8 A Perl program consists of a sequence of declarations and statements
9 which run from the top to the bottom. Loops, subroutines, and other
10 control structures allow you to jump around within the code.
12 Perl is a B<free-form> language: you can format and indent it however
13 you like. Whitespace serves mostly to separate tokens, unlike
14 languages like Python where it is an important part of the syntax,
15 or Fortran where it is immaterial.
17 Many of Perl's syntactic elements are B<optional>. Rather than
18 requiring you to put parentheses around every function call and
19 declare every variable, you can often leave such explicit elements off
20 and Perl will figure out what you meant. This is known as B<Do What I
21 Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
22 code in a style with which they are comfortable.
24 Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
25 Bourne Shell, Smalltalk, Lisp and even English. Other
26 languages have borrowed syntax from Perl, particularly its regular
27 expression extensions. So if you have programmed in another language
28 you will see familiar pieces in Perl. They often work the same, but
29 see L<perltrap> for information about how they differ.
32 X<declaration> X<undef> X<undefined> X<uninitialized>
34 The only things you need to declare in Perl are report formats and
35 subroutines (and sometimes not even subroutines). A scalar variable holds
36 the undefined value (C<undef>) until it has been assigned a defined
37 value, which is anything other than C<undef>. When used as a number,
38 C<undef> is treated as C<0>; when used as a string, it is treated as
39 the empty string, C<"">; and when used as a reference that isn't being
40 assigned to, it is treated as an error. If you enable warnings,
41 you'll be notified of an uninitialized value whenever you treat
42 C<undef> as a string or a number. Well, usually. Boolean contexts,
47 are exempt from warnings (because they care about truth rather than
48 definedness). Operators such as C<++>, C<-->, C<+=>,
49 C<-=>, and C<.=>, that operate on undefined variables such as:
54 are also always exempt from such warnings.
56 A declaration can be put anywhere a statement can, but has no effect on
57 the execution of the primary sequence of statements: declarations all
58 take effect at compile time. All declarations are typically put at
59 the beginning or the end of the script. However, if you're using
60 lexically-scoped private variables created with C<my()>,
61 C<state()>, or C<our()>, you'll have to make sure
62 your format or subroutine definition is within the same block scope
63 as the my if you expect to be able to access those private variables.
65 Declaring a subroutine allows a subroutine name to be used as if it were a
66 list operator from that point forward in the program. You can declare a
67 subroutine without defining it by saying C<sub name>, thus:
68 X<subroutine, declaration>
71 $me = myname $0 or die "can't get myname";
73 A bare declaration like that declares the function to be a list operator,
74 not a unary operator, so you have to be careful to use parentheses (or
75 C<or> instead of C<||>.) The C<||> operator binds too tightly to use after
76 list operators; it becomes part of the last element. You can always use
77 parentheses around the list operators arguments to turn the list operator
78 back into something that behaves more like a function call. Alternatively,
79 you can use the prototype C<($)> to turn the subroutine into a unary
83 $me = myname $0 || die "can't get myname";
85 That now parses as you'd expect, but you still ought to get in the habit of
86 using parentheses in that situation. For more on prototypes, see
89 Subroutines declarations can also be loaded up with the C<require> statement
90 or both loaded and imported into your namespace with a C<use> statement.
91 See L<perlmod> for details on this.
93 A statement sequence may contain declarations of lexically-scoped
94 variables, but apart from declaring a variable name, the declaration acts
95 like an ordinary statement, and is elaborated within the sequence of
96 statements as if it were an ordinary statement. That means it actually
97 has both compile-time and run-time effects.
102 Text from a C<"#"> character until the end of the line is a comment,
103 and is ignored. Exceptions include C<"#"> inside a string or regular
106 =head2 Simple Statements
107 X<statement> X<semicolon> X<expression> X<;>
109 The only kind of simple statement is an expression evaluated for its
110 side-effects. Every simple statement must be terminated with a
111 semicolon, unless it is the final statement in a block, in which case
112 the semicolon is optional. But put the semicolon in anyway if the
113 block takes up more than one line, because you may eventually add
114 another line. Note that there are operators like C<eval {}>, C<sub {}>, and
115 C<do {}> that I<look> like compound statements, but aren't--they're just
116 TERMs in an expression--and thus need an explicit termination when used
117 as the last item in a statement.
119 =head2 Truth and Falsehood
120 X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0>
122 The number 0, the strings C<'0'> and C<"">, the empty list C<()>, and
123 C<undef> are all false in a boolean context. All other values are true.
124 Negation of a true value by C<!> or C<not> returns a special false value.
125 When evaluated as a string it is treated as C<"">, but as a number, it
126 is treated as 0. Most Perl operators
127 that return true or false behave this way.
129 =head2 Statement Modifiers
130 X<statement modifier> X<modifier> X<if> X<unless> X<while>
131 X<until> X<when> X<foreach> X<for>
133 Any simple statement may optionally be followed by a I<SINGLE> modifier,
134 just before the terminating semicolon (or block ending). The possible
145 The C<EXPR> following the modifier is referred to as the "condition".
146 Its truth or falsehood determines how the modifier will behave.
148 C<if> executes the statement once I<if> and only if the condition is
149 true. C<unless> is the opposite, it executes the statement I<unless>
150 the condition is true (that is, if the condition is false).
152 print "Basset hounds got long ears" if length $ear >= 10;
153 go_outside() and play() unless $is_raining;
155 The C<for(each)> modifier is an iterator: it executes the statement once
156 for each item in the LIST (with C<$_> aliased to each item in turn).
158 print "Hello $_!\n" for qw(world Dolly nurse);
160 C<while> repeats the statement I<while> the condition is true.
161 C<until> does the opposite, it repeats the statement I<until> the
162 condition is true (or while the condition is false):
164 # Both of these count from 0 to 10.
165 print $i++ while $i <= 10;
166 print $j++ until $j > 10;
168 The C<while> and C<until> modifiers have the usual "C<while> loop"
169 semantics (conditional evaluated first), except when applied to a
170 C<do>-BLOCK (or to the Perl4 C<do>-SUBROUTINE statement), in
171 which case the block executes once before the conditional is
174 This is so that you can write loops like:
179 } until !defined($line) || $line eq ".\n"
181 See L<perlfunc/do>. Note also that the loop control statements described
182 later will I<NOT> work in this construct, because modifiers don't take
183 loop labels. Sorry. You can always put another block inside of it
184 (for C<next>) or around it (for C<last>) to do that sort of thing.
185 For C<next>, just double the braces:
186 X<next> X<last> X<redo>
193 For C<last>, you have to be more elaborate:
203 B<NOTE:> The behaviour of a C<my>, C<state>, or
204 C<our> modified with a statement modifier conditional
205 or loop construct (for example, C<my $x if ...>) is
206 B<undefined>. The value of the C<my> variable may be C<undef>, any
207 previously assigned value, or possibly anything else. Don't rely on
208 it. Future versions of perl might do something different from the
209 version of perl you try it out on. Here be dragons.
212 The C<when> modifier is an experimental feature that first appeared in Perl
213 5.14. To use it, you should include a C<use v5.14> declaration.
214 (Technically, it requires only the C<switch> feature, but that aspect of it
215 was not available before 5.14.) Operative only from within a C<foreach>
216 loop or a C<given> block, it executes the statement only if the smartmatch
217 C<< $_ ~~ I<EXPR> >> is true. If the statement executes, it is followed by
218 a C<next> from inside a C<foreach> and C<break> from inside a C<given>.
220 Under the current implementation, the C<foreach> loop can be
221 anywhere within the C<when> modifier's dynamic scope, but must be
222 within the C<given> block's lexical scope. This restricted may
223 be relaxed in a future release. See L<"Switch Statements"> below.
225 =head2 Compound Statements
226 X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
227 X<{> X<}> X<if> X<unless> X<given> X<while> X<until> X<foreach> X<for> X<continue>
229 In Perl, a sequence of statements that defines a scope is called a block.
230 Sometimes a block is delimited by the file containing it (in the case
231 of a required file, or the program as a whole), and sometimes a block
232 is delimited by the extent of a string (in the case of an eval).
234 But generally, a block is delimited by curly brackets, also known as braces.
235 We will call this syntactic construct a BLOCK.
237 The following compound statements may be used to control flow:
240 if (EXPR) BLOCK else BLOCK
241 if (EXPR) BLOCK elsif (EXPR) BLOCK ...
242 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
245 unless (EXPR) BLOCK else BLOCK
246 unless (EXPR) BLOCK elsif (EXPR) BLOCK ...
247 unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
251 LABEL while (EXPR) BLOCK
252 LABEL while (EXPR) BLOCK continue BLOCK
254 LABEL until (EXPR) BLOCK
255 LABEL until (EXPR) BLOCK continue BLOCK
257 LABEL for (EXPR; EXPR; EXPR) BLOCK
258 LABEL for VAR (LIST) BLOCK
259 LABEL for VAR (LIST) BLOCK continue BLOCK
261 LABEL foreach (EXPR; EXPR; EXPR) BLOCK
262 LABEL foreach VAR (LIST) BLOCK
263 LABEL foreach VAR (LIST) BLOCK continue BLOCK
266 LABEL BLOCK continue BLOCK
270 The experimental C<given> statement is I<not automatically enabled>; see
271 L</"Switch Statements"> below for how to do so, and the attendant caveats.
273 Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs,
274 not statements. This means that the curly brackets are I<required>--no
275 dangling statements allowed. If you want to write conditionals without
276 curly brackets, there are several other ways to do it. The following
277 all do the same thing:
279 if (!open(FOO)) { die "Can't open $FOO: $!" }
280 die "Can't open $FOO: $!" unless open(FOO);
281 open(FOO) || die "Can't open $FOO: $!";
282 open(FOO) ? () : die "Can't open $FOO: $!";
283 # a bit exotic, that last one
285 The C<if> statement is straightforward. Because BLOCKs are always
286 bounded by curly brackets, there is never any ambiguity about which
287 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
288 the sense of the test is reversed. Like C<if>, C<unless> can be followed
289 by C<else>. C<unless> can even be followed by one or more C<elsif>
290 statements, though you may want to think twice before using that particular
291 language construct, as everyone reading your code will have to think at least
292 twice before they can understand what's going on.
294 The C<while> statement executes the block as long as the expression is
295 L<true|/"Truth and Falsehood">.
296 The C<until> statement executes the block as long as the expression is
298 The LABEL is optional, and if present, consists of an identifier followed
299 by a colon. The LABEL identifies the loop for the loop control
300 statements C<next>, C<last>, and C<redo>.
301 If the LABEL is omitted, the loop control statement
302 refers to the innermost enclosing loop. This may include dynamically
303 looking back your call-stack at run time to find the LABEL. Such
304 desperate behavior triggers a warning if you use the C<use warnings>
305 pragma or the B<-w> flag.
307 If there is a C<continue> BLOCK, it is always executed just before the
308 conditional is about to be evaluated again. Thus it can be used to
309 increment a loop variable, even when the loop has been continued via
310 the C<next> statement.
312 When a block is preceding by a compilation phase keyword such as C<BEGIN>,
313 C<END>, C<INIT>, C<CHECK>, or C<UNITCHECK>, then the block will run only
314 during the corresponding phase of execution. See L<perlmod> for more details.
316 Extension modules can also hook into the Perl parser to define new
317 kinds of compound statements. These are introduced by a keyword which
318 the extension recognizes, and the syntax following the keyword is
319 defined entirely by the extension. If you are an implementor, see
320 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
321 a module, see the module's documentation for details of the syntax that
325 X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
327 The C<next> command starts the next iteration of the loop:
329 LINE: while (<STDIN>) {
330 next LINE if /^#/; # discard comments
334 The C<last> command immediately exits the loop in question. The
335 C<continue> block, if any, is not executed:
337 LINE: while (<STDIN>) {
338 last LINE if /^$/; # exit when done with header
342 The C<redo> command restarts the loop block without evaluating the
343 conditional again. The C<continue> block, if any, is I<not> executed.
344 This command is normally used by programs that want to lie to themselves
345 about what was just input.
347 For example, when processing a file like F</etc/termcap>.
348 If your input lines might end in backslashes to indicate continuation, you
349 want to skip ahead and get the next record.
360 which is Perl shorthand for the more explicitly written version:
362 LINE: while (defined($line = <ARGV>)) {
364 if ($line =~ s/\\$//) {
366 redo LINE unless eof(); # not eof(ARGV)!
371 Note that if there were a C<continue> block on the above code, it would
372 get executed only on lines discarded by the regex (since redo skips the
373 continue block). A continue block is often used to reset line counters
374 or C<m?pat?> one-time matches:
376 # inspired by :1,$g/fred/s//WILMA/
378 m?(fred)? && s//WILMA $1 WILMA/;
379 m?(barney)? && s//BETTY $1 BETTY/;
380 m?(homer)? && s//MARGE $1 MARGE/;
382 print "$ARGV $.: $_";
383 close ARGV if eof; # reset $.
384 reset if eof; # reset ?pat?
387 If the word C<while> is replaced by the word C<until>, the sense of the
388 test is reversed, but the conditional is still tested before the first
391 Loop control statements don't work in an C<if> or C<unless>, since
392 they aren't loops. You can double the braces to make them such, though.
396 next if /barney/; # same effect as "last",
397 # but doesn't document as well
401 This is caused by the fact that a block by itself acts as a loop that
402 executes once, see L<"Basic BLOCKs">.
404 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
405 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
410 Perl's C-style C<for> loop works like the corresponding C<while> loop;
411 that means that this:
413 for ($i = 1; $i < 10; $i++) {
426 There is one minor difference: if variables are declared with C<my>
427 in the initialization section of the C<for>, the lexical scope of
428 those variables is exactly the C<for> loop (the body of the loop
429 and the control sections).
432 Besides the normal array index looping, C<for> can lend itself
433 to many other interesting applications. Here's one that avoids the
434 problem you get into if you explicitly test for end-of-file on
435 an interactive file descriptor causing your program to appear to
437 X<eof> X<end-of-file> X<end of file>
439 $on_a_tty = -t STDIN && -t STDOUT;
440 sub prompt { print "yes? " if $on_a_tty }
441 for ( prompt(); <STDIN>; prompt() ) {
445 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
446 conditional of a C<for> loop is shorthand for the following. This
447 behaviour is the same as a C<while> loop conditional.
448 X<readline> X<< <> >>
450 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
457 The C<foreach> loop iterates over a normal list value and sets the
458 variable VAR to be each element of the list in turn. If the variable
459 is preceded with the keyword C<my>, then it is lexically scoped, and
460 is therefore visible only within the loop. Otherwise, the variable is
461 implicitly local to the loop and regains its former value upon exiting
462 the loop. If the variable was previously declared with C<my>, it uses
463 that variable instead of the global one, but it's still localized to
464 the loop. This implicit localization occurs I<only> in a C<foreach>
468 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
469 you can use either. If VAR is omitted, C<$_> is set to each value.
472 If any element of LIST is an lvalue, you can modify it by modifying
473 VAR inside the loop. Conversely, if any element of LIST is NOT an
474 lvalue, any attempt to modify that element will fail. In other words,
475 the C<foreach> loop index variable is an implicit alias for each item
476 in the list that you're looping over.
479 If any part of LIST is an array, C<foreach> will get very confused if
480 you add or remove elements within the loop body, for example with
481 C<splice>. So don't do that.
484 C<foreach> probably won't do what you expect if VAR is a tied or other
485 special variable. Don't do that either.
489 for (@ary) { s/foo/bar/ }
491 for my $elem (@elements) {
495 for $count (reverse(1..10), "BOOM") {
500 for (1..15) { print "Merry Christmas\n"; }
502 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
503 print "Item: $item\n";
506 Here's how a C programmer might code up a particular algorithm in Perl:
508 for (my $i = 0; $i < @ary1; $i++) {
509 for (my $j = 0; $j < @ary2; $j++) {
510 if ($ary1[$i] > $ary2[$j]) {
511 last; # can't go to outer :-(
513 $ary1[$i] += $ary2[$j];
515 # this is where that last takes me
518 Whereas here's how a Perl programmer more comfortable with the idiom might
521 OUTER: for my $wid (@ary1) {
522 INNER: for my $jet (@ary2) {
523 next OUTER if $wid > $jet;
528 See how much easier this is? It's cleaner, safer, and faster. It's
529 cleaner because it's less noisy. It's safer because if code gets added
530 between the inner and outer loops later on, the new code won't be
531 accidentally executed. The C<next> explicitly iterates the other loop
532 rather than merely terminating the inner one. And it's faster because
533 Perl executes a C<foreach> statement more rapidly than it would the
534 equivalent C<for> loop.
539 A BLOCK by itself (labeled or not) is semantically equivalent to a
540 loop that executes once. Thus you can use any of the loop control
541 statements in it to leave or restart the block. (Note that this is
542 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
543 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
546 The BLOCK construct can be used to emulate case structures.
549 if (/^abc/) { $abc = 1; last SWITCH; }
550 if (/^def/) { $def = 1; last SWITCH; }
551 if (/^xyz/) { $xyz = 1; last SWITCH; }
555 You'll also find that C<foreach> loop used to create a topicalizer
560 if (/^abc/) { $abc = 1; last SWITCH; }
561 if (/^def/) { $def = 1; last SWITCH; }
562 if (/^xyz/) { $xyz = 1; last SWITCH; }
566 Such constructs are quite frequently used, both because older versions of
567 Perl had no official C<switch> statement, and also because the new version
568 described immediately below remains experimental and can sometimes be confusing.
570 =head2 Switch Statements
572 X<switch> X<case> X<given> X<when> X<default>
574 Starting from Perl 5.10.1 (well, 5.10.0, but it didn't work
577 use feature "switch";
579 to enable an experimental switch feature. This is loosely based on an
580 old version of a Perl 6 proposal, but it no longer resembles the Perl 6
581 construct. You also get the switch feature whenever you declare that your
582 code prefers to run under a version of Perl that is 5.10 or later. For
587 Under the "switch" feature, Perl gains the experimental keywords
588 C<given>, C<when>, C<default>, C<continue>, and C<break>.
589 Starting from Perl 5.16, one can prefix the switch
590 keywords with C<CORE::> to access the feature without a C<use feature>
591 statement. The keywords C<given> and
592 C<when> are analogous to C<switch> and
593 C<case> in other languages, so the code in the previous section could be
598 when (/^abc/) { $abc = 1 }
599 when (/^def/) { $def = 1 }
600 when (/^xyz/) { $xyz = 1 }
601 default { $nothing = 1 }
604 The C<foreach> is the non-experimental way to set a topicalizer.
605 If you wish to use the highly experimental C<given>, that could be
610 when (/^abc/) { $abc = 1 }
611 when (/^def/) { $def = 1 }
612 when (/^xyz/) { $xyz = 1 }
613 default { $nothing = 1 }
616 As of 5.14, that can also be written this way:
620 $abc = 1 when /^abc/;
621 $def = 1 when /^def/;
622 $xyz = 1 when /^xyz/;
623 default { $nothing = 1 }
626 Or if you don't care to play it safe, like this:
630 $abc = 1 when /^abc/;
631 $def = 1 when /^def/;
632 $xyz = 1 when /^xyz/;
633 default { $nothing = 1 }
636 The arguments to C<given> and C<when> are in scalar context,
637 and C<given> assigns the C<$_> variable its topic value.
639 Exactly what the I<EXPR> argument to C<when> does is hard to describe
640 precisely, but in general, it tries to guess what you want done. Sometimes
641 it is interpreted as C<< $_ ~~ I<EXPR> >>, and sometimes it is not. It
642 also behaves differently when lexically enclosed by a C<given> block than
643 it does when dynamically enclosed by a C<foreach> loop. The rules are far
644 too difficult to understand to be described here. See L</"Experimental Details
645 on given and when"> later on.
647 Due to an unfortunate bug in how C<given> was implemented between Perl 5.10
648 and 5.16, under those implementations the version of C<$_> governed by
649 C<given> is merely a lexically scoped copy of the original, not a
650 dynamically scoped alias to the original, as it would be if it were a
651 C<foreach> or under both the original and the current Perl 6 language
652 specification. This bug was fixed in Perl
653 5.18. If you really want a lexical C<$_>,
654 specify that explicitly:
656 given(my $_ = EXPR) { ... }
658 If your code still needs to run on older versions,
659 stick to C<foreach> for your topicalizer and
660 you will be less unhappy.
665 Although not for the faint of heart, Perl does support a C<goto>
666 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
667 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
668 a C<goto>; it's just the name of the loop.
670 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
671 execution there. It may not be used to go into any construct that
672 requires initialization, such as a subroutine or a C<foreach> loop. It
673 also can't be used to go into a construct that is optimized away. It
674 can be used to go almost anywhere else within the dynamic scope,
675 including out of subroutines, but it's usually better to use some other
676 construct such as C<last> or C<die>. The author of Perl has never felt the
677 need to use this form of C<goto> (in Perl, that is--C is another matter).
679 The C<goto>-EXPR form expects a label name, whose scope will be resolved
680 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
681 necessarily recommended if you're optimizing for maintainability:
683 goto(("FOO", "BAR", "GLARCH")[$i]);
685 The C<goto>-&NAME form is highly magical, and substitutes a call to the
686 named subroutine for the currently running subroutine. This is used by
687 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
688 pretend that the other subroutine had been called in the first place
689 (except that any modifications to C<@_> in the current subroutine are
690 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
691 will be able to tell that this routine was called first.
693 In almost all cases like this, it's usually a far, far better idea to use the
694 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
695 resorting to a C<goto>. For certain applications, the catch and throw pair of
696 C<eval{}> and die() for exception processing can also be a prudent approach.
698 =head2 The Ellipsis Statement
702 X<elliptical statement>
703 X<unimplemented statement>
704 X<unimplemented operator>
706 X<yada-yada operator>
709 X<triple-dot operator>
711 Beginning in Perl 5.12, Perl accepts an ellipsis, "C<...>", as a
712 placeholder for code that you haven't implemented yet. This form of
713 ellipsis, the unimplemented statement, should not be confused with the
714 binary flip-flop C<...> operator. One is a statement and the other an
715 operator. (Perl doesn't usually confuse them because usually Perl can tell
716 whether it wants an operator or a statement, but see below for exceptions.)
718 When Perl 5.12 or later encounters an ellipsis statement, it parses this
719 without error, but if and when you should actually try to execute it, Perl
720 throws an exception with the text C<Unimplemented>:
723 sub unimplemented { ... }
724 eval { unimplemented() };
725 if ($@ =~ /^Unimplemented at /) {
726 say "I found an ellipsis!";
729 You can only use the elliptical statement to stand in for a
730 complete statement. These examples of how the ellipsis works:
748 The elliptical statement cannot stand in for an expression that
749 is part of a larger statement, since the C<...> is also the three-dot
750 version of the flip-flop operator (see L<perlop/"Range Operators">).
752 These examples of attempts to use an ellipsis are syntax errors:
757 open(my $fh, ">", "/dev/passwd") or ...;
758 if ($condition && ... ) { say "Howdy" };
760 There are some cases where Perl can't immediately tell the difference
761 between an expression and a statement. For instance, the syntax for a
762 block and an anonymous hash reference constructor look the same unless
763 there's something in the braces to give Perl a hint. The ellipsis is a
764 syntax error if Perl doesn't guess that the C<{ ... }> is a block. In that
765 case, it doesn't think the C<...> is an ellipsis because it's expecting an
766 expression instead of a statement:
768 @transformed = map { ... } @input; # syntax error
770 You can use a C<;> inside your block to denote that the C<{ ... }> is a
771 block and not a hash reference constructor. Now the ellipsis works:
773 @transformed = map {; ... } @input; # ; disambiguates
775 @transformed = map { ...; } @input; # ; disambiguates
777 Note: Some folks colloquially refer to this bit of punctuation as a
778 "yada-yada" or "triple-dot", but its true name
779 is actually an ellipsis. Perl does not yet
780 accept the Unicode version, U+2026 HORIZONTAL ELLIPSIS, as an alias for
781 C<...>, but someday it may.
783 =head2 PODs: Embedded Documentation
784 X<POD> X<documentation>
786 Perl has a mechanism for intermixing documentation with source code.
787 While it's expecting the beginning of a new statement, if the compiler
788 encounters a line that begins with an equal sign and a word, like this
790 =head1 Here There Be Pods!
792 Then that text and all remaining text up through and including a line
793 beginning with C<=cut> will be ignored. The format of the intervening
794 text is described in L<perlpod>.
796 This allows you to intermix your source code
797 and your documentation text freely, as in
801 The snazzle() function will behave in the most spectacular
802 form that you can possibly imagine, not even excepting
803 cybernetic pyrotechnics.
805 =cut back to the compiler, nuff of this pod stuff!
812 Note that pod translators should look at only paragraphs beginning
813 with a pod directive (it makes parsing easier), whereas the compiler
814 actually knows to look for pod escapes even in the middle of a
815 paragraph. This means that the following secret stuff will be
816 ignored by both the compiler and the translators.
820 warn "Neither POD nor CODE!?"
824 You probably shouldn't rely upon the C<warn()> being podded out forever.
825 Not all pod translators are well-behaved in this regard, and perhaps
826 the compiler will become pickier.
828 One may also use pod directives to quickly comment out a section
831 =head2 Plain Old Comments (Not!)
832 X<comment> X<line> X<#> X<preprocessor> X<eval>
834 Perl can process line directives, much like the C preprocessor. Using
835 this, one can control Perl's idea of filenames and line numbers in
836 error or warning messages (especially for strings that are processed
837 with C<eval()>). The syntax for this mechanism is almost the same as for
838 most C preprocessors: it matches the regular expression
840 # example: '# line 42 "new_filename.plx"'
843 (?:\s("?)([^"]+)\g2)? \s*
846 with C<$1> being the line number for the next line, and C<$3> being
847 the optional filename (specified with or without quotes). Note that
848 no whitespace may precede the C<< # >>, unlike modern C preprocessors.
850 There is a fairly obvious gotcha included with the line directive:
851 Debuggers and profilers will only show the last source line to appear
852 at a particular line number in a given file. Care should be taken not
853 to cause line number collisions in code you'd like to debug later.
855 Here are some examples that you should be able to type into your command
860 # the '#' on the previous line must be the first char on line
863 foo at bzzzt line 201.
867 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
872 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
874 foo at foo bar line 200.
878 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
881 foo at goop line 345.
883 =head2 Experimental Details on given and when
885 As previously mentioned, the "switch" feature is considered highly
886 experimental; it is subject to change with little notice. In particular,
887 C<when> has tricky behaviours that are expected to change to become less
888 tricky in the future. Do not rely upon its current (mis)implementation.
889 Before Perl 5.18, C<given> also had tricky behaviours that you should still
890 beware of if your code must run on older versions of Perl.
892 Here is a longer example of C<given>:
897 say '$foo is undefined';
900 say '$foo is the string "foo"';
903 say '$foo is an odd digit';
904 continue; # Fall through
907 say '$foo is numerically less than 100';
909 when (\&complicated_check) {
910 say 'a complicated check for $foo is true';
913 die q(I don't know what to do with $foo);
917 Before Perl 5.18, C<given(EXPR)> assigned the value of I<EXPR> to
918 merely a lexically scoped I<B<copy>> (!) of C<$_>, not a dynamically
919 scoped alias the way C<foreach> does. That made it similar to
921 do { my $_ = EXPR; ... }
923 except that the block was automatically broken out of by a successful
924 C<when> or an explicit C<break>. Because it was only a copy, and because
925 it was only lexically scoped, not dynamically scoped, you could not do the
926 things with it that you are used to in a C<foreach> loop. In particular,
927 it did not work for arbitrary function calls if those functions might try
928 to access $_. Best stick to C<foreach> for that.
930 Most of the power comes from the implicit smartmatching that can
931 sometimes apply. Most of the time, C<when(EXPR)> is treated as an
932 implicit smartmatch of C<$_>, that is, C<$_ ~~ EXPR>. (See
933 L<perlop/"Smartmatch Operator"> for more information on smartmatching.)
934 But when I<EXPR> is one of the 10 exceptional cases (or things like them)
935 listed below, it is used directly as a boolean.
941 A user-defined subroutine call or a method invocation.
945 A regular expression match in the form of C</REGEX/>, C<$foo =~ /REGEX/>,
946 or C<$foo =~ EXPR>. Also, a negated regular expression match in
947 the form C<!/REGEX/>, C<$foo !~ /REGEX/>, or C<$foo !~ EXPR>.
951 A smart match that uses an explicit C<~~> operator, such as C<EXPR ~~ EXPR>.
955 A boolean comparison operator such as C<$_ E<lt> 10> or C<$x eq "abc"> The
956 relational operators that this applies to are the six numeric comparisons
957 (C<< < >>, C<< > >>, C<< <= >>, C<< >= >>, C<< == >>, and C<< != >>), and
958 the six string comparisons (C<lt>, C<gt>, C<le>, C<ge>, C<eq>, and C<ne>).
960 B<NOTE:> You will often have to use C<$c ~~ $_> because
961 the default case uses C<$_ ~~ $c> , which is frequently
962 the opposite of what you want.
966 At least the three builtin functions C<defined(...)>, C<exists(...)>, and
967 C<eof(...)>. We might someday add more of these later if we think of them.
971 A negated expression, whether C<!(EXPR)> or C<not(EXPR)>, or a logical
972 exclusive-or, C<(EXPR1) xor (EXPR2)>. The bitwise versions (C<~> and C<^>)
977 A filetest operator, with exactly 4 exceptions: C<-s>, C<-M>, C<-A>, and
978 C<-C>, as these return numerical values, not boolean ones. The C<-z>
979 filetest operator is not included in the exception list.
983 The C<..> and C<...> flip-flop operators. Note that the C<...> flip-flop
984 operator is completely different from the C<...> elliptical statement
989 In those 8 cases above, the value of EXPR is used directly as a boolean, so
990 no smartmatching is done. You may think of C<when> as a smartsmartmatch.
992 Furthermore, Perl inspects the operands of logical operators to
993 decide whether to use smartmatching for each one by applying the
994 above test to the operands:
1000 If EXPR is C<EXPR1 && EXPR2> or C<EXPR1 and EXPR2>, the test is applied
1001 I<recursively> to both EXPR1 and EXPR2.
1002 Only if I<both> operands also pass the
1003 test, I<recursively>, will the expression be treated as boolean. Otherwise,
1004 smartmatching is used.
1008 If EXPR is C<EXPR1 || EXPR2>, C<EXPR1 // EXPR2>, or C<EXPR1 or EXPR2>, the
1009 test is applied I<recursively> to EXPR1 only (which might itself be a
1010 higher-precedence AND operator, for example, and thus subject to the
1011 previous rule), not to EXPR2. If EXPR1 is to use smartmatching, then EXPR2
1012 also does so, no matter what EXPR2 contains. But if EXPR2 does not get to
1013 use smartmatching, then the second argument will not be either. This is
1014 quite different from the C<&&> case just described, so be careful.
1018 These rules are complicated, but the goal is for them to do what you want
1019 (even if you don't quite understand why they are doing it). For example:
1021 when (/^\d+$/ && $_ < 75) { ... }
1023 will be treated as a boolean match because the rules say both
1024 a regex match and an explicit test on C<$_> will be treated
1029 when ([qw(foo bar)] && /baz/) { ... }
1031 will use smartmatching because only I<one> of the operands is a boolean:
1032 the other uses smartmatching, and that wins.
1036 when ([qw(foo bar)] || /^baz/) { ... }
1038 will use smart matching (only the first operand is considered), whereas
1040 when (/^baz/ || [qw(foo bar)]) { ... }
1042 will test only the regex, which causes both operands to be
1043 treated as boolean. Watch out for this one, then, because an
1044 arrayref is always a true value, which makes it effectively
1045 redundant. Not a good idea.
1047 Tautologous boolean operators are still going to be optimized
1048 away. Don't be tempted to write
1050 when ("foo" or "bar") { ... }
1052 This will optimize down to C<"foo">, so C<"bar"> will never be considered (even
1053 though the rules say to use a smartmatch
1054 on C<"foo">). For an alternation like
1055 this, an array ref will work, because this will instigate smartmatching:
1057 when ([qw(foo bar)] { ... }
1059 This is somewhat equivalent to the C-style switch statement's fallthrough
1060 functionality (not to be confused with I<Perl's> fallthrough
1061 functionality--see below), wherein the same block is used for several
1064 Another useful shortcut is that, if you use a literal array or hash as the
1065 argument to C<given>, it is turned into a reference. So C<given(@foo)> is
1066 the same as C<given(\@foo)>, for example.
1068 C<default> behaves exactly like C<when(1 == 1)>, which is
1069 to say that it always matches.
1073 You can use the C<break> keyword to break out of the enclosing
1074 C<given> block. Every C<when> block is implicitly ended with
1079 You can use the C<continue> keyword to fall through from one
1083 when (/x/) { say '$foo contains an x'; continue }
1084 when (/y/) { say '$foo contains a y' }
1085 default { say '$foo does not contain a y' }
1090 When a C<given> statement is also a valid expression (for example,
1091 when it's the last statement of a block), it evaluates to:
1097 An empty list as soon as an explicit C<break> is encountered.
1101 The value of the last evaluated expression of the successful
1102 C<when>/C<default> clause, if there happens to be one.
1106 The value of the last evaluated expression of the C<given> block if no
1111 In both last cases, the last expression is evaluated in the context that
1112 was applied to the C<given> block.
1114 Note that, unlike C<if> and C<unless>, failed C<when> statements always
1115 evaluate to an empty list.
1119 when (["pear", "apple"]) { 1 }
1120 break when "vote"; # My vote cannot be bought
1121 1e10 when /Mona Lisa/;
1126 Currently, C<given> blocks can't always
1127 be used as proper expressions. This
1128 may be addressed in a future version of Perl.
1130 =head3 Switching in a loop
1132 Instead of using C<given()>, you can use a C<foreach()> loop.
1133 For example, here's one way to count how many times a particular
1134 string occurs in an array:
1139 when ("foo") { ++$count }
1141 print "\@array contains $count copies of 'foo'\n";
1143 Or in a more recent version:
1148 ++$count when "foo";
1150 print "\@array contains $count copies of 'foo'\n";
1152 At the end of all C<when> blocks, there is an implicit C<next>.
1153 You can override that with an explicit C<last> if you're
1154 interested in only the first match alone.
1156 This doesn't work if you explicitly specify a loop variable, as
1157 in C<for $item (@array)>. You have to use the default variable C<$_>.
1159 =head3 Differences from Perl 6
1161 The Perl 5 smartmatch and C<given>/C<when> constructs are not compatible
1162 with their Perl 6 analogues. The most visible difference and least
1163 important difference is that, in Perl 5, parentheses are required around
1164 the argument to C<given()> and C<when()> (except when this last one is used
1165 as a statement modifier). Parentheses in Perl 6 are always optional in a
1166 control construct such as C<if()>, C<while()>, or C<when()>; they can't be
1167 made optional in Perl 5 without a great deal of potential confusion,
1168 because Perl 5 would parse the expression
1174 as though the argument to C<given> were an element of the hash
1175 C<%foo>, interpreting the braces as hash-element syntax.
1177 However, their are many, many other differences. For example,
1178 this works in Perl 5:
1181 my @primary = ("red", "blue", "green");
1183 if (@primary ~~ "red") {
1184 say "primary smartmatches red";
1187 if ("red" ~~ @primary) {
1188 say "red smartmatches primary";
1191 say "that's all, folks!";
1193 But it doesn't work at all in Perl 6. Instead, you should
1194 use the (parallelizable) C<any> operator instead:
1196 if any(@primary) eq "red" {
1197 say "primary smartmatches red";
1200 if "red" eq any(@primary) {
1201 say "red smartmatches primary";
1204 The table of smartmatches in L<perlop/"Smartmatch Operator"> is not
1205 identical to that proposed by the Perl 6 specification, mainly due to
1206 differences between Perl 6's and Perl 5's data models, but also because
1207 the Perl 6 spec has changed since Perl 5 rushed into early adoption.
1209 In Perl 6, C<when()> will always do an implicit smartmatch with its
1210 argument, while in Perl 5 it is convenient (albeit potentially confusing) to
1211 suppress this implicit smartmatch in various rather loosely-defined
1212 situations, as roughly outlined above. (The difference is largely because
1213 Perl 5 does not have, even internally, a boolean type.)