Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<syntax> |
a0d0e21e LW |
3 | |
4 | perlsyn - Perl syntax | |
5 | ||
6 | =head1 DESCRIPTION | |
7 | ||
6014d0cb MS |
8 | A Perl program consists of a sequence of declarations and statements |
9 | which run from the top to the bottom. Loops, subroutines and other | |
10 | control structures allow you to jump around within the code. | |
11 | ||
12 | Perl is a B<free-form> language, you can format and indent it however | |
13 | you like. Whitespace mostly serves to separate tokens, unlike | |
14 | languages like Python where it is an important part of the syntax. | |
15 | ||
16 | Many of Perl's syntactic elements are B<optional>. Rather than | |
110b9c83 | 17 | requiring you to put parentheses around every function call and |
6014d0cb MS |
18 | declare every variable, you can often leave such explicit elements off |
19 | and Perl will figure out what you meant. This is known as B<Do What I | |
20 | Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to | |
110b9c83 | 21 | code in a style with which they are comfortable. |
6014d0cb MS |
22 | |
23 | Perl B<borrows syntax> and concepts from many languages: awk, sed, C, | |
24 | Bourne Shell, Smalltalk, Lisp and even English. Other | |
25 | languages have borrowed syntax from Perl, particularly its regular | |
26 | expression extensions. So if you have programmed in another language | |
27 | you will see familiar pieces in Perl. They often work the same, but | |
28 | see L<perltrap> for information about how they differ. | |
a0d0e21e | 29 | |
0b8d69e9 | 30 | =head2 Declarations |
d74e8afc | 31 | X<declaration> X<undef> X<undefined> X<uninitialized> |
0b8d69e9 | 32 | |
cf48932e SF |
33 | The only things you need to declare in Perl are report formats and |
34 | subroutines (and sometimes not even subroutines). A variable holds | |
35 | the undefined value (C<undef>) until it has been assigned a defined | |
36 | value, which is anything other than C<undef>. When used as a number, | |
37 | C<undef> is treated as C<0>; when used as a string, it is treated as | |
38 | the empty string, C<"">; and when used as a reference that isn't being | |
39 | assigned to, it is treated as an error. If you enable warnings, | |
40 | you'll be notified of an uninitialized value whenever you treat | |
41 | C<undef> as a string or a number. Well, usually. Boolean contexts, | |
42 | such as: | |
7bd1983c EM |
43 | |
44 | my $a; | |
45 | if ($a) {} | |
46 | ||
a6b1f6d8 RGS |
47 | are exempt from warnings (because they care about truth rather than |
48 | definedness). Operators such as C<++>, C<-->, C<+=>, | |
7bd1983c EM |
49 | C<-=>, and C<.=>, that operate on undefined left values such as: |
50 | ||
51 | my $a; | |
52 | $a++; | |
53 | ||
54 | are also always exempt from such warnings. | |
0b8d69e9 | 55 | |
a0d0e21e LW |
56 | A declaration can be put anywhere a statement can, but has no effect on |
57 | the execution of the primary sequence of statements--declarations all | |
58 | take effect at compile time. Typically all the declarations are put at | |
54310121 | 59 | the beginning or the end of the script. However, if you're using |
0b8d69e9 GS |
60 | lexically-scoped private variables created with C<my()>, you'll |
61 | 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; |
a0d0e21e LW |
71 | $me = myname $0 or die "can't get myname"; |
72 | ||
1f950eb4 JB |
73 | Note that myname() functions as a list operator, not as a unary operator; |
74 | so be careful to use C<or> instead of C<||> in this case. However, if | |
54310121 | 75 | you were to declare the subroutine as C<sub myname ($)>, then |
02c45c47 | 76 | C<myname> would function as a unary operator, so either C<or> or |
54310121 | 77 | C<||> would work. |
a0d0e21e | 78 | |
4633a7c4 LW |
79 | Subroutines declarations can also be loaded up with the C<require> statement |
80 | or both loaded and imported into your namespace with a C<use> statement. | |
81 | See L<perlmod> for details on this. | |
a0d0e21e | 82 | |
4633a7c4 LW |
83 | A statement sequence may contain declarations of lexically-scoped |
84 | variables, but apart from declaring a variable name, the declaration acts | |
85 | like an ordinary statement, and is elaborated within the sequence of | |
86 | statements as if it were an ordinary statement. That means it actually | |
87 | has both compile-time and run-time effects. | |
a0d0e21e | 88 | |
6014d0cb | 89 | =head2 Comments |
d74e8afc | 90 | X<comment> X<#> |
6014d0cb MS |
91 | |
92 | Text from a C<"#"> character until the end of the line is a comment, | |
93 | and is ignored. Exceptions include C<"#"> inside a string or regular | |
94 | expression. | |
95 | ||
6ec4bd10 | 96 | =head2 Simple Statements |
d74e8afc | 97 | X<statement> X<semicolon> X<expression> X<;> |
a0d0e21e LW |
98 | |
99 | The only kind of simple statement is an expression evaluated for its | |
100 | side effects. Every simple statement must be terminated with a | |
101 | semicolon, unless it is the final statement in a block, in which case | |
f386e492 AMS |
102 | the semicolon is optional. (A semicolon is still encouraged if the |
103 | block takes up more than one line, because you may eventually add | |
cf48932e SF |
104 | another line.) Note that there are some operators like C<eval {}> and |
105 | C<do {}> that look like compound statements, but aren't (they're just | |
106 | TERMs in an expression), and thus need an explicit termination if used | |
107 | as the last item in a statement. | |
108 | ||
109 | =head2 Truth and Falsehood | |
d74e8afc | 110 | X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0> |
cf48932e | 111 | |
f92061c1 AMS |
112 | The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and |
113 | C<undef> are all false in a boolean context. All other values are true. | |
52ea55c9 SP |
114 | Negation of a true value by C<!> or C<not> returns a special false value. |
115 | When evaluated as a string it is treated as C<''>, but as a number, it | |
116 | is treated as 0. | |
cf48932e | 117 | |
cf48932e | 118 | =head2 Statement Modifiers |
d74e8afc | 119 | X<statement modifier> X<modifier> X<if> X<unless> X<while> |
4f8ea571 | 120 | X<until> X<when> X<foreach> X<for> |
a0d0e21e LW |
121 | |
122 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
123 | just before the terminating semicolon (or block ending). The possible | |
124 | modifiers are: | |
125 | ||
126 | if EXPR | |
127 | unless EXPR | |
128 | while EXPR | |
129 | until EXPR | |
4f8ea571 VP |
130 | when EXPR |
131 | for LIST | |
cf48932e SF |
132 | foreach LIST |
133 | ||
134 | The C<EXPR> following the modifier is referred to as the "condition". | |
135 | Its truth or falsehood determines how the modifier will behave. | |
136 | ||
137 | C<if> executes the statement once I<if> and only if the condition is | |
138 | true. C<unless> is the opposite, it executes the statement I<unless> | |
139 | the condition is true (i.e., if the condition is false). | |
140 | ||
141 | print "Basset hounds got long ears" if length $ear >= 10; | |
142 | go_outside() and play() unless $is_raining; | |
143 | ||
4f8ea571 VP |
144 | C<when> executes the statement I<when> C<$_> smart matches C<EXPR>, and |
145 | then either C<break>s out if it's enclosed in a C<given> scope or skips | |
146 | to the C<next> element when it lies directly inside a C<for> loop. | |
147 | See also L</"Switch statements">. | |
148 | ||
149 | given ($something) { | |
150 | $abc = 1 when /^abc/; | |
151 | $just_a = 1 when /^a/; | |
152 | $other = 1; | |
153 | } | |
154 | ||
155 | for (@names) { | |
156 | admin($_) when [ qw/Alice Bob/ ]; | |
157 | regular($_) when [ qw/Chris David Ellen/ ]; | |
158 | } | |
159 | ||
cf48932e SF |
160 | The C<foreach> modifier is an iterator: it executes the statement once |
161 | for each item in the LIST (with C<$_> aliased to each item in turn). | |
162 | ||
163 | print "Hello $_!\n" foreach qw(world Dolly nurse); | |
164 | ||
165 | C<while> repeats the statement I<while> the condition is true. | |
166 | C<until> does the opposite, it repeats the statement I<until> the | |
167 | condition is true (or while the condition is false): | |
168 | ||
169 | # Both of these count from 0 to 10. | |
170 | print $i++ while $i <= 10; | |
171 | print $j++ until $j > 10; | |
172 | ||
173 | The C<while> and C<until> modifiers have the usual "C<while> loop" | |
174 | semantics (conditional evaluated first), except when applied to a | |
175 | C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in | |
176 | which case the block executes once before the conditional is | |
177 | evaluated. This is so that you can write loops like: | |
a0d0e21e LW |
178 | |
179 | do { | |
4633a7c4 | 180 | $line = <STDIN>; |
a0d0e21e | 181 | ... |
4633a7c4 | 182 | } until $line eq ".\n"; |
a0d0e21e | 183 | |
5a964f20 TC |
184 | See L<perlfunc/do>. Note also that the loop control statements described |
185 | later will I<NOT> work in this construct, because modifiers don't take | |
186 | loop labels. Sorry. You can always put another block inside of it | |
187 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
f86cebdf | 188 | For C<next>, just double the braces: |
d74e8afc | 189 | X<next> X<last> X<redo> |
5a964f20 TC |
190 | |
191 | do {{ | |
192 | next if $x == $y; | |
193 | # do something here | |
194 | }} until $x++ > $z; | |
195 | ||
f86cebdf | 196 | For C<last>, you have to be more elaborate: |
d74e8afc | 197 | X<last> |
5a964f20 TC |
198 | |
199 | LOOP: { | |
200 | do { | |
201 | last if $x = $y**2; | |
202 | # do something here | |
203 | } while $x++ <= $z; | |
204 | } | |
a0d0e21e | 205 | |
457b36cb MV |
206 | B<NOTE:> The behaviour of a C<my> statement modified with a statement |
207 | modifier conditional or loop construct (e.g. C<my $x if ...>) is | |
208 | B<undefined>. The value of the C<my> variable may be C<undef>, any | |
209 | previously assigned value, or possibly anything else. Don't rely on | |
210 | it. Future versions of perl might do something different from the | |
211 | version of perl you try it out on. Here be dragons. | |
d74e8afc | 212 | X<my> |
457b36cb | 213 | |
6ec4bd10 | 214 | =head2 Compound Statements |
d74e8afc ITB |
215 | X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace> |
216 | X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue> | |
a0d0e21e LW |
217 | |
218 | In Perl, a sequence of statements that defines a scope is called a block. | |
219 | Sometimes a block is delimited by the file containing it (in the case | |
220 | of a required file, or the program as a whole), and sometimes a block | |
221 | is delimited by the extent of a string (in the case of an eval). | |
222 | ||
223 | But generally, a block is delimited by curly brackets, also known as braces. | |
224 | We will call this syntactic construct a BLOCK. | |
225 | ||
226 | The following compound statements may be used to control flow: | |
227 | ||
228 | if (EXPR) BLOCK | |
229 | if (EXPR) BLOCK else BLOCK | |
230 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK | |
62d98eed RU |
231 | unless (EXPR) BLOCK |
232 | unless (EXPR) BLOCK else BLOCK | |
d27f8d4b | 233 | unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK |
a0d0e21e LW |
234 | LABEL while (EXPR) BLOCK |
235 | LABEL while (EXPR) BLOCK continue BLOCK | |
5ec6d87f EA |
236 | LABEL until (EXPR) BLOCK |
237 | LABEL until (EXPR) BLOCK continue BLOCK | |
a0d0e21e | 238 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
748a9306 | 239 | LABEL foreach VAR (LIST) BLOCK |
b303ae78 | 240 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
a0d0e21e | 241 | LABEL BLOCK continue BLOCK |
43f66a76 | 242 | PHASE BLOCK |
a0d0e21e LW |
243 | |
244 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, | |
245 | not statements. This means that the curly brackets are I<required>--no | |
246 | dangling statements allowed. If you want to write conditionals without | |
247 | curly brackets there are several other ways to do it. The following | |
248 | all do the same thing: | |
249 | ||
250 | if (!open(FOO)) { die "Can't open $FOO: $!"; } | |
251 | die "Can't open $FOO: $!" unless open(FOO); | |
252 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! | |
253 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; | |
254 | # a bit exotic, that last one | |
255 | ||
5f05dabc | 256 | The C<if> statement is straightforward. Because BLOCKs are always |
a0d0e21e LW |
257 | bounded by curly brackets, there is never any ambiguity about which |
258 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
d27f8d4b JV |
259 | the sense of the test is reversed. Like C<if>, C<unless> can be followed |
260 | by C<else>. C<unless> can even be followed by one or more C<elsif> | |
261 | statements, though you may want to think twice before using that particular | |
262 | language construct, as everyone reading your code will have to think at least | |
263 | twice before they can understand what's going on. | |
a0d0e21e LW |
264 | |
265 | The C<while> statement executes the block as long as the expression is | |
e17b7802 | 266 | L<true|/"Truth and Falsehood">. |
1d5653dd RGS |
267 | The C<until> statement executes the block as long as the expression is |
268 | false. | |
b78218b7 GS |
269 | The LABEL is optional, and if present, consists of an identifier followed |
270 | by a colon. The LABEL identifies the loop for the loop control | |
271 | statements C<next>, C<last>, and C<redo>. | |
272 | If the LABEL is omitted, the loop control statement | |
4633a7c4 LW |
273 | refers to the innermost enclosing loop. This may include dynamically |
274 | looking back your call-stack at run time to find the LABEL. Such | |
9f1b1f2d | 275 | desperate behavior triggers a warning if you use the C<use warnings> |
a2293a43 | 276 | pragma or the B<-w> flag. |
4633a7c4 LW |
277 | |
278 | If there is a C<continue> BLOCK, it is always executed just before the | |
6ec4bd10 MS |
279 | conditional is about to be evaluated again. Thus it can be used to |
280 | increment a loop variable, even when the loop has been continued via | |
281 | the C<next> statement. | |
4633a7c4 | 282 | |
43f66a76 DG |
283 | When a block is preceding by a compilation phase keyword such as C<BEGIN>, |
284 | C<END>, C<INIT>, C<CHECK>, or C<UNITCHECK>, then the block will run only | |
285 | during the corresponding phase of execution. See L<perlmod> for more details. | |
286 | ||
88e1f1a2 JV |
287 | Extension modules can also hook into the Perl parser to define new |
288 | kinds of compound statement. These are introduced by a keyword which | |
6a0969e5 | 289 | the extension recognizes, and the syntax following the keyword is |
88e1f1a2 JV |
290 | defined entirely by the extension. If you are an implementor, see |
291 | L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such | |
292 | a module, see the module's documentation for details of the syntax that | |
293 | it defines. | |
294 | ||
4633a7c4 | 295 | =head2 Loop Control |
d74e8afc | 296 | X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue> |
4633a7c4 | 297 | |
6ec4bd10 | 298 | The C<next> command starts the next iteration of the loop: |
4633a7c4 LW |
299 | |
300 | LINE: while (<STDIN>) { | |
301 | next LINE if /^#/; # discard comments | |
302 | ... | |
303 | } | |
304 | ||
6ec4bd10 | 305 | The C<last> command immediately exits the loop in question. The |
4633a7c4 LW |
306 | C<continue> block, if any, is not executed: |
307 | ||
308 | LINE: while (<STDIN>) { | |
309 | last LINE if /^$/; # exit when done with header | |
310 | ... | |
311 | } | |
312 | ||
313 | The C<redo> command restarts the loop block without evaluating the | |
314 | conditional again. The C<continue> block, if any, is I<not> executed. | |
315 | This command is normally used by programs that want to lie to themselves | |
316 | about what was just input. | |
317 | ||
318 | For example, when processing a file like F</etc/termcap>. | |
319 | If your input lines might end in backslashes to indicate continuation, you | |
320 | want to skip ahead and get the next record. | |
321 | ||
322 | while (<>) { | |
323 | chomp; | |
54310121 | 324 | if (s/\\$//) { |
325 | $_ .= <>; | |
4633a7c4 LW |
326 | redo unless eof(); |
327 | } | |
328 | # now process $_ | |
54310121 | 329 | } |
4633a7c4 LW |
330 | |
331 | which is Perl short-hand for the more explicitly written version: | |
332 | ||
54310121 | 333 | LINE: while (defined($line = <ARGV>)) { |
4633a7c4 | 334 | chomp($line); |
54310121 | 335 | if ($line =~ s/\\$//) { |
336 | $line .= <ARGV>; | |
4633a7c4 LW |
337 | redo LINE unless eof(); # not eof(ARGV)! |
338 | } | |
339 | # now process $line | |
54310121 | 340 | } |
4633a7c4 | 341 | |
36e7a065 AMS |
342 | Note that if there were a C<continue> block on the above code, it would |
343 | get executed only on lines discarded by the regex (since redo skips the | |
344 | continue block). A continue block is often used to reset line counters | |
499a640d | 345 | or C<m?pat?> one-time matches: |
4633a7c4 | 346 | |
5a964f20 TC |
347 | # inspired by :1,$g/fred/s//WILMA/ |
348 | while (<>) { | |
499a640d TC |
349 | m?(fred)? && s//WILMA $1 WILMA/; |
350 | m?(barney)? && s//BETTY $1 BETTY/; | |
351 | m?(homer)? && s//MARGE $1 MARGE/; | |
5a964f20 TC |
352 | } continue { |
353 | print "$ARGV $.: $_"; | |
499a640d TC |
354 | close ARGV if eof; # reset $. |
355 | reset if eof; # reset ?pat? | |
4633a7c4 LW |
356 | } |
357 | ||
a0d0e21e LW |
358 | If the word C<while> is replaced by the word C<until>, the sense of the |
359 | test is reversed, but the conditional is still tested before the first | |
360 | iteration. | |
361 | ||
5a964f20 TC |
362 | The loop control statements don't work in an C<if> or C<unless>, since |
363 | they aren't loops. You can double the braces to make them such, though. | |
364 | ||
365 | if (/pattern/) {{ | |
7bd1983c EM |
366 | last if /fred/; |
367 | next if /barney/; # same effect as "last", but doesn't document as well | |
368 | # do something here | |
5a964f20 TC |
369 | }} |
370 | ||
7bd1983c | 371 | This is caused by the fact that a block by itself acts as a loop that |
27cec4bd | 372 | executes once, see L<"Basic BLOCKs">. |
7bd1983c | 373 | |
5b23ba8b MG |
374 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
375 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
4633a7c4 | 376 | |
cb1a09d0 | 377 | =head2 For Loops |
d74e8afc | 378 | X<for> X<foreach> |
a0d0e21e | 379 | |
b78df5de | 380 | Perl's C-style C<for> loop works like the corresponding C<while> loop; |
cb1a09d0 | 381 | that means that this: |
a0d0e21e LW |
382 | |
383 | for ($i = 1; $i < 10; $i++) { | |
384 | ... | |
385 | } | |
386 | ||
cb1a09d0 | 387 | is the same as this: |
a0d0e21e LW |
388 | |
389 | $i = 1; | |
390 | while ($i < 10) { | |
391 | ... | |
392 | } continue { | |
393 | $i++; | |
394 | } | |
395 | ||
b78df5de JA |
396 | There is one minor difference: if variables are declared with C<my> |
397 | in the initialization section of the C<for>, the lexical scope of | |
398 | those variables is exactly the C<for> loop (the body of the loop | |
399 | and the control sections). | |
d74e8afc | 400 | X<my> |
55497cff | 401 | |
cb1a09d0 AD |
402 | Besides the normal array index looping, C<for> can lend itself |
403 | to many other interesting applications. Here's one that avoids the | |
54310121 | 404 | problem you get into if you explicitly test for end-of-file on |
405 | an interactive file descriptor causing your program to appear to | |
cb1a09d0 | 406 | hang. |
d74e8afc | 407 | X<eof> X<end-of-file> X<end of file> |
cb1a09d0 AD |
408 | |
409 | $on_a_tty = -t STDIN && -t STDOUT; | |
410 | sub prompt { print "yes? " if $on_a_tty } | |
411 | for ( prompt(); <STDIN>; prompt() ) { | |
412 | # do something | |
54310121 | 413 | } |
cb1a09d0 | 414 | |
00cb5da1 CW |
415 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the |
416 | conditional of a C<for> loop is shorthand for the following. This | |
417 | behaviour is the same as a C<while> loop conditional. | |
d74e8afc | 418 | X<readline> X<< <> >> |
00cb5da1 CW |
419 | |
420 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { | |
421 | # do something | |
422 | } | |
423 | ||
cb1a09d0 | 424 | =head2 Foreach Loops |
d74e8afc | 425 | X<for> X<foreach> |
cb1a09d0 | 426 | |
4633a7c4 | 427 | The C<foreach> loop iterates over a normal list value and sets the |
55497cff | 428 | variable VAR to be each element of the list in turn. If the variable |
429 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
430 | is therefore visible only within the loop. Otherwise, the variable is | |
431 | implicitly local to the loop and regains its former value upon exiting | |
432 | the loop. If the variable was previously declared with C<my>, it uses | |
433 | that variable instead of the global one, but it's still localized to | |
6a0969e5 | 434 | the loop. This implicit localization occurs I<only> in a C<foreach> |
5c502d37 | 435 | loop. |
d74e8afc | 436 | X<my> X<local> |
4633a7c4 LW |
437 | |
438 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
5a964f20 TC |
439 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
440 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> | |
f86cebdf | 441 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
d74e8afc | 442 | X<$_> |
c5674021 |
443 | |
444 | If any element of LIST is an lvalue, you can modify it by modifying | |
445 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
446 | lvalue, any attempt to modify that element will fail. In other words, | |
447 | the C<foreach> loop index variable is an implicit alias for each item | |
448 | in the list that you're looping over. | |
d74e8afc | 449 | X<alias> |
302617ea MG |
450 | |
451 | If any part of LIST is an array, C<foreach> will get very confused if | |
452 | you add or remove elements within the loop body, for example with | |
453 | C<splice>. So don't do that. | |
d74e8afc | 454 | X<splice> |
302617ea MG |
455 | |
456 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
457 | special variable. Don't do that either. | |
4633a7c4 | 458 | |
748a9306 | 459 | Examples: |
a0d0e21e | 460 | |
4633a7c4 | 461 | for (@ary) { s/foo/bar/ } |
a0d0e21e | 462 | |
96f2dc66 | 463 | for my $elem (@elements) { |
a0d0e21e LW |
464 | $elem *= 2; |
465 | } | |
466 | ||
4633a7c4 LW |
467 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
468 | print $count, "\n"; sleep(1); | |
a0d0e21e LW |
469 | } |
470 | ||
471 | for (1..15) { print "Merry Christmas\n"; } | |
472 | ||
4633a7c4 | 473 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
a0d0e21e LW |
474 | print "Item: $item\n"; |
475 | } | |
476 | ||
4633a7c4 LW |
477 | Here's how a C programmer might code up a particular algorithm in Perl: |
478 | ||
55497cff | 479 | for (my $i = 0; $i < @ary1; $i++) { |
480 | for (my $j = 0; $j < @ary2; $j++) { | |
4633a7c4 LW |
481 | if ($ary1[$i] > $ary2[$j]) { |
482 | last; # can't go to outer :-( | |
483 | } | |
484 | $ary1[$i] += $ary2[$j]; | |
485 | } | |
cb1a09d0 | 486 | # this is where that last takes me |
4633a7c4 LW |
487 | } |
488 | ||
184e9718 | 489 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 | 490 | do it: |
4633a7c4 | 491 | |
96f2dc66 GS |
492 | OUTER: for my $wid (@ary1) { |
493 | INNER: for my $jet (@ary2) { | |
cb1a09d0 AD |
494 | next OUTER if $wid > $jet; |
495 | $wid += $jet; | |
54310121 | 496 | } |
497 | } | |
4633a7c4 | 498 | |
cb1a09d0 AD |
499 | See how much easier this is? It's cleaner, safer, and faster. It's |
500 | cleaner because it's less noisy. It's safer because if code gets added | |
c07a80fd | 501 | between the inner and outer loops later on, the new code won't be |
5f05dabc | 502 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd | 503 | rather than merely terminating the inner one. And it's faster because |
504 | Perl executes a C<foreach> statement more rapidly than it would the | |
505 | equivalent C<for> loop. | |
4633a7c4 | 506 | |
0d863452 RH |
507 | =head2 Basic BLOCKs |
508 | X<block> | |
4633a7c4 | 509 | |
55497cff | 510 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
511 | loop that executes once. Thus you can use any of the loop control | |
512 | statements in it to leave or restart the block. (Note that this is | |
513 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
514 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
515 | block is optional. | |
4633a7c4 | 516 | |
27cec4bd | 517 | The BLOCK construct can be used to emulate case structures. |
a0d0e21e LW |
518 | |
519 | SWITCH: { | |
520 | if (/^abc/) { $abc = 1; last SWITCH; } | |
521 | if (/^def/) { $def = 1; last SWITCH; } | |
522 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
523 | $nothing = 1; | |
524 | } | |
525 | ||
0d863452 RH |
526 | Such constructs are quite frequently used, because older versions |
527 | of Perl had no official C<switch> statement. | |
83df6a1d | 528 | |
0d863452 | 529 | =head2 Switch statements |
fd4f5766 | 530 | |
0d863452 | 531 | X<switch> X<case> X<given> X<when> X<default> |
83df6a1d | 532 | |
27cec4bd | 533 | Starting from Perl 5.10, you can say |
83df6a1d | 534 | |
27cec4bd | 535 | use feature "switch"; |
a0d0e21e | 536 | |
0d863452 | 537 | which enables a switch feature that is closely based on the |
4a904372 FC |
538 | Perl 6 proposal. Starting from Perl 5.16, one can prefix the switch |
539 | keywords with C<CORE::> to access the feature without a C<use feature> | |
540 | statement. | |
0d863452 RH |
541 | |
542 | The keywords C<given> and C<when> are analogous | |
543 | to C<switch> and C<case> in other languages, so the code | |
544 | above could be written as | |
545 | ||
27cec4bd RGS |
546 | given($_) { |
547 | when (/^abc/) { $abc = 1; } | |
548 | when (/^def/) { $def = 1; } | |
549 | when (/^xyz/) { $xyz = 1; } | |
550 | default { $nothing = 1; } | |
a0d0e21e LW |
551 | } |
552 | ||
0d863452 | 553 | This construct is very flexible and powerful. For example: |
a0d0e21e | 554 | |
4b7b0ae4 RH |
555 | use feature ":5.10"; |
556 | given($foo) { | |
557 | when (undef) { | |
558 | say '$foo is undefined'; | |
559 | } | |
4b7b0ae4 RH |
560 | when ("foo") { |
561 | say '$foo is the string "foo"'; | |
562 | } | |
4b7b0ae4 RH |
563 | when ([1,3,5,7,9]) { |
564 | say '$foo is an odd digit'; | |
565 | continue; # Fall through | |
9f435386 | 566 | } |
4b7b0ae4 RH |
567 | when ($_ < 100) { |
568 | say '$foo is numerically less than 100'; | |
569 | } | |
4b7b0ae4 | 570 | when (\&complicated_check) { |
f92e1a16 | 571 | say 'a complicated check for $foo is true'; |
4b7b0ae4 | 572 | } |
4b7b0ae4 RH |
573 | default { |
574 | die q(I don't know what to do with $foo); | |
575 | } | |
576 | } | |
577 | ||
578 | C<given(EXPR)> will assign the value of EXPR to C<$_> | |
579 | within the lexical scope of the block, so it's similar to | |
580 | ||
581 | do { my $_ = EXPR; ... } | |
582 | ||
583 | except that the block is automatically broken out of by a | |
584 | successful C<when> or an explicit C<break>. | |
585 | ||
586 | Most of the power comes from implicit smart matching: | |
a0d0e21e | 587 | |
4b7b0ae4 | 588 | when($foo) |
a0d0e21e | 589 | |
0d863452 | 590 | is exactly equivalent to |
a0d0e21e | 591 | |
4b7b0ae4 | 592 | when($_ ~~ $foo) |
a0d0e21e | 593 | |
b3ed409d CS |
594 | Most of the time, C<when(EXPR)> is treated as an implicit smart match of |
595 | C<$_>, i.e. C<$_ ~~ EXPR>. (See L</"Smart matching in detail"> for more | |
596 | information on smart matching.) But when EXPR is one of the below | |
597 | exceptional cases, it is used directly as a boolean: | |
0d863452 RH |
598 | |
599 | =over 4 | |
600 | ||
d991eed6 | 601 | =item * |
0d863452 RH |
602 | |
603 | a subroutine or method call | |
604 | ||
d991eed6 | 605 | =item * |
0d863452 RH |
606 | |
607 | a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>, | |
f92e1a16 | 608 | or a negated regular expression match (C<!/REGEX/> or C<$foo !~ /REGEX/>). |
0d863452 | 609 | |
d991eed6 | 610 | =item * |
0d863452 | 611 | |
4b7b0ae4 RH |
612 | a comparison such as C<$_ E<lt> 10> or C<$x eq "abc"> |
613 | (or of course C<$_ ~~ $c>) | |
0d863452 | 614 | |
d991eed6 | 615 | =item * |
0d863452 RH |
616 | |
617 | C<defined(...)>, C<exists(...)>, or C<eof(...)> | |
618 | ||
d991eed6 | 619 | =item * |
4633a7c4 | 620 | |
f92e1a16 | 621 | a negated expression C<!(...)> or C<not (...)>, or a logical |
0d863452 | 622 | exclusive-or C<(...) xor (...)>. |
cb1a09d0 | 623 | |
516817b4 RGS |
624 | =item * |
625 | ||
626 | a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>, | |
627 | that return numerical values, not boolean ones. | |
628 | ||
202d7cbd RGS |
629 | =item * |
630 | ||
f118ea0d | 631 | the C<..> and C<...> flip-flop operators. |
202d7cbd | 632 | |
0d863452 RH |
633 | =back |
634 | ||
f92e1a16 RGS |
635 | In those cases the value of EXPR is used directly as a boolean. |
636 | ||
a4fce065 AD |
637 | Furthermore, Perl inspects the operands of the binary boolean operators to |
638 | decide whether to use smart matching for each one by applying the above test to | |
639 | the operands: | |
0d863452 RH |
640 | |
641 | =over 4 | |
642 | ||
f92e1a16 | 643 | =item * |
0d863452 RH |
644 | |
645 | If EXPR is C<... && ...> or C<... and ...>, the test | |
a4fce065 AD |
646 | is applied recursively to both operands. If I<both> |
647 | operands pass the test, then the expression is treated | |
648 | as boolean; otherwise, smart matching is used. | |
0d863452 | 649 | |
f92e1a16 | 650 | =item * |
0d863452 | 651 | |
f92e1a16 | 652 | If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test |
a4fce065 AD |
653 | is applied recursively to the first operand (which may be a |
654 | higher-precedence AND operator, for example). If the first operand | |
655 | is to use smart matching, then both operands will do so; if it is | |
656 | not, then the second argument will not be either. | |
0d863452 RH |
657 | |
658 | =back | |
659 | ||
660 | These rules look complicated, but usually they will do what | |
a4fce065 | 661 | you want. For example: |
0d863452 | 662 | |
f849b90f | 663 | when (/^\d+$/ && $_ < 75) { ... } |
0d863452 | 664 | |
a4fce065 AD |
665 | will be treated as a boolean match because the rules say both a regex match and |
666 | an explicit test on $_ will be treated as boolean. | |
667 | ||
668 | Also: | |
669 | ||
670 | when ([qw(foo bar)] && /baz/) { ... } | |
671 | ||
672 | will use smart matching because only I<one> of the operands is a boolean; the | |
673 | other uses smart matching, and that wins. | |
674 | ||
675 | Further: | |
676 | ||
677 | when ([qw(foo bar)] || /^baz/) { ... } | |
678 | ||
679 | will use smart matching (only the first operand is considered), whereas | |
680 | ||
681 | when (/^baz/ || [qw(foo bar)]) { ... } | |
682 | ||
683 | will test only the regex, which causes both operands to be treated as boolean. | |
684 | Watch out for this one, then, because an arrayref is always a true value, which | |
685 | makes it effectively redundant. | |
686 | ||
6a0969e5 | 687 | Tautologous boolean operators are still going to be optimized away. Don't be |
a4fce065 AD |
688 | tempted to write |
689 | ||
690 | when ('foo' or 'bar') { ... } | |
691 | ||
6a0969e5 | 692 | This will optimize down to C<'foo'>, so C<'bar'> will never be considered (even |
a4fce065 AD |
693 | though the rules say to use a smart match on C<'foo'>). For an alternation like |
694 | this, an array ref will work, because this will instigate smart matching: | |
695 | ||
696 | when ([qw(foo bar)] { ... } | |
697 | ||
698 | This is somewhat equivalent to the C-style switch statement's fallthrough | |
699 | functionality (not to be confused with I<Perl's> fallthrough functionality - see | |
700 | below), wherein the same block is used for several C<case> statements. | |
701 | ||
4b7b0ae4 | 702 | Another useful shortcut is that, if you use a literal array |
107bd117 | 703 | or hash as the argument to C<given>, it is turned into a |
4b7b0ae4 RH |
704 | reference. So C<given(@foo)> is the same as C<given(\@foo)>, |
705 | for example. | |
706 | ||
0d863452 RH |
707 | C<default> behaves exactly like C<when(1 == 1)>, which is |
708 | to say that it always matches. | |
709 | ||
4b7b0ae4 RH |
710 | =head3 Breaking out |
711 | ||
712 | You can use the C<break> keyword to break out of the enclosing | |
713 | C<given> block. Every C<when> block is implicitly ended with | |
714 | a C<break>. | |
715 | ||
0d863452 RH |
716 | =head3 Fall-through |
717 | ||
718 | You can use the C<continue> keyword to fall through from one | |
719 | case to the next: | |
720 | ||
27cec4bd | 721 | given($foo) { |
4b7b0ae4 RH |
722 | when (/x/) { say '$foo contains an x'; continue } |
723 | when (/y/) { say '$foo contains a y' } | |
02e7afe2 | 724 | default { say '$foo does not contain a y' } |
27cec4bd | 725 | } |
0d863452 | 726 | |
25b991bf VP |
727 | =head3 Return value |
728 | ||
729 | When a C<given> statement is also a valid expression (e.g. | |
06b608b9 | 730 | when it's the last statement of a block), it evaluates to : |
25b991bf VP |
731 | |
732 | =over 4 | |
733 | ||
734 | =item * | |
735 | ||
06b608b9 | 736 | an empty list as soon as an explicit C<break> is encountered. |
25b991bf VP |
737 | |
738 | =item * | |
739 | ||
06b608b9 | 740 | the value of the last evaluated expression of the successful |
25b991bf VP |
741 | C<when>/C<default> clause, if there's one. |
742 | ||
743 | =item * | |
744 | ||
06b608b9 VP |
745 | the value of the last evaluated expression of the C<given> block if no |
746 | condition is true. | |
25b991bf VP |
747 | |
748 | =back | |
749 | ||
06b608b9 VP |
750 | In both last cases, the last expression is evaluated in the context that |
751 | was applied to the C<given> block. | |
752 | ||
753 | Note that, unlike C<if> and C<unless>, failed C<when> statements always | |
754 | evaluate to an empty list. | |
25b991bf VP |
755 | |
756 | my $price = do { given ($item) { | |
757 | when ([ 'pear', 'apple' ]) { 1 } | |
758 | break when 'vote'; # My vote cannot be bought | |
759 | 1e10 when /Mona Lisa/; | |
760 | 'unknown'; | |
761 | } }; | |
762 | ||
06b608b9 | 763 | Currently, C<given> blocks can't always be used as proper expressions. This |
25b991bf VP |
764 | may be addressed in a future version of perl. |
765 | ||
0d863452 RH |
766 | =head3 Switching in a loop |
767 | ||
768 | Instead of using C<given()>, you can use a C<foreach()> loop. | |
769 | For example, here's one way to count how many times a particular | |
770 | string occurs in an array: | |
771 | ||
27cec4bd RGS |
772 | my $count = 0; |
773 | for (@array) { | |
774 | when ("foo") { ++$count } | |
5a964f20 | 775 | } |
27cec4bd | 776 | print "\@array contains $count copies of 'foo'\n"; |
0d863452 | 777 | |
54091fc3 | 778 | At the end of all C<when> blocks, there is an implicit C<next>. |
0d863452 RH |
779 | You can override that with an explicit C<last> if you're only |
780 | interested in the first match. | |
781 | ||
782 | This doesn't work if you explicitly specify a loop variable, | |
783 | as in C<for $item (@array)>. You have to use the default | |
784 | variable C<$_>. (You can use C<for my $_ (@array)>.) | |
785 | ||
786 | =head3 Smart matching in detail | |
787 | ||
202d7cbd RGS |
788 | The behaviour of a smart match depends on what type of thing its arguments |
789 | are. The behaviour is determined by the following table: the first row | |
790 | that applies determines the match behaviour (which is thus mostly | |
791 | determined by the type of the right operand). Note that the smart match | |
d0b243e3 RGS |
792 | implicitly dereferences any non-blessed hash or array ref, so the "Hash" |
793 | and "Array" entries apply in those cases. (For blessed references, the | |
c6ebb512 | 794 | "Object" entries apply.) |
4b7b0ae4 | 795 | |
b3ed409d CS |
796 | Note that the "Matching Code" column is not always an exact rendition. For |
797 | example, the smart match operator short-circuits whenever possible, but | |
798 | C<grep> does not. | |
799 | ||
4b7b0ae4 RH |
800 | $a $b Type of Match Implied Matching Code |
801 | ====== ===== ===================== ============= | |
202d7cbd RGS |
802 | Any undef undefined !defined $a |
803 | ||
c6ebb512 | 804 | Any Object invokes ~~ overloading on $object, or dies |
4b7b0ae4 | 805 | |
168ff818 RGS |
806 | Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a |
807 | Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a | |
808 | Any CodeRef scalar sub truth $b->($a) | |
4b7b0ae4 | 809 | |
6f76d139 | 810 | Hash Hash hash keys identical (every key is found in both hashes) |
a8b2c106 | 811 | Array Hash hash keys intersection grep { exists $b->{$_} } @$a |
07edf497 | 812 | Regex Hash hash key grep grep /$a/, keys %$b |
202d7cbd RGS |
813 | undef Hash always false (undef can't be a key) |
814 | Any Hash hash entry existence exists $b->{$a} | |
815 | ||
a8b2c106 | 816 | Hash Array hash keys intersection grep { exists $a->{$_} } @$b |
168ff818 | 817 | Array Array arrays are comparable[2] |
c3886e8b RGS |
818 | Regex Array array grep grep /$a/, @$b |
819 | undef Array array contains undef grep !defined, @$b | |
168ff818 | 820 | Any Array match against an array element[3] |
c3886e8b | 821 | grep $a ~~ $_, @$b |
4b7b0ae4 | 822 | |
202d7cbd | 823 | Hash Regex hash key grep grep /$b/, keys %$a |
4b7b0ae4 | 824 | Array Regex array grep grep /$b/, @$a |
4b7b0ae4 | 825 | Any Regex pattern match $a =~ /$b/ |
202d7cbd | 826 | |
2c9d2554 | 827 | Object Any invokes ~~ overloading on $object, or falls back: |
0d7c3953 | 828 | undef Any undefined !defined($b) |
4b7b0ae4 | 829 | Any Num numeric equality $a == $b |
f118ea0d | 830 | Num numish[4] numeric equality $a == $b |
4b7b0ae4 RH |
831 | Any Any string equality $a eq $b |
832 | ||
07edf497 | 833 | 1 - empty hashes or arrays will match. |
329802ba RGS |
834 | 2 - that is, each element smart-matches the element of same index in the |
835 | other array. [3] | |
168ff818 | 836 | 3 - If a circular reference is found, we fall back to referential equality. |
f118ea0d | 837 | 4 - either a real number, or a string that looks like a number |
0d863452 | 838 | |
0d863452 | 839 | =head3 Custom matching via overloading |
5a964f20 | 840 | |
0d863452 | 841 | You can change the way that an object is matched by overloading |
0de1c906 | 842 | the C<~~> operator. This may alter the usual smart match semantics. |
5a964f20 | 843 | |
202d7cbd RGS |
844 | It should be noted that C<~~> will refuse to work on objects that |
845 | don't overload it (in order to avoid relying on the object's | |
2da5311b | 846 | underlying structure). |
202d7cbd | 847 | |
0de1c906 DM |
848 | Note also that smart match's matching rules take precedence over |
849 | overloading, so if C<$obj> has smart match overloading, then | |
850 | ||
851 | $obj ~~ X | |
852 | ||
853 | will not automatically invoke the overload method with X as an argument; | |
854 | instead the table above is consulted as normal, and based in the type of X, | |
855 | overloading may or may not be invoked. | |
856 | ||
857 | See L<overload>. | |
858 | ||
54a85b95 RH |
859 | =head3 Differences from Perl 6 |
860 | ||
861 | The Perl 5 smart match and C<given>/C<when> constructs are not | |
862 | absolutely identical to their Perl 6 analogues. The most visible | |
863 | difference is that, in Perl 5, parentheses are required around | |
4f8ea571 VP |
864 | the argument to C<given()> and C<when()> (except when this last |
865 | one is used as a statement modifier). Parentheses in Perl 6 | |
54a85b95 RH |
866 | are always optional in a control construct such as C<if()>, |
867 | C<while()>, or C<when()>; they can't be made optional in Perl | |
868 | 5 without a great deal of potential confusion, because Perl 5 | |
869 | would parse the expression | |
870 | ||
871 | given $foo { | |
872 | ... | |
873 | } | |
874 | ||
875 | as though the argument to C<given> were an element of the hash | |
876 | C<%foo>, interpreting the braces as hash-element syntax. | |
877 | ||
ccc668fa RGS |
878 | The table of smart matches is not identical to that proposed by the |
879 | Perl 6 specification, mainly due to the differences between Perl 6's | |
880 | and Perl 5's data models. | |
54a85b95 RH |
881 | |
882 | In Perl 6, C<when()> will always do an implicit smart match | |
883 | with its argument, whilst it is convenient in Perl 5 to | |
884 | suppress this implicit smart match in certain situations, | |
885 | as documented above. (The difference is largely because Perl 5 | |
886 | does not, even internally, have a boolean type.) | |
887 | ||
4633a7c4 | 888 | =head2 Goto |
d74e8afc | 889 | X<goto> |
4633a7c4 | 890 | |
19799a22 GS |
891 | Although not for the faint of heart, Perl does support a C<goto> |
892 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
893 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
894 | a C<goto>; it's just the name of the loop. | |
4633a7c4 | 895 | |
f86cebdf | 896 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 | 897 | execution there. It may not be used to go into any construct that |
f86cebdf | 898 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 LW |
899 | also can't be used to go into a construct that is optimized away. It |
900 | can be used to go almost anywhere else within the dynamic scope, | |
901 | including out of subroutines, but it's usually better to use some other | |
f86cebdf GS |
902 | construct such as C<last> or C<die>. The author of Perl has never felt the |
903 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
4633a7c4 | 904 | |
f86cebdf GS |
905 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
906 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
4633a7c4 LW |
907 | necessarily recommended if you're optimizing for maintainability: |
908 | ||
96f2dc66 | 909 | goto(("FOO", "BAR", "GLARCH")[$i]); |
4633a7c4 | 910 | |
f86cebdf | 911 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 | 912 | named subroutine for the currently running subroutine. This is used by |
f86cebdf | 913 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 | 914 | pretend that the other subroutine had been called in the first place |
f86cebdf GS |
915 | (except that any modifications to C<@_> in the current subroutine are |
916 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
4633a7c4 LW |
917 | will be able to tell that this routine was called first. |
918 | ||
c07a80fd | 919 | In almost all cases like this, it's usually a far, far better idea to use the |
920 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
4633a7c4 LW |
921 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
922 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
cb1a09d0 AD |
923 | |
924 | =head2 PODs: Embedded Documentation | |
d74e8afc | 925 | X<POD> X<documentation> |
cb1a09d0 AD |
926 | |
927 | Perl has a mechanism for intermixing documentation with source code. | |
c07a80fd | 928 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 AD |
929 | encounters a line that begins with an equal sign and a word, like this |
930 | ||
931 | =head1 Here There Be Pods! | |
932 | ||
933 | Then that text and all remaining text up through and including a line | |
934 | beginning with C<=cut> will be ignored. The format of the intervening | |
54310121 | 935 | text is described in L<perlpod>. |
cb1a09d0 AD |
936 | |
937 | This allows you to intermix your source code | |
938 | and your documentation text freely, as in | |
939 | ||
940 | =item snazzle($) | |
941 | ||
54310121 | 942 | The snazzle() function will behave in the most spectacular |
cb1a09d0 AD |
943 | form that you can possibly imagine, not even excepting |
944 | cybernetic pyrotechnics. | |
945 | ||
946 | =cut back to the compiler, nuff of this pod stuff! | |
947 | ||
948 | sub snazzle($) { | |
949 | my $thingie = shift; | |
950 | ......... | |
54310121 | 951 | } |
cb1a09d0 | 952 | |
54310121 | 953 | Note that pod translators should look at only paragraphs beginning |
184e9718 | 954 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 | 955 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 AD |
956 | paragraph. This means that the following secret stuff will be |
957 | ignored by both the compiler and the translators. | |
958 | ||
959 | $a=3; | |
960 | =secret stuff | |
961 | warn "Neither POD nor CODE!?" | |
962 | =cut back | |
963 | print "got $a\n"; | |
964 | ||
f86cebdf | 965 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 AD |
966 | Not all pod translators are well-behaved in this regard, and perhaps |
967 | the compiler will become pickier. | |
774d564b | 968 | |
969 | One may also use pod directives to quickly comment out a section | |
970 | of code. | |
971 | ||
972 | =head2 Plain Old Comments (Not!) | |
d74e8afc | 973 | X<comment> X<line> X<#> X<preprocessor> X<eval> |
774d564b | 974 | |
6ec4bd10 | 975 | Perl can process line directives, much like the C preprocessor. Using |
5a964f20 | 976 | this, one can control Perl's idea of filenames and line numbers in |
774d564b | 977 | error or warning messages (especially for strings that are processed |
24802a74 A |
978 | with C<eval()>). The syntax for this mechanism is almost the same as for |
979 | most C preprocessors: it matches the regular expression | |
6ec4bd10 MS |
980 | |
981 | # example: '# line 42 "new_filename.plx"' | |
82d4537c | 982 | /^\# \s* |
6ec4bd10 | 983 | line \s+ (\d+) \s* |
d8b950dc | 984 | (?:\s("?)([^"]+)\g2)? \s* |
6ec4bd10 MS |
985 | $/x |
986 | ||
7b6e93a8 | 987 | with C<$1> being the line number for the next line, and C<$3> being |
24802a74 | 988 | the optional filename (specified with or without quotes). Note that |
c69ca1d4 | 989 | no whitespace may precede the C<< # >>, unlike modern C preprocessors. |
774d564b | 990 | |
003183f2 GS |
991 | There is a fairly obvious gotcha included with the line directive: |
992 | Debuggers and profilers will only show the last source line to appear | |
993 | at a particular line number in a given file. Care should be taken not | |
994 | to cause line number collisions in code you'd like to debug later. | |
995 | ||
774d564b | 996 | Here are some examples that you should be able to type into your command |
997 | shell: | |
998 | ||
999 | % perl | |
1000 | # line 200 "bzzzt" | |
1001 | # the `#' on the previous line must be the first char on line | |
1002 | die 'foo'; | |
1003 | __END__ | |
1004 | foo at bzzzt line 201. | |
54310121 | 1005 | |
774d564b | 1006 | % perl |
1007 | # line 200 "bzzzt" | |
1008 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
1009 | __END__ | |
1010 | foo at - line 2001. | |
54310121 | 1011 | |
774d564b | 1012 | % perl |
1013 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
1014 | __END__ | |
1015 | foo at foo bar line 200. | |
54310121 | 1016 | |
774d564b | 1017 | % perl |
1018 | # line 345 "goop" | |
1019 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
1020 | print $@; | |
1021 | __END__ | |
1022 | foo at goop line 345. | |
1023 | ||
1024 | =cut |