Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<syntax> |
a0d0e21e LW |
3 | |
4 | perlsyn - Perl syntax | |
5 | ||
6 | =head1 DESCRIPTION | |
7 | ||
6014d0cb | 8 | A Perl program consists of a sequence of declarations and statements |
c2f1e229 | 9 | which run from the top to the bottom. Loops, subroutines, and other |
6014d0cb MS |
10 | control structures allow you to jump around within the code. |
11 | ||
c2f1e229 TC |
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. | |
6014d0cb MS |
16 | |
17 | Many of Perl's syntactic elements are B<optional>. Rather than | |
110b9c83 | 18 | requiring you to put parentheses around every function call and |
6014d0cb MS |
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 | |
110b9c83 | 22 | code in a style with which they are comfortable. |
6014d0cb MS |
23 | |
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. | |
a0d0e21e | 30 | |
0b8d69e9 | 31 | =head2 Declarations |
d74e8afc | 32 | X<declaration> X<undef> X<undefined> X<uninitialized> |
0b8d69e9 | 33 | |
cf48932e | 34 | The only things you need to declare in Perl are report formats and |
c2f1e229 | 35 | subroutines (and sometimes not even subroutines). A scalar variable holds |
cf48932e SF |
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, | |
43 | such as: | |
7bd1983c | 44 | |
7bd1983c EM |
45 | if ($a) {} |
46 | ||
a6b1f6d8 RGS |
47 | are exempt from warnings (because they care about truth rather than |
48 | definedness). Operators such as C<++>, C<-->, C<+=>, | |
c2f1e229 | 49 | C<-=>, and C<.=>, that operate on undefined variables such as: |
7bd1983c | 50 | |
c2f1e229 | 51 | undef $a; |
7bd1983c EM |
52 | $a++; |
53 | ||
54 | are also always exempt from such warnings. | |
0b8d69e9 | 55 | |
a0d0e21e | 56 | A declaration can be put anywhere a statement can, but has no effect on |
c2f1e229 TC |
57 | the execution of the primary sequence of statements: declarations all |
58 | take effect at compile time. All declarations are typically put at | |
54310121 | 59 | the beginning or the end of the script. However, if you're using |
c2f1e229 TC |
60 | lexically-scoped private variables created with C<my()>, |
61 | C<state()>, or C<our()>, you'll have to make sure | |
4633a7c4 | 62 | your format or subroutine definition is within the same block scope |
5f05dabc | 63 | as the my if you expect to be able to access those private variables. |
a0d0e21e | 64 | |
4633a7c4 LW |
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 | |
54310121 | 67 | subroutine without defining it by saying C<sub name>, thus: |
d74e8afc | 68 | X<subroutine, declaration> |
a0d0e21e | 69 | |
54310121 | 70 | sub myname; |
15faabe4 | 71 | $me = myname $0 or die "can't get myname"; |
a0d0e21e | 72 | |
c2f1e229 TC |
73 | A bare declaration like that declares the function to be a list operator, |
74 | not a unary operator, so you have to be careful to use parentheses (or | |
89a3b501 | 75 | C<or> instead of C<||>.) The C<||> operator binds too tightly to use after |
c2f1e229 TC |
76 | list operators; it becomes part of the last element. You can always use |
77 | parentheses around the list operators arguments to turn the list operator | |
78 | back into something that behaves more like a function call. Alternatively, | |
79 | you can use the prototype C<($)> to turn the subroutine into a unary | |
80 | operator: | |
81 | ||
82 | sub myname ($); | |
83 | $me = myname $0 || die "can't get myname"; | |
84 | ||
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 | |
368fb018 | 87 | L<perlsub>. |
a0d0e21e | 88 | |
4633a7c4 LW |
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. | |
a0d0e21e | 92 | |
4633a7c4 LW |
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. | |
a0d0e21e | 98 | |
6014d0cb | 99 | =head2 Comments |
d74e8afc | 100 | X<comment> X<#> |
6014d0cb MS |
101 | |
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 | |
104 | expression. | |
105 | ||
6ec4bd10 | 106 | =head2 Simple Statements |
d74e8afc | 107 | X<statement> X<semicolon> X<expression> X<;> |
a0d0e21e LW |
108 | |
109 | The only kind of simple statement is an expression evaluated for its | |
c2f1e229 | 110 | side-effects. Every simple statement must be terminated with a |
a0d0e21e | 111 | semicolon, unless it is the final statement in a block, in which case |
c2f1e229 | 112 | the semicolon is optional. But put the semicolon in anyway if the |
f386e492 | 113 | block takes up more than one line, because you may eventually add |
c2f1e229 TC |
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 | |
cf48932e SF |
117 | as the last item in a statement. |
118 | ||
cf48932e | 119 | =head2 Statement Modifiers |
d74e8afc | 120 | X<statement modifier> X<modifier> X<if> X<unless> X<while> |
7896dde7 | 121 | X<until> X<when> X<foreach> X<for> |
a0d0e21e LW |
122 | |
123 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
124 | just before the terminating semicolon (or block ending). The possible | |
125 | modifiers are: | |
126 | ||
127 | if EXPR | |
128 | unless EXPR | |
129 | while EXPR | |
130 | until EXPR | |
4f8ea571 | 131 | for LIST |
cf48932e | 132 | foreach LIST |
7896dde7 | 133 | when EXPR |
cf48932e SF |
134 | |
135 | The C<EXPR> following the modifier is referred to as the "condition". | |
136 | Its truth or falsehood determines how the modifier will behave. | |
137 | ||
138 | C<if> executes the statement once I<if> and only if the condition is | |
139 | true. C<unless> is the opposite, it executes the statement I<unless> | |
c2f1e229 | 140 | the condition is true (that is, if the condition is false). |
cf48932e SF |
141 | |
142 | print "Basset hounds got long ears" if length $ear >= 10; | |
143 | go_outside() and play() unless $is_raining; | |
144 | ||
c2f1e229 | 145 | The C<for(each)> modifier is an iterator: it executes the statement once |
cf48932e SF |
146 | for each item in the LIST (with C<$_> aliased to each item in turn). |
147 | ||
c2f1e229 | 148 | print "Hello $_!\n" for qw(world Dolly nurse); |
cf48932e SF |
149 | |
150 | C<while> repeats the statement I<while> the condition is true. | |
5e979393 Z |
151 | Postfix C<while> has the same magic treatment of some kinds of condition |
152 | that prefix C<while> has. | |
cf48932e SF |
153 | C<until> does the opposite, it repeats the statement I<until> the |
154 | condition is true (or while the condition is false): | |
155 | ||
156 | # Both of these count from 0 to 10. | |
157 | print $i++ while $i <= 10; | |
158 | print $j++ until $j > 10; | |
159 | ||
160 | The C<while> and C<until> modifiers have the usual "C<while> loop" | |
161 | semantics (conditional evaluated first), except when applied to a | |
c2f1e229 | 162 | C<do>-BLOCK (or to the Perl4 C<do>-SUBROUTINE statement), in |
cf48932e | 163 | which case the block executes once before the conditional is |
c2f1e229 TC |
164 | evaluated. |
165 | ||
166 | This is so that you can write loops like: | |
a0d0e21e LW |
167 | |
168 | do { | |
15faabe4 SF |
169 | $line = <STDIN>; |
170 | ... | |
c2f1e229 | 171 | } until !defined($line) || $line eq ".\n" |
a0d0e21e | 172 | |
5a964f20 TC |
173 | See L<perlfunc/do>. Note also that the loop control statements described |
174 | later will I<NOT> work in this construct, because modifiers don't take | |
175 | loop labels. Sorry. You can always put another block inside of it | |
015aa1a8 | 176 | (for C<next>/C<redo>) or around it (for C<last>) to do that sort of thing. |
d74e8afc | 177 | X<next> X<last> X<redo> |
5a964f20 | 178 | |
015aa1a8 LM |
179 | For C<next> or C<redo>, just double the braces: |
180 | ||
5a964f20 | 181 | do {{ |
15faabe4 SF |
182 | next if $x == $y; |
183 | # do something here | |
5a964f20 TC |
184 | }} until $x++ > $z; |
185 | ||
015aa1a8 | 186 | For C<last>, you have to be more elaborate and put braces around it: |
d74e8afc | 187 | X<last> |
5a964f20 | 188 | |
015aa1a8 LM |
189 | { |
190 | do { | |
191 | last if $x == $y**2; | |
192 | # do something here | |
193 | } while $x++ <= $z; | |
194 | } | |
195 | ||
196 | If you need both C<next> and C<last>, you have to do both and also use a | |
197 | loop label: | |
198 | ||
15faabe4 | 199 | LOOP: { |
015aa1a8 LM |
200 | do {{ |
201 | next if $x == $y; | |
202 | last LOOP if $x == $y**2; | |
203 | # do something here | |
204 | }} until $x++ > $z; | |
5a964f20 | 205 | } |
a0d0e21e | 206 | |
c2f1e229 TC |
207 | B<NOTE:> The behaviour of a C<my>, C<state>, or |
208 | C<our> modified with a statement modifier conditional | |
209 | or loop construct (for example, C<my $x if ...>) is | |
457b36cb MV |
210 | B<undefined>. The value of the C<my> variable may be C<undef>, any |
211 | previously assigned value, or possibly anything else. Don't rely on | |
212 | it. Future versions of perl might do something different from the | |
213 | version of perl you try it out on. Here be dragons. | |
d74e8afc | 214 | X<my> |
457b36cb | 215 | |
7896dde7 Z |
216 | The C<when> modifier is an experimental feature that first appeared in Perl |
217 | 5.14. To use it, you should include a C<use v5.14> declaration. | |
218 | (Technically, it requires only the C<switch> feature, but that aspect of it | |
219 | was not available before 5.14.) Operative only from within a C<foreach> | |
220 | loop or a C<given> block, it executes the statement only if the smartmatch | |
221 | C<< $_ ~~ I<EXPR> >> is true. If the statement executes, it is followed by | |
222 | a C<next> from inside a C<foreach> and C<break> from inside a C<given>. | |
223 | ||
224 | Under the current implementation, the C<foreach> loop can be | |
225 | anywhere within the C<when> modifier's dynamic scope, but must be | |
226 | within the C<given> block's lexical scope. This restriction may | |
227 | be relaxed in a future release. See L</"Switch Statements"> below. | |
c2f1e229 | 228 | |
6ec4bd10 | 229 | =head2 Compound Statements |
d74e8afc | 230 | X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace> |
c2f1e229 | 231 | X<{> X<}> X<if> X<unless> X<given> X<while> X<until> X<foreach> X<for> X<continue> |
a0d0e21e LW |
232 | |
233 | In Perl, a sequence of statements that defines a scope is called a block. | |
234 | Sometimes a block is delimited by the file containing it (in the case | |
235 | of a required file, or the program as a whole), and sometimes a block | |
236 | is delimited by the extent of a string (in the case of an eval). | |
237 | ||
55771418 Z |
238 | But generally, a block is delimited by curly brackets, also known as |
239 | braces. We will call this syntactic construct a BLOCK. Because enclosing | |
240 | braces are also the syntax for hash reference constructor expressions | |
241 | (see L<perlref>), you may occasionally need to disambiguate by placing a | |
242 | C<;> immediately after an opening brace so that Perl realises the brace | |
243 | is the start of a block. You will more frequently need to disambiguate | |
244 | the other way, by placing a C<+> immediately before an opening brace to | |
245 | force it to be interpreted as a hash reference constructor expression. | |
246 | It is considered good style to use these disambiguating mechanisms | |
247 | liberally, not only when Perl would otherwise guess incorrectly. | |
a0d0e21e LW |
248 | |
249 | The following compound statements may be used to control flow: | |
250 | ||
251 | if (EXPR) BLOCK | |
252 | if (EXPR) BLOCK else BLOCK | |
c2f1e229 | 253 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... |
a0d0e21e | 254 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
c2f1e229 | 255 | |
62d98eed RU |
256 | unless (EXPR) BLOCK |
257 | unless (EXPR) BLOCK else BLOCK | |
c2f1e229 | 258 | unless (EXPR) BLOCK elsif (EXPR) BLOCK ... |
d27f8d4b | 259 | unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
c2f1e229 TC |
260 | |
261 | given (EXPR) BLOCK | |
262 | ||
a0d0e21e LW |
263 | LABEL while (EXPR) BLOCK |
264 | LABEL while (EXPR) BLOCK continue BLOCK | |
c2f1e229 | 265 | |
5ec6d87f EA |
266 | LABEL until (EXPR) BLOCK |
267 | LABEL until (EXPR) BLOCK continue BLOCK | |
c2f1e229 | 268 | |
a0d0e21e | 269 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
7808b687 AB |
270 | LABEL for VAR (LIST) BLOCK |
271 | LABEL for VAR (LIST) BLOCK continue BLOCK | |
c2f1e229 | 272 | |
7808b687 | 273 | LABEL foreach (EXPR; EXPR; EXPR) BLOCK |
748a9306 | 274 | LABEL foreach VAR (LIST) BLOCK |
b303ae78 | 275 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
c2f1e229 TC |
276 | |
277 | LABEL BLOCK | |
a0d0e21e | 278 | LABEL BLOCK continue BLOCK |
c2f1e229 | 279 | |
43f66a76 | 280 | PHASE BLOCK |
a0d0e21e | 281 | |
7896dde7 Z |
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. | |
c2f1e229 TC |
284 | |
285 | Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs, | |
a0d0e21e LW |
286 | not statements. This means that the curly brackets are I<required>--no |
287 | dangling statements allowed. If you want to write conditionals without | |
c2f1e229 | 288 | curly brackets, there are several other ways to do it. The following |
a0d0e21e LW |
289 | all do the same thing: |
290 | ||
c2f1e229 | 291 | if (!open(FOO)) { die "Can't open $FOO: $!" } |
a0d0e21e | 292 | die "Can't open $FOO: $!" unless open(FOO); |
c2f1e229 TC |
293 | open(FOO) || die "Can't open $FOO: $!"; |
294 | open(FOO) ? () : die "Can't open $FOO: $!"; | |
15faabe4 | 295 | # a bit exotic, that last one |
a0d0e21e | 296 | |
5f05dabc | 297 | The C<if> statement is straightforward. Because BLOCKs are always |
a0d0e21e LW |
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>, | |
89a3b501 FC |
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> | |
d27f8d4b JV |
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. | |
a0d0e21e LW |
305 | |
306 | The C<while> statement executes the block as long as the expression is | |
77fae439 | 307 | true. |
1d5653dd RGS |
308 | The C<until> statement executes the block as long as the expression is |
309 | false. | |
b78218b7 GS |
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 | |
4633a7c4 LW |
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 | |
9f1b1f2d | 316 | desperate behavior triggers a warning if you use the C<use warnings> |
a2293a43 | 317 | pragma or the B<-w> flag. |
4633a7c4 | 318 | |
5e979393 Z |
319 | If the condition expression of a C<while> statement is based |
320 | on any of a group of iterative expression types then it gets | |
321 | some magic treatment. The affected iterative expression types | |
322 | are L<C<readline>|perlfunc/readline EXPR>, the L<C<< <FILEHANDLE> | |
323 | >>|perlop/"I/O Operators"> input operator, L<C<readdir>|perlfunc/readdir | |
324 | DIRHANDLE>, L<C<glob>|perlfunc/glob EXPR>, the L<C<< <PATTERN> | |
325 | >>|perlop/"I/O Operators"> globbing operator, and L<C<each>|perlfunc/each | |
326 | HASH>. If the condition expression is one of these expression types, then | |
327 | the value yielded by the iterative operator will be implicitly assigned | |
328 | to C<$_>. If the condition expression is one of these expression types | |
329 | or an explicit assignment of one of them to a scalar, then the condition | |
330 | actually tests for definedness of the expression's value, not for its | |
331 | regular truth value. | |
332 | ||
4633a7c4 | 333 | If there is a C<continue> BLOCK, it is always executed just before the |
6ec4bd10 MS |
334 | conditional is about to be evaluated again. Thus it can be used to |
335 | increment a loop variable, even when the loop has been continued via | |
336 | the C<next> statement. | |
4633a7c4 | 337 | |
43f66a76 DG |
338 | When a block is preceding by a compilation phase keyword such as C<BEGIN>, |
339 | C<END>, C<INIT>, C<CHECK>, or C<UNITCHECK>, then the block will run only | |
340 | during the corresponding phase of execution. See L<perlmod> for more details. | |
341 | ||
88e1f1a2 | 342 | Extension modules can also hook into the Perl parser to define new |
c2f1e229 | 343 | kinds of compound statements. These are introduced by a keyword which |
6a0969e5 | 344 | the extension recognizes, and the syntax following the keyword is |
88e1f1a2 JV |
345 | defined entirely by the extension. If you are an implementor, see |
346 | L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such | |
347 | a module, see the module's documentation for details of the syntax that | |
348 | it defines. | |
349 | ||
4633a7c4 | 350 | =head2 Loop Control |
d74e8afc | 351 | X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue> |
4633a7c4 | 352 | |
6ec4bd10 | 353 | The C<next> command starts the next iteration of the loop: |
4633a7c4 LW |
354 | |
355 | LINE: while (<STDIN>) { | |
15faabe4 SF |
356 | next LINE if /^#/; # discard comments |
357 | ... | |
4633a7c4 LW |
358 | } |
359 | ||
6ec4bd10 | 360 | The C<last> command immediately exits the loop in question. The |
4633a7c4 LW |
361 | C<continue> block, if any, is not executed: |
362 | ||
363 | LINE: while (<STDIN>) { | |
15faabe4 SF |
364 | last LINE if /^$/; # exit when done with header |
365 | ... | |
4633a7c4 LW |
366 | } |
367 | ||
368 | The C<redo> command restarts the loop block without evaluating the | |
369 | conditional again. The C<continue> block, if any, is I<not> executed. | |
370 | This command is normally used by programs that want to lie to themselves | |
371 | about what was just input. | |
372 | ||
373 | For example, when processing a file like F</etc/termcap>. | |
374 | If your input lines might end in backslashes to indicate continuation, you | |
375 | want to skip ahead and get the next record. | |
376 | ||
377 | while (<>) { | |
15faabe4 SF |
378 | chomp; |
379 | if (s/\\$//) { | |
380 | $_ .= <>; | |
381 | redo unless eof(); | |
382 | } | |
383 | # now process $_ | |
54310121 | 384 | } |
4633a7c4 | 385 | |
c2f1e229 | 386 | which is Perl shorthand for the more explicitly written version: |
4633a7c4 | 387 | |
54310121 | 388 | LINE: while (defined($line = <ARGV>)) { |
15faabe4 SF |
389 | chomp($line); |
390 | if ($line =~ s/\\$//) { | |
391 | $line .= <ARGV>; | |
392 | redo LINE unless eof(); # not eof(ARGV)! | |
393 | } | |
394 | # now process $line | |
54310121 | 395 | } |
4633a7c4 | 396 | |
36e7a065 AMS |
397 | Note that if there were a C<continue> block on the above code, it would |
398 | get executed only on lines discarded by the regex (since redo skips the | |
89a3b501 | 399 | continue block). A continue block is often used to reset line counters |
499a640d | 400 | or C<m?pat?> one-time matches: |
4633a7c4 | 401 | |
5a964f20 TC |
402 | # inspired by :1,$g/fred/s//WILMA/ |
403 | while (<>) { | |
15faabe4 SF |
404 | m?(fred)? && s//WILMA $1 WILMA/; |
405 | m?(barney)? && s//BETTY $1 BETTY/; | |
406 | m?(homer)? && s//MARGE $1 MARGE/; | |
5a964f20 | 407 | } continue { |
15faabe4 SF |
408 | print "$ARGV $.: $_"; |
409 | close ARGV if eof; # reset $. | |
410 | reset if eof; # reset ?pat? | |
4633a7c4 LW |
411 | } |
412 | ||
a0d0e21e LW |
413 | If the word C<while> is replaced by the word C<until>, the sense of the |
414 | test is reversed, but the conditional is still tested before the first | |
415 | iteration. | |
416 | ||
c2f1e229 | 417 | Loop control statements don't work in an C<if> or C<unless>, since |
5a964f20 TC |
418 | they aren't loops. You can double the braces to make them such, though. |
419 | ||
420 | if (/pattern/) {{ | |
15faabe4 SF |
421 | last if /fred/; |
422 | next if /barney/; # same effect as "last", | |
423 | # but doesn't document as well | |
424 | # do something here | |
5a964f20 TC |
425 | }} |
426 | ||
7bd1983c | 427 | This is caused by the fact that a block by itself acts as a loop that |
88bd7502 | 428 | executes once, see L</"Basic BLOCKs">. |
7bd1983c | 429 | |
5b23ba8b MG |
430 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
431 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
4633a7c4 | 432 | |
cb1a09d0 | 433 | =head2 For Loops |
d74e8afc | 434 | X<for> X<foreach> |
a0d0e21e | 435 | |
b78df5de | 436 | Perl's C-style C<for> loop works like the corresponding C<while> loop; |
cb1a09d0 | 437 | that means that this: |
a0d0e21e LW |
438 | |
439 | for ($i = 1; $i < 10; $i++) { | |
15faabe4 | 440 | ... |
a0d0e21e LW |
441 | } |
442 | ||
cb1a09d0 | 443 | is the same as this: |
a0d0e21e LW |
444 | |
445 | $i = 1; | |
446 | while ($i < 10) { | |
15faabe4 | 447 | ... |
a0d0e21e | 448 | } continue { |
15faabe4 | 449 | $i++; |
a0d0e21e LW |
450 | } |
451 | ||
b78df5de JA |
452 | There is one minor difference: if variables are declared with C<my> |
453 | in the initialization section of the C<for>, the lexical scope of | |
454 | those variables is exactly the C<for> loop (the body of the loop | |
455 | and the control sections). | |
d74e8afc | 456 | X<my> |
55497cff | 457 | |
62bd8e4f PM |
458 | As a special case, if the test in the C<for> loop (or the corresponding |
459 | C<while> loop) is empty, it is treated as true. That is, both | |
460 | ||
461 | for (;;) { | |
15faabe4 | 462 | ... |
62bd8e4f PM |
463 | } |
464 | ||
465 | and | |
466 | ||
467 | while () { | |
15faabe4 | 468 | ... |
62bd8e4f PM |
469 | } |
470 | ||
471 | are treated as infinite loops. | |
472 | ||
cb1a09d0 AD |
473 | Besides the normal array index looping, C<for> can lend itself |
474 | to many other interesting applications. Here's one that avoids the | |
54310121 | 475 | problem you get into if you explicitly test for end-of-file on |
476 | an interactive file descriptor causing your program to appear to | |
cb1a09d0 | 477 | hang. |
d74e8afc | 478 | X<eof> X<end-of-file> X<end of file> |
cb1a09d0 AD |
479 | |
480 | $on_a_tty = -t STDIN && -t STDOUT; | |
481 | sub prompt { print "yes? " if $on_a_tty } | |
482 | for ( prompt(); <STDIN>; prompt() ) { | |
15faabe4 | 483 | # do something |
54310121 | 484 | } |
cb1a09d0 | 485 | |
5e979393 Z |
486 | The condition expression of a C<for> loop gets the same magic treatment of |
487 | C<readline> et al that the condition expression of a C<while> loop gets. | |
00cb5da1 | 488 | |
cb1a09d0 | 489 | =head2 Foreach Loops |
d74e8afc | 490 | X<for> X<foreach> |
cb1a09d0 | 491 | |
82848c10 | 492 | The C<foreach> loop iterates over a normal list value and sets the scalar |
55497cff | 493 | variable VAR to be each element of the list in turn. If the variable |
494 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
495 | is therefore visible only within the loop. Otherwise, the variable is | |
496 | implicitly local to the loop and regains its former value upon exiting | |
497 | the loop. If the variable was previously declared with C<my>, it uses | |
498 | that variable instead of the global one, but it's still localized to | |
6a0969e5 | 499 | the loop. This implicit localization occurs I<only> in a C<foreach> |
5c502d37 | 500 | loop. |
d74e8afc | 501 | X<my> X<local> |
4633a7c4 LW |
502 | |
503 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
c2f1e229 | 504 | you can use either. If VAR is omitted, C<$_> is set to each value. |
d74e8afc | 505 | X<$_> |
c5674021 |
506 | |
507 | If any element of LIST is an lvalue, you can modify it by modifying | |
508 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
509 | lvalue, any attempt to modify that element will fail. In other words, | |
510 | the C<foreach> loop index variable is an implicit alias for each item | |
511 | in the list that you're looping over. | |
d74e8afc | 512 | X<alias> |
302617ea MG |
513 | |
514 | If any part of LIST is an array, C<foreach> will get very confused if | |
515 | you add or remove elements within the loop body, for example with | |
516 | C<splice>. So don't do that. | |
d74e8afc | 517 | X<splice> |
302617ea MG |
518 | |
519 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
520 | special variable. Don't do that either. | |
4633a7c4 | 521 | |
82848c10 FC |
522 | As of Perl 5.22, there is an experimental variant of this loop that accepts |
523 | a variable preceded by a backslash for VAR, in which case the items in the | |
524 | LIST must be references. The backslashed variable will become an alias | |
525 | to each referenced item in the LIST, which must be of the correct type. | |
526 | The variable needn't be a scalar in this case, and the backslash may be | |
baabe3fb | 527 | followed by C<my>. To use this form, you must enable the C<refaliasing> |
82848c10 FC |
528 | feature via C<use feature>. (See L<feature>. See also L<perlref/Assigning |
529 | to References>.) | |
530 | ||
748a9306 | 531 | Examples: |
a0d0e21e | 532 | |
4633a7c4 | 533 | for (@ary) { s/foo/bar/ } |
a0d0e21e | 534 | |
96f2dc66 | 535 | for my $elem (@elements) { |
15faabe4 | 536 | $elem *= 2; |
a0d0e21e LW |
537 | } |
538 | ||
c2f1e229 | 539 | for $count (reverse(1..10), "BOOM") { |
15faabe4 SF |
540 | print $count, "\n"; |
541 | sleep(1); | |
a0d0e21e LW |
542 | } |
543 | ||
544 | for (1..15) { print "Merry Christmas\n"; } | |
545 | ||
4633a7c4 | 546 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
15faabe4 | 547 | print "Item: $item\n"; |
a0d0e21e LW |
548 | } |
549 | ||
baabe3fb FC |
550 | use feature "refaliasing"; |
551 | no warnings "experimental::refaliasing"; | |
82848c10 | 552 | foreach \my %hash (@array_of_hash_references) { |
15faabe4 | 553 | # do something which each %hash |
82848c10 FC |
554 | } |
555 | ||
4633a7c4 LW |
556 | Here's how a C programmer might code up a particular algorithm in Perl: |
557 | ||
55497cff | 558 | for (my $i = 0; $i < @ary1; $i++) { |
15faabe4 SF |
559 | for (my $j = 0; $j < @ary2; $j++) { |
560 | if ($ary1[$i] > $ary2[$j]) { | |
561 | last; # can't go to outer :-( | |
562 | } | |
563 | $ary1[$i] += $ary2[$j]; | |
564 | } | |
565 | # this is where that last takes me | |
4633a7c4 LW |
566 | } |
567 | ||
184e9718 | 568 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 | 569 | do it: |
4633a7c4 | 570 | |
96f2dc66 GS |
571 | OUTER: for my $wid (@ary1) { |
572 | INNER: for my $jet (@ary2) { | |
15faabe4 SF |
573 | next OUTER if $wid > $jet; |
574 | $wid += $jet; | |
575 | } | |
576 | } | |
4633a7c4 | 577 | |
cb1a09d0 AD |
578 | See how much easier this is? It's cleaner, safer, and faster. It's |
579 | cleaner because it's less noisy. It's safer because if code gets added | |
c07a80fd | 580 | between the inner and outer loops later on, the new code won't be |
5f05dabc | 581 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd | 582 | rather than merely terminating the inner one. And it's faster because |
583 | Perl executes a C<foreach> statement more rapidly than it would the | |
0d86a399 | 584 | equivalent C-style C<for> loop. |
4633a7c4 | 585 | |
739ba955 RS |
586 | Perceptive Perl hackers may have noticed that a C<for> loop has a return |
587 | value, and that this value can be captured by wrapping the loop in a C<do> | |
588 | block. The reward for this discovery is this cautionary advice: The | |
589 | return value of a C<for> loop is unspecified and may change without notice. | |
590 | Do not rely on it. | |
591 | ||
0d863452 RH |
592 | =head2 Basic BLOCKs |
593 | X<block> | |
4633a7c4 | 594 | |
55497cff | 595 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
596 | loop that executes once. Thus you can use any of the loop control | |
597 | statements in it to leave or restart the block. (Note that this is | |
598 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
599 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
600 | block is optional. | |
4633a7c4 | 601 | |
27cec4bd | 602 | The BLOCK construct can be used to emulate case structures. |
a0d0e21e LW |
603 | |
604 | SWITCH: { | |
15faabe4 SF |
605 | if (/^abc/) { $abc = 1; last SWITCH; } |
606 | if (/^def/) { $def = 1; last SWITCH; } | |
607 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
608 | $nothing = 1; | |
a0d0e21e LW |
609 | } |
610 | ||
7896dde7 Z |
611 | You'll also find that C<foreach> loop used to create a topicalizer |
612 | and a switch: | |
c2f1e229 TC |
613 | |
614 | SWITCH: | |
615 | for ($var) { | |
15faabe4 SF |
616 | if (/^abc/) { $abc = 1; last SWITCH; } |
617 | if (/^def/) { $def = 1; last SWITCH; } | |
618 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
619 | $nothing = 1; | |
c2f1e229 TC |
620 | } |
621 | ||
622 | Such constructs are quite frequently used, both because older versions of | |
623 | Perl had no official C<switch> statement, and also because the new version | |
7896dde7 | 624 | described immediately below remains experimental and can sometimes be confusing. |
83df6a1d | 625 | |
c2f1e229 | 626 | =head2 Switch Statements |
fd4f5766 | 627 | |
7896dde7 | 628 | X<switch> X<case> X<given> X<when> X<default> |
83df6a1d | 629 | |
7896dde7 Z |
630 | Starting from Perl 5.10.1 (well, 5.10.0, but it didn't work |
631 | right), you can say | |
83df6a1d | 632 | |
27cec4bd | 633 | use feature "switch"; |
a0d0e21e | 634 | |
7896dde7 Z |
635 | to enable an experimental switch feature. This is loosely based on an |
636 | old version of a Perl 6 proposal, but it no longer resembles the Perl 6 | |
637 | construct. You also get the switch feature whenever you declare that your | |
c2f1e229 TC |
638 | code prefers to run under a version of Perl that is 5.10 or later. For |
639 | example: | |
640 | ||
641 | use v5.14; | |
642 | ||
7896dde7 Z |
643 | Under the "switch" feature, Perl gains the experimental keywords |
644 | C<given>, C<when>, C<default>, C<continue>, and C<break>. | |
645 | Starting from Perl 5.16, one can prefix the switch | |
646 | keywords with C<CORE::> to access the feature without a C<use feature> | |
647 | statement. The keywords C<given> and | |
648 | C<when> are analogous to C<switch> and | |
649 | C<case> in other languages -- though C<continue> is not -- so the code | |
650 | in the previous section could be rewritten as | |
c2f1e229 TC |
651 | |
652 | use v5.10.1; | |
7896dde7 Z |
653 | for ($var) { |
654 | when (/^abc/) { $abc = 1 } | |
655 | when (/^def/) { $def = 1 } | |
656 | when (/^xyz/) { $xyz = 1 } | |
657 | default { $nothing = 1 } | |
c2f1e229 TC |
658 | } |
659 | ||
7896dde7 Z |
660 | The C<foreach> is the non-experimental way to set a topicalizer. |
661 | If you wish to use the highly experimental C<given>, that could be | |
662 | written like this: | |
0d863452 | 663 | |
7896dde7 | 664 | use v5.10.1; |
c2f1e229 | 665 | given ($var) { |
7896dde7 Z |
666 | when (/^abc/) { $abc = 1 } |
667 | when (/^def/) { $def = 1 } | |
668 | when (/^xyz/) { $xyz = 1 } | |
669 | default { $nothing = 1 } | |
c2f1e229 TC |
670 | } |
671 | ||
7896dde7 | 672 | As of 5.14, that can also be written this way: |
0d863452 | 673 | |
7896dde7 Z |
674 | use v5.14; |
675 | for ($var) { | |
676 | $abc = 1 when /^abc/; | |
677 | $def = 1 when /^def/; | |
678 | $xyz = 1 when /^xyz/; | |
679 | default { $nothing = 1 } | |
a0d0e21e LW |
680 | } |
681 | ||
7896dde7 | 682 | Or if you don't care to play it safe, like this: |
c2f1e229 | 683 | |
7896dde7 Z |
684 | use v5.14; |
685 | given ($var) { | |
686 | $abc = 1 when /^abc/; | |
687 | $def = 1 when /^def/; | |
688 | $xyz = 1 when /^xyz/; | |
689 | default { $nothing = 1 } | |
690 | } | |
64805db9 | 691 | |
7896dde7 Z |
692 | The arguments to C<given> and C<when> are in scalar context, |
693 | and C<given> assigns the C<$_> variable its topic value. | |
694 | ||
695 | Exactly what the I<EXPR> argument to C<when> does is hard to describe | |
696 | precisely, but in general, it tries to guess what you want done. Sometimes | |
697 | it is interpreted as C<< $_ ~~ I<EXPR> >>, and sometimes it is not. It | |
698 | also behaves differently when lexically enclosed by a C<given> block than | |
699 | it does when dynamically enclosed by a C<foreach> loop. The rules are far | |
700 | too difficult to understand to be described here. See L</"Experimental Details | |
701 | on given and when"> later on. | |
702 | ||
703 | Due to an unfortunate bug in how C<given> was implemented between Perl 5.10 | |
704 | and 5.16, under those implementations the version of C<$_> governed by | |
705 | C<given> is merely a lexically scoped copy of the original, not a | |
706 | dynamically scoped alias to the original, as it would be if it were a | |
707 | C<foreach> or under both the original and the current Perl 6 language | |
708 | specification. This bug was fixed in Perl 5.18 (and lexicalized C<$_> itself | |
709 | was removed in Perl 5.24). | |
710 | ||
711 | If your code still needs to run on older versions, | |
712 | stick to C<foreach> for your topicalizer and | |
713 | you will be less unhappy. | |
c2f1e229 TC |
714 | |
715 | =head2 Goto | |
716 | X<goto> | |
717 | ||
718 | Although not for the faint of heart, Perl does support a C<goto> | |
719 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
720 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
721 | a C<goto>; it's just the name of the loop. | |
722 | ||
723 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes | |
724 | execution there. It may not be used to go into any construct that | |
725 | requires initialization, such as a subroutine or a C<foreach> loop. It | |
726 | also can't be used to go into a construct that is optimized away. It | |
727 | can be used to go almost anywhere else within the dynamic scope, | |
728 | including out of subroutines, but it's usually better to use some other | |
729 | construct such as C<last> or C<die>. The author of Perl has never felt the | |
730 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
731 | ||
732 | The C<goto>-EXPR form expects a label name, whose scope will be resolved | |
733 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
734 | necessarily recommended if you're optimizing for maintainability: | |
735 | ||
736 | goto(("FOO", "BAR", "GLARCH")[$i]); | |
737 | ||
738 | The C<goto>-&NAME form is highly magical, and substitutes a call to the | |
739 | named subroutine for the currently running subroutine. This is used by | |
740 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then | |
741 | pretend that the other subroutine had been called in the first place | |
742 | (except that any modifications to C<@_> in the current subroutine are | |
743 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
744 | will be able to tell that this routine was called first. | |
745 | ||
746 | In almost all cases like this, it's usually a far, far better idea to use the | |
747 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
748 | resorting to a C<goto>. For certain applications, the catch and throw pair of | |
749 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
750 | ||
751 | =head2 The Ellipsis Statement | |
752 | X<...> | |
753 | X<... statement> | |
754 | X<ellipsis operator> | |
755 | X<elliptical statement> | |
756 | X<unimplemented statement> | |
757 | X<unimplemented operator> | |
758 | X<yada-yada> | |
81104cdf FC |
759 | X<yada-yada operator> |
760 | X<... operator> | |
761 | X<whatever operator> | |
762 | X<triple-dot operator> | |
c2f1e229 TC |
763 | |
764 | Beginning in Perl 5.12, Perl accepts an ellipsis, "C<...>", as a | |
29d69c3c | 765 | placeholder for code that you haven't implemented yet. |
b735b77b | 766 | When Perl 5.12 or later encounters an ellipsis statement, it parses this |
c2f1e229 TC |
767 | without error, but if and when you should actually try to execute it, Perl |
768 | throws an exception with the text C<Unimplemented>: | |
769 | ||
9dba9ce0 | 770 | use v5.12; |
c2f1e229 TC |
771 | sub unimplemented { ... } |
772 | eval { unimplemented() }; | |
9dba9ce0 | 773 | if ($@ =~ /^Unimplemented at /) { |
15faabe4 | 774 | say "I found an ellipsis!"; |
c2f1e229 TC |
775 | } |
776 | ||
29d69c3c Z |
777 | You can only use the elliptical statement to stand in for a complete |
778 | statement. Syntactically, "C<...;>" is a complete statement, but, | |
779 | as with other kinds of semicolon-terminated statement, the semicolon | |
780 | may be omitted if "C<...>" appears immediately before a closing brace. | |
781 | These examples show how the ellipsis works: | |
c2f1e229 TC |
782 | |
783 | use v5.12; | |
784 | { ... } | |
785 | sub foo { ... } | |
786 | ...; | |
787 | eval { ... }; | |
788 | sub somemeth { | |
15faabe4 SF |
789 | my $self = shift; |
790 | ...; | |
c2f1e229 TC |
791 | } |
792 | $x = do { | |
15faabe4 SF |
793 | my $n; |
794 | ...; | |
795 | say "Hurrah!"; | |
796 | $n; | |
c2f1e229 TC |
797 | }; |
798 | ||
799 | The elliptical statement cannot stand in for an expression that | |
29d69c3c | 800 | is part of a larger statement. |
c2f1e229 TC |
801 | These examples of attempts to use an ellipsis are syntax errors: |
802 | ||
803 | use v5.12; | |
804 | ||
805 | print ...; | |
806 | open(my $fh, ">", "/dev/passwd") or ...; | |
807 | if ($condition && ... ) { say "Howdy" }; | |
29d69c3c Z |
808 | ... if $a > $b; |
809 | say "Cromulent" if ...; | |
810 | $flub = 5 + ...; | |
c2f1e229 TC |
811 | |
812 | There are some cases where Perl can't immediately tell the difference | |
813 | between an expression and a statement. For instance, the syntax for a | |
814 | block and an anonymous hash reference constructor look the same unless | |
815 | there's something in the braces to give Perl a hint. The ellipsis is a | |
29d69c3c | 816 | syntax error if Perl doesn't guess that the C<{ ... }> is a block. |
12d22d1f | 817 | Inside your block, you can use a C<;> before the ellipsis to denote that the |
29d69c3c | 818 | C<{ ... }> is a block and not a hash reference constructor. |
c2f1e229 TC |
819 | |
820 | Note: Some folks colloquially refer to this bit of punctuation as a | |
81104cdf | 821 | "yada-yada" or "triple-dot", but its true name |
84539229 | 822 | is actually an ellipsis. |
c2f1e229 TC |
823 | |
824 | =head2 PODs: Embedded Documentation | |
825 | X<POD> X<documentation> | |
826 | ||
827 | Perl has a mechanism for intermixing documentation with source code. | |
828 | While it's expecting the beginning of a new statement, if the compiler | |
829 | encounters a line that begins with an equal sign and a word, like this | |
830 | ||
831 | =head1 Here There Be Pods! | |
832 | ||
833 | Then that text and all remaining text up through and including a line | |
834 | beginning with C<=cut> will be ignored. The format of the intervening | |
835 | text is described in L<perlpod>. | |
836 | ||
837 | This allows you to intermix your source code | |
838 | and your documentation text freely, as in | |
839 | ||
840 | =item snazzle($) | |
841 | ||
842 | The snazzle() function will behave in the most spectacular | |
843 | form that you can possibly imagine, not even excepting | |
844 | cybernetic pyrotechnics. | |
845 | ||
846 | =cut back to the compiler, nuff of this pod stuff! | |
847 | ||
848 | sub snazzle($) { | |
15faabe4 SF |
849 | my $thingie = shift; |
850 | ......... | |
c2f1e229 TC |
851 | } |
852 | ||
853 | Note that pod translators should look at only paragraphs beginning | |
854 | with a pod directive (it makes parsing easier), whereas the compiler | |
855 | actually knows to look for pod escapes even in the middle of a | |
856 | paragraph. This means that the following secret stuff will be | |
857 | ignored by both the compiler and the translators. | |
858 | ||
859 | $a=3; | |
860 | =secret stuff | |
861 | warn "Neither POD nor CODE!?" | |
862 | =cut back | |
863 | print "got $a\n"; | |
864 | ||
865 | You probably shouldn't rely upon the C<warn()> being podded out forever. | |
866 | Not all pod translators are well-behaved in this regard, and perhaps | |
867 | the compiler will become pickier. | |
868 | ||
869 | One may also use pod directives to quickly comment out a section | |
870 | of code. | |
871 | ||
872 | =head2 Plain Old Comments (Not!) | |
873 | X<comment> X<line> X<#> X<preprocessor> X<eval> | |
874 | ||
875 | Perl can process line directives, much like the C preprocessor. Using | |
876 | this, one can control Perl's idea of filenames and line numbers in | |
877 | error or warning messages (especially for strings that are processed | |
878 | with C<eval()>). The syntax for this mechanism is almost the same as for | |
879 | most C preprocessors: it matches the regular expression | |
880 | ||
881 | # example: '# line 42 "new_filename.plx"' | |
882 | /^\# \s* | |
883 | line \s+ (\d+) \s* | |
884 | (?:\s("?)([^"]+)\g2)? \s* | |
885 | $/x | |
886 | ||
887 | with C<$1> being the line number for the next line, and C<$3> being | |
89a3b501 | 888 | the optional filename (specified with or without quotes). Note that |
c2f1e229 TC |
889 | no whitespace may precede the C<< # >>, unlike modern C preprocessors. |
890 | ||
891 | There is a fairly obvious gotcha included with the line directive: | |
892 | Debuggers and profilers will only show the last source line to appear | |
893 | at a particular line number in a given file. Care should be taken not | |
894 | to cause line number collisions in code you'd like to debug later. | |
895 | ||
896 | Here are some examples that you should be able to type into your command | |
897 | shell: | |
898 | ||
899 | % perl | |
900 | # line 200 "bzzzt" | |
901 | # the '#' on the previous line must be the first char on line | |
902 | die 'foo'; | |
903 | __END__ | |
904 | foo at bzzzt line 201. | |
905 | ||
906 | % perl | |
907 | # line 200 "bzzzt" | |
908 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
909 | __END__ | |
910 | foo at - line 2001. | |
911 | ||
912 | % perl | |
913 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
914 | __END__ | |
915 | foo at foo bar line 200. | |
916 | ||
917 | % perl | |
918 | # line 345 "goop" | |
919 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
920 | print $@; | |
921 | __END__ | |
922 | foo at goop line 345. | |
923 | ||
7896dde7 Z |
924 | =head2 Experimental Details on given and when |
925 | ||
926 | As previously mentioned, the "switch" feature is considered highly | |
927 | experimental; it is subject to change with little notice. In particular, | |
928 | C<when> has tricky behaviours that are expected to change to become less | |
929 | tricky in the future. Do not rely upon its current (mis)implementation. | |
930 | Before Perl 5.18, C<given> also had tricky behaviours that you should still | |
931 | beware of if your code must run on older versions of Perl. | |
932 | ||
933 | Here is a longer example of C<given>: | |
934 | ||
935 | use feature ":5.10"; | |
936 | given ($foo) { | |
937 | when (undef) { | |
938 | say '$foo is undefined'; | |
939 | } | |
940 | when ("foo") { | |
941 | say '$foo is the string "foo"'; | |
942 | } | |
943 | when ([1,3,5,7,9]) { | |
944 | say '$foo is an odd digit'; | |
945 | continue; # Fall through | |
946 | } | |
947 | when ($_ < 100) { | |
948 | say '$foo is numerically less than 100'; | |
949 | } | |
950 | when (\&complicated_check) { | |
951 | say 'a complicated check for $foo is true'; | |
952 | } | |
953 | default { | |
954 | die q(I don't know what to do with $foo); | |
955 | } | |
956 | } | |
957 | ||
958 | Before Perl 5.18, C<given(EXPR)> assigned the value of I<EXPR> to | |
959 | merely a lexically scoped I<B<copy>> (!) of C<$_>, not a dynamically | |
960 | scoped alias the way C<foreach> does. That made it similar to | |
961 | ||
962 | do { my $_ = EXPR; ... } | |
963 | ||
964 | except that the block was automatically broken out of by a successful | |
965 | C<when> or an explicit C<break>. Because it was only a copy, and because | |
966 | it was only lexically scoped, not dynamically scoped, you could not do the | |
967 | things with it that you are used to in a C<foreach> loop. In particular, | |
968 | it did not work for arbitrary function calls if those functions might try | |
969 | to access $_. Best stick to C<foreach> for that. | |
970 | ||
971 | Most of the power comes from the implicit smartmatching that can | |
972 | sometimes apply. Most of the time, C<when(EXPR)> is treated as an | |
973 | implicit smartmatch of C<$_>, that is, C<$_ ~~ EXPR>. (See | |
974 | L<perlop/"Smartmatch Operator"> for more information on smartmatching.) | |
975 | But when I<EXPR> is one of the 10 exceptional cases (or things like them) | |
976 | listed below, it is used directly as a boolean. | |
977 | ||
978 | =over 4 | |
979 | ||
980 | =item Z<>1. | |
981 | ||
982 | A user-defined subroutine call or a method invocation. | |
983 | ||
984 | =item Z<>2. | |
985 | ||
986 | A regular expression match in the form of C</REGEX/>, C<$foo =~ /REGEX/>, | |
987 | or C<$foo =~ EXPR>. Also, a negated regular expression match in | |
988 | the form C<!/REGEX/>, C<$foo !~ /REGEX/>, or C<$foo !~ EXPR>. | |
989 | ||
990 | =item Z<>3. | |
991 | ||
992 | A smart match that uses an explicit C<~~> operator, such as C<EXPR ~~ EXPR>. | |
993 | ||
994 | B<NOTE:> You will often have to use C<$c ~~ $_> because the default case | |
dabde021 | 995 | uses C<$_ ~~ $c> , which is frequently the opposite of what you want. |
7896dde7 Z |
996 | |
997 | =item Z<>4. | |
998 | ||
999 | A boolean comparison operator such as C<$_ E<lt> 10> or C<$x eq "abc">. The | |
1000 | relational operators that this applies to are the six numeric comparisons | |
1001 | (C<< < >>, C<< > >>, C<< <= >>, C<< >= >>, C<< == >>, and C<< != >>), and | |
1002 | the six string comparisons (C<lt>, C<gt>, C<le>, C<ge>, C<eq>, and C<ne>). | |
1003 | ||
1004 | =item Z<>5. | |
1005 | ||
1006 | At least the three builtin functions C<defined(...)>, C<exists(...)>, and | |
1007 | C<eof(...)>. We might someday add more of these later if we think of them. | |
1008 | ||
1009 | =item Z<>6. | |
1010 | ||
1011 | A negated expression, whether C<!(EXPR)> or C<not(EXPR)>, or a logical | |
1012 | exclusive-or, C<(EXPR1) xor (EXPR2)>. The bitwise versions (C<~> and C<^>) | |
1013 | are not included. | |
1014 | ||
1015 | =item Z<>7. | |
1016 | ||
1017 | A filetest operator, with exactly 4 exceptions: C<-s>, C<-M>, C<-A>, and | |
1018 | C<-C>, as these return numerical values, not boolean ones. The C<-z> | |
1019 | filetest operator is not included in the exception list. | |
1020 | ||
1021 | =item Z<>8. | |
1022 | ||
1023 | The C<..> and C<...> flip-flop operators. Note that the C<...> flip-flop | |
1024 | operator is completely different from the C<...> elliptical statement | |
1025 | just described. | |
1026 | ||
1027 | =back | |
1028 | ||
1029 | In those 8 cases above, the value of EXPR is used directly as a boolean, so | |
1030 | no smartmatching is done. You may think of C<when> as a smartsmartmatch. | |
1031 | ||
1032 | Furthermore, Perl inspects the operands of logical operators to | |
1033 | decide whether to use smartmatching for each one by applying the | |
1034 | above test to the operands: | |
1035 | ||
1036 | =over 4 | |
1037 | ||
1038 | =item Z<>9. | |
1039 | ||
1040 | If EXPR is C<EXPR1 && EXPR2> or C<EXPR1 and EXPR2>, the test is applied | |
1041 | I<recursively> to both EXPR1 and EXPR2. | |
1042 | Only if I<both> operands also pass the | |
1043 | test, I<recursively>, will the expression be treated as boolean. Otherwise, | |
1044 | smartmatching is used. | |
1045 | ||
1046 | =item Z<>10. | |
1047 | ||
1048 | If EXPR is C<EXPR1 || EXPR2>, C<EXPR1 // EXPR2>, or C<EXPR1 or EXPR2>, the | |
1049 | test is applied I<recursively> to EXPR1 only (which might itself be a | |
1050 | higher-precedence AND operator, for example, and thus subject to the | |
1051 | previous rule), not to EXPR2. If EXPR1 is to use smartmatching, then EXPR2 | |
1052 | also does so, no matter what EXPR2 contains. But if EXPR2 does not get to | |
1053 | use smartmatching, then the second argument will not be either. This is | |
1054 | quite different from the C<&&> case just described, so be careful. | |
1055 | ||
1056 | =back | |
1057 | ||
1058 | These rules are complicated, but the goal is for them to do what you want | |
1059 | (even if you don't quite understand why they are doing it). For example: | |
1060 | ||
1061 | when (/^\d+$/ && $_ < 75) { ... } | |
1062 | ||
1063 | will be treated as a boolean match because the rules say both | |
1064 | a regex match and an explicit test on C<$_> will be treated | |
1065 | as boolean. | |
1066 | ||
1067 | Also: | |
1068 | ||
1069 | when ([qw(foo bar)] && /baz/) { ... } | |
1070 | ||
1071 | will use smartmatching because only I<one> of the operands is a boolean: | |
1072 | the other uses smartmatching, and that wins. | |
1073 | ||
1074 | Further: | |
1075 | ||
1076 | when ([qw(foo bar)] || /^baz/) { ... } | |
1077 | ||
1078 | will use smart matching (only the first operand is considered), whereas | |
1079 | ||
1080 | when (/^baz/ || [qw(foo bar)]) { ... } | |
1081 | ||
1082 | will test only the regex, which causes both operands to be | |
1083 | treated as boolean. Watch out for this one, then, because an | |
1084 | arrayref is always a true value, which makes it effectively | |
1085 | redundant. Not a good idea. | |
1086 | ||
1087 | Tautologous boolean operators are still going to be optimized | |
1088 | away. Don't be tempted to write | |
1089 | ||
1090 | when ("foo" or "bar") { ... } | |
1091 | ||
1092 | This will optimize down to C<"foo">, so C<"bar"> will never be considered (even | |
1093 | though the rules say to use a smartmatch | |
1094 | on C<"foo">). For an alternation like | |
1095 | this, an array ref will work, because this will instigate smartmatching: | |
1096 | ||
1097 | when ([qw(foo bar)] { ... } | |
1098 | ||
1099 | This is somewhat equivalent to the C-style switch statement's fallthrough | |
1100 | functionality (not to be confused with I<Perl's> fallthrough | |
1101 | functionality--see below), wherein the same block is used for several | |
1102 | C<case> statements. | |
1103 | ||
1104 | Another useful shortcut is that, if you use a literal array or hash as the | |
1105 | argument to C<given>, it is turned into a reference. So C<given(@foo)> is | |
1106 | the same as C<given(\@foo)>, for example. | |
1107 | ||
1108 | C<default> behaves exactly like C<when(1 == 1)>, which is | |
1109 | to say that it always matches. | |
1110 | ||
1111 | =head3 Breaking out | |
1112 | ||
1113 | You can use the C<break> keyword to break out of the enclosing | |
1114 | C<given> block. Every C<when> block is implicitly ended with | |
1115 | a C<break>. | |
1116 | ||
1117 | =head3 Fall-through | |
1118 | ||
1119 | You can use the C<continue> keyword to fall through from one | |
1120 | case to the next immediate C<when> or C<default>: | |
1121 | ||
1122 | given($foo) { | |
1123 | when (/x/) { say '$foo contains an x'; continue } | |
1124 | when (/y/) { say '$foo contains a y' } | |
1125 | default { say '$foo does not contain a y' } | |
1126 | } | |
1127 | ||
1128 | =head3 Return value | |
1129 | ||
1130 | When a C<given> statement is also a valid expression (for example, | |
1131 | when it's the last statement of a block), it evaluates to: | |
1132 | ||
1133 | =over 4 | |
1134 | ||
1135 | =item * | |
1136 | ||
1137 | An empty list as soon as an explicit C<break> is encountered. | |
1138 | ||
1139 | =item * | |
1140 | ||
1141 | The value of the last evaluated expression of the successful | |
1142 | C<when>/C<default> clause, if there happens to be one. | |
1143 | ||
1144 | =item * | |
1145 | ||
1146 | The value of the last evaluated expression of the C<given> block if no | |
1147 | condition is true. | |
1148 | ||
1149 | =back | |
1150 | ||
1151 | In both last cases, the last expression is evaluated in the context that | |
1152 | was applied to the C<given> block. | |
1153 | ||
1154 | Note that, unlike C<if> and C<unless>, failed C<when> statements always | |
1155 | evaluate to an empty list. | |
1156 | ||
1157 | my $price = do { | |
1158 | given ($item) { | |
1159 | when (["pear", "apple"]) { 1 } | |
1160 | break when "vote"; # My vote cannot be bought | |
1161 | 1e10 when /Mona Lisa/; | |
1162 | "unknown"; | |
1163 | } | |
1164 | }; | |
1165 | ||
1166 | Currently, C<given> blocks can't always | |
1167 | be used as proper expressions. This | |
1168 | may be addressed in a future version of Perl. | |
1169 | ||
1170 | =head3 Switching in a loop | |
1171 | ||
1172 | Instead of using C<given()>, you can use a C<foreach()> loop. | |
1173 | For example, here's one way to count how many times a particular | |
1174 | string occurs in an array: | |
1175 | ||
1176 | use v5.10.1; | |
1177 | my $count = 0; | |
1178 | for (@array) { | |
1179 | when ("foo") { ++$count } | |
1180 | } | |
1181 | print "\@array contains $count copies of 'foo'\n"; | |
1182 | ||
1183 | Or in a more recent version: | |
1184 | ||
1185 | use v5.14; | |
1186 | my $count = 0; | |
1187 | for (@array) { | |
1188 | ++$count when "foo"; | |
1189 | } | |
1190 | print "\@array contains $count copies of 'foo'\n"; | |
1191 | ||
1192 | At the end of all C<when> blocks, there is an implicit C<next>. | |
1193 | You can override that with an explicit C<last> if you're | |
1194 | interested in only the first match alone. | |
1195 | ||
1196 | This doesn't work if you explicitly specify a loop variable, as | |
1197 | in C<for $item (@array)>. You have to use the default variable C<$_>. | |
1198 | ||
1199 | =head3 Differences from Perl 6 | |
1200 | ||
1201 | The Perl 5 smartmatch and C<given>/C<when> constructs are not compatible | |
1202 | with their Perl 6 analogues. The most visible difference and least | |
1203 | important difference is that, in Perl 5, parentheses are required around | |
1204 | the argument to C<given()> and C<when()> (except when this last one is used | |
1205 | as a statement modifier). Parentheses in Perl 6 are always optional in a | |
1206 | control construct such as C<if()>, C<while()>, or C<when()>; they can't be | |
1207 | made optional in Perl 5 without a great deal of potential confusion, | |
1208 | because Perl 5 would parse the expression | |
1209 | ||
1210 | given $foo { | |
1211 | ... | |
1212 | } | |
1213 | ||
1214 | as though the argument to C<given> were an element of the hash | |
1215 | C<%foo>, interpreting the braces as hash-element syntax. | |
1216 | ||
1217 | However, their are many, many other differences. For example, | |
1218 | this works in Perl 5: | |
1219 | ||
1220 | use v5.12; | |
1221 | my @primary = ("red", "blue", "green"); | |
1222 | ||
1223 | if (@primary ~~ "red") { | |
1224 | say "primary smartmatches red"; | |
1225 | } | |
1226 | ||
1227 | if ("red" ~~ @primary) { | |
1228 | say "red smartmatches primary"; | |
1229 | } | |
1230 | ||
1231 | say "that's all, folks!"; | |
1232 | ||
1233 | But it doesn't work at all in Perl 6. Instead, you should | |
1234 | use the (parallelizable) C<any> operator: | |
1235 | ||
1236 | if any(@primary) eq "red" { | |
1237 | say "primary smartmatches red"; | |
1238 | } | |
1239 | ||
1240 | if "red" eq any(@primary) { | |
1241 | say "red smartmatches primary"; | |
1242 | } | |
1243 | ||
1244 | The table of smartmatches in L<perlop/"Smartmatch Operator"> is not | |
1245 | identical to that proposed by the Perl 6 specification, mainly due to | |
1246 | differences between Perl 6's and Perl 5's data models, but also because | |
1247 | the Perl 6 spec has changed since Perl 5 rushed into early adoption. | |
1248 | ||
1249 | In Perl 6, C<when()> will always do an implicit smartmatch with its | |
1250 | argument, while in Perl 5 it is convenient (albeit potentially confusing) to | |
1251 | suppress this implicit smartmatch in various rather loosely-defined | |
1252 | situations, as roughly outlined above. (The difference is largely because | |
1253 | Perl 5 does not have, even internally, a boolean type.) | |
1254 | ||
774d564b | 1255 | =cut |