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a0d0e21e LW |
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 only things that need to be declared in Perl are report formats | |
9 | and subroutines. See the sections below for more information on those | |
10 | declarations. All uninitialized user-created objects are assumed to | |
f86cebdf | 11 | start with a C<null> or C<0> value until they are defined by some explicit |
a0d0e21e LW |
12 | operation such as assignment. (Though you can get warnings about the |
13 | use of undefined values if you like.) The sequence of statements is | |
14 | executed just once, unlike in B<sed> and B<awk> scripts, where the | |
15 | sequence of statements is executed for each input line. While this means | |
16 | that you must explicitly loop over the lines of your input file (or | |
17 | files), it also means you have much more control over which files and | |
18 | which lines you look at. (Actually, I'm lying--it is possible to do an | |
19 | implicit loop with either the B<-n> or B<-p> switch. It's just not the | |
20 | mandatory default like it is in B<sed> and B<awk>.) | |
21 | ||
4633a7c4 LW |
22 | =head2 Declarations |
23 | ||
f00f6914 GS |
24 | Perl is, for the most part, a free-form language. (The only exception |
25 | to this is format declarations, for obvious reasons.) Text from a | |
26 | C<"#"> character until the end of the line is a comment, and is | |
27 | ignored. If you attempt to use C</* */> C-style comments, it will be | |
28 | interpreted either as division or pattern matching, depending on the | |
29 | context, and C++ C<//> comments just look like a null regular | |
30 | expression, so don't do that. | |
a0d0e21e LW |
31 | |
32 | A declaration can be put anywhere a statement can, but has no effect on | |
33 | the execution of the primary sequence of statements--declarations all | |
34 | take effect at compile time. Typically all the declarations are put at | |
54310121 | 35 | the beginning or the end of the script. However, if you're using |
f86cebdf | 36 | lexically-scoped private variables created with C<my()>, you'll have to make sure |
4633a7c4 | 37 | your format or subroutine definition is within the same block scope |
5f05dabc | 38 | as the my if you expect to be able to access those private variables. |
a0d0e21e | 39 | |
4633a7c4 LW |
40 | Declaring a subroutine allows a subroutine name to be used as if it were a |
41 | list operator from that point forward in the program. You can declare a | |
54310121 | 42 | subroutine without defining it by saying C<sub name>, thus: |
a0d0e21e | 43 | |
54310121 | 44 | sub myname; |
a0d0e21e LW |
45 | $me = myname $0 or die "can't get myname"; |
46 | ||
19799a22 | 47 | Note that my() functions as a list operator, not as a unary operator; so |
54310121 | 48 | be careful to use C<or> instead of C<||> in this case. However, if |
49 | you were to declare the subroutine as C<sub myname ($)>, then | |
02c45c47 | 50 | C<myname> would function as a unary operator, so either C<or> or |
54310121 | 51 | C<||> would work. |
a0d0e21e | 52 | |
4633a7c4 LW |
53 | Subroutines declarations can also be loaded up with the C<require> statement |
54 | or both loaded and imported into your namespace with a C<use> statement. | |
55 | See L<perlmod> for details on this. | |
a0d0e21e | 56 | |
4633a7c4 LW |
57 | A statement sequence may contain declarations of lexically-scoped |
58 | variables, but apart from declaring a variable name, the declaration acts | |
59 | like an ordinary statement, and is elaborated within the sequence of | |
60 | statements as if it were an ordinary statement. That means it actually | |
61 | has both compile-time and run-time effects. | |
a0d0e21e LW |
62 | |
63 | =head2 Simple statements | |
64 | ||
65 | The only kind of simple statement is an expression evaluated for its | |
66 | side effects. Every simple statement must be terminated with a | |
67 | semicolon, unless it is the final statement in a block, in which case | |
68 | the semicolon is optional. (A semicolon is still encouraged there if the | |
5f05dabc | 69 | block takes up more than one line, because you may eventually add another line.) |
a0d0e21e | 70 | Note that there are some operators like C<eval {}> and C<do {}> that look |
54310121 | 71 | like compound statements, but aren't (they're just TERMs in an expression), |
4633a7c4 | 72 | and thus need an explicit termination if used as the last item in a statement. |
a0d0e21e LW |
73 | |
74 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
75 | just before the terminating semicolon (or block ending). The possible | |
76 | modifiers are: | |
77 | ||
78 | if EXPR | |
79 | unless EXPR | |
80 | while EXPR | |
81 | until EXPR | |
ecca16b0 | 82 | foreach EXPR |
a0d0e21e LW |
83 | |
84 | The C<if> and C<unless> modifiers have the expected semantics, | |
ecca16b0 | 85 | presuming you're a speaker of English. The C<foreach> modifier is an |
f86cebdf | 86 | iterator: For each value in EXPR, it aliases C<$_> to the value and |
ecca16b0 | 87 | executes the statement. The C<while> and C<until> modifiers have the |
f86cebdf | 88 | usual "C<while> loop" semantics (conditional evaluated first), except |
19799a22 | 89 | when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE |
ecca16b0 CS |
90 | statement), in which case the block executes once before the |
91 | conditional is evaluated. This is so that you can write loops like: | |
a0d0e21e LW |
92 | |
93 | do { | |
4633a7c4 | 94 | $line = <STDIN>; |
a0d0e21e | 95 | ... |
4633a7c4 | 96 | } until $line eq ".\n"; |
a0d0e21e | 97 | |
5a964f20 TC |
98 | See L<perlfunc/do>. Note also that the loop control statements described |
99 | later will I<NOT> work in this construct, because modifiers don't take | |
100 | loop labels. Sorry. You can always put another block inside of it | |
101 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
f86cebdf | 102 | For C<next>, just double the braces: |
5a964f20 TC |
103 | |
104 | do {{ | |
105 | next if $x == $y; | |
106 | # do something here | |
107 | }} until $x++ > $z; | |
108 | ||
f86cebdf | 109 | For C<last>, you have to be more elaborate: |
5a964f20 TC |
110 | |
111 | LOOP: { | |
112 | do { | |
113 | last if $x = $y**2; | |
114 | # do something here | |
115 | } while $x++ <= $z; | |
116 | } | |
a0d0e21e LW |
117 | |
118 | =head2 Compound statements | |
119 | ||
120 | In Perl, a sequence of statements that defines a scope is called a block. | |
121 | Sometimes a block is delimited by the file containing it (in the case | |
122 | of a required file, or the program as a whole), and sometimes a block | |
123 | is delimited by the extent of a string (in the case of an eval). | |
124 | ||
125 | But generally, a block is delimited by curly brackets, also known as braces. | |
126 | We will call this syntactic construct a BLOCK. | |
127 | ||
128 | The following compound statements may be used to control flow: | |
129 | ||
130 | if (EXPR) BLOCK | |
131 | if (EXPR) BLOCK else BLOCK | |
132 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK | |
133 | LABEL while (EXPR) BLOCK | |
134 | LABEL while (EXPR) BLOCK continue BLOCK | |
135 | LABEL for (EXPR; EXPR; EXPR) BLOCK | |
748a9306 | 136 | LABEL foreach VAR (LIST) BLOCK |
b303ae78 | 137 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
a0d0e21e LW |
138 | LABEL BLOCK continue BLOCK |
139 | ||
140 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, | |
141 | not statements. This means that the curly brackets are I<required>--no | |
142 | dangling statements allowed. If you want to write conditionals without | |
143 | curly brackets there are several other ways to do it. The following | |
144 | all do the same thing: | |
145 | ||
146 | if (!open(FOO)) { die "Can't open $FOO: $!"; } | |
147 | die "Can't open $FOO: $!" unless open(FOO); | |
148 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! | |
149 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; | |
150 | # a bit exotic, that last one | |
151 | ||
5f05dabc | 152 | The C<if> statement is straightforward. Because BLOCKs are always |
a0d0e21e LW |
153 | bounded by curly brackets, there is never any ambiguity about which |
154 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
155 | the sense of the test is reversed. | |
156 | ||
157 | The C<while> statement executes the block as long as the expression is | |
f86cebdf | 158 | true (does not evaluate to the null string (C<"">) or C<0> or C<"0")>. The LABEL is |
4633a7c4 LW |
159 | optional, and if present, consists of an identifier followed by a colon. |
160 | The LABEL identifies the loop for the loop control statements C<next>, | |
161 | C<last>, and C<redo>. If the LABEL is omitted, the loop control statement | |
162 | refers to the innermost enclosing loop. This may include dynamically | |
163 | looking back your call-stack at run time to find the LABEL. Such | |
164 | desperate behavior triggers a warning if you use the B<-w> flag. | |
165 | ||
166 | If there is a C<continue> BLOCK, it is always executed just before the | |
167 | conditional is about to be evaluated again, just like the third part of a | |
168 | C<for> loop in C. Thus it can be used to increment a loop variable, even | |
169 | when the loop has been continued via the C<next> statement (which is | |
170 | similar to the C C<continue> statement). | |
171 | ||
172 | =head2 Loop Control | |
173 | ||
174 | The C<next> command is like the C<continue> statement in C; it starts | |
175 | the next iteration of the loop: | |
176 | ||
177 | LINE: while (<STDIN>) { | |
178 | next LINE if /^#/; # discard comments | |
179 | ... | |
180 | } | |
181 | ||
182 | The C<last> command is like the C<break> statement in C (as used in | |
183 | loops); it immediately exits the loop in question. The | |
184 | C<continue> block, if any, is not executed: | |
185 | ||
186 | LINE: while (<STDIN>) { | |
187 | last LINE if /^$/; # exit when done with header | |
188 | ... | |
189 | } | |
190 | ||
191 | The C<redo> command restarts the loop block without evaluating the | |
192 | conditional again. The C<continue> block, if any, is I<not> executed. | |
193 | This command is normally used by programs that want to lie to themselves | |
194 | about what was just input. | |
195 | ||
196 | For example, when processing a file like F</etc/termcap>. | |
197 | If your input lines might end in backslashes to indicate continuation, you | |
198 | want to skip ahead and get the next record. | |
199 | ||
200 | while (<>) { | |
201 | chomp; | |
54310121 | 202 | if (s/\\$//) { |
203 | $_ .= <>; | |
4633a7c4 LW |
204 | redo unless eof(); |
205 | } | |
206 | # now process $_ | |
54310121 | 207 | } |
4633a7c4 LW |
208 | |
209 | which is Perl short-hand for the more explicitly written version: | |
210 | ||
54310121 | 211 | LINE: while (defined($line = <ARGV>)) { |
4633a7c4 | 212 | chomp($line); |
54310121 | 213 | if ($line =~ s/\\$//) { |
214 | $line .= <ARGV>; | |
4633a7c4 LW |
215 | redo LINE unless eof(); # not eof(ARGV)! |
216 | } | |
217 | # now process $line | |
54310121 | 218 | } |
4633a7c4 | 219 | |
5a964f20 TC |
220 | Note that if there were a C<continue> block on the above code, it would get |
221 | executed even on discarded lines. This is often used to reset line counters | |
222 | or C<?pat?> one-time matches. | |
4633a7c4 | 223 | |
5a964f20 TC |
224 | # inspired by :1,$g/fred/s//WILMA/ |
225 | while (<>) { | |
226 | ?(fred)? && s//WILMA $1 WILMA/; | |
227 | ?(barney)? && s//BETTY $1 BETTY/; | |
228 | ?(homer)? && s//MARGE $1 MARGE/; | |
229 | } continue { | |
230 | print "$ARGV $.: $_"; | |
231 | close ARGV if eof(); # reset $. | |
232 | reset if eof(); # reset ?pat? | |
4633a7c4 LW |
233 | } |
234 | ||
a0d0e21e LW |
235 | If the word C<while> is replaced by the word C<until>, the sense of the |
236 | test is reversed, but the conditional is still tested before the first | |
237 | iteration. | |
238 | ||
5a964f20 TC |
239 | The loop control statements don't work in an C<if> or C<unless>, since |
240 | they aren't loops. You can double the braces to make them such, though. | |
241 | ||
242 | if (/pattern/) {{ | |
243 | next if /fred/; | |
244 | next if /barney/; | |
245 | # so something here | |
246 | }} | |
247 | ||
5b23ba8b MG |
248 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
249 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
4633a7c4 | 250 | |
cb1a09d0 | 251 | =head2 For Loops |
a0d0e21e | 252 | |
cb1a09d0 AD |
253 | Perl's C-style C<for> loop works exactly like the corresponding C<while> loop; |
254 | that means that this: | |
a0d0e21e LW |
255 | |
256 | for ($i = 1; $i < 10; $i++) { | |
257 | ... | |
258 | } | |
259 | ||
cb1a09d0 | 260 | is the same as this: |
a0d0e21e LW |
261 | |
262 | $i = 1; | |
263 | while ($i < 10) { | |
264 | ... | |
265 | } continue { | |
266 | $i++; | |
267 | } | |
268 | ||
55497cff | 269 | (There is one minor difference: The first form implies a lexical scope |
270 | for variables declared with C<my> in the initialization expression.) | |
271 | ||
cb1a09d0 AD |
272 | Besides the normal array index looping, C<for> can lend itself |
273 | to many other interesting applications. Here's one that avoids the | |
54310121 | 274 | problem you get into if you explicitly test for end-of-file on |
275 | an interactive file descriptor causing your program to appear to | |
cb1a09d0 AD |
276 | hang. |
277 | ||
278 | $on_a_tty = -t STDIN && -t STDOUT; | |
279 | sub prompt { print "yes? " if $on_a_tty } | |
280 | for ( prompt(); <STDIN>; prompt() ) { | |
281 | # do something | |
54310121 | 282 | } |
cb1a09d0 AD |
283 | |
284 | =head2 Foreach Loops | |
285 | ||
4633a7c4 | 286 | The C<foreach> loop iterates over a normal list value and sets the |
55497cff | 287 | variable VAR to be each element of the list in turn. If the variable |
288 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
289 | is therefore visible only within the loop. Otherwise, the variable is | |
290 | implicitly local to the loop and regains its former value upon exiting | |
291 | the loop. If the variable was previously declared with C<my>, it uses | |
292 | that variable instead of the global one, but it's still localized to | |
19799a22 | 293 | the loop. |
4633a7c4 LW |
294 | |
295 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
5a964f20 TC |
296 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
297 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> | |
f86cebdf | 298 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
5a964f20 TC |
299 | If any element of LIST is an lvalue, you can modify it by modifying VAR |
300 | inside the loop. That's because the C<foreach> loop index variable is | |
301 | an implicit alias for each item in the list that you're looping over. | |
302617ea MG |
302 | |
303 | If any part of LIST is an array, C<foreach> will get very confused if | |
304 | you add or remove elements within the loop body, for example with | |
305 | C<splice>. So don't do that. | |
306 | ||
307 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
308 | special variable. Don't do that either. | |
4633a7c4 | 309 | |
748a9306 | 310 | Examples: |
a0d0e21e | 311 | |
4633a7c4 | 312 | for (@ary) { s/foo/bar/ } |
a0d0e21e | 313 | |
55497cff | 314 | foreach my $elem (@elements) { |
a0d0e21e LW |
315 | $elem *= 2; |
316 | } | |
317 | ||
4633a7c4 LW |
318 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
319 | print $count, "\n"; sleep(1); | |
a0d0e21e LW |
320 | } |
321 | ||
322 | for (1..15) { print "Merry Christmas\n"; } | |
323 | ||
4633a7c4 | 324 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
a0d0e21e LW |
325 | print "Item: $item\n"; |
326 | } | |
327 | ||
4633a7c4 LW |
328 | Here's how a C programmer might code up a particular algorithm in Perl: |
329 | ||
55497cff | 330 | for (my $i = 0; $i < @ary1; $i++) { |
331 | for (my $j = 0; $j < @ary2; $j++) { | |
4633a7c4 LW |
332 | if ($ary1[$i] > $ary2[$j]) { |
333 | last; # can't go to outer :-( | |
334 | } | |
335 | $ary1[$i] += $ary2[$j]; | |
336 | } | |
cb1a09d0 | 337 | # this is where that last takes me |
4633a7c4 LW |
338 | } |
339 | ||
184e9718 | 340 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 | 341 | do it: |
4633a7c4 | 342 | |
54310121 | 343 | OUTER: foreach my $wid (@ary1) { |
55497cff | 344 | INNER: foreach my $jet (@ary2) { |
cb1a09d0 AD |
345 | next OUTER if $wid > $jet; |
346 | $wid += $jet; | |
54310121 | 347 | } |
348 | } | |
4633a7c4 | 349 | |
cb1a09d0 AD |
350 | See how much easier this is? It's cleaner, safer, and faster. It's |
351 | cleaner because it's less noisy. It's safer because if code gets added | |
c07a80fd | 352 | between the inner and outer loops later on, the new code won't be |
5f05dabc | 353 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd | 354 | rather than merely terminating the inner one. And it's faster because |
355 | Perl executes a C<foreach> statement more rapidly than it would the | |
356 | equivalent C<for> loop. | |
4633a7c4 LW |
357 | |
358 | =head2 Basic BLOCKs and Switch Statements | |
359 | ||
55497cff | 360 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
361 | loop that executes once. Thus you can use any of the loop control | |
362 | statements in it to leave or restart the block. (Note that this is | |
363 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
364 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
365 | block is optional. | |
4633a7c4 LW |
366 | |
367 | The BLOCK construct is particularly nice for doing case | |
a0d0e21e LW |
368 | structures. |
369 | ||
370 | SWITCH: { | |
371 | if (/^abc/) { $abc = 1; last SWITCH; } | |
372 | if (/^def/) { $def = 1; last SWITCH; } | |
373 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
374 | $nothing = 1; | |
375 | } | |
376 | ||
f86cebdf | 377 | There is no official C<switch> statement in Perl, because there are |
a0d0e21e LW |
378 | already several ways to write the equivalent. In addition to the |
379 | above, you could write | |
380 | ||
381 | SWITCH: { | |
382 | $abc = 1, last SWITCH if /^abc/; | |
383 | $def = 1, last SWITCH if /^def/; | |
384 | $xyz = 1, last SWITCH if /^xyz/; | |
385 | $nothing = 1; | |
386 | } | |
387 | ||
cb1a09d0 | 388 | (That's actually not as strange as it looks once you realize that you can |
a0d0e21e LW |
389 | use loop control "operators" within an expression, That's just the normal |
390 | C comma operator.) | |
391 | ||
392 | or | |
393 | ||
394 | SWITCH: { | |
395 | /^abc/ && do { $abc = 1; last SWITCH; }; | |
396 | /^def/ && do { $def = 1; last SWITCH; }; | |
397 | /^xyz/ && do { $xyz = 1; last SWITCH; }; | |
398 | $nothing = 1; | |
399 | } | |
400 | ||
f86cebdf | 401 | or formatted so it stands out more as a "proper" C<switch> statement: |
a0d0e21e LW |
402 | |
403 | SWITCH: { | |
54310121 | 404 | /^abc/ && do { |
405 | $abc = 1; | |
406 | last SWITCH; | |
a0d0e21e LW |
407 | }; |
408 | ||
54310121 | 409 | /^def/ && do { |
410 | $def = 1; | |
411 | last SWITCH; | |
a0d0e21e LW |
412 | }; |
413 | ||
54310121 | 414 | /^xyz/ && do { |
415 | $xyz = 1; | |
416 | last SWITCH; | |
a0d0e21e LW |
417 | }; |
418 | $nothing = 1; | |
419 | } | |
420 | ||
421 | or | |
422 | ||
423 | SWITCH: { | |
424 | /^abc/ and $abc = 1, last SWITCH; | |
425 | /^def/ and $def = 1, last SWITCH; | |
426 | /^xyz/ and $xyz = 1, last SWITCH; | |
427 | $nothing = 1; | |
428 | } | |
429 | ||
430 | or even, horrors, | |
431 | ||
432 | if (/^abc/) | |
433 | { $abc = 1 } | |
434 | elsif (/^def/) | |
435 | { $def = 1 } | |
436 | elsif (/^xyz/) | |
437 | { $xyz = 1 } | |
438 | else | |
439 | { $nothing = 1 } | |
440 | ||
f86cebdf GS |
441 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make |
442 | a temporary assignment to C<$_> for convenient matching: | |
4633a7c4 LW |
443 | |
444 | SWITCH: for ($where) { | |
445 | /In Card Names/ && do { push @flags, '-e'; last; }; | |
446 | /Anywhere/ && do { push @flags, '-h'; last; }; | |
447 | /In Rulings/ && do { last; }; | |
448 | die "unknown value for form variable where: `$where'"; | |
54310121 | 449 | } |
4633a7c4 | 450 | |
cb1a09d0 AD |
451 | Another interesting approach to a switch statement is arrange |
452 | for a C<do> block to return the proper value: | |
453 | ||
454 | $amode = do { | |
5a964f20 | 455 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? |
54310121 | 456 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
cb1a09d0 AD |
457 | elsif ($flag & O_RDWR) { |
458 | if ($flag & O_CREAT) { "w+" } | |
c07a80fd | 459 | else { ($flag & O_APPEND) ? "a+" : "r+" } |
cb1a09d0 AD |
460 | } |
461 | }; | |
462 | ||
5a964f20 TC |
463 | Or |
464 | ||
465 | print do { | |
466 | ($flags & O_WRONLY) ? "write-only" : | |
467 | ($flags & O_RDWR) ? "read-write" : | |
468 | "read-only"; | |
469 | }; | |
470 | ||
471 | Or if you are certainly that all the C<&&> clauses are true, you can use | |
472 | something like this, which "switches" on the value of the | |
f86cebdf | 473 | C<HTTP_USER_AGENT> envariable. |
5a964f20 TC |
474 | |
475 | #!/usr/bin/perl | |
476 | # pick out jargon file page based on browser | |
477 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; | |
478 | for ($ENV{HTTP_USER_AGENT}) { | |
479 | $page = /Mac/ && 'm/Macintrash.html' | |
480 | || /Win(dows )?NT/ && 'e/evilandrude.html' | |
481 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' | |
482 | || /Linux/ && 'l/Linux.html' | |
483 | || /HP-UX/ && 'h/HP-SUX.html' | |
484 | || /SunOS/ && 's/ScumOS.html' | |
485 | || 'a/AppendixB.html'; | |
486 | } | |
487 | print "Location: $dir/$page\015\012\015\012"; | |
488 | ||
489 | That kind of switch statement only works when you know the C<&&> clauses | |
490 | will be true. If you don't, the previous C<?:> example should be used. | |
491 | ||
19799a22 GS |
492 | You might also consider writing a hash of subroutine references |
493 | instead of synthesizing a C<switch> statement. | |
5a964f20 | 494 | |
4633a7c4 LW |
495 | =head2 Goto |
496 | ||
19799a22 GS |
497 | Although not for the faint of heart, Perl does support a C<goto> |
498 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
499 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
500 | a C<goto>; it's just the name of the loop. | |
4633a7c4 | 501 | |
f86cebdf | 502 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 | 503 | execution there. It may not be used to go into any construct that |
f86cebdf | 504 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 LW |
505 | also can't be used to go into a construct that is optimized away. It |
506 | can be used to go almost anywhere else within the dynamic scope, | |
507 | including out of subroutines, but it's usually better to use some other | |
f86cebdf GS |
508 | construct such as C<last> or C<die>. The author of Perl has never felt the |
509 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
4633a7c4 | 510 | |
f86cebdf GS |
511 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
512 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
4633a7c4 LW |
513 | necessarily recommended if you're optimizing for maintainability: |
514 | ||
515 | goto ("FOO", "BAR", "GLARCH")[$i]; | |
516 | ||
f86cebdf | 517 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 | 518 | named subroutine for the currently running subroutine. This is used by |
f86cebdf | 519 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 | 520 | pretend that the other subroutine had been called in the first place |
f86cebdf GS |
521 | (except that any modifications to C<@_> in the current subroutine are |
522 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
4633a7c4 LW |
523 | will be able to tell that this routine was called first. |
524 | ||
c07a80fd | 525 | In almost all cases like this, it's usually a far, far better idea to use the |
526 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
4633a7c4 LW |
527 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
528 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
cb1a09d0 AD |
529 | |
530 | =head2 PODs: Embedded Documentation | |
531 | ||
532 | Perl has a mechanism for intermixing documentation with source code. | |
c07a80fd | 533 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 AD |
534 | encounters a line that begins with an equal sign and a word, like this |
535 | ||
536 | =head1 Here There Be Pods! | |
537 | ||
538 | Then that text and all remaining text up through and including a line | |
539 | beginning with C<=cut> will be ignored. The format of the intervening | |
54310121 | 540 | text is described in L<perlpod>. |
cb1a09d0 AD |
541 | |
542 | This allows you to intermix your source code | |
543 | and your documentation text freely, as in | |
544 | ||
545 | =item snazzle($) | |
546 | ||
54310121 | 547 | The snazzle() function will behave in the most spectacular |
cb1a09d0 AD |
548 | form that you can possibly imagine, not even excepting |
549 | cybernetic pyrotechnics. | |
550 | ||
551 | =cut back to the compiler, nuff of this pod stuff! | |
552 | ||
553 | sub snazzle($) { | |
554 | my $thingie = shift; | |
555 | ......... | |
54310121 | 556 | } |
cb1a09d0 | 557 | |
54310121 | 558 | Note that pod translators should look at only paragraphs beginning |
184e9718 | 559 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 | 560 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 AD |
561 | paragraph. This means that the following secret stuff will be |
562 | ignored by both the compiler and the translators. | |
563 | ||
564 | $a=3; | |
565 | =secret stuff | |
566 | warn "Neither POD nor CODE!?" | |
567 | =cut back | |
568 | print "got $a\n"; | |
569 | ||
f86cebdf | 570 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 AD |
571 | Not all pod translators are well-behaved in this regard, and perhaps |
572 | the compiler will become pickier. | |
774d564b | 573 | |
574 | One may also use pod directives to quickly comment out a section | |
575 | of code. | |
576 | ||
577 | =head2 Plain Old Comments (Not!) | |
578 | ||
5a964f20 TC |
579 | Much like the C preprocessor, Perl can process line directives. Using |
580 | this, one can control Perl's idea of filenames and line numbers in | |
774d564b | 581 | error or warning messages (especially for strings that are processed |
f86cebdf | 582 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b | 583 | C preprocessors: it matches the regular expression |
4b094ceb | 584 | C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line |
774d564b | 585 | number for the next line, and C<$2> being the optional filename |
586 | (specified within quotes). | |
587 | ||
588 | Here are some examples that you should be able to type into your command | |
589 | shell: | |
590 | ||
591 | % perl | |
592 | # line 200 "bzzzt" | |
593 | # the `#' on the previous line must be the first char on line | |
594 | die 'foo'; | |
595 | __END__ | |
596 | foo at bzzzt line 201. | |
54310121 | 597 | |
774d564b | 598 | % perl |
599 | # line 200 "bzzzt" | |
600 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
601 | __END__ | |
602 | foo at - line 2001. | |
54310121 | 603 | |
774d564b | 604 | % perl |
605 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
606 | __END__ | |
607 | foo at foo bar line 200. | |
54310121 | 608 | |
774d564b | 609 | % perl |
610 | # line 345 "goop" | |
611 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
612 | print $@; | |
613 | __END__ | |
614 | foo at goop line 345. | |
615 | ||
616 | =cut |