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1 | =head1 NAME |
2 | X<syntax> | |
3 | ||
4 | perlsyn - Perl syntax | |
5 | ||
6 | =head1 DESCRIPTION | |
7 | ||
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 | |
17 | requiring you to put parentheses around every function call and | |
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 | |
21 | code in a style with which they are comfortable. | |
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. | |
29 | ||
30 | =head2 Declarations | |
31 | X<declaration> X<undef> X<undefined> X<uninitialized> | |
32 | ||
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: | |
43 | ||
44 | my $a; | |
45 | if ($a) {} | |
46 | ||
47 | are exempt from warnings (because they care about truth rather than | |
48 | definedness). Operators such as C<++>, C<-->, C<+=>, | |
49 | C<-=>, and C<.=>, that operate on undefined left values such as: | |
50 | ||
51 | my $a; | |
52 | $a++; | |
53 | ||
54 | are also always exempt from such warnings. | |
55 | ||
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 | |
59 | the beginning or the end of the script. However, if you're using | |
60 | lexically-scoped private variables created with C<my()>, you'll | |
61 | have to make sure | |
62 | your format or subroutine definition is within the same block scope | |
63 | as the my if you expect to be able to access those private variables. | |
64 | ||
65 | Declaring a subroutine allows a subroutine name to be used as if it were a | |
66 | list operator from that point forward in the program. You can declare a | |
67 | subroutine without defining it by saying C<sub name>, thus: | |
68 | X<subroutine, declaration> | |
69 | ||
70 | sub myname; | |
71 | $me = myname $0 or die "can't get myname"; | |
72 | ||
73 | Note that myname() functions as a list operator, not as a unary operator; | |
74 | so be careful to use C<or> instead of C<||> in this case. However, if | |
75 | you were to declare the subroutine as C<sub myname ($)>, then | |
76 | C<myname> would function as a unary operator, so either C<or> or | |
77 | C<||> would work. | |
78 | ||
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. | |
82 | ||
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. | |
88 | ||
89 | =head2 Comments | |
90 | X<comment> X<#> | |
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 | ||
96 | =head2 Simple Statements | |
97 | X<statement> X<semicolon> X<expression> X<;> | |
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 | |
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 | |
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 | |
110 | X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0> | |
111 | ||
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. | |
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. | |
117 | ||
118 | =head2 Statement Modifiers | |
119 | X<statement modifier> X<modifier> X<if> X<unless> X<while> | |
120 | X<until> X<foreach> X<for> | |
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 | |
130 | foreach LIST | |
131 | ||
132 | The C<EXPR> following the modifier is referred to as the "condition". | |
133 | Its truth or falsehood determines how the modifier will behave. | |
134 | ||
135 | C<if> executes the statement once I<if> and only if the condition is | |
136 | true. C<unless> is the opposite, it executes the statement I<unless> | |
137 | the condition is true (i.e., if the condition is false). | |
138 | ||
139 | print "Basset hounds got long ears" if length $ear >= 10; | |
140 | go_outside() and play() unless $is_raining; | |
141 | ||
142 | The C<foreach> modifier is an iterator: it executes the statement once | |
143 | for each item in the LIST (with C<$_> aliased to each item in turn). | |
144 | ||
145 | print "Hello $_!\n" foreach qw(world Dolly nurse); | |
146 | ||
147 | C<while> repeats the statement I<while> the condition is true. | |
148 | C<until> does the opposite, it repeats the statement I<until> the | |
149 | condition is true (or while the condition is false): | |
150 | ||
151 | # Both of these count from 0 to 10. | |
152 | print $i++ while $i <= 10; | |
153 | print $j++ until $j > 10; | |
154 | ||
155 | The C<while> and C<until> modifiers have the usual "C<while> loop" | |
156 | semantics (conditional evaluated first), except when applied to a | |
157 | C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in | |
158 | which case the block executes once before the conditional is | |
159 | evaluated. This is so that you can write loops like: | |
160 | ||
161 | do { | |
162 | $line = <STDIN>; | |
163 | ... | |
164 | } until $line eq ".\n"; | |
165 | ||
166 | See L<perlfunc/do>. Note also that the loop control statements described | |
167 | later will I<NOT> work in this construct, because modifiers don't take | |
168 | loop labels. Sorry. You can always put another block inside of it | |
169 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
170 | For C<next>, just double the braces: | |
171 | X<next> X<last> X<redo> | |
172 | ||
173 | do {{ | |
174 | next if $x == $y; | |
175 | # do something here | |
176 | }} until $x++ > $z; | |
177 | ||
178 | For C<last>, you have to be more elaborate: | |
179 | X<last> | |
180 | ||
181 | LOOP: { | |
182 | do { | |
183 | last if $x = $y**2; | |
184 | # do something here | |
185 | } while $x++ <= $z; | |
186 | } | |
187 | ||
188 | B<NOTE:> The behaviour of a C<my> statement modified with a statement | |
189 | modifier conditional or loop construct (e.g. C<my $x if ...>) is | |
190 | B<undefined>. The value of the C<my> variable may be C<undef>, any | |
191 | previously assigned value, or possibly anything else. Don't rely on | |
192 | it. Future versions of perl might do something different from the | |
193 | version of perl you try it out on. Here be dragons. | |
194 | X<my> | |
195 | ||
196 | =head2 Compound Statements | |
197 | X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace> | |
198 | X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue> | |
199 | ||
200 | In Perl, a sequence of statements that defines a scope is called a block. | |
201 | Sometimes a block is delimited by the file containing it (in the case | |
202 | of a required file, or the program as a whole), and sometimes a block | |
203 | is delimited by the extent of a string (in the case of an eval). | |
204 | ||
205 | But generally, a block is delimited by curly brackets, also known as braces. | |
206 | We will call this syntactic construct a BLOCK. | |
207 | ||
208 | The following compound statements may be used to control flow: | |
209 | ||
210 | if (EXPR) BLOCK | |
211 | if (EXPR) BLOCK else BLOCK | |
212 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK | |
213 | LABEL while (EXPR) BLOCK | |
214 | LABEL while (EXPR) BLOCK continue BLOCK | |
215 | LABEL until (EXPR) BLOCK | |
216 | LABEL until (EXPR) BLOCK continue BLOCK | |
217 | LABEL for (EXPR; EXPR; EXPR) BLOCK | |
218 | LABEL foreach VAR (LIST) BLOCK | |
219 | LABEL foreach VAR (LIST) BLOCK continue BLOCK | |
220 | LABEL BLOCK continue BLOCK | |
221 | ||
222 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, | |
223 | not statements. This means that the curly brackets are I<required>--no | |
224 | dangling statements allowed. If you want to write conditionals without | |
225 | curly brackets there are several other ways to do it. The following | |
226 | all do the same thing: | |
227 | ||
228 | if (!open(FOO)) { die "Can't open $FOO: $!"; } | |
229 | die "Can't open $FOO: $!" unless open(FOO); | |
230 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! | |
231 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; | |
232 | # a bit exotic, that last one | |
233 | ||
234 | The C<if> statement is straightforward. Because BLOCKs are always | |
235 | bounded by curly brackets, there is never any ambiguity about which | |
236 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
237 | the sense of the test is reversed. | |
238 | ||
239 | The C<while> statement executes the block as long as the expression is | |
240 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). | |
241 | The C<until> statement executes the block as long as the expression is | |
242 | false. | |
243 | The LABEL is optional, and if present, consists of an identifier followed | |
244 | by a colon. The LABEL identifies the loop for the loop control | |
245 | statements C<next>, C<last>, and C<redo>. | |
246 | If the LABEL is omitted, the loop control statement | |
247 | refers to the innermost enclosing loop. This may include dynamically | |
248 | looking back your call-stack at run time to find the LABEL. Such | |
249 | desperate behavior triggers a warning if you use the C<use warnings> | |
250 | pragma or the B<-w> flag. | |
251 | ||
252 | If there is a C<continue> BLOCK, it is always executed just before the | |
253 | conditional is about to be evaluated again. Thus it can be used to | |
254 | increment a loop variable, even when the loop has been continued via | |
255 | the C<next> statement. | |
256 | ||
257 | =head2 Loop Control | |
258 | X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue> | |
259 | ||
260 | The C<next> command starts the next iteration of the loop: | |
261 | ||
262 | LINE: while (<STDIN>) { | |
263 | next LINE if /^#/; # discard comments | |
264 | ... | |
265 | } | |
266 | ||
267 | The C<last> command immediately exits the loop in question. The | |
268 | C<continue> block, if any, is not executed: | |
269 | ||
270 | LINE: while (<STDIN>) { | |
271 | last LINE if /^$/; # exit when done with header | |
272 | ... | |
273 | } | |
274 | ||
275 | The C<redo> command restarts the loop block without evaluating the | |
276 | conditional again. The C<continue> block, if any, is I<not> executed. | |
277 | This command is normally used by programs that want to lie to themselves | |
278 | about what was just input. | |
279 | ||
280 | For example, when processing a file like F</etc/termcap>. | |
281 | If your input lines might end in backslashes to indicate continuation, you | |
282 | want to skip ahead and get the next record. | |
283 | ||
284 | while (<>) { | |
285 | chomp; | |
286 | if (s/\\$//) { | |
287 | $_ .= <>; | |
288 | redo unless eof(); | |
289 | } | |
290 | # now process $_ | |
291 | } | |
292 | ||
293 | which is Perl short-hand for the more explicitly written version: | |
294 | ||
295 | LINE: while (defined($line = <ARGV>)) { | |
296 | chomp($line); | |
297 | if ($line =~ s/\\$//) { | |
298 | $line .= <ARGV>; | |
299 | redo LINE unless eof(); # not eof(ARGV)! | |
300 | } | |
301 | # now process $line | |
302 | } | |
303 | ||
304 | Note that if there were a C<continue> block on the above code, it would | |
305 | get executed only on lines discarded by the regex (since redo skips the | |
306 | continue block). A continue block is often used to reset line counters | |
307 | or C<?pat?> one-time matches: | |
308 | ||
309 | # inspired by :1,$g/fred/s//WILMA/ | |
310 | while (<>) { | |
311 | ?(fred)? && s//WILMA $1 WILMA/; | |
312 | ?(barney)? && s//BETTY $1 BETTY/; | |
313 | ?(homer)? && s//MARGE $1 MARGE/; | |
314 | } continue { | |
315 | print "$ARGV $.: $_"; | |
316 | close ARGV if eof(); # reset $. | |
317 | reset if eof(); # reset ?pat? | |
318 | } | |
319 | ||
320 | If the word C<while> is replaced by the word C<until>, the sense of the | |
321 | test is reversed, but the conditional is still tested before the first | |
322 | iteration. | |
323 | ||
324 | The loop control statements don't work in an C<if> or C<unless>, since | |
325 | they aren't loops. You can double the braces to make them such, though. | |
326 | ||
327 | if (/pattern/) {{ | |
328 | last if /fred/; | |
329 | next if /barney/; # same effect as "last", but doesn't document as well | |
330 | # do something here | |
331 | }} | |
332 | ||
333 | This is caused by the fact that a block by itself acts as a loop that | |
334 | executes once, see L<"Basic BLOCKs and Switch Statements">. | |
335 | ||
336 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer | |
337 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
338 | ||
339 | =head2 For Loops | |
340 | X<for> X<foreach> | |
341 | ||
342 | Perl's C-style C<for> loop works like the corresponding C<while> loop; | |
343 | that means that this: | |
344 | ||
345 | for ($i = 1; $i < 10; $i++) { | |
346 | ... | |
347 | } | |
348 | ||
349 | is the same as this: | |
350 | ||
351 | $i = 1; | |
352 | while ($i < 10) { | |
353 | ... | |
354 | } continue { | |
355 | $i++; | |
356 | } | |
357 | ||
358 | There is one minor difference: if variables are declared with C<my> | |
359 | in the initialization section of the C<for>, the lexical scope of | |
360 | those variables is exactly the C<for> loop (the body of the loop | |
361 | and the control sections). | |
362 | X<my> | |
363 | ||
364 | Besides the normal array index looping, C<for> can lend itself | |
365 | to many other interesting applications. Here's one that avoids the | |
366 | problem you get into if you explicitly test for end-of-file on | |
367 | an interactive file descriptor causing your program to appear to | |
368 | hang. | |
369 | X<eof> X<end-of-file> X<end of file> | |
370 | ||
371 | $on_a_tty = -t STDIN && -t STDOUT; | |
372 | sub prompt { print "yes? " if $on_a_tty } | |
373 | for ( prompt(); <STDIN>; prompt() ) { | |
374 | # do something | |
375 | } | |
376 | ||
377 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the | |
378 | conditional of a C<for> loop is shorthand for the following. This | |
379 | behaviour is the same as a C<while> loop conditional. | |
380 | X<readline> X<< <> >> | |
381 | ||
382 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { | |
383 | # do something | |
384 | } | |
385 | ||
386 | =head2 Foreach Loops | |
387 | X<for> X<foreach> | |
388 | ||
389 | The C<foreach> loop iterates over a normal list value and sets the | |
390 | variable VAR to be each element of the list in turn. If the variable | |
391 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
392 | is therefore visible only within the loop. Otherwise, the variable is | |
393 | implicitly local to the loop and regains its former value upon exiting | |
394 | the loop. If the variable was previously declared with C<my>, it uses | |
395 | that variable instead of the global one, but it's still localized to | |
396 | the loop. This implicit localisation occurs I<only> in a C<foreach> | |
397 | loop. | |
398 | X<my> X<local> | |
399 | ||
400 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
401 | you can use C<foreach> for readability or C<for> for brevity. (Or because | |
402 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> | |
403 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. | |
404 | X<$_> | |
405 | ||
406 | If any element of LIST is an lvalue, you can modify it by modifying | |
407 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
408 | lvalue, any attempt to modify that element will fail. In other words, | |
409 | the C<foreach> loop index variable is an implicit alias for each item | |
410 | in the list that you're looping over. | |
411 | X<alias> | |
412 | ||
413 | If any part of LIST is an array, C<foreach> will get very confused if | |
414 | you add or remove elements within the loop body, for example with | |
415 | C<splice>. So don't do that. | |
416 | X<splice> | |
417 | ||
418 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
419 | special variable. Don't do that either. | |
420 | ||
421 | Examples: | |
422 | ||
423 | for (@ary) { s/foo/bar/ } | |
424 | ||
425 | for my $elem (@elements) { | |
426 | $elem *= 2; | |
427 | } | |
428 | ||
429 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { | |
430 | print $count, "\n"; sleep(1); | |
431 | } | |
432 | ||
433 | for (1..15) { print "Merry Christmas\n"; } | |
434 | ||
435 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { | |
436 | print "Item: $item\n"; | |
437 | } | |
438 | ||
439 | Here's how a C programmer might code up a particular algorithm in Perl: | |
440 | ||
441 | for (my $i = 0; $i < @ary1; $i++) { | |
442 | for (my $j = 0; $j < @ary2; $j++) { | |
443 | if ($ary1[$i] > $ary2[$j]) { | |
444 | last; # can't go to outer :-( | |
445 | } | |
446 | $ary1[$i] += $ary2[$j]; | |
447 | } | |
448 | # this is where that last takes me | |
449 | } | |
450 | ||
451 | Whereas here's how a Perl programmer more comfortable with the idiom might | |
452 | do it: | |
453 | ||
454 | OUTER: for my $wid (@ary1) { | |
455 | INNER: for my $jet (@ary2) { | |
456 | next OUTER if $wid > $jet; | |
457 | $wid += $jet; | |
458 | } | |
459 | } | |
460 | ||
461 | See how much easier this is? It's cleaner, safer, and faster. It's | |
462 | cleaner because it's less noisy. It's safer because if code gets added | |
463 | between the inner and outer loops later on, the new code won't be | |
464 | accidentally executed. The C<next> explicitly iterates the other loop | |
465 | rather than merely terminating the inner one. And it's faster because | |
466 | Perl executes a C<foreach> statement more rapidly than it would the | |
467 | equivalent C<for> loop. | |
468 | ||
469 | =head2 Basic BLOCKs and Switch Statements | |
470 | X<switch> X<block> X<case> | |
471 | ||
472 | A BLOCK by itself (labeled or not) is semantically equivalent to a | |
473 | loop that executes once. Thus you can use any of the loop control | |
474 | statements in it to leave or restart the block. (Note that this is | |
475 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
476 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
477 | block is optional. | |
478 | ||
479 | The BLOCK construct is particularly nice for doing case | |
480 | structures. | |
481 | ||
482 | SWITCH: { | |
483 | if (/^abc/) { $abc = 1; last SWITCH; } | |
484 | if (/^def/) { $def = 1; last SWITCH; } | |
485 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
486 | $nothing = 1; | |
487 | } | |
488 | ||
489 | There is no official C<switch> statement in Perl, because there are | |
490 | already several ways to write the equivalent. | |
491 | ||
492 | However, starting from Perl 5.8 to get switch and case one can use | |
493 | the Switch extension and say: | |
494 | ||
495 | use Switch; | |
496 | ||
497 | after which one has switch and case. It is not as fast as it could be | |
498 | because it's not really part of the language (it's done using source | |
499 | filters) but it is available, and it's very flexible. | |
500 | ||
501 | In addition to the above BLOCK construct, you could write | |
502 | ||
503 | SWITCH: { | |
504 | $abc = 1, last SWITCH if /^abc/; | |
505 | $def = 1, last SWITCH if /^def/; | |
506 | $xyz = 1, last SWITCH if /^xyz/; | |
507 | $nothing = 1; | |
508 | } | |
509 | ||
510 | (That's actually not as strange as it looks once you realize that you can | |
511 | use loop control "operators" within an expression. That's just the binary | |
512 | comma operator in scalar context. See L<perlop/"Comma Operator">.) | |
513 | ||
514 | or | |
515 | ||
516 | SWITCH: { | |
517 | /^abc/ && do { $abc = 1; last SWITCH; }; | |
518 | /^def/ && do { $def = 1; last SWITCH; }; | |
519 | /^xyz/ && do { $xyz = 1; last SWITCH; }; | |
520 | $nothing = 1; | |
521 | } | |
522 | ||
523 | or formatted so it stands out more as a "proper" C<switch> statement: | |
524 | ||
525 | SWITCH: { | |
526 | /^abc/ && do { | |
527 | $abc = 1; | |
528 | last SWITCH; | |
529 | }; | |
530 | ||
531 | /^def/ && do { | |
532 | $def = 1; | |
533 | last SWITCH; | |
534 | }; | |
535 | ||
536 | /^xyz/ && do { | |
537 | $xyz = 1; | |
538 | last SWITCH; | |
539 | }; | |
540 | $nothing = 1; | |
541 | } | |
542 | ||
543 | or | |
544 | ||
545 | SWITCH: { | |
546 | /^abc/ and $abc = 1, last SWITCH; | |
547 | /^def/ and $def = 1, last SWITCH; | |
548 | /^xyz/ and $xyz = 1, last SWITCH; | |
549 | $nothing = 1; | |
550 | } | |
551 | ||
552 | or even, horrors, | |
553 | ||
554 | if (/^abc/) | |
555 | { $abc = 1 } | |
556 | elsif (/^def/) | |
557 | { $def = 1 } | |
558 | elsif (/^xyz/) | |
559 | { $xyz = 1 } | |
560 | else | |
561 | { $nothing = 1 } | |
562 | ||
563 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make | |
564 | a temporary assignment to C<$_> for convenient matching: | |
565 | ||
566 | SWITCH: for ($where) { | |
567 | /In Card Names/ && do { push @flags, '-e'; last; }; | |
568 | /Anywhere/ && do { push @flags, '-h'; last; }; | |
569 | /In Rulings/ && do { last; }; | |
570 | die "unknown value for form variable where: `$where'"; | |
571 | } | |
572 | ||
573 | Another interesting approach to a switch statement is arrange | |
574 | for a C<do> block to return the proper value: | |
575 | ||
576 | $amode = do { | |
577 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? | |
578 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } | |
579 | elsif ($flag & O_RDWR) { | |
580 | if ($flag & O_CREAT) { "w+" } | |
581 | else { ($flag & O_APPEND) ? "a+" : "r+" } | |
582 | } | |
583 | }; | |
584 | ||
585 | Or | |
586 | ||
587 | print do { | |
588 | ($flags & O_WRONLY) ? "write-only" : | |
589 | ($flags & O_RDWR) ? "read-write" : | |
590 | "read-only"; | |
591 | }; | |
592 | ||
593 | Or if you are certain that all the C<&&> clauses are true, you can use | |
594 | something like this, which "switches" on the value of the | |
595 | C<HTTP_USER_AGENT> environment variable. | |
596 | ||
597 | #!/usr/bin/perl | |
598 | # pick out jargon file page based on browser | |
599 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; | |
600 | for ($ENV{HTTP_USER_AGENT}) { | |
601 | $page = /Mac/ && 'm/Macintrash.html' | |
602 | || /Win(dows )?NT/ && 'e/evilandrude.html' | |
603 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' | |
604 | || /Linux/ && 'l/Linux.html' | |
605 | || /HP-UX/ && 'h/HP-SUX.html' | |
606 | || /SunOS/ && 's/ScumOS.html' | |
607 | || 'a/AppendixB.html'; | |
608 | } | |
609 | print "Location: $dir/$page\015\012\015\012"; | |
610 | ||
611 | That kind of switch statement only works when you know the C<&&> clauses | |
612 | will be true. If you don't, the previous C<?:> example should be used. | |
613 | ||
614 | You might also consider writing a hash of subroutine references | |
615 | instead of synthesizing a C<switch> statement. | |
616 | ||
617 | =head2 Goto | |
618 | X<goto> | |
619 | ||
620 | Although not for the faint of heart, Perl does support a C<goto> | |
621 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
622 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
623 | a C<goto>; it's just the name of the loop. | |
624 | ||
625 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes | |
626 | execution there. It may not be used to go into any construct that | |
627 | requires initialization, such as a subroutine or a C<foreach> loop. It | |
628 | also can't be used to go into a construct that is optimized away. It | |
629 | can be used to go almost anywhere else within the dynamic scope, | |
630 | including out of subroutines, but it's usually better to use some other | |
631 | construct such as C<last> or C<die>. The author of Perl has never felt the | |
632 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
633 | ||
634 | The C<goto>-EXPR form expects a label name, whose scope will be resolved | |
635 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
636 | necessarily recommended if you're optimizing for maintainability: | |
637 | ||
638 | goto(("FOO", "BAR", "GLARCH")[$i]); | |
639 | ||
640 | The C<goto>-&NAME form is highly magical, and substitutes a call to the | |
641 | named subroutine for the currently running subroutine. This is used by | |
642 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then | |
643 | pretend that the other subroutine had been called in the first place | |
644 | (except that any modifications to C<@_> in the current subroutine are | |
645 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
646 | will be able to tell that this routine was called first. | |
647 | ||
648 | In almost all cases like this, it's usually a far, far better idea to use the | |
649 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
650 | resorting to a C<goto>. For certain applications, the catch and throw pair of | |
651 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
652 | ||
653 | =head2 PODs: Embedded Documentation | |
654 | X<POD> X<documentation> | |
655 | ||
656 | Perl has a mechanism for intermixing documentation with source code. | |
657 | While it's expecting the beginning of a new statement, if the compiler | |
658 | encounters a line that begins with an equal sign and a word, like this | |
659 | ||
660 | =head1 Here There Be Pods! | |
661 | ||
662 | Then that text and all remaining text up through and including a line | |
663 | beginning with C<=cut> will be ignored. The format of the intervening | |
664 | text is described in L<perlpod>. | |
665 | ||
666 | This allows you to intermix your source code | |
667 | and your documentation text freely, as in | |
668 | ||
669 | =item snazzle($) | |
670 | ||
671 | The snazzle() function will behave in the most spectacular | |
672 | form that you can possibly imagine, not even excepting | |
673 | cybernetic pyrotechnics. | |
674 | ||
675 | =cut back to the compiler, nuff of this pod stuff! | |
676 | ||
677 | sub snazzle($) { | |
678 | my $thingie = shift; | |
679 | ......... | |
680 | } | |
681 | ||
682 | Note that pod translators should look at only paragraphs beginning | |
683 | with a pod directive (it makes parsing easier), whereas the compiler | |
684 | actually knows to look for pod escapes even in the middle of a | |
685 | paragraph. This means that the following secret stuff will be | |
686 | ignored by both the compiler and the translators. | |
687 | ||
688 | $a=3; | |
689 | =secret stuff | |
690 | warn "Neither POD nor CODE!?" | |
691 | =cut back | |
692 | print "got $a\n"; | |
693 | ||
694 | You probably shouldn't rely upon the C<warn()> being podded out forever. | |
695 | Not all pod translators are well-behaved in this regard, and perhaps | |
696 | the compiler will become pickier. | |
697 | ||
698 | One may also use pod directives to quickly comment out a section | |
699 | of code. | |
700 | ||
701 | =head2 Plain Old Comments (Not!) | |
702 | X<comment> X<line> X<#> X<preprocessor> X<eval> | |
703 | ||
704 | Perl can process line directives, much like the C preprocessor. Using | |
705 | this, one can control Perl's idea of filenames and line numbers in | |
706 | error or warning messages (especially for strings that are processed | |
707 | with C<eval()>). The syntax for this mechanism is the same as for most | |
708 | C preprocessors: it matches the regular expression | |
709 | ||
710 | # example: '# line 42 "new_filename.plx"' | |
711 | /^\# \s* | |
712 | line \s+ (\d+) \s* | |
713 | (?:\s("?)([^"]+)\2)? \s* | |
714 | $/x | |
715 | ||
716 | with C<$1> being the line number for the next line, and C<$3> being | |
717 | the optional filename (specified with or without quotes). | |
718 | ||
719 | There is a fairly obvious gotcha included with the line directive: | |
720 | Debuggers and profilers will only show the last source line to appear | |
721 | at a particular line number in a given file. Care should be taken not | |
722 | to cause line number collisions in code you'd like to debug later. | |
723 | ||
724 | Here are some examples that you should be able to type into your command | |
725 | shell: | |
726 | ||
727 | % perl | |
728 | # line 200 "bzzzt" | |
729 | # the `#' on the previous line must be the first char on line | |
730 | die 'foo'; | |
731 | __END__ | |
732 | foo at bzzzt line 201. | |
733 | ||
734 | % perl | |
735 | # line 200 "bzzzt" | |
736 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
737 | __END__ | |
738 | foo at - line 2001. | |
739 | ||
740 | % perl | |
741 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
742 | __END__ | |
743 | foo at foo bar line 200. | |
744 | ||
745 | % perl | |
746 | # line 345 "goop" | |
747 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
748 | print $@; | |
749 | __END__ | |
750 | foo at goop line 345. | |
751 | ||
752 | =cut |