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 | ||
119 | =head2 Truth and Falsehood | |
d74e8afc | 120 | X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0> |
cf48932e | 121 | |
c2f1e229 TC |
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. | |
52ea55c9 | 124 | Negation of a true value by C<!> or C<not> returns a special false value. |
c2f1e229 TC |
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. | |
cf48932e | 128 | |
cf48932e | 129 | =head2 Statement Modifiers |
d74e8afc | 130 | X<statement modifier> X<modifier> X<if> X<unless> X<while> |
4f8ea571 | 131 | X<until> X<when> X<foreach> X<for> |
a0d0e21e LW |
132 | |
133 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
134 | just before the terminating semicolon (or block ending). The possible | |
135 | modifiers are: | |
136 | ||
137 | if EXPR | |
138 | unless EXPR | |
139 | while EXPR | |
140 | until EXPR | |
4f8ea571 | 141 | for LIST |
cf48932e | 142 | foreach LIST |
c2f1e229 | 143 | when EXPR |
cf48932e SF |
144 | |
145 | The C<EXPR> following the modifier is referred to as the "condition". | |
146 | Its truth or falsehood determines how the modifier will behave. | |
147 | ||
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> | |
c2f1e229 | 150 | the condition is true (that is, if the condition is false). |
cf48932e SF |
151 | |
152 | print "Basset hounds got long ears" if length $ear >= 10; | |
153 | go_outside() and play() unless $is_raining; | |
154 | ||
c2f1e229 | 155 | The C<for(each)> modifier is an iterator: it executes the statement once |
cf48932e SF |
156 | for each item in the LIST (with C<$_> aliased to each item in turn). |
157 | ||
c2f1e229 | 158 | print "Hello $_!\n" for qw(world Dolly nurse); |
cf48932e SF |
159 | |
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): | |
163 | ||
164 | # Both of these count from 0 to 10. | |
165 | print $i++ while $i <= 10; | |
166 | print $j++ until $j > 10; | |
167 | ||
168 | The C<while> and C<until> modifiers have the usual "C<while> loop" | |
169 | semantics (conditional evaluated first), except when applied to a | |
c2f1e229 | 170 | C<do>-BLOCK (or to the Perl4 C<do>-SUBROUTINE statement), in |
cf48932e | 171 | which case the block executes once before the conditional is |
c2f1e229 TC |
172 | evaluated. |
173 | ||
174 | This is so that you can write loops like: | |
a0d0e21e LW |
175 | |
176 | do { | |
15faabe4 SF |
177 | $line = <STDIN>; |
178 | ... | |
c2f1e229 | 179 | } until !defined($line) || $line eq ".\n" |
a0d0e21e | 180 | |
5a964f20 TC |
181 | See L<perlfunc/do>. Note also that the loop control statements described |
182 | later will I<NOT> work in this construct, because modifiers don't take | |
183 | loop labels. Sorry. You can always put another block inside of it | |
184 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
f86cebdf | 185 | For C<next>, just double the braces: |
d74e8afc | 186 | X<next> X<last> X<redo> |
5a964f20 TC |
187 | |
188 | do {{ | |
15faabe4 SF |
189 | next if $x == $y; |
190 | # do something here | |
5a964f20 TC |
191 | }} until $x++ > $z; |
192 | ||
f86cebdf | 193 | For C<last>, you have to be more elaborate: |
d74e8afc | 194 | X<last> |
5a964f20 | 195 | |
15faabe4 SF |
196 | LOOP: { |
197 | do { | |
198 | last if $x = $y**2; | |
199 | # do something here | |
200 | } while $x++ <= $z; | |
5a964f20 | 201 | } |
a0d0e21e | 202 | |
c2f1e229 TC |
203 | B<NOTE:> The behaviour of a C<my>, C<state>, or |
204 | C<our> modified with a statement modifier conditional | |
205 | or loop construct (for example, C<my $x if ...>) is | |
457b36cb MV |
206 | B<undefined>. The value of the C<my> variable may be C<undef>, any |
207 | previously assigned value, or possibly anything else. Don't rely on | |
208 | it. Future versions of perl might do something different from the | |
209 | version of perl you try it out on. Here be dragons. | |
d74e8afc | 210 | X<my> |
457b36cb | 211 | |
c2f1e229 TC |
212 | The C<when> modifier is an experimental feature that first appeared in Perl |
213 | 5.14. To use it, you should include a C<use v5.14> declaration. | |
214 | (Technically, it requires only the C<switch> feature, but that aspect of it | |
215 | was not available before 5.14.) Operative only from within a C<foreach> | |
216 | loop or a C<given> block, it executes the statement only if the smartmatch | |
217 | C<< $_ ~~ I<EXPR> >> is true. If the statement executes, it is followed by | |
218 | a C<next> from inside a C<foreach> and C<break> from inside a C<given>. | |
219 | ||
220 | Under the current implementation, the C<foreach> loop can be | |
221 | anywhere within the C<when> modifier's dynamic scope, but must be | |
222 | within the C<given> block's lexical scope. This restricted may | |
48238296 | 223 | be relaxed in a future release. See L<"Switch Statements"> below. |
c2f1e229 | 224 | |
6ec4bd10 | 225 | =head2 Compound Statements |
d74e8afc | 226 | X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace> |
c2f1e229 | 227 | X<{> X<}> X<if> X<unless> X<given> X<while> X<until> X<foreach> X<for> X<continue> |
a0d0e21e LW |
228 | |
229 | In Perl, a sequence of statements that defines a scope is called a block. | |
230 | Sometimes a block is delimited by the file containing it (in the case | |
231 | of a required file, or the program as a whole), and sometimes a block | |
232 | is delimited by the extent of a string (in the case of an eval). | |
233 | ||
234 | But generally, a block is delimited by curly brackets, also known as braces. | |
235 | We will call this syntactic construct a BLOCK. | |
236 | ||
237 | The following compound statements may be used to control flow: | |
238 | ||
239 | if (EXPR) BLOCK | |
240 | if (EXPR) BLOCK else BLOCK | |
c2f1e229 | 241 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... |
a0d0e21e | 242 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
c2f1e229 | 243 | |
62d98eed RU |
244 | unless (EXPR) BLOCK |
245 | unless (EXPR) BLOCK else BLOCK | |
c2f1e229 | 246 | unless (EXPR) BLOCK elsif (EXPR) BLOCK ... |
d27f8d4b | 247 | unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
c2f1e229 TC |
248 | |
249 | given (EXPR) BLOCK | |
250 | ||
a0d0e21e LW |
251 | LABEL while (EXPR) BLOCK |
252 | LABEL while (EXPR) BLOCK continue BLOCK | |
c2f1e229 | 253 | |
5ec6d87f EA |
254 | LABEL until (EXPR) BLOCK |
255 | LABEL until (EXPR) BLOCK continue BLOCK | |
c2f1e229 | 256 | |
a0d0e21e | 257 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
7808b687 AB |
258 | LABEL for VAR (LIST) BLOCK |
259 | LABEL for VAR (LIST) BLOCK continue BLOCK | |
c2f1e229 | 260 | |
7808b687 | 261 | LABEL foreach (EXPR; EXPR; EXPR) BLOCK |
748a9306 | 262 | LABEL foreach VAR (LIST) BLOCK |
b303ae78 | 263 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
c2f1e229 TC |
264 | |
265 | LABEL BLOCK | |
a0d0e21e | 266 | LABEL BLOCK continue BLOCK |
c2f1e229 | 267 | |
43f66a76 | 268 | PHASE BLOCK |
a0d0e21e | 269 | |
15faabe4 | 270 | The experimental C<given> statement is I<not automatically enabled>; see |
c2f1e229 TC |
271 | L</"Switch Statements"> below for how to do so, and the attendant caveats. |
272 | ||
273 | Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs, | |
a0d0e21e LW |
274 | not statements. This means that the curly brackets are I<required>--no |
275 | dangling statements allowed. If you want to write conditionals without | |
c2f1e229 | 276 | curly brackets, there are several other ways to do it. The following |
a0d0e21e LW |
277 | all do the same thing: |
278 | ||
c2f1e229 | 279 | if (!open(FOO)) { die "Can't open $FOO: $!" } |
a0d0e21e | 280 | die "Can't open $FOO: $!" unless open(FOO); |
c2f1e229 TC |
281 | open(FOO) || die "Can't open $FOO: $!"; |
282 | open(FOO) ? () : die "Can't open $FOO: $!"; | |
15faabe4 | 283 | # a bit exotic, that last one |
a0d0e21e | 284 | |
5f05dabc | 285 | The C<if> statement is straightforward. Because BLOCKs are always |
a0d0e21e LW |
286 | bounded by curly brackets, there is never any ambiguity about which |
287 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
89a3b501 FC |
288 | the sense of the test is reversed. Like C<if>, C<unless> can be followed |
289 | by C<else>. C<unless> can even be followed by one or more C<elsif> | |
d27f8d4b JV |
290 | statements, though you may want to think twice before using that particular |
291 | language construct, as everyone reading your code will have to think at least | |
292 | twice before they can understand what's going on. | |
a0d0e21e LW |
293 | |
294 | The C<while> statement executes the block as long as the expression is | |
e17b7802 | 295 | L<true|/"Truth and Falsehood">. |
1d5653dd RGS |
296 | The C<until> statement executes the block as long as the expression is |
297 | false. | |
b78218b7 GS |
298 | The LABEL is optional, and if present, consists of an identifier followed |
299 | by a colon. The LABEL identifies the loop for the loop control | |
300 | statements C<next>, C<last>, and C<redo>. | |
301 | If the LABEL is omitted, the loop control statement | |
4633a7c4 LW |
302 | refers to the innermost enclosing loop. This may include dynamically |
303 | looking back your call-stack at run time to find the LABEL. Such | |
9f1b1f2d | 304 | desperate behavior triggers a warning if you use the C<use warnings> |
a2293a43 | 305 | pragma or the B<-w> flag. |
4633a7c4 LW |
306 | |
307 | If there is a C<continue> BLOCK, it is always executed just before the | |
6ec4bd10 MS |
308 | conditional is about to be evaluated again. Thus it can be used to |
309 | increment a loop variable, even when the loop has been continued via | |
310 | the C<next> statement. | |
4633a7c4 | 311 | |
43f66a76 DG |
312 | When a block is preceding by a compilation phase keyword such as C<BEGIN>, |
313 | C<END>, C<INIT>, C<CHECK>, or C<UNITCHECK>, then the block will run only | |
314 | during the corresponding phase of execution. See L<perlmod> for more details. | |
315 | ||
88e1f1a2 | 316 | Extension modules can also hook into the Perl parser to define new |
c2f1e229 | 317 | kinds of compound statements. These are introduced by a keyword which |
6a0969e5 | 318 | the extension recognizes, and the syntax following the keyword is |
88e1f1a2 JV |
319 | defined entirely by the extension. If you are an implementor, see |
320 | L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such | |
321 | a module, see the module's documentation for details of the syntax that | |
322 | it defines. | |
323 | ||
4633a7c4 | 324 | =head2 Loop Control |
d74e8afc | 325 | X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue> |
4633a7c4 | 326 | |
6ec4bd10 | 327 | The C<next> command starts the next iteration of the loop: |
4633a7c4 LW |
328 | |
329 | LINE: while (<STDIN>) { | |
15faabe4 SF |
330 | next LINE if /^#/; # discard comments |
331 | ... | |
4633a7c4 LW |
332 | } |
333 | ||
6ec4bd10 | 334 | The C<last> command immediately exits the loop in question. The |
4633a7c4 LW |
335 | C<continue> block, if any, is not executed: |
336 | ||
337 | LINE: while (<STDIN>) { | |
15faabe4 SF |
338 | last LINE if /^$/; # exit when done with header |
339 | ... | |
4633a7c4 LW |
340 | } |
341 | ||
342 | The C<redo> command restarts the loop block without evaluating the | |
343 | conditional again. The C<continue> block, if any, is I<not> executed. | |
344 | This command is normally used by programs that want to lie to themselves | |
345 | about what was just input. | |
346 | ||
347 | For example, when processing a file like F</etc/termcap>. | |
348 | If your input lines might end in backslashes to indicate continuation, you | |
349 | want to skip ahead and get the next record. | |
350 | ||
351 | while (<>) { | |
15faabe4 SF |
352 | chomp; |
353 | if (s/\\$//) { | |
354 | $_ .= <>; | |
355 | redo unless eof(); | |
356 | } | |
357 | # now process $_ | |
54310121 | 358 | } |
4633a7c4 | 359 | |
c2f1e229 | 360 | which is Perl shorthand for the more explicitly written version: |
4633a7c4 | 361 | |
54310121 | 362 | LINE: while (defined($line = <ARGV>)) { |
15faabe4 SF |
363 | chomp($line); |
364 | if ($line =~ s/\\$//) { | |
365 | $line .= <ARGV>; | |
366 | redo LINE unless eof(); # not eof(ARGV)! | |
367 | } | |
368 | # now process $line | |
54310121 | 369 | } |
4633a7c4 | 370 | |
36e7a065 AMS |
371 | Note that if there were a C<continue> block on the above code, it would |
372 | get executed only on lines discarded by the regex (since redo skips the | |
89a3b501 | 373 | continue block). A continue block is often used to reset line counters |
499a640d | 374 | or C<m?pat?> one-time matches: |
4633a7c4 | 375 | |
5a964f20 TC |
376 | # inspired by :1,$g/fred/s//WILMA/ |
377 | while (<>) { | |
15faabe4 SF |
378 | m?(fred)? && s//WILMA $1 WILMA/; |
379 | m?(barney)? && s//BETTY $1 BETTY/; | |
380 | m?(homer)? && s//MARGE $1 MARGE/; | |
5a964f20 | 381 | } continue { |
15faabe4 SF |
382 | print "$ARGV $.: $_"; |
383 | close ARGV if eof; # reset $. | |
384 | reset if eof; # reset ?pat? | |
4633a7c4 LW |
385 | } |
386 | ||
a0d0e21e LW |
387 | If the word C<while> is replaced by the word C<until>, the sense of the |
388 | test is reversed, but the conditional is still tested before the first | |
389 | iteration. | |
390 | ||
c2f1e229 | 391 | Loop control statements don't work in an C<if> or C<unless>, since |
5a964f20 TC |
392 | they aren't loops. You can double the braces to make them such, though. |
393 | ||
394 | if (/pattern/) {{ | |
15faabe4 SF |
395 | last if /fred/; |
396 | next if /barney/; # same effect as "last", | |
397 | # but doesn't document as well | |
398 | # do something here | |
5a964f20 TC |
399 | }} |
400 | ||
7bd1983c | 401 | This is caused by the fact that a block by itself acts as a loop that |
27cec4bd | 402 | executes once, see L<"Basic BLOCKs">. |
7bd1983c | 403 | |
5b23ba8b MG |
404 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
405 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
4633a7c4 | 406 | |
cb1a09d0 | 407 | =head2 For Loops |
d74e8afc | 408 | X<for> X<foreach> |
a0d0e21e | 409 | |
b78df5de | 410 | Perl's C-style C<for> loop works like the corresponding C<while> loop; |
cb1a09d0 | 411 | that means that this: |
a0d0e21e LW |
412 | |
413 | for ($i = 1; $i < 10; $i++) { | |
15faabe4 | 414 | ... |
a0d0e21e LW |
415 | } |
416 | ||
cb1a09d0 | 417 | is the same as this: |
a0d0e21e LW |
418 | |
419 | $i = 1; | |
420 | while ($i < 10) { | |
15faabe4 | 421 | ... |
a0d0e21e | 422 | } continue { |
15faabe4 | 423 | $i++; |
a0d0e21e LW |
424 | } |
425 | ||
b78df5de JA |
426 | There is one minor difference: if variables are declared with C<my> |
427 | in the initialization section of the C<for>, the lexical scope of | |
428 | those variables is exactly the C<for> loop (the body of the loop | |
429 | and the control sections). | |
d74e8afc | 430 | X<my> |
55497cff | 431 | |
62bd8e4f PM |
432 | As a special case, if the test in the C<for> loop (or the corresponding |
433 | C<while> loop) is empty, it is treated as true. That is, both | |
434 | ||
435 | for (;;) { | |
15faabe4 | 436 | ... |
62bd8e4f PM |
437 | } |
438 | ||
439 | and | |
440 | ||
441 | while () { | |
15faabe4 | 442 | ... |
62bd8e4f PM |
443 | } |
444 | ||
445 | are treated as infinite loops. | |
446 | ||
cb1a09d0 AD |
447 | Besides the normal array index looping, C<for> can lend itself |
448 | to many other interesting applications. Here's one that avoids the | |
54310121 | 449 | problem you get into if you explicitly test for end-of-file on |
450 | an interactive file descriptor causing your program to appear to | |
cb1a09d0 | 451 | hang. |
d74e8afc | 452 | X<eof> X<end-of-file> X<end of file> |
cb1a09d0 AD |
453 | |
454 | $on_a_tty = -t STDIN && -t STDOUT; | |
455 | sub prompt { print "yes? " if $on_a_tty } | |
456 | for ( prompt(); <STDIN>; prompt() ) { | |
15faabe4 | 457 | # do something |
54310121 | 458 | } |
cb1a09d0 | 459 | |
00cb5da1 CW |
460 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the |
461 | conditional of a C<for> loop is shorthand for the following. This | |
462 | behaviour is the same as a C<while> loop conditional. | |
d74e8afc | 463 | X<readline> X<< <> >> |
00cb5da1 CW |
464 | |
465 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { | |
466 | # do something | |
467 | } | |
468 | ||
cb1a09d0 | 469 | =head2 Foreach Loops |
d74e8afc | 470 | X<for> X<foreach> |
cb1a09d0 | 471 | |
82848c10 | 472 | The C<foreach> loop iterates over a normal list value and sets the scalar |
55497cff | 473 | variable VAR to be each element of the list in turn. If the variable |
474 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
475 | is therefore visible only within the loop. Otherwise, the variable is | |
476 | implicitly local to the loop and regains its former value upon exiting | |
477 | the loop. If the variable was previously declared with C<my>, it uses | |
478 | that variable instead of the global one, but it's still localized to | |
6a0969e5 | 479 | the loop. This implicit localization occurs I<only> in a C<foreach> |
5c502d37 | 480 | loop. |
d74e8afc | 481 | X<my> X<local> |
4633a7c4 LW |
482 | |
483 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
c2f1e229 | 484 | you can use either. If VAR is omitted, C<$_> is set to each value. |
d74e8afc | 485 | X<$_> |
c5674021 |
486 | |
487 | If any element of LIST is an lvalue, you can modify it by modifying | |
488 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
489 | lvalue, any attempt to modify that element will fail. In other words, | |
490 | the C<foreach> loop index variable is an implicit alias for each item | |
491 | in the list that you're looping over. | |
d74e8afc | 492 | X<alias> |
302617ea MG |
493 | |
494 | If any part of LIST is an array, C<foreach> will get very confused if | |
495 | you add or remove elements within the loop body, for example with | |
496 | C<splice>. So don't do that. | |
d74e8afc | 497 | X<splice> |
302617ea MG |
498 | |
499 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
500 | special variable. Don't do that either. | |
4633a7c4 | 501 | |
82848c10 FC |
502 | As of Perl 5.22, there is an experimental variant of this loop that accepts |
503 | a variable preceded by a backslash for VAR, in which case the items in the | |
504 | LIST must be references. The backslashed variable will become an alias | |
505 | to each referenced item in the LIST, which must be of the correct type. | |
506 | The variable needn't be a scalar in this case, and the backslash may be | |
baabe3fb | 507 | followed by C<my>. To use this form, you must enable the C<refaliasing> |
82848c10 FC |
508 | feature via C<use feature>. (See L<feature>. See also L<perlref/Assigning |
509 | to References>.) | |
510 | ||
748a9306 | 511 | Examples: |
a0d0e21e | 512 | |
4633a7c4 | 513 | for (@ary) { s/foo/bar/ } |
a0d0e21e | 514 | |
96f2dc66 | 515 | for my $elem (@elements) { |
15faabe4 | 516 | $elem *= 2; |
a0d0e21e LW |
517 | } |
518 | ||
c2f1e229 | 519 | for $count (reverse(1..10), "BOOM") { |
15faabe4 SF |
520 | print $count, "\n"; |
521 | sleep(1); | |
a0d0e21e LW |
522 | } |
523 | ||
524 | for (1..15) { print "Merry Christmas\n"; } | |
525 | ||
4633a7c4 | 526 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
15faabe4 | 527 | print "Item: $item\n"; |
a0d0e21e LW |
528 | } |
529 | ||
baabe3fb FC |
530 | use feature "refaliasing"; |
531 | no warnings "experimental::refaliasing"; | |
82848c10 | 532 | foreach \my %hash (@array_of_hash_references) { |
15faabe4 | 533 | # do something which each %hash |
82848c10 FC |
534 | } |
535 | ||
4633a7c4 LW |
536 | Here's how a C programmer might code up a particular algorithm in Perl: |
537 | ||
55497cff | 538 | for (my $i = 0; $i < @ary1; $i++) { |
15faabe4 SF |
539 | for (my $j = 0; $j < @ary2; $j++) { |
540 | if ($ary1[$i] > $ary2[$j]) { | |
541 | last; # can't go to outer :-( | |
542 | } | |
543 | $ary1[$i] += $ary2[$j]; | |
544 | } | |
545 | # this is where that last takes me | |
4633a7c4 LW |
546 | } |
547 | ||
184e9718 | 548 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 | 549 | do it: |
4633a7c4 | 550 | |
96f2dc66 GS |
551 | OUTER: for my $wid (@ary1) { |
552 | INNER: for my $jet (@ary2) { | |
15faabe4 SF |
553 | next OUTER if $wid > $jet; |
554 | $wid += $jet; | |
555 | } | |
556 | } | |
4633a7c4 | 557 | |
cb1a09d0 AD |
558 | See how much easier this is? It's cleaner, safer, and faster. It's |
559 | cleaner because it's less noisy. It's safer because if code gets added | |
c07a80fd | 560 | between the inner and outer loops later on, the new code won't be |
5f05dabc | 561 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd | 562 | rather than merely terminating the inner one. And it's faster because |
563 | Perl executes a C<foreach> statement more rapidly than it would the | |
564 | equivalent C<for> loop. | |
4633a7c4 | 565 | |
739ba955 RS |
566 | Perceptive Perl hackers may have noticed that a C<for> loop has a return |
567 | value, and that this value can be captured by wrapping the loop in a C<do> | |
568 | block. The reward for this discovery is this cautionary advice: The | |
569 | return value of a C<for> loop is unspecified and may change without notice. | |
570 | Do not rely on it. | |
571 | ||
0d863452 RH |
572 | =head2 Basic BLOCKs |
573 | X<block> | |
4633a7c4 | 574 | |
55497cff | 575 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
576 | loop that executes once. Thus you can use any of the loop control | |
577 | statements in it to leave or restart the block. (Note that this is | |
578 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
579 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
580 | block is optional. | |
4633a7c4 | 581 | |
27cec4bd | 582 | The BLOCK construct can be used to emulate case structures. |
a0d0e21e LW |
583 | |
584 | SWITCH: { | |
15faabe4 SF |
585 | if (/^abc/) { $abc = 1; last SWITCH; } |
586 | if (/^def/) { $def = 1; last SWITCH; } | |
587 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
588 | $nothing = 1; | |
a0d0e21e LW |
589 | } |
590 | ||
c2f1e229 TC |
591 | You'll also find that C<foreach> loop used to create a topicalizer |
592 | and a switch: | |
593 | ||
594 | SWITCH: | |
595 | for ($var) { | |
15faabe4 SF |
596 | if (/^abc/) { $abc = 1; last SWITCH; } |
597 | if (/^def/) { $def = 1; last SWITCH; } | |
598 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
599 | $nothing = 1; | |
c2f1e229 TC |
600 | } |
601 | ||
602 | Such constructs are quite frequently used, both because older versions of | |
603 | Perl had no official C<switch> statement, and also because the new version | |
604 | described immediately below remains experimental and can sometimes be confusing. | |
83df6a1d | 605 | |
c2f1e229 | 606 | =head2 Switch Statements |
fd4f5766 | 607 | |
0d863452 | 608 | X<switch> X<case> X<given> X<when> X<default> |
83df6a1d | 609 | |
c2f1e229 TC |
610 | Starting from Perl 5.10.1 (well, 5.10.0, but it didn't work |
611 | right), you can say | |
83df6a1d | 612 | |
27cec4bd | 613 | use feature "switch"; |
a0d0e21e | 614 | |
c2f1e229 TC |
615 | to enable an experimental switch feature. This is loosely based on an |
616 | old version of a Perl 6 proposal, but it no longer resembles the Perl 6 | |
617 | construct. You also get the switch feature whenever you declare that your | |
618 | code prefers to run under a version of Perl that is 5.10 or later. For | |
619 | example: | |
620 | ||
621 | use v5.14; | |
622 | ||
623 | Under the "switch" feature, Perl gains the experimental keywords | |
624 | C<given>, C<when>, C<default>, C<continue>, and C<break>. | |
625 | Starting from Perl 5.16, one can prefix the switch | |
4a904372 | 626 | keywords with C<CORE::> to access the feature without a C<use feature> |
89a3b501 FC |
627 | statement. The keywords C<given> and |
628 | C<when> are analogous to C<switch> and | |
c2f1e229 TC |
629 | C<case> in other languages, so the code in the previous section could be |
630 | rewritten as | |
631 | ||
632 | use v5.10.1; | |
633 | for ($var) { | |
15faabe4 SF |
634 | when (/^abc/) { $abc = 1 } |
635 | when (/^def/) { $def = 1 } | |
636 | when (/^xyz/) { $xyz = 1 } | |
637 | default { $nothing = 1 } | |
c2f1e229 TC |
638 | } |
639 | ||
640 | The C<foreach> is the non-experimental way to set a topicalizer. | |
641 | If you wish to use the highly experimental C<given>, that could be | |
642 | written like this: | |
0d863452 | 643 | |
c2f1e229 TC |
644 | use v5.10.1; |
645 | given ($var) { | |
15faabe4 SF |
646 | when (/^abc/) { $abc = 1 } |
647 | when (/^def/) { $def = 1 } | |
648 | when (/^xyz/) { $xyz = 1 } | |
649 | default { $nothing = 1 } | |
c2f1e229 TC |
650 | } |
651 | ||
8e15b189 | 652 | As of 5.14, that can also be written this way: |
0d863452 | 653 | |
c2f1e229 TC |
654 | use v5.14; |
655 | for ($var) { | |
15faabe4 SF |
656 | $abc = 1 when /^abc/; |
657 | $def = 1 when /^def/; | |
658 | $xyz = 1 when /^xyz/; | |
659 | default { $nothing = 1 } | |
a0d0e21e LW |
660 | } |
661 | ||
c2f1e229 TC |
662 | Or if you don't care to play it safe, like this: |
663 | ||
664 | use v5.14; | |
665 | given ($var) { | |
15faabe4 SF |
666 | $abc = 1 when /^abc/; |
667 | $def = 1 when /^def/; | |
668 | $xyz = 1 when /^xyz/; | |
669 | default { $nothing = 1 } | |
c2f1e229 TC |
670 | } |
671 | ||
672 | The arguments to C<given> and C<when> are in scalar context, | |
673 | and C<given> assigns the C<$_> variable its topic value. | |
674 | ||
675 | Exactly what the I<EXPR> argument to C<when> does is hard to describe | |
676 | precisely, but in general, it tries to guess what you want done. Sometimes | |
c74de2fb | 677 | it is interpreted as C<< $_ ~~ I<EXPR> >>, and sometimes it is not. It |
c2f1e229 TC |
678 | also behaves differently when lexically enclosed by a C<given> block than |
679 | it does when dynamically enclosed by a C<foreach> loop. The rules are far | |
680 | too difficult to understand to be described here. See L</"Experimental Details | |
681 | on given and when"> later on. | |
682 | ||
683 | Due to an unfortunate bug in how C<given> was implemented between Perl 5.10 | |
c74de2fb | 684 | and 5.16, under those implementations the version of C<$_> governed by |
c2f1e229 TC |
685 | C<given> is merely a lexically scoped copy of the original, not a |
686 | dynamically scoped alias to the original, as it would be if it were a | |
687 | C<foreach> or under both the original and the current Perl 6 language | |
c74de2fb FC |
688 | specification. This bug was fixed in Perl |
689 | 5.18. If you really want a lexical C<$_>, | |
c071e214 FC |
690 | specify that explicitly, but note that C<my $_> |
691 | is now deprecated and will warn unless warnings | |
692 | have been disabled: | |
c2f1e229 TC |
693 | |
694 | given(my $_ = EXPR) { ... } | |
695 | ||
c74de2fb FC |
696 | If your code still needs to run on older versions, |
697 | stick to C<foreach> for your topicalizer and | |
c2f1e229 TC |
698 | you will be less unhappy. |
699 | ||
700 | =head2 Goto | |
701 | X<goto> | |
702 | ||
703 | Although not for the faint of heart, Perl does support a C<goto> | |
704 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
705 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
706 | a C<goto>; it's just the name of the loop. | |
707 | ||
708 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes | |
709 | execution there. It may not be used to go into any construct that | |
710 | requires initialization, such as a subroutine or a C<foreach> loop. It | |
711 | also can't be used to go into a construct that is optimized away. It | |
712 | can be used to go almost anywhere else within the dynamic scope, | |
713 | including out of subroutines, but it's usually better to use some other | |
714 | construct such as C<last> or C<die>. The author of Perl has never felt the | |
715 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
716 | ||
717 | The C<goto>-EXPR form expects a label name, whose scope will be resolved | |
718 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
719 | necessarily recommended if you're optimizing for maintainability: | |
720 | ||
721 | goto(("FOO", "BAR", "GLARCH")[$i]); | |
722 | ||
723 | The C<goto>-&NAME form is highly magical, and substitutes a call to the | |
724 | named subroutine for the currently running subroutine. This is used by | |
725 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then | |
726 | pretend that the other subroutine had been called in the first place | |
727 | (except that any modifications to C<@_> in the current subroutine are | |
728 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
729 | will be able to tell that this routine was called first. | |
730 | ||
731 | In almost all cases like this, it's usually a far, far better idea to use the | |
732 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
733 | resorting to a C<goto>. For certain applications, the catch and throw pair of | |
734 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
735 | ||
736 | =head2 The Ellipsis Statement | |
737 | X<...> | |
738 | X<... statement> | |
739 | X<ellipsis operator> | |
740 | X<elliptical statement> | |
741 | X<unimplemented statement> | |
742 | X<unimplemented operator> | |
743 | X<yada-yada> | |
81104cdf FC |
744 | X<yada-yada operator> |
745 | X<... operator> | |
746 | X<whatever operator> | |
747 | X<triple-dot operator> | |
c2f1e229 TC |
748 | |
749 | Beginning in Perl 5.12, Perl accepts an ellipsis, "C<...>", as a | |
750 | placeholder for code that you haven't implemented yet. This form of | |
751 | ellipsis, the unimplemented statement, should not be confused with the | |
752 | binary flip-flop C<...> operator. One is a statement and the other an | |
753 | operator. (Perl doesn't usually confuse them because usually Perl can tell | |
754 | whether it wants an operator or a statement, but see below for exceptions.) | |
755 | ||
b735b77b | 756 | When Perl 5.12 or later encounters an ellipsis statement, it parses this |
c2f1e229 TC |
757 | without error, but if and when you should actually try to execute it, Perl |
758 | throws an exception with the text C<Unimplemented>: | |
759 | ||
9dba9ce0 | 760 | use v5.12; |
c2f1e229 TC |
761 | sub unimplemented { ... } |
762 | eval { unimplemented() }; | |
9dba9ce0 | 763 | if ($@ =~ /^Unimplemented at /) { |
15faabe4 | 764 | say "I found an ellipsis!"; |
c2f1e229 TC |
765 | } |
766 | ||
767 | You can only use the elliptical statement to stand in for a | |
768 | complete statement. These examples of how the ellipsis works: | |
769 | ||
770 | use v5.12; | |
771 | { ... } | |
772 | sub foo { ... } | |
773 | ...; | |
774 | eval { ... }; | |
775 | sub somemeth { | |
15faabe4 SF |
776 | my $self = shift; |
777 | ...; | |
c2f1e229 TC |
778 | } |
779 | $x = do { | |
15faabe4 SF |
780 | my $n; |
781 | ...; | |
782 | say "Hurrah!"; | |
783 | $n; | |
c2f1e229 TC |
784 | }; |
785 | ||
786 | The elliptical statement cannot stand in for an expression that | |
787 | is part of a larger statement, since the C<...> is also the three-dot | |
788 | version of the flip-flop operator (see L<perlop/"Range Operators">). | |
789 | ||
790 | These examples of attempts to use an ellipsis are syntax errors: | |
791 | ||
792 | use v5.12; | |
793 | ||
794 | print ...; | |
795 | open(my $fh, ">", "/dev/passwd") or ...; | |
796 | if ($condition && ... ) { say "Howdy" }; | |
797 | ||
798 | There are some cases where Perl can't immediately tell the difference | |
799 | between an expression and a statement. For instance, the syntax for a | |
800 | block and an anonymous hash reference constructor look the same unless | |
801 | there's something in the braces to give Perl a hint. The ellipsis is a | |
802 | syntax error if Perl doesn't guess that the C<{ ... }> is a block. In that | |
803 | case, it doesn't think the C<...> is an ellipsis because it's expecting an | |
804 | expression instead of a statement: | |
805 | ||
12d22d1f | 806 | @transformed = map { ... } @input; # syntax error |
c2f1e229 | 807 | |
12d22d1f JK |
808 | Inside your block, you can use a C<;> before the ellipsis to denote that the |
809 | C<{ ... }> is a block and not a hash reference constructor. Now the ellipsis | |
810 | works: | |
c2f1e229 | 811 | |
12d22d1f | 812 | @transformed = map {; ... } @input; # ';' disambiguates |
c2f1e229 TC |
813 | |
814 | Note: Some folks colloquially refer to this bit of punctuation as a | |
81104cdf | 815 | "yada-yada" or "triple-dot", but its true name |
84539229 | 816 | is actually an ellipsis. |
c2f1e229 TC |
817 | |
818 | =head2 PODs: Embedded Documentation | |
819 | X<POD> X<documentation> | |
820 | ||
821 | Perl has a mechanism for intermixing documentation with source code. | |
822 | While it's expecting the beginning of a new statement, if the compiler | |
823 | encounters a line that begins with an equal sign and a word, like this | |
824 | ||
825 | =head1 Here There Be Pods! | |
826 | ||
827 | Then that text and all remaining text up through and including a line | |
828 | beginning with C<=cut> will be ignored. The format of the intervening | |
829 | text is described in L<perlpod>. | |
830 | ||
831 | This allows you to intermix your source code | |
832 | and your documentation text freely, as in | |
833 | ||
834 | =item snazzle($) | |
835 | ||
836 | The snazzle() function will behave in the most spectacular | |
837 | form that you can possibly imagine, not even excepting | |
838 | cybernetic pyrotechnics. | |
839 | ||
840 | =cut back to the compiler, nuff of this pod stuff! | |
841 | ||
842 | sub snazzle($) { | |
15faabe4 SF |
843 | my $thingie = shift; |
844 | ......... | |
c2f1e229 TC |
845 | } |
846 | ||
847 | Note that pod translators should look at only paragraphs beginning | |
848 | with a pod directive (it makes parsing easier), whereas the compiler | |
849 | actually knows to look for pod escapes even in the middle of a | |
850 | paragraph. This means that the following secret stuff will be | |
851 | ignored by both the compiler and the translators. | |
852 | ||
853 | $a=3; | |
854 | =secret stuff | |
855 | warn "Neither POD nor CODE!?" | |
856 | =cut back | |
857 | print "got $a\n"; | |
858 | ||
859 | You probably shouldn't rely upon the C<warn()> being podded out forever. | |
860 | Not all pod translators are well-behaved in this regard, and perhaps | |
861 | the compiler will become pickier. | |
862 | ||
863 | One may also use pod directives to quickly comment out a section | |
864 | of code. | |
865 | ||
866 | =head2 Plain Old Comments (Not!) | |
867 | X<comment> X<line> X<#> X<preprocessor> X<eval> | |
868 | ||
869 | Perl can process line directives, much like the C preprocessor. Using | |
870 | this, one can control Perl's idea of filenames and line numbers in | |
871 | error or warning messages (especially for strings that are processed | |
872 | with C<eval()>). The syntax for this mechanism is almost the same as for | |
873 | most C preprocessors: it matches the regular expression | |
874 | ||
875 | # example: '# line 42 "new_filename.plx"' | |
876 | /^\# \s* | |
877 | line \s+ (\d+) \s* | |
878 | (?:\s("?)([^"]+)\g2)? \s* | |
879 | $/x | |
880 | ||
881 | with C<$1> being the line number for the next line, and C<$3> being | |
89a3b501 | 882 | the optional filename (specified with or without quotes). Note that |
c2f1e229 TC |
883 | no whitespace may precede the C<< # >>, unlike modern C preprocessors. |
884 | ||
885 | There is a fairly obvious gotcha included with the line directive: | |
886 | Debuggers and profilers will only show the last source line to appear | |
887 | at a particular line number in a given file. Care should be taken not | |
888 | to cause line number collisions in code you'd like to debug later. | |
889 | ||
890 | Here are some examples that you should be able to type into your command | |
891 | shell: | |
892 | ||
893 | % perl | |
894 | # line 200 "bzzzt" | |
895 | # the '#' on the previous line must be the first char on line | |
896 | die 'foo'; | |
897 | __END__ | |
898 | foo at bzzzt line 201. | |
899 | ||
900 | % perl | |
901 | # line 200 "bzzzt" | |
902 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
903 | __END__ | |
904 | foo at - line 2001. | |
905 | ||
906 | % perl | |
907 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
908 | __END__ | |
909 | foo at foo bar line 200. | |
910 | ||
911 | % perl | |
912 | # line 345 "goop" | |
913 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
914 | print $@; | |
915 | __END__ | |
916 | foo at goop line 345. | |
917 | ||
918 | =head2 Experimental Details on given and when | |
919 | ||
920 | As previously mentioned, the "switch" feature is considered highly | |
921 | experimental; it is subject to change with little notice. In particular, | |
c74de2fb FC |
922 | C<when> has tricky behaviours that are expected to change to become less |
923 | tricky in the future. Do not rely upon its current (mis)implementation. | |
924 | Before Perl 5.18, C<given> also had tricky behaviours that you should still | |
925 | beware of if your code must run on older versions of Perl. | |
c2f1e229 TC |
926 | |
927 | Here is a longer example of C<given>: | |
a0d0e21e | 928 | |
4b7b0ae4 | 929 | use feature ":5.10"; |
c2f1e229 | 930 | given ($foo) { |
15faabe4 SF |
931 | when (undef) { |
932 | say '$foo is undefined'; | |
933 | } | |
934 | when ("foo") { | |
935 | say '$foo is the string "foo"'; | |
936 | } | |
937 | when ([1,3,5,7,9]) { | |
938 | say '$foo is an odd digit'; | |
939 | continue; # Fall through | |
940 | } | |
941 | when ($_ < 100) { | |
942 | say '$foo is numerically less than 100'; | |
943 | } | |
944 | when (\&complicated_check) { | |
945 | say 'a complicated check for $foo is true'; | |
946 | } | |
947 | default { | |
948 | die q(I don't know what to do with $foo); | |
949 | } | |
4b7b0ae4 RH |
950 | } |
951 | ||
c74de2fb | 952 | Before Perl 5.18, C<given(EXPR)> assigned the value of I<EXPR> to |
c2f1e229 | 953 | merely a lexically scoped I<B<copy>> (!) of C<$_>, not a dynamically |
c74de2fb | 954 | scoped alias the way C<foreach> does. That made it similar to |
4b7b0ae4 | 955 | |
15faabe4 | 956 | do { my $_ = EXPR; ... } |
4b7b0ae4 | 957 | |
c74de2fb FC |
958 | except that the block was automatically broken out of by a successful |
959 | C<when> or an explicit C<break>. Because it was only a copy, and because | |
960 | it was only lexically scoped, not dynamically scoped, you could not do the | |
961 | things with it that you are used to in a C<foreach> loop. In particular, | |
962 | it did not work for arbitrary function calls if those functions might try | |
963 | to access $_. Best stick to C<foreach> for that. | |
c2f1e229 TC |
964 | |
965 | Most of the power comes from the implicit smartmatching that can | |
966 | sometimes apply. Most of the time, C<when(EXPR)> is treated as an | |
89a3b501 | 967 | implicit smartmatch of C<$_>, that is, C<$_ ~~ EXPR>. (See |
c2f1e229 TC |
968 | L<perlop/"Smartmatch Operator"> for more information on smartmatching.) |
969 | But when I<EXPR> is one of the 10 exceptional cases (or things like them) | |
970 | listed below, it is used directly as a boolean. | |
4b7b0ae4 | 971 | |
c2f1e229 | 972 | =over 4 |
a0d0e21e | 973 | |
c9dde696 | 974 | =item Z<>1. |
a0d0e21e | 975 | |
c2f1e229 | 976 | A user-defined subroutine call or a method invocation. |
a0d0e21e | 977 | |
c9dde696 | 978 | =item Z<>2. |
a0d0e21e | 979 | |
c2f1e229 TC |
980 | A regular expression match in the form of C</REGEX/>, C<$foo =~ /REGEX/>, |
981 | or C<$foo =~ EXPR>. Also, a negated regular expression match in | |
982 | the form C<!/REGEX/>, C<$foo !~ /REGEX/>, or C<$foo !~ EXPR>. | |
0d863452 | 983 | |
c9dde696 | 984 | =item Z<>3. |
0d863452 | 985 | |
c2f1e229 | 986 | A smart match that uses an explicit C<~~> operator, such as C<EXPR ~~ EXPR>. |
0d863452 | 987 | |
15faabe4 | 988 | B<NOTE:> You will often have to use C<$c ~~ $_> because the default case |
e10c9f69 DB |
989 | uses C<$_ ~~ $c> , which is frequentlythe opposite of what you want. |
990 | ||
c9dde696 | 991 | =item Z<>4. |
0d863452 | 992 | |
46391258 | 993 | A boolean comparison operator such as C<$_ E<lt> 10> or C<$x eq "abc">. The |
c2f1e229 TC |
994 | relational operators that this applies to are the six numeric comparisons |
995 | (C<< < >>, C<< > >>, C<< <= >>, C<< >= >>, C<< == >>, and C<< != >>), and | |
996 | the six string comparisons (C<lt>, C<gt>, C<le>, C<ge>, C<eq>, and C<ne>). | |
0d863452 | 997 | |
c9dde696 | 998 | =item Z<>5. |
0d863452 | 999 | |
c2f1e229 | 1000 | At least the three builtin functions C<defined(...)>, C<exists(...)>, and |
89a3b501 | 1001 | C<eof(...)>. We might someday add more of these later if we think of them. |
0d863452 | 1002 | |
c9dde696 | 1003 | =item Z<>6. |
0d863452 | 1004 | |
c2f1e229 TC |
1005 | A negated expression, whether C<!(EXPR)> or C<not(EXPR)>, or a logical |
1006 | exclusive-or, C<(EXPR1) xor (EXPR2)>. The bitwise versions (C<~> and C<^>) | |
1007 | are not included. | |
0d863452 | 1008 | |
c9dde696 | 1009 | =item Z<>7. |
4633a7c4 | 1010 | |
c2f1e229 TC |
1011 | A filetest operator, with exactly 4 exceptions: C<-s>, C<-M>, C<-A>, and |
1012 | C<-C>, as these return numerical values, not boolean ones. The C<-z> | |
1013 | filetest operator is not included in the exception list. | |
cb1a09d0 | 1014 | |
c9dde696 | 1015 | =item Z<>8. |
516817b4 | 1016 | |
c2f1e229 TC |
1017 | The C<..> and C<...> flip-flop operators. Note that the C<...> flip-flop |
1018 | operator is completely different from the C<...> elliptical statement | |
1019 | just described. | |
202d7cbd | 1020 | |
0d863452 RH |
1021 | =back |
1022 | ||
c2f1e229 TC |
1023 | In those 8 cases above, the value of EXPR is used directly as a boolean, so |
1024 | no smartmatching is done. You may think of C<when> as a smartsmartmatch. | |
f92e1a16 | 1025 | |
c2f1e229 TC |
1026 | Furthermore, Perl inspects the operands of logical operators to |
1027 | decide whether to use smartmatching for each one by applying the | |
1028 | above test to the operands: | |
0d863452 RH |
1029 | |
1030 | =over 4 | |
1031 | ||
c9dde696 | 1032 | =item Z<>9. |
0d863452 | 1033 | |
c2f1e229 | 1034 | If EXPR is C<EXPR1 && EXPR2> or C<EXPR1 and EXPR2>, the test is applied |
89a3b501 FC |
1035 | I<recursively> to both EXPR1 and EXPR2. |
1036 | Only if I<both> operands also pass the | |
c2f1e229 TC |
1037 | test, I<recursively>, will the expression be treated as boolean. Otherwise, |
1038 | smartmatching is used. | |
0d863452 | 1039 | |
c9dde696 | 1040 | =item Z<>10. |
0d863452 | 1041 | |
c2f1e229 TC |
1042 | If EXPR is C<EXPR1 || EXPR2>, C<EXPR1 // EXPR2>, or C<EXPR1 or EXPR2>, the |
1043 | test is applied I<recursively> to EXPR1 only (which might itself be a | |
1044 | higher-precedence AND operator, for example, and thus subject to the | |
89a3b501 | 1045 | previous rule), not to EXPR2. If EXPR1 is to use smartmatching, then EXPR2 |
c2f1e229 TC |
1046 | also does so, no matter what EXPR2 contains. But if EXPR2 does not get to |
1047 | use smartmatching, then the second argument will not be either. This is | |
1048 | quite different from the C<&&> case just described, so be careful. | |
0d863452 RH |
1049 | |
1050 | =back | |
1051 | ||
c2f1e229 TC |
1052 | These rules are complicated, but the goal is for them to do what you want |
1053 | (even if you don't quite understand why they are doing it). For example: | |
0d863452 | 1054 | |
f849b90f | 1055 | when (/^\d+$/ && $_ < 75) { ... } |
0d863452 | 1056 | |
c2f1e229 TC |
1057 | will be treated as a boolean match because the rules say both |
1058 | a regex match and an explicit test on C<$_> will be treated | |
1059 | as boolean. | |
a4fce065 AD |
1060 | |
1061 | Also: | |
1062 | ||
1063 | when ([qw(foo bar)] && /baz/) { ... } | |
1064 | ||
c2f1e229 TC |
1065 | will use smartmatching because only I<one> of the operands is a boolean: |
1066 | the other uses smartmatching, and that wins. | |
a4fce065 AD |
1067 | |
1068 | Further: | |
1069 | ||
1070 | when ([qw(foo bar)] || /^baz/) { ... } | |
1071 | ||
1072 | will use smart matching (only the first operand is considered), whereas | |
1073 | ||
1074 | when (/^baz/ || [qw(foo bar)]) { ... } | |
1075 | ||
c2f1e229 | 1076 | will test only the regex, which causes both operands to be |
89a3b501 | 1077 | treated as boolean. Watch out for this one, then, because an |
c2f1e229 TC |
1078 | arrayref is always a true value, which makes it effectively |
1079 | redundant. Not a good idea. | |
a4fce065 | 1080 | |
c2f1e229 | 1081 | Tautologous boolean operators are still going to be optimized |
89a3b501 | 1082 | away. Don't be tempted to write |
a4fce065 | 1083 | |
c2f1e229 | 1084 | when ("foo" or "bar") { ... } |
a4fce065 | 1085 | |
c2f1e229 | 1086 | This will optimize down to C<"foo">, so C<"bar"> will never be considered (even |
89a3b501 FC |
1087 | though the rules say to use a smartmatch |
1088 | on C<"foo">). For an alternation like | |
c2f1e229 | 1089 | this, an array ref will work, because this will instigate smartmatching: |
a4fce065 AD |
1090 | |
1091 | when ([qw(foo bar)] { ... } | |
1092 | ||
1093 | This is somewhat equivalent to the C-style switch statement's fallthrough | |
c2f1e229 TC |
1094 | functionality (not to be confused with I<Perl's> fallthrough |
1095 | functionality--see below), wherein the same block is used for several | |
1096 | C<case> statements. | |
a4fce065 | 1097 | |
c2f1e229 | 1098 | Another useful shortcut is that, if you use a literal array or hash as the |
89a3b501 | 1099 | argument to C<given>, it is turned into a reference. So C<given(@foo)> is |
c2f1e229 | 1100 | the same as C<given(\@foo)>, for example. |
4b7b0ae4 | 1101 | |
0d863452 RH |
1102 | C<default> behaves exactly like C<when(1 == 1)>, which is |
1103 | to say that it always matches. | |
1104 | ||
4b7b0ae4 RH |
1105 | =head3 Breaking out |
1106 | ||
1107 | You can use the C<break> keyword to break out of the enclosing | |
1108 | C<given> block. Every C<when> block is implicitly ended with | |
1109 | a C<break>. | |
1110 | ||
0d863452 RH |
1111 | =head3 Fall-through |
1112 | ||
1113 | You can use the C<continue> keyword to fall through from one | |
1114 | case to the next: | |
1115 | ||
27cec4bd | 1116 | given($foo) { |
15faabe4 SF |
1117 | when (/x/) { say '$foo contains an x'; continue } |
1118 | when (/y/) { say '$foo contains a y' } | |
1119 | default { say '$foo does not contain a y' } | |
27cec4bd | 1120 | } |
0d863452 | 1121 | |
25b991bf VP |
1122 | =head3 Return value |
1123 | ||
c2f1e229 TC |
1124 | When a C<given> statement is also a valid expression (for example, |
1125 | when it's the last statement of a block), it evaluates to: | |
25b991bf VP |
1126 | |
1127 | =over 4 | |
1128 | ||
1129 | =item * | |
1130 | ||
c2f1e229 | 1131 | An empty list as soon as an explicit C<break> is encountered. |
25b991bf VP |
1132 | |
1133 | =item * | |
1134 | ||
c2f1e229 TC |
1135 | The value of the last evaluated expression of the successful |
1136 | C<when>/C<default> clause, if there happens to be one. | |
25b991bf VP |
1137 | |
1138 | =item * | |
1139 | ||
c2f1e229 | 1140 | The value of the last evaluated expression of the C<given> block if no |
06b608b9 | 1141 | condition is true. |
25b991bf VP |
1142 | |
1143 | =back | |
1144 | ||
06b608b9 VP |
1145 | In both last cases, the last expression is evaluated in the context that |
1146 | was applied to the C<given> block. | |
1147 | ||
1148 | Note that, unlike C<if> and C<unless>, failed C<when> statements always | |
1149 | evaluate to an empty list. | |
25b991bf | 1150 | |
c2f1e229 | 1151 | my $price = do { |
15faabe4 SF |
1152 | given ($item) { |
1153 | when (["pear", "apple"]) { 1 } | |
1154 | break when "vote"; # My vote cannot be bought | |
1155 | 1e10 when /Mona Lisa/; | |
1156 | "unknown"; | |
1157 | } | |
c2f1e229 | 1158 | }; |
25b991bf | 1159 | |
89a3b501 FC |
1160 | Currently, C<given> blocks can't always |
1161 | be used as proper expressions. This | |
c2f1e229 | 1162 | may be addressed in a future version of Perl. |
25b991bf | 1163 | |
0d863452 RH |
1164 | =head3 Switching in a loop |
1165 | ||
1166 | Instead of using C<given()>, you can use a C<foreach()> loop. | |
1167 | For example, here's one way to count how many times a particular | |
1168 | string occurs in an array: | |
1169 | ||
c2f1e229 | 1170 | use v5.10.1; |
27cec4bd RGS |
1171 | my $count = 0; |
1172 | for (@array) { | |
15faabe4 | 1173 | when ("foo") { ++$count } |
5a964f20 | 1174 | } |
27cec4bd | 1175 | print "\@array contains $count copies of 'foo'\n"; |
0d863452 | 1176 | |
c2f1e229 | 1177 | Or in a more recent version: |
0de1c906 | 1178 | |
c2f1e229 TC |
1179 | use v5.14; |
1180 | my $count = 0; | |
1181 | for (@array) { | |
15faabe4 | 1182 | ++$count when "foo"; |
c2f1e229 TC |
1183 | } |
1184 | print "\@array contains $count copies of 'foo'\n"; | |
0de1c906 | 1185 | |
c2f1e229 TC |
1186 | At the end of all C<when> blocks, there is an implicit C<next>. |
1187 | You can override that with an explicit C<last> if you're | |
1188 | interested in only the first match alone. | |
0de1c906 | 1189 | |
c2f1e229 | 1190 | This doesn't work if you explicitly specify a loop variable, as |
89a3b501 | 1191 | in C<for $item (@array)>. You have to use the default variable C<$_>. |
0de1c906 | 1192 | |
54a85b95 RH |
1193 | =head3 Differences from Perl 6 |
1194 | ||
c2f1e229 TC |
1195 | The Perl 5 smartmatch and C<given>/C<when> constructs are not compatible |
1196 | with their Perl 6 analogues. The most visible difference and least | |
1197 | important difference is that, in Perl 5, parentheses are required around | |
1198 | the argument to C<given()> and C<when()> (except when this last one is used | |
1199 | as a statement modifier). Parentheses in Perl 6 are always optional in a | |
1200 | control construct such as C<if()>, C<while()>, or C<when()>; they can't be | |
1201 | made optional in Perl 5 without a great deal of potential confusion, | |
1202 | because Perl 5 would parse the expression | |
54a85b95 | 1203 | |
c2f1e229 | 1204 | given $foo { |
15faabe4 | 1205 | ... |
c2f1e229 | 1206 | } |
54a85b95 RH |
1207 | |
1208 | as though the argument to C<given> were an element of the hash | |
1209 | C<%foo>, interpreting the braces as hash-element syntax. | |
1210 | ||
c2f1e229 TC |
1211 | However, their are many, many other differences. For example, |
1212 | this works in Perl 5: | |
cb1a09d0 | 1213 | |
c2f1e229 TC |
1214 | use v5.12; |
1215 | my @primary = ("red", "blue", "green"); | |
cb1a09d0 | 1216 | |
c2f1e229 TC |
1217 | if (@primary ~~ "red") { |
1218 | say "primary smartmatches red"; | |
54310121 | 1219 | } |
cb1a09d0 | 1220 | |
c2f1e229 TC |
1221 | if ("red" ~~ @primary) { |
1222 | say "red smartmatches primary"; | |
1223 | } | |
6ec4bd10 | 1224 | |
c2f1e229 | 1225 | say "that's all, folks!"; |
6ec4bd10 | 1226 | |
c2f1e229 | 1227 | But it doesn't work at all in Perl 6. Instead, you should |
368fb018 | 1228 | use the (parallelizable) C<any> operator: |
774d564b | 1229 | |
c2f1e229 TC |
1230 | if any(@primary) eq "red" { |
1231 | say "primary smartmatches red"; | |
1232 | } | |
003183f2 | 1233 | |
c2f1e229 TC |
1234 | if "red" eq any(@primary) { |
1235 | say "red smartmatches primary"; | |
1236 | } | |
774d564b | 1237 | |
c2f1e229 TC |
1238 | The table of smartmatches in L<perlop/"Smartmatch Operator"> is not |
1239 | identical to that proposed by the Perl 6 specification, mainly due to | |
1240 | differences between Perl 6's and Perl 5's data models, but also because | |
1241 | the Perl 6 spec has changed since Perl 5 rushed into early adoption. | |
54310121 | 1242 | |
c2f1e229 | 1243 | In Perl 6, C<when()> will always do an implicit smartmatch with its |
b735b77b | 1244 | argument, while in Perl 5 it is convenient (albeit potentially confusing) to |
c2f1e229 | 1245 | suppress this implicit smartmatch in various rather loosely-defined |
89a3b501 | 1246 | situations, as roughly outlined above. (The difference is largely because |
c2f1e229 | 1247 | Perl 5 does not have, even internally, a boolean type.) |
774d564b | 1248 | |
1249 | =cut |