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>/C<redo>) or around it (for C<last>) to do that sort of thing.
185 X<next> X<last> X<redo>
187 For C<next> or C<redo>, just double the braces:
194 For C<last>, you have to be more elaborate and put braces around it:
204 If you need both C<next> and C<last>, you have to do both and also use a
210 last LOOP if $x == $y**2;
215 B<NOTE:> The behaviour of a C<my>, C<state>, or
216 C<our> modified with a statement modifier conditional
217 or loop construct (for example, C<my $x if ...>) is
218 B<undefined>. The value of the C<my> variable may be C<undef>, any
219 previously assigned value, or possibly anything else. Don't rely on
220 it. Future versions of perl might do something different from the
221 version of perl you try it out on. Here be dragons.
224 The C<when> modifier is an experimental feature that first appeared in Perl
225 5.14. To use it, you should include a C<use v5.14> declaration.
226 (Technically, it requires only the C<switch> feature, but that aspect of it
227 was not available before 5.14.) Operative only from within a C<foreach>
228 loop or a C<given> block, it executes the statement only if the smartmatch
229 C<< $_ ~~ I<EXPR> >> is true. If the statement executes, it is followed by
230 a C<next> from inside a C<foreach> and C<break> from inside a C<given>.
232 Under the current implementation, the C<foreach> loop can be
233 anywhere within the C<when> modifier's dynamic scope, but must be
234 within the C<given> block's lexical scope. This restriction may
235 be relaxed in a future release. See L</"Switch Statements"> below.
237 =head2 Compound Statements
238 X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
239 X<{> X<}> X<if> X<unless> X<given> X<while> X<until> X<foreach> X<for> X<continue>
241 In Perl, a sequence of statements that defines a scope is called a block.
242 Sometimes a block is delimited by the file containing it (in the case
243 of a required file, or the program as a whole), and sometimes a block
244 is delimited by the extent of a string (in the case of an eval).
246 But generally, a block is delimited by curly brackets, also known as braces.
247 We will call this syntactic construct a BLOCK.
249 The following compound statements may be used to control flow:
252 if (EXPR) BLOCK else BLOCK
253 if (EXPR) BLOCK elsif (EXPR) BLOCK ...
254 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
257 unless (EXPR) BLOCK else BLOCK
258 unless (EXPR) BLOCK elsif (EXPR) BLOCK ...
259 unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
263 LABEL while (EXPR) BLOCK
264 LABEL while (EXPR) BLOCK continue BLOCK
266 LABEL until (EXPR) BLOCK
267 LABEL until (EXPR) BLOCK continue BLOCK
269 LABEL for (EXPR; EXPR; EXPR) BLOCK
270 LABEL for VAR (LIST) BLOCK
271 LABEL for VAR (LIST) BLOCK continue BLOCK
273 LABEL foreach (EXPR; EXPR; EXPR) BLOCK
274 LABEL foreach VAR (LIST) BLOCK
275 LABEL foreach VAR (LIST) BLOCK continue BLOCK
278 LABEL BLOCK continue BLOCK
282 The experimental C<given> statement is I<not automatically enabled>; see
283 L</"Switch Statements"> below for how to do so, and the attendant caveats.
285 Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs,
286 not statements. This means that the curly brackets are I<required>--no
287 dangling statements allowed. If you want to write conditionals without
288 curly brackets, there are several other ways to do it. The following
289 all do the same thing:
291 if (!open(FOO)) { die "Can't open $FOO: $!" }
292 die "Can't open $FOO: $!" unless open(FOO);
293 open(FOO) || die "Can't open $FOO: $!";
294 open(FOO) ? () : die "Can't open $FOO: $!";
295 # a bit exotic, that last one
297 The C<if> statement is straightforward. Because BLOCKs are always
298 bounded by curly brackets, there is never any ambiguity about which
299 C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
300 the sense of the test is reversed. Like C<if>, C<unless> can be followed
301 by C<else>. C<unless> can even be followed by one or more C<elsif>
302 statements, though you may want to think twice before using that particular
303 language construct, as everyone reading your code will have to think at least
304 twice before they can understand what's going on.
306 The C<while> statement executes the block as long as the expression is
307 L<true|/"Truth and Falsehood">.
308 The C<until> statement executes the block as long as the expression is
310 The LABEL is optional, and if present, consists of an identifier followed
311 by a colon. The LABEL identifies the loop for the loop control
312 statements C<next>, C<last>, and C<redo>.
313 If the LABEL is omitted, the loop control statement
314 refers to the innermost enclosing loop. This may include dynamically
315 looking back your call-stack at run time to find the LABEL. Such
316 desperate behavior triggers a warning if you use the C<use warnings>
317 pragma or the B<-w> flag.
319 If there is a C<continue> BLOCK, it is always executed just before the
320 conditional is about to be evaluated again. Thus it can be used to
321 increment a loop variable, even when the loop has been continued via
322 the C<next> statement.
324 When a block is preceding by a compilation phase keyword such as C<BEGIN>,
325 C<END>, C<INIT>, C<CHECK>, or C<UNITCHECK>, then the block will run only
326 during the corresponding phase of execution. See L<perlmod> for more details.
328 Extension modules can also hook into the Perl parser to define new
329 kinds of compound statements. These are introduced by a keyword which
330 the extension recognizes, and the syntax following the keyword is
331 defined entirely by the extension. If you are an implementor, see
332 L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
333 a module, see the module's documentation for details of the syntax that
337 X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
339 The C<next> command starts the next iteration of the loop:
341 LINE: while (<STDIN>) {
342 next LINE if /^#/; # discard comments
346 The C<last> command immediately exits the loop in question. The
347 C<continue> block, if any, is not executed:
349 LINE: while (<STDIN>) {
350 last LINE if /^$/; # exit when done with header
354 The C<redo> command restarts the loop block without evaluating the
355 conditional again. The C<continue> block, if any, is I<not> executed.
356 This command is normally used by programs that want to lie to themselves
357 about what was just input.
359 For example, when processing a file like F</etc/termcap>.
360 If your input lines might end in backslashes to indicate continuation, you
361 want to skip ahead and get the next record.
372 which is Perl shorthand for the more explicitly written version:
374 LINE: while (defined($line = <ARGV>)) {
376 if ($line =~ s/\\$//) {
378 redo LINE unless eof(); # not eof(ARGV)!
383 Note that if there were a C<continue> block on the above code, it would
384 get executed only on lines discarded by the regex (since redo skips the
385 continue block). A continue block is often used to reset line counters
386 or C<m?pat?> one-time matches:
388 # inspired by :1,$g/fred/s//WILMA/
390 m?(fred)? && s//WILMA $1 WILMA/;
391 m?(barney)? && s//BETTY $1 BETTY/;
392 m?(homer)? && s//MARGE $1 MARGE/;
394 print "$ARGV $.: $_";
395 close ARGV if eof; # reset $.
396 reset if eof; # reset ?pat?
399 If the word C<while> is replaced by the word C<until>, the sense of the
400 test is reversed, but the conditional is still tested before the first
403 Loop control statements don't work in an C<if> or C<unless>, since
404 they aren't loops. You can double the braces to make them such, though.
408 next if /barney/; # same effect as "last",
409 # but doesn't document as well
413 This is caused by the fact that a block by itself acts as a loop that
414 executes once, see L</"Basic BLOCKs">.
416 The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
417 available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
422 Perl's C-style C<for> loop works like the corresponding C<while> loop;
423 that means that this:
425 for ($i = 1; $i < 10; $i++) {
438 There is one minor difference: if variables are declared with C<my>
439 in the initialization section of the C<for>, the lexical scope of
440 those variables is exactly the C<for> loop (the body of the loop
441 and the control sections).
444 As a special case, if the test in the C<for> loop (or the corresponding
445 C<while> loop) is empty, it is treated as true. That is, both
457 are treated as infinite loops.
459 Besides the normal array index looping, C<for> can lend itself
460 to many other interesting applications. Here's one that avoids the
461 problem you get into if you explicitly test for end-of-file on
462 an interactive file descriptor causing your program to appear to
464 X<eof> X<end-of-file> X<end of file>
466 $on_a_tty = -t STDIN && -t STDOUT;
467 sub prompt { print "yes? " if $on_a_tty }
468 for ( prompt(); <STDIN>; prompt() ) {
472 Using C<readline> (or the operator form, C<< <EXPR> >>) as the
473 conditional of a C<for> loop is shorthand for the following. This
474 behaviour is the same as a C<while> loop conditional.
475 X<readline> X<< <> >>
477 for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
484 The C<foreach> loop iterates over a normal list value and sets the scalar
485 variable VAR to be each element of the list in turn. If the variable
486 is preceded with the keyword C<my>, then it is lexically scoped, and
487 is therefore visible only within the loop. Otherwise, the variable is
488 implicitly local to the loop and regains its former value upon exiting
489 the loop. If the variable was previously declared with C<my>, it uses
490 that variable instead of the global one, but it's still localized to
491 the loop. This implicit localization occurs I<only> in a C<foreach>
495 The C<foreach> keyword is actually a synonym for the C<for> keyword, so
496 you can use either. If VAR is omitted, C<$_> is set to each value.
499 If any element of LIST is an lvalue, you can modify it by modifying
500 VAR inside the loop. Conversely, if any element of LIST is NOT an
501 lvalue, any attempt to modify that element will fail. In other words,
502 the C<foreach> loop index variable is an implicit alias for each item
503 in the list that you're looping over.
506 If any part of LIST is an array, C<foreach> will get very confused if
507 you add or remove elements within the loop body, for example with
508 C<splice>. So don't do that.
511 C<foreach> probably won't do what you expect if VAR is a tied or other
512 special variable. Don't do that either.
514 As of Perl 5.22, there is an experimental variant of this loop that accepts
515 a variable preceded by a backslash for VAR, in which case the items in the
516 LIST must be references. The backslashed variable will become an alias
517 to each referenced item in the LIST, which must be of the correct type.
518 The variable needn't be a scalar in this case, and the backslash may be
519 followed by C<my>. To use this form, you must enable the C<refaliasing>
520 feature via C<use feature>. (See L<feature>. See also L<perlref/Assigning
525 for (@ary) { s/foo/bar/ }
527 for my $elem (@elements) {
531 for $count (reverse(1..10), "BOOM") {
536 for (1..15) { print "Merry Christmas\n"; }
538 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
539 print "Item: $item\n";
542 use feature "refaliasing";
543 no warnings "experimental::refaliasing";
544 foreach \my %hash (@array_of_hash_references) {
545 # do something which each %hash
548 Here's how a C programmer might code up a particular algorithm in Perl:
550 for (my $i = 0; $i < @ary1; $i++) {
551 for (my $j = 0; $j < @ary2; $j++) {
552 if ($ary1[$i] > $ary2[$j]) {
553 last; # can't go to outer :-(
555 $ary1[$i] += $ary2[$j];
557 # this is where that last takes me
560 Whereas here's how a Perl programmer more comfortable with the idiom might
563 OUTER: for my $wid (@ary1) {
564 INNER: for my $jet (@ary2) {
565 next OUTER if $wid > $jet;
570 See how much easier this is? It's cleaner, safer, and faster. It's
571 cleaner because it's less noisy. It's safer because if code gets added
572 between the inner and outer loops later on, the new code won't be
573 accidentally executed. The C<next> explicitly iterates the other loop
574 rather than merely terminating the inner one. And it's faster because
575 Perl executes a C<foreach> statement more rapidly than it would the
576 equivalent C<for> loop.
578 Perceptive Perl hackers may have noticed that a C<for> loop has a return
579 value, and that this value can be captured by wrapping the loop in a C<do>
580 block. The reward for this discovery is this cautionary advice: The
581 return value of a C<for> loop is unspecified and may change without notice.
587 A BLOCK by itself (labeled or not) is semantically equivalent to a
588 loop that executes once. Thus you can use any of the loop control
589 statements in it to leave or restart the block. (Note that this is
590 I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
591 C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
594 The BLOCK construct can be used to emulate case structures.
597 if (/^abc/) { $abc = 1; last SWITCH; }
598 if (/^def/) { $def = 1; last SWITCH; }
599 if (/^xyz/) { $xyz = 1; last SWITCH; }
603 You'll also find that C<foreach> loop used to create a topicalizer
608 if (/^abc/) { $abc = 1; last SWITCH; }
609 if (/^def/) { $def = 1; last SWITCH; }
610 if (/^xyz/) { $xyz = 1; last SWITCH; }
614 Such constructs are quite frequently used, both because older versions of
615 Perl had no official C<switch> statement, and also because the new version
616 described immediately below remains experimental and can sometimes be confusing.
618 =head2 Switch Statements
620 X<switch> X<case> X<given> X<when> X<default>
622 Starting from Perl 5.10.1 (well, 5.10.0, but it didn't work
625 use feature "switch";
627 to enable an experimental switch feature. This is loosely based on an
628 old version of a Perl 6 proposal, but it no longer resembles the Perl 6
629 construct. You also get the switch feature whenever you declare that your
630 code prefers to run under a version of Perl that is 5.10 or later. For
635 Under the "switch" feature, Perl gains the experimental keywords
636 C<given>, C<when>, C<default>, C<continue>, and C<break>.
637 Starting from Perl 5.16, one can prefix the switch
638 keywords with C<CORE::> to access the feature without a C<use feature>
639 statement. The keywords C<given> and
640 C<when> are analogous to C<switch> and
641 C<case> in other languages -- though C<continue> is not -- so the code
642 in the previous section could be rewritten as
646 when (/^abc/) { $abc = 1 }
647 when (/^def/) { $def = 1 }
648 when (/^xyz/) { $xyz = 1 }
649 default { $nothing = 1 }
652 The C<foreach> is the non-experimental way to set a topicalizer.
653 If you wish to use the highly experimental C<given>, that could be
658 when (/^abc/) { $abc = 1 }
659 when (/^def/) { $def = 1 }
660 when (/^xyz/) { $xyz = 1 }
661 default { $nothing = 1 }
664 As of 5.14, that can also be written this way:
668 $abc = 1 when /^abc/;
669 $def = 1 when /^def/;
670 $xyz = 1 when /^xyz/;
671 default { $nothing = 1 }
674 Or if you don't care to play it safe, like this:
678 $abc = 1 when /^abc/;
679 $def = 1 when /^def/;
680 $xyz = 1 when /^xyz/;
681 default { $nothing = 1 }
684 The arguments to C<given> and C<when> are in scalar context,
685 and C<given> assigns the C<$_> variable its topic value.
687 Exactly what the I<EXPR> argument to C<when> does is hard to describe
688 precisely, but in general, it tries to guess what you want done. Sometimes
689 it is interpreted as C<< $_ ~~ I<EXPR> >>, and sometimes it is not. It
690 also behaves differently when lexically enclosed by a C<given> block than
691 it does when dynamically enclosed by a C<foreach> loop. The rules are far
692 too difficult to understand to be described here. See L</"Experimental Details
693 on given and when"> later on.
695 Due to an unfortunate bug in how C<given> was implemented between Perl 5.10
696 and 5.16, under those implementations the version of C<$_> governed by
697 C<given> is merely a lexically scoped copy of the original, not a
698 dynamically scoped alias to the original, as it would be if it were a
699 C<foreach> or under both the original and the current Perl 6 language
700 specification. This bug was fixed in Perl 5.18 (and lexicalized C<$_> itself
701 was removed in Perl 5.24).
703 If your code still needs to run on older versions,
704 stick to C<foreach> for your topicalizer and
705 you will be less unhappy.
710 Although not for the faint of heart, Perl does support a C<goto>
711 statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
712 C<goto>-&NAME. A loop's LABEL is not actually a valid target for
713 a C<goto>; it's just the name of the loop.
715 The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
716 execution there. It may not be used to go into any construct that
717 requires initialization, such as a subroutine or a C<foreach> loop. It
718 also can't be used to go into a construct that is optimized away. It
719 can be used to go almost anywhere else within the dynamic scope,
720 including out of subroutines, but it's usually better to use some other
721 construct such as C<last> or C<die>. The author of Perl has never felt the
722 need to use this form of C<goto> (in Perl, that is--C is another matter).
724 The C<goto>-EXPR form expects a label name, whose scope will be resolved
725 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
726 necessarily recommended if you're optimizing for maintainability:
728 goto(("FOO", "BAR", "GLARCH")[$i]);
730 The C<goto>-&NAME form is highly magical, and substitutes a call to the
731 named subroutine for the currently running subroutine. This is used by
732 C<AUTOLOAD()> subroutines that wish to load another subroutine and then
733 pretend that the other subroutine had been called in the first place
734 (except that any modifications to C<@_> in the current subroutine are
735 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
736 will be able to tell that this routine was called first.
738 In almost all cases like this, it's usually a far, far better idea to use the
739 structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
740 resorting to a C<goto>. For certain applications, the catch and throw pair of
741 C<eval{}> and die() for exception processing can also be a prudent approach.
743 =head2 The Ellipsis Statement
747 X<elliptical statement>
748 X<unimplemented statement>
749 X<unimplemented operator>
751 X<yada-yada operator>
754 X<triple-dot operator>
756 Beginning in Perl 5.12, Perl accepts an ellipsis, "C<...>", as a
757 placeholder for code that you haven't implemented yet. This form of
758 ellipsis, the unimplemented statement, should not be confused with the
759 binary flip-flop C<...> operator. One is a statement and the other an
760 operator. (Perl doesn't usually confuse them because usually Perl can tell
761 whether it wants an operator or a statement, but see below for exceptions.)
763 When Perl 5.12 or later encounters an ellipsis statement, it parses this
764 without error, but if and when you should actually try to execute it, Perl
765 throws an exception with the text C<Unimplemented>:
768 sub unimplemented { ... }
769 eval { unimplemented() };
770 if ($@ =~ /^Unimplemented at /) {
771 say "I found an ellipsis!";
774 You can only use the elliptical statement to stand in for a
775 complete statement. These examples of how the ellipsis works:
793 The elliptical statement cannot stand in for an expression that
794 is part of a larger statement, since the C<...> is also the three-dot
795 version of the flip-flop operator (see L<perlop/"Range Operators">).
797 These examples of attempts to use an ellipsis are syntax errors:
802 open(my $fh, ">", "/dev/passwd") or ...;
803 if ($condition && ... ) { say "Howdy" };
805 There are some cases where Perl can't immediately tell the difference
806 between an expression and a statement. For instance, the syntax for a
807 block and an anonymous hash reference constructor look the same unless
808 there's something in the braces to give Perl a hint. The ellipsis is a
809 syntax error if Perl doesn't guess that the C<{ ... }> is a block. In that
810 case, it doesn't think the C<...> is an ellipsis because it's expecting an
811 expression instead of a statement:
813 @transformed = map { ... } @input; # syntax error
815 Inside your block, you can use a C<;> before the ellipsis to denote that the
816 C<{ ... }> is a block and not a hash reference constructor. Now the ellipsis
819 @transformed = map {; ... } @input; # ';' disambiguates
821 Note: Some folks colloquially refer to this bit of punctuation as a
822 "yada-yada" or "triple-dot", but its true name
823 is actually an ellipsis.
825 =head2 PODs: Embedded Documentation
826 X<POD> X<documentation>
828 Perl has a mechanism for intermixing documentation with source code.
829 While it's expecting the beginning of a new statement, if the compiler
830 encounters a line that begins with an equal sign and a word, like this
832 =head1 Here There Be Pods!
834 Then that text and all remaining text up through and including a line
835 beginning with C<=cut> will be ignored. The format of the intervening
836 text is described in L<perlpod>.
838 This allows you to intermix your source code
839 and your documentation text freely, as in
843 The snazzle() function will behave in the most spectacular
844 form that you can possibly imagine, not even excepting
845 cybernetic pyrotechnics.
847 =cut back to the compiler, nuff of this pod stuff!
854 Note that pod translators should look at only paragraphs beginning
855 with a pod directive (it makes parsing easier), whereas the compiler
856 actually knows to look for pod escapes even in the middle of a
857 paragraph. This means that the following secret stuff will be
858 ignored by both the compiler and the translators.
862 warn "Neither POD nor CODE!?"
866 You probably shouldn't rely upon the C<warn()> being podded out forever.
867 Not all pod translators are well-behaved in this regard, and perhaps
868 the compiler will become pickier.
870 One may also use pod directives to quickly comment out a section
873 =head2 Plain Old Comments (Not!)
874 X<comment> X<line> X<#> X<preprocessor> X<eval>
876 Perl can process line directives, much like the C preprocessor. Using
877 this, one can control Perl's idea of filenames and line numbers in
878 error or warning messages (especially for strings that are processed
879 with C<eval()>). The syntax for this mechanism is almost the same as for
880 most C preprocessors: it matches the regular expression
882 # example: '# line 42 "new_filename.plx"'
885 (?:\s("?)([^"]+)\g2)? \s*
888 with C<$1> being the line number for the next line, and C<$3> being
889 the optional filename (specified with or without quotes). Note that
890 no whitespace may precede the C<< # >>, unlike modern C preprocessors.
892 There is a fairly obvious gotcha included with the line directive:
893 Debuggers and profilers will only show the last source line to appear
894 at a particular line number in a given file. Care should be taken not
895 to cause line number collisions in code you'd like to debug later.
897 Here are some examples that you should be able to type into your command
902 # the '#' on the previous line must be the first char on line
905 foo at bzzzt line 201.
909 eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
914 eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
916 foo at foo bar line 200.
920 eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
923 foo at goop line 345.
925 =head2 Experimental Details on given and when
927 As previously mentioned, the "switch" feature is considered highly
928 experimental; it is subject to change with little notice. In particular,
929 C<when> has tricky behaviours that are expected to change to become less
930 tricky in the future. Do not rely upon its current (mis)implementation.
931 Before Perl 5.18, C<given> also had tricky behaviours that you should still
932 beware of if your code must run on older versions of Perl.
934 Here is a longer example of C<given>:
939 say '$foo is undefined';
942 say '$foo is the string "foo"';
945 say '$foo is an odd digit';
946 continue; # Fall through
949 say '$foo is numerically less than 100';
951 when (\&complicated_check) {
952 say 'a complicated check for $foo is true';
955 die q(I don't know what to do with $foo);
959 Before Perl 5.18, C<given(EXPR)> assigned the value of I<EXPR> to
960 merely a lexically scoped I<B<copy>> (!) of C<$_>, not a dynamically
961 scoped alias the way C<foreach> does. That made it similar to
963 do { my $_ = EXPR; ... }
965 except that the block was automatically broken out of by a successful
966 C<when> or an explicit C<break>. Because it was only a copy, and because
967 it was only lexically scoped, not dynamically scoped, you could not do the
968 things with it that you are used to in a C<foreach> loop. In particular,
969 it did not work for arbitrary function calls if those functions might try
970 to access $_. Best stick to C<foreach> for that.
972 Most of the power comes from the implicit smartmatching that can
973 sometimes apply. Most of the time, C<when(EXPR)> is treated as an
974 implicit smartmatch of C<$_>, that is, C<$_ ~~ EXPR>. (See
975 L<perlop/"Smartmatch Operator"> for more information on smartmatching.)
976 But when I<EXPR> is one of the 10 exceptional cases (or things like them)
977 listed below, it is used directly as a boolean.
983 A user-defined subroutine call or a method invocation.
987 A regular expression match in the form of C</REGEX/>, C<$foo =~ /REGEX/>,
988 or C<$foo =~ EXPR>. Also, a negated regular expression match in
989 the form C<!/REGEX/>, C<$foo !~ /REGEX/>, or C<$foo !~ EXPR>.
993 A smart match that uses an explicit C<~~> operator, such as C<EXPR ~~ EXPR>.
995 B<NOTE:> You will often have to use C<$c ~~ $_> because the default case
996 uses C<$_ ~~ $c> , which is frequentlythe opposite of what you want.
1000 A boolean comparison operator such as C<$_ E<lt> 10> or C<$x eq "abc">. The
1001 relational operators that this applies to are the six numeric comparisons
1002 (C<< < >>, C<< > >>, C<< <= >>, C<< >= >>, C<< == >>, and C<< != >>), and
1003 the six string comparisons (C<lt>, C<gt>, C<le>, C<ge>, C<eq>, and C<ne>).
1007 At least the three builtin functions C<defined(...)>, C<exists(...)>, and
1008 C<eof(...)>. We might someday add more of these later if we think of them.
1012 A negated expression, whether C<!(EXPR)> or C<not(EXPR)>, or a logical
1013 exclusive-or, C<(EXPR1) xor (EXPR2)>. The bitwise versions (C<~> and C<^>)
1018 A filetest operator, with exactly 4 exceptions: C<-s>, C<-M>, C<-A>, and
1019 C<-C>, as these return numerical values, not boolean ones. The C<-z>
1020 filetest operator is not included in the exception list.
1024 The C<..> and C<...> flip-flop operators. Note that the C<...> flip-flop
1025 operator is completely different from the C<...> elliptical statement
1030 In those 8 cases above, the value of EXPR is used directly as a boolean, so
1031 no smartmatching is done. You may think of C<when> as a smartsmartmatch.
1033 Furthermore, Perl inspects the operands of logical operators to
1034 decide whether to use smartmatching for each one by applying the
1035 above test to the operands:
1041 If EXPR is C<EXPR1 && EXPR2> or C<EXPR1 and EXPR2>, the test is applied
1042 I<recursively> to both EXPR1 and EXPR2.
1043 Only if I<both> operands also pass the
1044 test, I<recursively>, will the expression be treated as boolean. Otherwise,
1045 smartmatching is used.
1049 If EXPR is C<EXPR1 || EXPR2>, C<EXPR1 // EXPR2>, or C<EXPR1 or EXPR2>, the
1050 test is applied I<recursively> to EXPR1 only (which might itself be a
1051 higher-precedence AND operator, for example, and thus subject to the
1052 previous rule), not to EXPR2. If EXPR1 is to use smartmatching, then EXPR2
1053 also does so, no matter what EXPR2 contains. But if EXPR2 does not get to
1054 use smartmatching, then the second argument will not be either. This is
1055 quite different from the C<&&> case just described, so be careful.
1059 These rules are complicated, but the goal is for them to do what you want
1060 (even if you don't quite understand why they are doing it). For example:
1062 when (/^\d+$/ && $_ < 75) { ... }
1064 will be treated as a boolean match because the rules say both
1065 a regex match and an explicit test on C<$_> will be treated
1070 when ([qw(foo bar)] && /baz/) { ... }
1072 will use smartmatching because only I<one> of the operands is a boolean:
1073 the other uses smartmatching, and that wins.
1077 when ([qw(foo bar)] || /^baz/) { ... }
1079 will use smart matching (only the first operand is considered), whereas
1081 when (/^baz/ || [qw(foo bar)]) { ... }
1083 will test only the regex, which causes both operands to be
1084 treated as boolean. Watch out for this one, then, because an
1085 arrayref is always a true value, which makes it effectively
1086 redundant. Not a good idea.
1088 Tautologous boolean operators are still going to be optimized
1089 away. Don't be tempted to write
1091 when ("foo" or "bar") { ... }
1093 This will optimize down to C<"foo">, so C<"bar"> will never be considered (even
1094 though the rules say to use a smartmatch
1095 on C<"foo">). For an alternation like
1096 this, an array ref will work, because this will instigate smartmatching:
1098 when ([qw(foo bar)] { ... }
1100 This is somewhat equivalent to the C-style switch statement's fallthrough
1101 functionality (not to be confused with I<Perl's> fallthrough
1102 functionality--see below), wherein the same block is used for several
1105 Another useful shortcut is that, if you use a literal array or hash as the
1106 argument to C<given>, it is turned into a reference. So C<given(@foo)> is
1107 the same as C<given(\@foo)>, for example.
1109 C<default> behaves exactly like C<when(1 == 1)>, which is
1110 to say that it always matches.
1114 You can use the C<break> keyword to break out of the enclosing
1115 C<given> block. Every C<when> block is implicitly ended with
1120 You can use the C<continue> keyword to fall through from one
1121 case to the next immediate C<when> or C<default>:
1124 when (/x/) { say '$foo contains an x'; continue }
1125 when (/y/) { say '$foo contains a y' }
1126 default { say '$foo does not contain a y' }
1131 When a C<given> statement is also a valid expression (for example,
1132 when it's the last statement of a block), it evaluates to:
1138 An empty list as soon as an explicit C<break> is encountered.
1142 The value of the last evaluated expression of the successful
1143 C<when>/C<default> clause, if there happens to be one.
1147 The value of the last evaluated expression of the C<given> block if no
1152 In both last cases, the last expression is evaluated in the context that
1153 was applied to the C<given> block.
1155 Note that, unlike C<if> and C<unless>, failed C<when> statements always
1156 evaluate to an empty list.
1160 when (["pear", "apple"]) { 1 }
1161 break when "vote"; # My vote cannot be bought
1162 1e10 when /Mona Lisa/;
1167 Currently, C<given> blocks can't always
1168 be used as proper expressions. This
1169 may be addressed in a future version of Perl.
1171 =head3 Switching in a loop
1173 Instead of using C<given()>, you can use a C<foreach()> loop.
1174 For example, here's one way to count how many times a particular
1175 string occurs in an array:
1180 when ("foo") { ++$count }
1182 print "\@array contains $count copies of 'foo'\n";
1184 Or in a more recent version:
1189 ++$count when "foo";
1191 print "\@array contains $count copies of 'foo'\n";
1193 At the end of all C<when> blocks, there is an implicit C<next>.
1194 You can override that with an explicit C<last> if you're
1195 interested in only the first match alone.
1197 This doesn't work if you explicitly specify a loop variable, as
1198 in C<for $item (@array)>. You have to use the default variable C<$_>.
1200 =head3 Differences from Perl 6
1202 The Perl 5 smartmatch and C<given>/C<when> constructs are not compatible
1203 with their Perl 6 analogues. The most visible difference and least
1204 important difference is that, in Perl 5, parentheses are required around
1205 the argument to C<given()> and C<when()> (except when this last one is used
1206 as a statement modifier). Parentheses in Perl 6 are always optional in a
1207 control construct such as C<if()>, C<while()>, or C<when()>; they can't be
1208 made optional in Perl 5 without a great deal of potential confusion,
1209 because Perl 5 would parse the expression
1215 as though the argument to C<given> were an element of the hash
1216 C<%foo>, interpreting the braces as hash-element syntax.
1218 However, their are many, many other differences. For example,
1219 this works in Perl 5:
1222 my @primary = ("red", "blue", "green");
1224 if (@primary ~~ "red") {
1225 say "primary smartmatches red";
1228 if ("red" ~~ @primary) {
1229 say "red smartmatches primary";
1232 say "that's all, folks!";
1234 But it doesn't work at all in Perl 6. Instead, you should
1235 use the (parallelizable) C<any> operator:
1237 if any(@primary) eq "red" {
1238 say "primary smartmatches red";
1241 if "red" eq any(@primary) {
1242 say "red smartmatches primary";
1245 The table of smartmatches in L<perlop/"Smartmatch Operator"> is not
1246 identical to that proposed by the Perl 6 specification, mainly due to
1247 differences between Perl 6's and Perl 5's data models, but also because
1248 the Perl 6 spec has changed since Perl 5 rushed into early adoption.
1250 In Perl 6, C<when()> will always do an implicit smartmatch with its
1251 argument, while in Perl 5 it is convenient (albeit potentially confusing) to
1252 suppress this implicit smartmatch in various rather loosely-defined
1253 situations, as roughly outlined above. (The difference is largely because
1254 Perl 5 does not have, even internally, a boolean type.)