and how do I know when to use them?

They are type specifiers, as detailed in L<perldata>:

	$ for scalar values (number, string or reference)
	@ for arrays
	% for hashes (associative arrays)
	& for subroutines (aka functions, procedures, methods)
	* for all types of that symbol name.  In version 4 you used them like
	  pointers, but in modern perls you can just use references.

There are couple of other symbols that you're likely to encounter that aren't
really type specifiers:

	<> are used for inputting a record from a filehandle.
	\  takes a reference to something.

Note that <FILE> is I<neither> the type specifier for files
nor the name of the handle.  It is the C<< <> >> operator applied
to the handle FILE.  It reads one line (well, record--see
L<perlvar/$E<sol>>) from the handle FILE in scalar context, or I<all> lines
in list context.  When performing open, close, or any other operation
besides C<< <> >> on files, or even when talking about the handle, do
I<not> use the brackets.  These are correct: C<eof(FH)>, C<seek(FH, 0,
2)> and "copying from STDIN to FILE".

=head2 Do I always/never have to quote my strings or use semicolons and commas?

Normally, a bareword doesn't need to be quoted, but in most cases
probably should be (and must be under C<use strict>).  But a hash key
consisting of a simple word (that isn't the name of a defined
subroutine) and the left-hand operand to the C<< => >> operator both
count as though they were quoted:

	This                    is like this
	------------            ---------------
	$foo{line}              $foo{'line'}
	bar => stuff            'bar' => stuff

The final semicolon in a block is optional, as is the final comma in a
list.  Good style (see L<perlstyle>) says to put them in except for
one-liners:

	if ($whoops) { exit 1 }
	@nums = (1, 2, 3);

	if ($whoops) {
		exit 1;
	}

	@lines = (
	"There Beren came from mountains cold",
	"And lost he wandered under leaves",
	);

=head2 How do I skip some return values?

One way is to treat the return values as a list and index into it:

	$dir = (getpwnam($user))[7];

Another way is to use undef as an element on the left-hand-side:

	($dev, $ino, undef, undef, $uid, $gid) = stat($file);

You can also use a list slice to select only the elements that
you need:

	($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];

=head2 How do I temporarily block warnings?

If you are running Perl 5.6.0 or better, the C<use warnings> pragma
allows fine control of what warning are produced.
See L<perllexwarn> for more details.

	{
	no warnings;          # temporarily turn off warnings
	$a = $b + $c;         # I know these might be undef
	}

Additionally, you can enable and disable categories of warnings.
You turn off the categories you want to ignore and you can still
get other categories of warnings.  See L<perllexwarn> for the
complete details, including the category names and hierarchy.

	{
	no warnings 'uninitialized';
	$a = $b + $c;
	}

If you have an older version of Perl, the C<$^W> variable (documented
in L<perlvar>) controls runtime warnings for a block:

	{
	local $^W = 0;        # temporarily turn off warnings
	$a = $b + $c;         # I know these might be undef
	}

Note that like all the punctuation variables, you cannot currently
use my() on C<$^W>, only local().

=head2 What's an extension?

An extension is a way of calling compiled C code from Perl.  Reading
L<perlxstut> is a good place to learn more about extensions.

=head2 Why do Perl operators have different precedence than C operators?

Actually, they don't.  All C operators that Perl copies have the same
precedence in Perl as they do in C.  The problem is with operators that C
doesn't have, especially functions that give a list context to everything
on their right, eg. print, chmod, exec, and so on.  Such functions are
called "list operators" and appear as such in the precedence table in
L<perlop>.

A common mistake is to write:

	unlink $file || die "snafu";

This gets interpreted as:

	unlink ($file || die "snafu");

To avoid this problem, either put in extra parentheses or use the
super low precedence C<or> operator:

	(unlink $file) || die "snafu";
	unlink $file or die "snafu";

The "English" operators (C<and>, C<or>, C<xor>, and C<not>)
deliberately have precedence lower than that of list operators for
just such situations as the one above.

Another operator with surprising precedence is exponentiation.  It
binds more tightly even than unary minus, making C<-2**2> produce a
negative not a positive four.  It is also right-associating, meaning
that C<2**3**2> is two raised to the ninth power, not eight squared.

Although it has the same precedence as in C, Perl's C<?:> operator
produces an lvalue.  This assigns $x to either $a or $b, depending
on the trueness of $maybe:

	($maybe ? $a : $b) = $x;

=head2 How do I declare/create a structure?

In general, you don't "declare" a structure.  Just use a (probably
anonymous) hash reference.  See L<perlref> and L<perldsc> for details.
Here's an example:

	$person = {};                   # new anonymous hash
	$person->{AGE}  = 24;           # set field AGE to 24
	$person->{NAME} = "Nat";        # set field NAME to "Nat"

If you're looking for something a bit more rigorous, try L<perltoot>.

=head2 How do I create a module?

(contributed by brian d foy)

L<perlmod>, L<perlmodlib>, L<perlmodstyle> explain modules
in all the gory details. L<perlnewmod> gives a brief
overview of the process along with a couple of suggestions
about style.

If you need to include C code or C library interfaces in
your module, you'll need h2xs.  h2xs will create the module
distribution structure and the initial interface files
you'll need.  L<perlxs> and L<perlxstut> explain the details.

If you don't need to use C code, other tools such as
ExtUtils::ModuleMaker and Module::Starter, can help you
create a skeleton module distribution.

You may also want to see Sam Tregar's "Writing Perl Modules
for CPAN" ( http://apress.com/book/bookDisplay.html?bID=14 )
which is the best hands-on guide to creating module
distributions.

=head2 How do I adopt or take over a module already on CPAN?

(contributed by brian d foy)

The easiest way to take over a module is to have the current
module maintainer either make you a co-maintainer or transfer
the module to you.

If you can't reach the author for some reason (e.g. email bounces),
the PAUSE admins at modules@perl.org can help. The PAUSE admins
treat each case individually.

=over 4

=item

Get a login for the Perl Authors Upload Server (PAUSE) if you don't
already have one: http://pause.perl.org

=item

Write to modules@perl.org explaining what you did to contact the
current maintainer. The PAUSE admins will also try to reach the
maintainer.

=item

Post a public message in a heavily trafficked site announcing your
intention to take over the module.

=item

Wait a bit. The PAUSE admins don't want to act too quickly in case
the current maintainer is on holiday. If there's no response to
private communication or the public post, a PAUSE admin can transfer
it to you.

=back

=head2 How do I create a class?
X<class, creation> X<package>

(contributed by brian d foy)

In Perl, a class is just a package, and methods are just subroutines.
Perl doesn't get more formal than that and lets you set up the package
just the way that you like it (that is, it doesn't set up anything for
you).

The Perl documentation has several tutorials that cover class
creation, including L<perlboot> (Barnyard Object Oriented Tutorial),
L<perltoot> (Tom's Object Oriented Tutorial), L<perlbot> (Bag o'
Object Tricks), and L<perlobj>.

=head2 How can I tell if a variable is tainted?

You can use the tainted() function of the Scalar::Util module, available
from CPAN (or included with Perl since release 5.8.0).
See also L<perlsec/"Laundering and Detecting Tainted Data">.

=head2 What's a closure?

Closures are documented in L<perlref>.

I<Closure> is a computer science term with a precise but
hard-to-explain meaning. Usually, closures are implemented in Perl as
anonymous subroutines with lasting references to lexical variables
outside their own scopes. These lexicals magically refer to the
variables that were around when the subroutine was defined (deep
binding).

Closures are most often used in programming languages where you can
have the return value of a function be itself a function, as you can
in Perl. Note that some languages provide anonymous functions but are
not capable of providing proper closures: the Python language, for
example.  For more information on closures, check out any textbook on
functional programming.  Scheme is a language that not only supports
but encourages closures.

Here's a classic non-closure function-generating function:

	sub add_function_generator {
		return sub { shift() + shift() };
		}

	$add_sub = add_function_generator();
	$sum = $add_sub->(4,5);                # $sum is 9 now.

The anonymous subroutine returned by add_function_generator() isn't
technically a closure because it refers to no lexicals outside its own
scope.  Using a closure gives you a I<function template> with some
customization slots left out to be filled later.

Contrast this with the following make_adder() function, in which the
returned anonymous function contains a reference to a lexical variable
outside the scope of that function itself.  Such a reference requires
that Perl return a proper closure, thus locking in for all time the
value that the lexical had when the function was created.

	sub make_adder {
		my $addpiece = shift;
		return sub { shift() + $addpiece };
	}

	$f1 = make_adder(20);
	$f2 = make_adder(555);

Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas
C<&$f2($n)> is always 555 plus whatever $n you pass in.  The $addpiece
in the closure sticks around.

Closures are often used for less esoteric purposes.  For example, when
you want to pass in a bit of code into a function:

	my $line;
	timeout( 30, sub { $line = <STDIN> } );

If the code to execute had been passed in as a string,
C<< '$line = <STDIN>' >>, there would have been no way for the
hypothetical timeout() function to access the lexical variable
$line back in its caller's scope.

Another use for a closure is to make a variable I<private> to a
named subroutine, e.g. a counter that gets initialized at creation
time of the sub and can only be modified from within the sub.
This is sometimes used with a BEGIN block in package files to make
sure a variable doesn't get meddled with during the lifetime of the
package:

	BEGIN {
		my $id = 0;
		sub next_id { ++$id }
	}

This is discussed in more detail in L<perlsub>, see the entry on
I<Persistent Private Variables>.

=head2 What is variable suicide and how can I prevent it?

This problem was fixed in perl 5.004_05, so preventing it means upgrading
your version of perl. ;)

Variable suicide is when you (temporarily or permanently) lose the value
of a variable.  It is caused by scoping through my() and local()
interacting with either closures or aliased foreach() iterator variables
and subroutine arguments.  It used to be easy to inadvertently lose a
variable's value this way, but now it's much harder.  Take this code:

	my $f = 'foo';
	sub T {
		while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
		}

	T;
	print "Finally $f\n";

If you are experiencing variable suicide, that C<my $f> in the subroutine
doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output
shows that inside the subroutine the value of C<$f> leaks through when it
shouldn't, as in this output:

	foobar
	foobarbar
	foobarbarbar
	Finally foo

The $f that has "bar" added to it three times should be a new C<$f>
C<my $f> should create a new lexical variable each time through the loop.
The expected output is:

	foobar
	foobar
	foobar
	Finally foo

=head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?

With the exception of regexes, you need to pass references to these
objects.  See L<perlsub/"Pass by Reference"> for this particular
question, and L<perlref> for information on references.

See "Passing Regexes", later in L<perlfaq7>, for information on
passing regular expressions.

=over 4

=item Passing Variables and Functions

Regular variables and functions are quite easy to pass: just pass in a
reference to an existing or anonymous variable or function:

	func( \$some_scalar );

	func( \@some_array  );
	func( [ 1 .. 10 ]   );

	func( \%some_hash   );
	func( { this => 10, that => 20 }   );

	func( \&some_func   );
	func( sub { $_[0] ** $_[1] }   );

=item Passing Filehandles

As of Perl 5.6, you can represent filehandles with scalar variables
which you treat as any other scalar.

	open my $fh, $filename or die "Cannot open $filename! $!";
	func( $fh );

	sub func {
		my $passed_fh = shift;

		my $line = <$passed_fh>;
		}

Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations.
These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles">
and especially L<perlsub/"Pass by Reference"> for more information.

=item Passing Regexes

To pass regexes around, you'll need to be using a release of Perl
sufficiently recent as to support the C<qr//> construct, pass around
strings and use an exception-trapping eval, or else be very, very clever.

Here's an example of how to pass in a string to be regex compared
using C<qr//>:

	sub compare($$) {
		my ($val1, $regex) = @_;
		my $retval = $val1 =~ /$regex/;
	return $retval;
	}
	$match = compare("old McDonald", qr/d.*D/i);

=item Passing Methods

To pass an object method into a subroutine, you can do this:

	call_a_lot(10, $some_obj, "methname")
	sub call_a_lot {
		my ($count, $widget, $trick) = @_;
		for (my $i = 0; $i < $count; $i++) {
			$widget->$trick();
		}
	}

Or, you can use a closure to bundle up the object, its
method call, and arguments:

	my $whatnot =  sub { $some_obj->obfuscate(@args) };
	func($whatnot);
	sub func {
		my $code = shift;
		&$code();
	}

You could also investigate the can() method in the UNIVERSAL class
(part of the standard perl distribution).

=back

=head2 How do I create a static variable?

(contributed by brian d foy)

In Perl 5.10, declare the variable with C<state>. The C<state>
declaration creates the lexical variable that persists between calls
to the subroutine:

	sub counter { state $count = 1; $counter++ }

You can fake a static variable by using a lexical variable which goes
out of scope. In this example, you define the subroutine C<counter>, and
it uses the lexical variable C<$count>. Since you wrap this in a BEGIN
block, C<$count> is defined at compile-time, but also goes out of
scope at the end of the BEGIN block. The BEGIN block also ensures that
the subroutine and the value it uses is defined at compile-time so the
subroutine is ready to use just like any other subroutine, and you can
put this code in the same place as other subroutines in the program
text (i.e. at the end of the code, typically). The subroutine
C<counter> still has a reference to the data, and is the only way you
can access the value (and each time you do, you increment the value).
The data in chunk of memory defined by C<$count> is private to
C<counter>.

	BEGIN {
		my $count = 1;
		sub counter { $count++ }
	}

	my $start = counter();

	.... # code that calls counter();

	my $end = counter();

In the previous example, you created a function-private variable
because only one function remembered its reference. You could define
multiple functions while the variable is in scope, and each function
can share the "private" variable. It's not really "static" because you
can access it outside the function while the lexical variable is in
scope, and even create references to it. In this example,
C<increment_count> and C<return_count> share the variable. One
function adds to the value and the other simply returns the value.
They can both access C<$count>, and since it has gone out of scope,
there is no other way to access it.

	BEGIN {
		my $count = 1;
		sub increment_count { $count++ }
		sub return_count    { $count }
	}

To declare a file-private variable, you still use a lexical variable.
A file is also a scope, so a lexical variable defined in the file
cannot be seen from any other file.

See L<perlsub/"Persistent Private Variables"> for more information.
The discussion of closures in L<perlref> may help you even though we
did not use anonymous subroutines in this answer. See
L<perlsub/"Persistent Private Variables"> for details.

=head2 What's the difference between dynamic and lexical (static) scoping?  Between local() and my()?

C<local($x)> saves away the old value of the global variable C<$x>
and assigns a new value for the duration of the subroutine I<which is
visible in other functions called from that subroutine>.  This is done
at run-time, so is called dynamic scoping.  local() always affects global
variables, also called package variables or dynamic variables.

C<my($x)> creates a new variable that is only visible in the current
subroutine.  This is done at compile-time, so it is called lexical or
static scoping.  my() always affects private variables, also called
lexical variables or (improperly) static(ly scoped) variables.

For instance:

	sub visible {
		print "var has value $var\n";
		}

	sub dynamic {
		local $var = 'local';	# new temporary value for the still-global
		visible();              #   variable called $var
		}

	sub lexical {
		my $var = 'private';    # new private variable, $var
		visible();              # (invisible outside of sub scope)
		}

	$var = 'global';

	visible();      		# prints global
	dynamic();      		# prints local
	lexical();      		# prints global

Notice how at no point does the value "private" get printed.  That's
because $var only has that value within the block of the lexical()
function, and it is hidden from called subroutine.

In summary, local() doesn't make what you think of as private, local
variables.  It gives a global variable a temporary value.  my() is
what you're looking for if you want private variables.

See L<perlsub/"Private Variables via my()"> and
L<perlsub/"Temporary Values via local()"> for excruciating details.

=head2 How can I access a dynamic variable while a similarly named lexical is in scope?

If you know your package, you can just mention it explicitly, as in
$Some_Pack::var. Note that the notation $::var is B<not> the dynamic $var
in the current package, but rather the one in the "main" package, as
though you had written $main::var.

	use vars '$var';
	local $var = "global";
	my    $var = "lexical";

	print "lexical is $var\n";
	print "global  is $main::var\n";

Alternatively you can use the compiler directive our() to bring a
dynamic variable into the current lexical scope.

	require 5.006; # our() did not exist before 5.6
	use vars '$var';

	local $var = "global";
	my $var    = "lexical";

	print "lexical is $var\n";

	{
		our $var;
		print "global  is $var\n";
	}

=head2 What's the difference between deep and shallow binding?

In deep binding, lexical variables mentioned in anonymous subroutines
are the same ones that were in scope when the subroutine was created.
In shallow binding, they are whichever variables with the same names
happen to be in scope when the subroutine is called.  Perl always uses
deep binding of lexical variables (i.e., those created with my()).
However, dynamic variables (aka global, local, or package variables)
are effectively shallowly bound.  Consider this just one more reason
not to use them.  See the answer to L<"What's a closure?">.

=head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?

C<my()> and C<local()> give list context to the right hand side
of C<=>.  The <FH> read operation, like so many of Perl's
functions and operators, can tell which context it was called in and
behaves appropriately.  In general, the scalar() function can help.
This function does nothing to the data itself (contrary to popular myth)
but rather tells its argument to behave in whatever its scalar fashion is.
If that function doesn't have a defined scalar behavior, this of course
doesn't help you (such as with sort()).

To enforce scalar context in this particular case, however, you need
merely omit the parentheses:

	local($foo) = <FILE>;	    # WRONG
	local($foo) = scalar(<FILE>);   # ok
	local $foo  = <FILE>;	    # right

You should probably be using lexical variables anyway, although the
issue is the same here:

	my($foo) = <FILE>;	# WRONG
	my $foo  = <FILE>;	# right

=head2 How do I redefine a builtin function, operator, or method?

Why do you want to do that? :-)

If you want to override a predefined function, such as open(),
then you'll have to import the new definition from a different
module.  See L<perlsub/"Overriding Built-in Functions">.  There's
also an example in L<perltoot/"Class::Template">.

If you want to overload a Perl operator, such as C<+> or C<**>,
then you'll want to use the C<use overload> pragma, documented
in L<overload>.

If you're talking about obscuring method calls in parent classes,
see L<perltoot/"Overridden Methods">.

=head2 What's the difference between calling a function as &foo and foo()?

(contributed by brian d foy)

Calling a subroutine as C<&foo> with no trailing parentheses ignores
the prototype of C<foo> and passes it the current value of the argument
list, C<@_>. Here's an example; the C<bar> subroutine calls C<&foo>,
which prints its arguments list:

	sub bar { &foo }

	sub foo { print "Args in foo are: @_\n" }

	bar( qw( a b c ) );

When you call C<bar> with arguments, you see that C<foo> got the same C<@_>:

	Args in foo are: a b c

Calling the subroutine with trailing parentheses, with or without arguments,
does not use the current C<@_> and respects the subroutine prototype. Changing
the example to put parentheses after the call to C<foo> changes the program:

	sub bar { &foo() }

	sub foo { print "Args in foo are: @_\n" }

	bar( qw( a b c ) );

Now the output shows that C<foo> doesn't get the C<@_> from its caller.

	Args in foo are:

The main use of the C<@_> pass-through feature is to write subroutines
whose main job it is to call other subroutines for you. For further
details, see L<perlsub>.

=head2 How do I create a switch or case statement?

In Perl 5.10, use the C<given-when> construct described in L<perlsyn>:

	use 5.010;

	given ( $string ) {
		when( 'Fred' )        { say "I found Fred!" }
		when( 'Barney' )      { say "I found Barney!" }
		when( /Bamm-?Bamm/ )  { say "I found Bamm-Bamm!" }
		default               { say "I don't recognize the name!" }
		};

If one wants to use pure Perl and to be compatible with Perl versions
prior to 5.10, the general answer is to use C<if-elsif-else>:

	for ($variable_to_test) {
		if    (/pat1/)  { }     # do something
		elsif (/pat2/)  { }     # do something else
		elsif (/pat3/)  { }     # do something else
		else            { }     # default
		}

Here's a simple example of a switch based on pattern matching,
lined up in a way to make it look more like a switch statement.
We'll do a multiway conditional based on the type of reference stored
in $whatchamacallit:

    SWITCH: for (ref $whatchamacallit) {

	/^$/		&& die "not a reference";

	/SCALAR/	&& do {
				print_scalar($$ref);
				last SWITCH;
			};

	/ARRAY/		&& do {
				print_array(@$ref);
				last SWITCH;
			};

	/HASH/		&& do {
				print_hash(%$ref);
				last SWITCH;
			};

	/CODE/		&& do {
				warn "can't print function ref";
				last SWITCH;
			};

	# DEFAULT

	warn "User defined type skipped";

    }

See L<perlsyn> for other examples in this style.

Sometimes you should change the positions of the constant and the variable.
For example, let's say you wanted to test which of many answers you were
given, but in a case-insensitive way that also allows abbreviations.
You can use the following technique if the strings all start with
different characters or if you want to arrange the matches so that
one takes precedence over another, as C<"SEND"> has precedence over
C<"STOP"> here:

	chomp($answer = <>);
	if    ("SEND"  =~ /^\Q$answer/i) { print "Action is send\n"  }
	elsif ("STOP"  =~ /^\Q$answer/i) { print "Action is stop\n"  }
	elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
	elsif ("LIST"  =~ /^\Q$answer/i) { print "Action is list\n"  }
	elsif ("EDIT"  =~ /^\Q$answer/i) { print "Action is edit\n"  }

A totally different approach is to create a hash of function references.

	my %commands = (
		"happy" => \&joy,
		"sad",  => \&sullen,
		"done"  => sub { die "See ya!" },
		"mad"   => \&angry,
	);

	print "How are you? ";
	chomp($string = <STDIN>);
	if ($commands{$string}) {
		$commands{$string}->();
	} else {
		print "No such command: $string\n";
	}

Starting from Perl 5.8, a source filter module, C<Switch>, can also be
used to get switch and case. Its use is now discouraged, because it's
not fully compatible with the native switch of Perl 5.10, and because,
as it's implemented as a source filter, it doesn't always work as intended
when complex syntax is involved.

=head2 How can I catch accesses to undefined variables, functions, or methods?

The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and
L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to
undefined functions and methods.

When it comes to undefined variables that would trigger a warning
under C<use warnings>, you can promote the warning to an error.

	use warnings FATAL => qw(uninitialized);

=head2 Why can't a method included in this same file be found?

Some possible reasons: your inheritance is getting confused, you've
misspelled the method name, or the object is of the wrong type.  Check
out L<perltoot> for details about any of the above cases.  You may
also use C<print ref($object)> to find out the class C<$object> was
blessed into.

Another possible reason for problems is because you've used the
indirect object syntax (eg, C<find Guru "Samy">) on a class name
before Perl has seen that such a package exists.  It's wisest to make
sure your packages are all defined before you start using them, which
will be taken care of if you use the C<use> statement instead of
C<require>.  If not, make sure to use arrow notation (eg.,
C<< Guru->find("Samy") >>) instead.  Object notation is explained in
L<perlobj>.

Make sure to read about creating modules in L<perlmod> and
the perils of indirect objects in L<perlobj/"Method Invocation">.

=head2 How can I find out my current or calling package?

(contributed by brian d foy)

To find the package you are currently in, use the special literal
C<__PACKAGE__>, as documented in L<perldata>. You can only use the
special literals as separate tokens, so you can't interpolate them
into strings like you can with variables:

	my $current_package = __PACKAGE__;
	print "I am in package $current_package\n";

This is different from finding out the package an object is blessed
into, which might not be the current package. For that, use C<blessed>
from C<Scalar::Util>, part of the Standard Library since Perl 5.8:

	use Scalar::Util qw(blessed);
	my $object_package = blessed( $object );

Most of the time, you shouldn't care what package an object is blessed
into, however, as long as it claims to inherit from that class:

	my $is_right_class = eval { $object->isa( $package ) }; # true or false

If you want to find the package calling your code, perhaps to give better
diagnostics as C<Carp> does, use the C<caller> built-in:

	sub foo {
		my @args = ...;
		my( $package, $filename, $line ) = caller;

		print "I was called from package $package\n";
		);

By default, your program starts in package C<main>, so you should
always be in some package unless someone uses the C<package> built-in
with no namespace. See the C<package> entry in L<perlfunc> for the
details of empty packages.

=head2 How can I comment out a large block of Perl code?

(contributed by brian d foy)

The quick-and-dirty way to comment out more than one line of Perl is
to surround those lines with Pod directives. You have to put these
directives at the beginning of the line and somewhere where Perl
expects a new statement (so not in the middle of statements like the #
comments). You end the comment with C<=cut>, ending the Pod section:

	=pod

	my $object = NotGonnaHappen->new();

	ignored_sub();

	$wont_be_assigned = 37;

	=cut

The quick-and-dirty method only works well when you don't plan to 
leave the commented code in the source. If a Pod parser comes along,
you're multiline comment is going to show up in the Pod translation.
A better way hides it from Pod parsers as well. 

The C<=begin> directive can mark a section for a particular purpose.
If the Pod parser doesn't want to handle it, it just ignores it. Label
the comments with C<comment>. End the comment using C<=end> with the
same label. You still need the C<=cut> to go back to Perl code from
the Pod comment:

	=begin comment

	my $object = NotGonnaHappen->new();

	ignored_sub();

	$wont_be_assigned = 37;

	=end comment

	=cut

For more information on Pod, check out L<perlpod> and L<perlpodspec>.

=head2 How do I clear a package?

Use this code, provided by Mark-Jason Dominus:

	sub scrub_package {
		no strict 'refs';
		my $pack = shift;
		die "Shouldn't delete main package"
			if $pack eq "" || $pack eq "main";
		my $stash = *{$pack . '::'}{HASH};
		my $name;
		foreach $name (keys %$stash) {
			my $fullname = $pack . '::' . $name;
			# Get rid of everything with that name.
			undef $$fullname;
			undef @$fullname;
			undef %$fullname;
			undef &$fullname;
			undef *$fullname;
	}
	}

Or, if you're using a recent release of Perl, you can
just use the Symbol::delete_package() function instead.

=head2 How can I use a variable as a variable name?

Beginners often think they want to have a variable contain the name
of a variable.

	$fred    = 23;
	$varname = "fred";
	++$$varname;         # $fred now 24

This works I<sometimes>, but it is a very bad idea for two reasons.

The first reason is that this technique I<only works on global
variables>.  That means that if $fred is a lexical variable created
with my() in the above example, the code wouldn't work at all: you'd
accidentally access the global and skip right over the private lexical
altogether.  Global variables are bad because they can easily collide
accidentally and in general make for non-scalable and confusing code.

Symbolic references are forbidden under the C<use strict> pragma.
They are not true references and consequently are not reference counted
or garbage collected.

The other reason why using a variable to hold the name of another
variable is a bad idea is that the question often stems from a lack of
understanding of Perl data structures, particularly hashes.  By using
symbolic references, you are just using the package's symbol-table hash
(like C<%main::>) instead of a user-defined hash.  The solution is to
use your own hash or a real reference instead.

	$USER_VARS{"fred"} = 23;
	$varname = "fred";
	$USER_VARS{$varname}++;  # not $$varname++

There we're using the %USER_VARS hash instead of symbolic references.
Sometimes this comes up in reading strings from the user with variable
references and wanting to expand them to the values of your perl
program's variables.  This is also a bad idea because it conflates the
program-addressable namespace and the user-addressable one.  Instead of
reading a string and expanding it to the actual contents of your program's
own variables:

	$str = 'this has a $fred and $barney in it';
	$str =~ s/(\$\w+)/$1/eeg;		  # need double eval

it would be better to keep a hash around like %USER_VARS and have
variable references actually refer to entries in that hash:

	$str =~ s/\$(\w+)/$USER_VARS{$1}/g;   # no /e here at all

That's faster, cleaner, and safer than the previous approach.  Of course,
you don't need to use a dollar sign.  You could use your own scheme to
make it less confusing, like bracketed percent symbols, etc.

	$str = 'this has a %fred% and %barney% in it';
	$str =~ s/%(\w+)%/$USER_VARS{$1}/g;   # no /e here at all

Another reason that folks sometimes think they want a variable to
contain the name of a variable is because they don't know how to build
proper data structures using hashes.  For example, let's say they
wanted two hashes in their program: %fred and %barney, and that they
wanted to use another scalar variable to refer to those by name.

	$name = "fred";
	$$name{WIFE} = "wilma";     # set %fred

	$name = "barney";
	$$name{WIFE} = "betty";	# set %barney

This is still a symbolic reference, and is still saddled with the
problems enumerated above.  It would be far better to write:

	$folks{"fred"}{WIFE}   = "wilma";
	$folks{"barney"}{WIFE} = "betty";

And just use a multilevel hash to start with.

The only times that you absolutely I<must> use symbolic references are
when you really must refer to the symbol table.  This may be because it's
something that can't take a real reference to, such as a format name.
Doing so may also be important for method calls, since these always go
through the symbol table for resolution.

In those cases, you would turn off C<strict 'refs'> temporarily so you
can play around with the symbol table.  For example:

	@colors = qw(red blue green yellow orange purple violet);
	for my $name (@colors) {
		no strict 'refs';  # renege for the block
		*$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
	}

All those functions (red(), blue(), green(), etc.) appear to be separate,
but the real code in the closure actually was compiled only once.

So, sometimes you might want to use symbolic references to directly
manipulate the symbol table.  This doesn't matter for formats, handles, and
subroutines, because they are always global--you can't use my() on them.
For scalars, arrays, and hashes, though--and usually for subroutines--
you probably only want to use hard references.

=head2 What does "bad interpreter" mean?

(contributed by brian d foy)

The "bad interpreter" message comes from the shell, not perl.  The
actual message may vary depending on your platform, shell, and locale
settings.

If you see "bad interpreter - no such file or directory", the first
line in your perl script (the "shebang" line) does not contain the
right path to perl (or any other program capable of running scripts).
Sometimes this happens when you move the script from one machine to
another and each machine has a different path to perl--/usr/bin/perl
versus /usr/local/bin/perl for instance. It may also indicate
that the source machine has CRLF line terminators and the
destination machine has LF only: the shell tries to find
/usr/bin/perl<CR>, but can't.

If you see "bad interpreter: Permission denied", you need to make your
script executable.

In either case, you should still be able to run the scripts with perl
explicitly:

	% perl script.pl

If you get a message like "perl: command not found", perl is not in
your PATH, which might also mean that the location of perl is not
where you expect it so you need to adjust your shebang line.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.

This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.

Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain.  You are permitted and
encouraged to use this code in your own programs for fun
or for profit as you see fit.  A simple comment in the code giving
credit would be courteous but is not required.