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1 | =head1 NAME |
2 | ||
3 | perlsyn - Perl syntax | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | A Perl script consists of a sequence of declarations and statements. | |
8 | The sequence of statements is executed just once, unlike in B<sed> | |
9 | and B<awk> scripts, where the sequence of statements is executed | |
10 | for each input line. While this means that you must explicitly | |
11 | loop over the lines of your input file (or files), it also means | |
12 | you have much more control over which files and which lines you look at. | |
13 | (Actually, I'm lying--it is possible to do an implicit loop with | |
14 | either the B<-n> or B<-p> switch. It's just not the mandatory | |
15 | default like it is in B<sed> and B<awk>.) | |
16 | ||
17 | Perl is, for the most part, a free-form language. (The only exception | |
18 | to this is format declarations, for obvious reasons.) Text from a | |
19 | C<"#"> character until the end of the line is a comment, and is | |
20 | ignored. If you attempt to use C</* */> C-style comments, it will be | |
21 | interpreted either as division or pattern matching, depending on the | |
22 | context, and C++ C<//> comments just look like a null regular | |
23 | expression, so don't do that. | |
24 | ||
25 | =head2 Declarations | |
26 | ||
27 | The only things you need to declare in Perl are report formats | |
28 | and subroutines--and even undefined subroutines can be handled | |
29 | through AUTOLOAD. A variable holds the undefined value (C<undef>) | |
30 | until it has been assigned a defined value, which is anything | |
31 | other than C<undef>. When used as a number, C<undef> is treated | |
32 | as C<0>; when used as a string, it is treated the empty string, | |
33 | C<"">; and when used as a reference that isn't being assigned | |
34 | to, it is treated as an error. If you enable warnings, you'll | |
35 | be notified of an uninitialized value whenever you treat C<undef> | |
36 | as a string or a number. Well, usually. Boolean contexts, such as: | |
37 | ||
38 | my $a; | |
39 | if ($a) {} | |
40 | ||
41 | are exempt from warnings (because they care about truth rather than | |
42 | definedness). Operators such as C<++>, C<-->, C<+=>, | |
43 | C<-=>, and C<.=>, that operate on undefined left values such as: | |
44 | ||
45 | my $a; | |
46 | $a++; | |
47 | ||
48 | are also always exempt from such warnings. | |
49 | ||
50 | A declaration can be put anywhere a statement can, but has no effect on | |
51 | the execution of the primary sequence of statements--declarations all | |
52 | take effect at compile time. Typically all the declarations are put at | |
53 | the beginning or the end of the script. However, if you're using | |
54 | lexically-scoped private variables created with C<my()>, you'll | |
55 | have to make sure | |
56 | your format or subroutine definition is within the same block scope | |
57 | as the my if you expect to be able to access those private variables. | |
58 | ||
59 | Declaring a subroutine allows a subroutine name to be used as if it were a | |
60 | list operator from that point forward in the program. You can declare a | |
61 | subroutine without defining it by saying C<sub name>, thus: | |
62 | ||
63 | sub myname; | |
64 | $me = myname $0 or die "can't get myname"; | |
65 | ||
66 | Note that myname() functions as a list operator, not as a unary operator; | |
67 | so be careful to use C<or> instead of C<||> in this case. However, if | |
68 | you were to declare the subroutine as C<sub myname ($)>, then | |
69 | C<myname> would function as a unary operator, so either C<or> or | |
70 | C<||> would work. | |
71 | ||
72 | Subroutines declarations can also be loaded up with the C<require> statement | |
73 | or both loaded and imported into your namespace with a C<use> statement. | |
74 | See L<perlmod> for details on this. | |
75 | ||
76 | A statement sequence may contain declarations of lexically-scoped | |
77 | variables, but apart from declaring a variable name, the declaration acts | |
78 | like an ordinary statement, and is elaborated within the sequence of | |
79 | statements as if it were an ordinary statement. That means it actually | |
80 | has both compile-time and run-time effects. | |
81 | ||
82 | =head2 Simple statements | |
83 | ||
84 | The only kind of simple statement is an expression evaluated for its | |
85 | side effects. Every simple statement must be terminated with a | |
86 | semicolon, unless it is the final statement in a block, in which case | |
87 | the semicolon is optional. (A semicolon is still encouraged there if the | |
88 | block takes up more than one line, because you may eventually add another line.) | |
89 | Note that there are some operators like C<eval {}> and C<do {}> that look | |
90 | like compound statements, but aren't (they're just TERMs in an expression), | |
91 | and thus need an explicit termination if used as the last item in a statement. | |
92 | ||
93 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
94 | just before the terminating semicolon (or block ending). The possible | |
95 | modifiers are: | |
96 | ||
97 | if EXPR | |
98 | unless EXPR | |
99 | while EXPR | |
100 | until EXPR | |
101 | foreach EXPR | |
102 | ||
103 | The C<if> and C<unless> modifiers have the expected semantics, | |
104 | presuming you're a speaker of English. The C<foreach> modifier is an | |
105 | iterator: For each value in EXPR, it aliases C<$_> to the value and | |
106 | executes the statement. The C<while> and C<until> modifiers have the | |
107 | usual "C<while> loop" semantics (conditional evaluated first), except | |
108 | when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE | |
109 | statement), in which case the block executes once before the | |
110 | conditional is evaluated. This is so that you can write loops like: | |
111 | ||
112 | do { | |
113 | $line = <STDIN>; | |
114 | ... | |
115 | } until $line eq ".\n"; | |
116 | ||
117 | See L<perlfunc/do>. Note also that the loop control statements described | |
118 | later will I<NOT> work in this construct, because modifiers don't take | |
119 | loop labels. Sorry. You can always put another block inside of it | |
120 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
121 | For C<next>, just double the braces: | |
122 | ||
123 | do {{ | |
124 | next if $x == $y; | |
125 | # do something here | |
126 | }} until $x++ > $z; | |
127 | ||
128 | For C<last>, you have to be more elaborate: | |
129 | ||
130 | LOOP: { | |
131 | do { | |
132 | last if $x = $y**2; | |
133 | # do something here | |
134 | } while $x++ <= $z; | |
135 | } | |
136 | ||
137 | =head2 Compound statements | |
138 | ||
139 | In Perl, a sequence of statements that defines a scope is called a block. | |
140 | Sometimes a block is delimited by the file containing it (in the case | |
141 | of a required file, or the program as a whole), and sometimes a block | |
142 | is delimited by the extent of a string (in the case of an eval). | |
143 | ||
144 | But generally, a block is delimited by curly brackets, also known as braces. | |
145 | We will call this syntactic construct a BLOCK. | |
146 | ||
147 | The following compound statements may be used to control flow: | |
148 | ||
149 | if (EXPR) BLOCK | |
150 | if (EXPR) BLOCK else BLOCK | |
151 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK | |
152 | LABEL while (EXPR) BLOCK | |
153 | LABEL while (EXPR) BLOCK continue BLOCK | |
154 | LABEL for (EXPR; EXPR; EXPR) BLOCK | |
155 | LABEL foreach VAR (LIST) BLOCK | |
156 | LABEL foreach VAR (LIST) BLOCK continue BLOCK | |
157 | LABEL BLOCK continue BLOCK | |
158 | ||
159 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, | |
160 | not statements. This means that the curly brackets are I<required>--no | |
161 | dangling statements allowed. If you want to write conditionals without | |
162 | curly brackets there are several other ways to do it. The following | |
163 | all do the same thing: | |
164 | ||
165 | if (!open(FOO)) { die "Can't open $FOO: $!"; } | |
166 | die "Can't open $FOO: $!" unless open(FOO); | |
167 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! | |
168 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; | |
169 | # a bit exotic, that last one | |
170 | ||
171 | The C<if> statement is straightforward. Because BLOCKs are always | |
172 | bounded by curly brackets, there is never any ambiguity about which | |
173 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
174 | the sense of the test is reversed. | |
175 | ||
176 | The C<while> statement executes the block as long as the expression is | |
177 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). | |
178 | The LABEL is optional, and if present, consists of an identifier followed | |
179 | by a colon. The LABEL identifies the loop for the loop control | |
180 | statements C<next>, C<last>, and C<redo>. | |
181 | If the LABEL is omitted, the loop control statement | |
182 | refers to the innermost enclosing loop. This may include dynamically | |
183 | looking back your call-stack at run time to find the LABEL. Such | |
184 | desperate behavior triggers a warning if you use the C<use warnings> | |
185 | pragma or the B<-w> flag. | |
186 | Unlike a C<foreach> statement, a C<while> statement never implicitly | |
187 | localises any variables. | |
188 | ||
189 | If there is a C<continue> BLOCK, it is always executed just before the | |
190 | conditional is about to be evaluated again, just like the third part of a | |
191 | C<for> loop in C. Thus it can be used to increment a loop variable, even | |
192 | when the loop has been continued via the C<next> statement (which is | |
193 | similar to the C C<continue> statement). | |
194 | ||
195 | =head2 Loop Control | |
196 | ||
197 | The C<next> command is like the C<continue> statement in C; it starts | |
198 | the next iteration of the loop: | |
199 | ||
200 | LINE: while (<STDIN>) { | |
201 | next LINE if /^#/; # discard comments | |
202 | ... | |
203 | } | |
204 | ||
205 | The C<last> command is like the C<break> statement in C (as used in | |
206 | loops); it immediately exits the loop in question. The | |
207 | C<continue> block, if any, is not executed: | |
208 | ||
209 | LINE: while (<STDIN>) { | |
210 | last LINE if /^$/; # exit when done with header | |
211 | ... | |
212 | } | |
213 | ||
214 | The C<redo> command restarts the loop block without evaluating the | |
215 | conditional again. The C<continue> block, if any, is I<not> executed. | |
216 | This command is normally used by programs that want to lie to themselves | |
217 | about what was just input. | |
218 | ||
219 | For example, when processing a file like F</etc/termcap>. | |
220 | If your input lines might end in backslashes to indicate continuation, you | |
221 | want to skip ahead and get the next record. | |
222 | ||
223 | while (<>) { | |
224 | chomp; | |
225 | if (s/\\$//) { | |
226 | $_ .= <>; | |
227 | redo unless eof(); | |
228 | } | |
229 | # now process $_ | |
230 | } | |
231 | ||
232 | which is Perl short-hand for the more explicitly written version: | |
233 | ||
234 | LINE: while (defined($line = <ARGV>)) { | |
235 | chomp($line); | |
236 | if ($line =~ s/\\$//) { | |
237 | $line .= <ARGV>; | |
238 | redo LINE unless eof(); # not eof(ARGV)! | |
239 | } | |
240 | # now process $line | |
241 | } | |
242 | ||
243 | Note that if there were a C<continue> block on the above code, it would | |
244 | get executed only on lines discarded by the regex (since redo skips the | |
245 | continue block). A continue block is often used to reset line counters | |
246 | or C<?pat?> one-time matches: | |
247 | ||
248 | # inspired by :1,$g/fred/s//WILMA/ | |
249 | while (<>) { | |
250 | ?(fred)? && s//WILMA $1 WILMA/; | |
251 | ?(barney)? && s//BETTY $1 BETTY/; | |
252 | ?(homer)? && s//MARGE $1 MARGE/; | |
253 | } continue { | |
254 | print "$ARGV $.: $_"; | |
255 | close ARGV if eof(); # reset $. | |
256 | reset if eof(); # reset ?pat? | |
257 | } | |
258 | ||
259 | If the word C<while> is replaced by the word C<until>, the sense of the | |
260 | test is reversed, but the conditional is still tested before the first | |
261 | iteration. | |
262 | ||
263 | The loop control statements don't work in an C<if> or C<unless>, since | |
264 | they aren't loops. You can double the braces to make them such, though. | |
265 | ||
266 | if (/pattern/) {{ | |
267 | last if /fred/; | |
268 | next if /barney/; # same effect as "last", but doesn't document as well | |
269 | # do something here | |
270 | }} | |
271 | ||
272 | This is caused by the fact that a block by itself acts as a loop that | |
273 | executes once, see L<"Basic BLOCKs and Switch Statements">. | |
274 | ||
275 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer | |
276 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
277 | ||
278 | =head2 For Loops | |
279 | ||
280 | Perl's C-style C<for> loop works like the corresponding C<while> loop; | |
281 | that means that this: | |
282 | ||
283 | for ($i = 1; $i < 10; $i++) { | |
284 | ... | |
285 | } | |
286 | ||
287 | is the same as this: | |
288 | ||
289 | $i = 1; | |
290 | while ($i < 10) { | |
291 | ... | |
292 | } continue { | |
293 | $i++; | |
294 | } | |
295 | ||
296 | There is one minor difference: if variables are declared with C<my> | |
297 | in the initialization section of the C<for>, the lexical scope of | |
298 | those variables is exactly the C<for> loop (the body of the loop | |
299 | and the control sections). | |
300 | ||
301 | Besides the normal array index looping, C<for> can lend itself | |
302 | to many other interesting applications. Here's one that avoids the | |
303 | problem you get into if you explicitly test for end-of-file on | |
304 | an interactive file descriptor causing your program to appear to | |
305 | hang. | |
306 | ||
307 | $on_a_tty = -t STDIN && -t STDOUT; | |
308 | sub prompt { print "yes? " if $on_a_tty } | |
309 | for ( prompt(); <STDIN>; prompt() ) { | |
310 | # do something | |
311 | } | |
312 | ||
313 | =head2 Foreach Loops | |
314 | ||
315 | The C<foreach> loop iterates over a normal list value and sets the | |
316 | variable VAR to be each element of the list in turn. If the variable | |
317 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
318 | is therefore visible only within the loop. Otherwise, the variable is | |
319 | implicitly local to the loop and regains its former value upon exiting | |
320 | the loop. If the variable was previously declared with C<my>, it uses | |
321 | that variable instead of the global one, but it's still localized to | |
322 | the loop. | |
323 | ||
324 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
325 | you can use C<foreach> for readability or C<for> for brevity. (Or because | |
326 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> | |
327 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. | |
328 | ||
329 | If any element of LIST is an lvalue, you can modify it by modifying | |
330 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
331 | lvalue, any attempt to modify that element will fail. In other words, | |
332 | the C<foreach> loop index variable is an implicit alias for each item | |
333 | in the list that you're looping over. | |
334 | ||
335 | If any part of LIST is an array, C<foreach> will get very confused if | |
336 | you add or remove elements within the loop body, for example with | |
337 | C<splice>. So don't do that. | |
338 | ||
339 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
340 | special variable. Don't do that either. | |
341 | ||
342 | Examples: | |
343 | ||
344 | for (@ary) { s/foo/bar/ } | |
345 | ||
346 | for my $elem (@elements) { | |
347 | $elem *= 2; | |
348 | } | |
349 | ||
350 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { | |
351 | print $count, "\n"; sleep(1); | |
352 | } | |
353 | ||
354 | for (1..15) { print "Merry Christmas\n"; } | |
355 | ||
356 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { | |
357 | print "Item: $item\n"; | |
358 | } | |
359 | ||
360 | Here's how a C programmer might code up a particular algorithm in Perl: | |
361 | ||
362 | for (my $i = 0; $i < @ary1; $i++) { | |
363 | for (my $j = 0; $j < @ary2; $j++) { | |
364 | if ($ary1[$i] > $ary2[$j]) { | |
365 | last; # can't go to outer :-( | |
366 | } | |
367 | $ary1[$i] += $ary2[$j]; | |
368 | } | |
369 | # this is where that last takes me | |
370 | } | |
371 | ||
372 | Whereas here's how a Perl programmer more comfortable with the idiom might | |
373 | do it: | |
374 | ||
375 | OUTER: for my $wid (@ary1) { | |
376 | INNER: for my $jet (@ary2) { | |
377 | next OUTER if $wid > $jet; | |
378 | $wid += $jet; | |
379 | } | |
380 | } | |
381 | ||
382 | See how much easier this is? It's cleaner, safer, and faster. It's | |
383 | cleaner because it's less noisy. It's safer because if code gets added | |
384 | between the inner and outer loops later on, the new code won't be | |
385 | accidentally executed. The C<next> explicitly iterates the other loop | |
386 | rather than merely terminating the inner one. And it's faster because | |
387 | Perl executes a C<foreach> statement more rapidly than it would the | |
388 | equivalent C<for> loop. | |
389 | ||
390 | =head2 Basic BLOCKs and Switch Statements | |
391 | ||
392 | A BLOCK by itself (labeled or not) is semantically equivalent to a | |
393 | loop that executes once. Thus you can use any of the loop control | |
394 | statements in it to leave or restart the block. (Note that this is | |
395 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
396 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
397 | block is optional. | |
398 | ||
399 | The BLOCK construct is particularly nice for doing case | |
400 | structures. | |
401 | ||
402 | SWITCH: { | |
403 | if (/^abc/) { $abc = 1; last SWITCH; } | |
404 | if (/^def/) { $def = 1; last SWITCH; } | |
405 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
406 | $nothing = 1; | |
407 | } | |
408 | ||
409 | There is no official C<switch> statement in Perl, because there are | |
410 | already several ways to write the equivalent. | |
411 | ||
412 | However, starting from Perl 5.8 to get switch and case one can use | |
413 | the Switch extension and say: | |
414 | ||
415 | use Switch; | |
416 | ||
417 | after which one has switch and case. It is not as fast as it could be | |
418 | because it's not really part of the language (it's done using source | |
419 | filters) but it is available, and it's very flexible. | |
420 | ||
421 | In addition to the above BLOCK construct, you could write | |
422 | ||
423 | SWITCH: { | |
424 | $abc = 1, last SWITCH if /^abc/; | |
425 | $def = 1, last SWITCH if /^def/; | |
426 | $xyz = 1, last SWITCH if /^xyz/; | |
427 | $nothing = 1; | |
428 | } | |
429 | ||
430 | (That's actually not as strange as it looks once you realize that you can | |
431 | use loop control "operators" within an expression, That's just the normal | |
432 | C comma operator.) | |
433 | ||
434 | or | |
435 | ||
436 | SWITCH: { | |
437 | /^abc/ && do { $abc = 1; last SWITCH; }; | |
438 | /^def/ && do { $def = 1; last SWITCH; }; | |
439 | /^xyz/ && do { $xyz = 1; last SWITCH; }; | |
440 | $nothing = 1; | |
441 | } | |
442 | ||
443 | or formatted so it stands out more as a "proper" C<switch> statement: | |
444 | ||
445 | SWITCH: { | |
446 | /^abc/ && do { | |
447 | $abc = 1; | |
448 | last SWITCH; | |
449 | }; | |
450 | ||
451 | /^def/ && do { | |
452 | $def = 1; | |
453 | last SWITCH; | |
454 | }; | |
455 | ||
456 | /^xyz/ && do { | |
457 | $xyz = 1; | |
458 | last SWITCH; | |
459 | }; | |
460 | $nothing = 1; | |
461 | } | |
462 | ||
463 | or | |
464 | ||
465 | SWITCH: { | |
466 | /^abc/ and $abc = 1, last SWITCH; | |
467 | /^def/ and $def = 1, last SWITCH; | |
468 | /^xyz/ and $xyz = 1, last SWITCH; | |
469 | $nothing = 1; | |
470 | } | |
471 | ||
472 | or even, horrors, | |
473 | ||
474 | if (/^abc/) | |
475 | { $abc = 1 } | |
476 | elsif (/^def/) | |
477 | { $def = 1 } | |
478 | elsif (/^xyz/) | |
479 | { $xyz = 1 } | |
480 | else | |
481 | { $nothing = 1 } | |
482 | ||
483 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make | |
484 | a temporary assignment to C<$_> for convenient matching: | |
485 | ||
486 | SWITCH: for ($where) { | |
487 | /In Card Names/ && do { push @flags, '-e'; last; }; | |
488 | /Anywhere/ && do { push @flags, '-h'; last; }; | |
489 | /In Rulings/ && do { last; }; | |
490 | die "unknown value for form variable where: `$where'"; | |
491 | } | |
492 | ||
493 | Another interesting approach to a switch statement is arrange | |
494 | for a C<do> block to return the proper value: | |
495 | ||
496 | $amode = do { | |
497 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? | |
498 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } | |
499 | elsif ($flag & O_RDWR) { | |
500 | if ($flag & O_CREAT) { "w+" } | |
501 | else { ($flag & O_APPEND) ? "a+" : "r+" } | |
502 | } | |
503 | }; | |
504 | ||
505 | Or | |
506 | ||
507 | print do { | |
508 | ($flags & O_WRONLY) ? "write-only" : | |
509 | ($flags & O_RDWR) ? "read-write" : | |
510 | "read-only"; | |
511 | }; | |
512 | ||
513 | Or if you are certain that all the C<&&> clauses are true, you can use | |
514 | something like this, which "switches" on the value of the | |
515 | C<HTTP_USER_AGENT> environment variable. | |
516 | ||
517 | #!/usr/bin/perl | |
518 | # pick out jargon file page based on browser | |
519 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; | |
520 | for ($ENV{HTTP_USER_AGENT}) { | |
521 | $page = /Mac/ && 'm/Macintrash.html' | |
522 | || /Win(dows )?NT/ && 'e/evilandrude.html' | |
523 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' | |
524 | || /Linux/ && 'l/Linux.html' | |
525 | || /HP-UX/ && 'h/HP-SUX.html' | |
526 | || /SunOS/ && 's/ScumOS.html' | |
527 | || 'a/AppendixB.html'; | |
528 | } | |
529 | print "Location: $dir/$page\015\012\015\012"; | |
530 | ||
531 | That kind of switch statement only works when you know the C<&&> clauses | |
532 | will be true. If you don't, the previous C<?:> example should be used. | |
533 | ||
534 | You might also consider writing a hash of subroutine references | |
535 | instead of synthesizing a C<switch> statement. | |
536 | ||
537 | =head2 Goto | |
538 | ||
539 | Although not for the faint of heart, Perl does support a C<goto> | |
540 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
541 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
542 | a C<goto>; it's just the name of the loop. | |
543 | ||
544 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes | |
545 | execution there. It may not be used to go into any construct that | |
546 | requires initialization, such as a subroutine or a C<foreach> loop. It | |
547 | also can't be used to go into a construct that is optimized away. It | |
548 | can be used to go almost anywhere else within the dynamic scope, | |
549 | including out of subroutines, but it's usually better to use some other | |
550 | construct such as C<last> or C<die>. The author of Perl has never felt the | |
551 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
552 | ||
553 | The C<goto>-EXPR form expects a label name, whose scope will be resolved | |
554 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
555 | necessarily recommended if you're optimizing for maintainability: | |
556 | ||
557 | goto(("FOO", "BAR", "GLARCH")[$i]); | |
558 | ||
559 | The C<goto>-&NAME form is highly magical, and substitutes a call to the | |
560 | named subroutine for the currently running subroutine. This is used by | |
561 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then | |
562 | pretend that the other subroutine had been called in the first place | |
563 | (except that any modifications to C<@_> in the current subroutine are | |
564 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
565 | will be able to tell that this routine was called first. | |
566 | ||
567 | In almost all cases like this, it's usually a far, far better idea to use the | |
568 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
569 | resorting to a C<goto>. For certain applications, the catch and throw pair of | |
570 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
571 | ||
572 | =head2 PODs: Embedded Documentation | |
573 | ||
574 | Perl has a mechanism for intermixing documentation with source code. | |
575 | While it's expecting the beginning of a new statement, if the compiler | |
576 | encounters a line that begins with an equal sign and a word, like this | |
577 | ||
578 | =head1 Here There Be Pods! | |
579 | ||
580 | Then that text and all remaining text up through and including a line | |
581 | beginning with C<=cut> will be ignored. The format of the intervening | |
582 | text is described in L<perlpod>. | |
583 | ||
584 | This allows you to intermix your source code | |
585 | and your documentation text freely, as in | |
586 | ||
587 | =item snazzle($) | |
588 | ||
589 | The snazzle() function will behave in the most spectacular | |
590 | form that you can possibly imagine, not even excepting | |
591 | cybernetic pyrotechnics. | |
592 | ||
593 | =cut back to the compiler, nuff of this pod stuff! | |
594 | ||
595 | sub snazzle($) { | |
596 | my $thingie = shift; | |
597 | ......... | |
598 | } | |
599 | ||
600 | Note that pod translators should look at only paragraphs beginning | |
601 | with a pod directive (it makes parsing easier), whereas the compiler | |
602 | actually knows to look for pod escapes even in the middle of a | |
603 | paragraph. This means that the following secret stuff will be | |
604 | ignored by both the compiler and the translators. | |
605 | ||
606 | $a=3; | |
607 | =secret stuff | |
608 | warn "Neither POD nor CODE!?" | |
609 | =cut back | |
610 | print "got $a\n"; | |
611 | ||
612 | You probably shouldn't rely upon the C<warn()> being podded out forever. | |
613 | Not all pod translators are well-behaved in this regard, and perhaps | |
614 | the compiler will become pickier. | |
615 | ||
616 | One may also use pod directives to quickly comment out a section | |
617 | of code. | |
618 | ||
619 | =head2 Plain Old Comments (Not!) | |
620 | ||
621 | Much like the C preprocessor, Perl can process line directives. Using | |
622 | this, one can control Perl's idea of filenames and line numbers in | |
623 | error or warning messages (especially for strings that are processed | |
624 | with C<eval()>). The syntax for this mechanism is the same as for most | |
625 | C preprocessors: it matches the regular expression | |
626 | C</^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/> with C<$1> being the line | |
627 | number for the next line, and C<$2> being the optional filename | |
628 | (specified within quotes). | |
629 | ||
630 | There is a fairly obvious gotcha included with the line directive: | |
631 | Debuggers and profilers will only show the last source line to appear | |
632 | at a particular line number in a given file. Care should be taken not | |
633 | to cause line number collisions in code you'd like to debug later. | |
634 | ||
635 | Here are some examples that you should be able to type into your command | |
636 | shell: | |
637 | ||
638 | % perl | |
639 | # line 200 "bzzzt" | |
640 | # the `#' on the previous line must be the first char on line | |
641 | die 'foo'; | |
642 | __END__ | |
643 | foo at bzzzt line 201. | |
644 | ||
645 | % perl | |
646 | # line 200 "bzzzt" | |
647 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
648 | __END__ | |
649 | foo at - line 2001. | |
650 | ||
651 | % perl | |
652 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
653 | __END__ | |
654 | foo at foo bar line 200. | |
655 | ||
656 | % perl | |
657 | # line 345 "goop" | |
658 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
659 | print $@; | |
660 | __END__ | |
661 | foo at goop line 345. | |
662 | ||
663 | =cut |