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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. | |
0b8d69e9 GS |
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>.) | |
4633a7c4 | 16 | |
f00f6914 GS |
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. | |
a0d0e21e | 24 | |
0b8d69e9 GS |
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 ("don't-care") | |
37 | contexts and operators such as C<++>, C<-->, C<+=>, C<-=>, and | |
38 | C<.=> are always exempt from such warnings. | |
39 | ||
a0d0e21e LW |
40 | A declaration can be put anywhere a statement can, but has no effect on |
41 | the execution of the primary sequence of statements--declarations all | |
42 | take effect at compile time. Typically all the declarations are put at | |
54310121 | 43 | the beginning or the end of the script. However, if you're using |
0b8d69e9 GS |
44 | lexically-scoped private variables created with C<my()>, you'll |
45 | have to make sure | |
4633a7c4 | 46 | your format or subroutine definition is within the same block scope |
5f05dabc | 47 | as the my if you expect to be able to access those private variables. |
a0d0e21e | 48 | |
4633a7c4 LW |
49 | Declaring a subroutine allows a subroutine name to be used as if it were a |
50 | list operator from that point forward in the program. You can declare a | |
54310121 | 51 | subroutine without defining it by saying C<sub name>, thus: |
a0d0e21e | 52 | |
54310121 | 53 | sub myname; |
a0d0e21e LW |
54 | $me = myname $0 or die "can't get myname"; |
55 | ||
0dff5aa1 GS |
56 | Note that myname() functions as a list operator, not as a unary operator; |
57 | so be careful to use C<or> instead of C<||> in this case. However, if | |
54310121 | 58 | you were to declare the subroutine as C<sub myname ($)>, then |
02c45c47 | 59 | C<myname> would function as a unary operator, so either C<or> or |
54310121 | 60 | C<||> would work. |
a0d0e21e | 61 | |
4633a7c4 LW |
62 | Subroutines declarations can also be loaded up with the C<require> statement |
63 | or both loaded and imported into your namespace with a C<use> statement. | |
64 | See L<perlmod> for details on this. | |
a0d0e21e | 65 | |
4633a7c4 LW |
66 | A statement sequence may contain declarations of lexically-scoped |
67 | variables, but apart from declaring a variable name, the declaration acts | |
68 | like an ordinary statement, and is elaborated within the sequence of | |
69 | statements as if it were an ordinary statement. That means it actually | |
70 | has both compile-time and run-time effects. | |
a0d0e21e LW |
71 | |
72 | =head2 Simple statements | |
73 | ||
74 | The only kind of simple statement is an expression evaluated for its | |
75 | side effects. Every simple statement must be terminated with a | |
76 | semicolon, unless it is the final statement in a block, in which case | |
77 | the semicolon is optional. (A semicolon is still encouraged there if the | |
5f05dabc | 78 | block takes up more than one line, because you may eventually add another line.) |
a0d0e21e | 79 | Note that there are some operators like C<eval {}> and C<do {}> that look |
54310121 | 80 | like compound statements, but aren't (they're just TERMs in an expression), |
4633a7c4 | 81 | and thus need an explicit termination if used as the last item in a statement. |
a0d0e21e LW |
82 | |
83 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
84 | just before the terminating semicolon (or block ending). The possible | |
85 | modifiers are: | |
86 | ||
87 | if EXPR | |
88 | unless EXPR | |
89 | while EXPR | |
90 | until EXPR | |
ecca16b0 | 91 | foreach EXPR |
a0d0e21e LW |
92 | |
93 | The C<if> and C<unless> modifiers have the expected semantics, | |
ecca16b0 | 94 | presuming you're a speaker of English. The C<foreach> modifier is an |
f86cebdf | 95 | iterator: For each value in EXPR, it aliases C<$_> to the value and |
ecca16b0 | 96 | executes the statement. The C<while> and C<until> modifiers have the |
f86cebdf | 97 | usual "C<while> loop" semantics (conditional evaluated first), except |
19799a22 | 98 | when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE |
ecca16b0 CS |
99 | statement), in which case the block executes once before the |
100 | conditional is evaluated. This is so that you can write loops like: | |
a0d0e21e LW |
101 | |
102 | do { | |
4633a7c4 | 103 | $line = <STDIN>; |
a0d0e21e | 104 | ... |
4633a7c4 | 105 | } until $line eq ".\n"; |
a0d0e21e | 106 | |
5a964f20 TC |
107 | See L<perlfunc/do>. Note also that the loop control statements described |
108 | later will I<NOT> work in this construct, because modifiers don't take | |
109 | loop labels. Sorry. You can always put another block inside of it | |
110 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
f86cebdf | 111 | For C<next>, just double the braces: |
5a964f20 TC |
112 | |
113 | do {{ | |
114 | next if $x == $y; | |
115 | # do something here | |
116 | }} until $x++ > $z; | |
117 | ||
f86cebdf | 118 | For C<last>, you have to be more elaborate: |
5a964f20 TC |
119 | |
120 | LOOP: { | |
121 | do { | |
122 | last if $x = $y**2; | |
123 | # do something here | |
124 | } while $x++ <= $z; | |
125 | } | |
a0d0e21e LW |
126 | |
127 | =head2 Compound statements | |
128 | ||
129 | In Perl, a sequence of statements that defines a scope is called a block. | |
130 | Sometimes a block is delimited by the file containing it (in the case | |
131 | of a required file, or the program as a whole), and sometimes a block | |
132 | is delimited by the extent of a string (in the case of an eval). | |
133 | ||
134 | But generally, a block is delimited by curly brackets, also known as braces. | |
135 | We will call this syntactic construct a BLOCK. | |
136 | ||
137 | The following compound statements may be used to control flow: | |
138 | ||
139 | if (EXPR) BLOCK | |
140 | if (EXPR) BLOCK else BLOCK | |
141 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK | |
142 | LABEL while (EXPR) BLOCK | |
143 | LABEL while (EXPR) BLOCK continue BLOCK | |
144 | LABEL for (EXPR; EXPR; EXPR) BLOCK | |
748a9306 | 145 | LABEL foreach VAR (LIST) BLOCK |
b303ae78 | 146 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
a0d0e21e LW |
147 | LABEL BLOCK continue BLOCK |
148 | ||
149 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, | |
150 | not statements. This means that the curly brackets are I<required>--no | |
151 | dangling statements allowed. If you want to write conditionals without | |
152 | curly brackets there are several other ways to do it. The following | |
153 | all do the same thing: | |
154 | ||
155 | if (!open(FOO)) { die "Can't open $FOO: $!"; } | |
156 | die "Can't open $FOO: $!" unless open(FOO); | |
157 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! | |
158 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; | |
159 | # a bit exotic, that last one | |
160 | ||
5f05dabc | 161 | The C<if> statement is straightforward. Because BLOCKs are always |
a0d0e21e LW |
162 | bounded by curly brackets, there is never any ambiguity about which |
163 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
164 | the sense of the test is reversed. | |
165 | ||
166 | The C<while> statement executes the block as long as the expression is | |
0eb389d5 | 167 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). |
b78218b7 GS |
168 | The LABEL is optional, and if present, consists of an identifier followed |
169 | by a colon. The LABEL identifies the loop for the loop control | |
170 | statements C<next>, C<last>, and C<redo>. | |
171 | If the LABEL is omitted, the loop control statement | |
4633a7c4 LW |
172 | refers to the innermost enclosing loop. This may include dynamically |
173 | looking back your call-stack at run time to find the LABEL. Such | |
9f1b1f2d | 174 | desperate behavior triggers a warning if you use the C<use warnings> |
a2293a43 | 175 | pragma or the B<-w> flag. |
3ce0d271 GS |
176 | Unlike a C<foreach> statement, a C<while> statement never implicitly |
177 | localises any variables. | |
4633a7c4 LW |
178 | |
179 | If there is a C<continue> BLOCK, it is always executed just before the | |
180 | conditional is about to be evaluated again, just like the third part of a | |
181 | C<for> loop in C. Thus it can be used to increment a loop variable, even | |
182 | when the loop has been continued via the C<next> statement (which is | |
183 | similar to the C C<continue> statement). | |
184 | ||
185 | =head2 Loop Control | |
186 | ||
187 | The C<next> command is like the C<continue> statement in C; it starts | |
188 | the next iteration of the loop: | |
189 | ||
190 | LINE: while (<STDIN>) { | |
191 | next LINE if /^#/; # discard comments | |
192 | ... | |
193 | } | |
194 | ||
195 | The C<last> command is like the C<break> statement in C (as used in | |
196 | loops); it immediately exits the loop in question. The | |
197 | C<continue> block, if any, is not executed: | |
198 | ||
199 | LINE: while (<STDIN>) { | |
200 | last LINE if /^$/; # exit when done with header | |
201 | ... | |
202 | } | |
203 | ||
204 | The C<redo> command restarts the loop block without evaluating the | |
205 | conditional again. The C<continue> block, if any, is I<not> executed. | |
206 | This command is normally used by programs that want to lie to themselves | |
207 | about what was just input. | |
208 | ||
209 | For example, when processing a file like F</etc/termcap>. | |
210 | If your input lines might end in backslashes to indicate continuation, you | |
211 | want to skip ahead and get the next record. | |
212 | ||
213 | while (<>) { | |
214 | chomp; | |
54310121 | 215 | if (s/\\$//) { |
216 | $_ .= <>; | |
4633a7c4 LW |
217 | redo unless eof(); |
218 | } | |
219 | # now process $_ | |
54310121 | 220 | } |
4633a7c4 LW |
221 | |
222 | which is Perl short-hand for the more explicitly written version: | |
223 | ||
54310121 | 224 | LINE: while (defined($line = <ARGV>)) { |
4633a7c4 | 225 | chomp($line); |
54310121 | 226 | if ($line =~ s/\\$//) { |
227 | $line .= <ARGV>; | |
4633a7c4 LW |
228 | redo LINE unless eof(); # not eof(ARGV)! |
229 | } | |
230 | # now process $line | |
54310121 | 231 | } |
4633a7c4 | 232 | |
5a964f20 TC |
233 | Note that if there were a C<continue> block on the above code, it would get |
234 | executed even on discarded lines. This is often used to reset line counters | |
235 | or C<?pat?> one-time matches. | |
4633a7c4 | 236 | |
5a964f20 TC |
237 | # inspired by :1,$g/fred/s//WILMA/ |
238 | while (<>) { | |
239 | ?(fred)? && s//WILMA $1 WILMA/; | |
240 | ?(barney)? && s//BETTY $1 BETTY/; | |
241 | ?(homer)? && s//MARGE $1 MARGE/; | |
242 | } continue { | |
243 | print "$ARGV $.: $_"; | |
244 | close ARGV if eof(); # reset $. | |
245 | reset if eof(); # reset ?pat? | |
4633a7c4 LW |
246 | } |
247 | ||
a0d0e21e LW |
248 | If the word C<while> is replaced by the word C<until>, the sense of the |
249 | test is reversed, but the conditional is still tested before the first | |
250 | iteration. | |
251 | ||
5a964f20 TC |
252 | The loop control statements don't work in an C<if> or C<unless>, since |
253 | they aren't loops. You can double the braces to make them such, though. | |
254 | ||
255 | if (/pattern/) {{ | |
256 | next if /fred/; | |
257 | next if /barney/; | |
258 | # so something here | |
259 | }} | |
260 | ||
5b23ba8b MG |
261 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
262 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
4633a7c4 | 263 | |
cb1a09d0 | 264 | =head2 For Loops |
a0d0e21e | 265 | |
cb1a09d0 AD |
266 | Perl's C-style C<for> loop works exactly like the corresponding C<while> loop; |
267 | that means that this: | |
a0d0e21e LW |
268 | |
269 | for ($i = 1; $i < 10; $i++) { | |
270 | ... | |
271 | } | |
272 | ||
cb1a09d0 | 273 | is the same as this: |
a0d0e21e LW |
274 | |
275 | $i = 1; | |
276 | while ($i < 10) { | |
277 | ... | |
278 | } continue { | |
279 | $i++; | |
280 | } | |
281 | ||
55497cff | 282 | (There is one minor difference: The first form implies a lexical scope |
283 | for variables declared with C<my> in the initialization expression.) | |
284 | ||
cb1a09d0 AD |
285 | Besides the normal array index looping, C<for> can lend itself |
286 | to many other interesting applications. Here's one that avoids the | |
54310121 | 287 | problem you get into if you explicitly test for end-of-file on |
288 | an interactive file descriptor causing your program to appear to | |
cb1a09d0 AD |
289 | hang. |
290 | ||
291 | $on_a_tty = -t STDIN && -t STDOUT; | |
292 | sub prompt { print "yes? " if $on_a_tty } | |
293 | for ( prompt(); <STDIN>; prompt() ) { | |
294 | # do something | |
54310121 | 295 | } |
cb1a09d0 AD |
296 | |
297 | =head2 Foreach Loops | |
298 | ||
4633a7c4 | 299 | The C<foreach> loop iterates over a normal list value and sets the |
55497cff | 300 | variable VAR to be each element of the list in turn. If the variable |
301 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
302 | is therefore visible only within the loop. Otherwise, the variable is | |
303 | implicitly local to the loop and regains its former value upon exiting | |
304 | the loop. If the variable was previously declared with C<my>, it uses | |
305 | that variable instead of the global one, but it's still localized to | |
19799a22 | 306 | the loop. |
4633a7c4 LW |
307 | |
308 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
5a964f20 TC |
309 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
310 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> | |
f86cebdf | 311 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
3a6f1039 GS |
312 | |
313 | If any element of LIST is an lvalue, you can modify it by modifying | |
314 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
315 | lvalue, any attempt to modify that element will fail. In other words, | |
316 | the C<foreach> loop index variable is an implicit alias for each item | |
317 | in the list that you're looping over. | |
302617ea MG |
318 | |
319 | If any part of LIST is an array, C<foreach> will get very confused if | |
320 | you add or remove elements within the loop body, for example with | |
321 | C<splice>. So don't do that. | |
322 | ||
323 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
324 | special variable. Don't do that either. | |
4633a7c4 | 325 | |
748a9306 | 326 | Examples: |
a0d0e21e | 327 | |
4633a7c4 | 328 | for (@ary) { s/foo/bar/ } |
a0d0e21e | 329 | |
96f2dc66 | 330 | for my $elem (@elements) { |
a0d0e21e LW |
331 | $elem *= 2; |
332 | } | |
333 | ||
4633a7c4 LW |
334 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
335 | print $count, "\n"; sleep(1); | |
a0d0e21e LW |
336 | } |
337 | ||
338 | for (1..15) { print "Merry Christmas\n"; } | |
339 | ||
4633a7c4 | 340 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
a0d0e21e LW |
341 | print "Item: $item\n"; |
342 | } | |
343 | ||
4633a7c4 LW |
344 | Here's how a C programmer might code up a particular algorithm in Perl: |
345 | ||
55497cff | 346 | for (my $i = 0; $i < @ary1; $i++) { |
347 | for (my $j = 0; $j < @ary2; $j++) { | |
4633a7c4 LW |
348 | if ($ary1[$i] > $ary2[$j]) { |
349 | last; # can't go to outer :-( | |
350 | } | |
351 | $ary1[$i] += $ary2[$j]; | |
352 | } | |
cb1a09d0 | 353 | # this is where that last takes me |
4633a7c4 LW |
354 | } |
355 | ||
184e9718 | 356 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 | 357 | do it: |
4633a7c4 | 358 | |
96f2dc66 GS |
359 | OUTER: for my $wid (@ary1) { |
360 | INNER: for my $jet (@ary2) { | |
cb1a09d0 AD |
361 | next OUTER if $wid > $jet; |
362 | $wid += $jet; | |
54310121 | 363 | } |
364 | } | |
4633a7c4 | 365 | |
cb1a09d0 AD |
366 | See how much easier this is? It's cleaner, safer, and faster. It's |
367 | cleaner because it's less noisy. It's safer because if code gets added | |
c07a80fd | 368 | between the inner and outer loops later on, the new code won't be |
5f05dabc | 369 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd | 370 | rather than merely terminating the inner one. And it's faster because |
371 | Perl executes a C<foreach> statement more rapidly than it would the | |
372 | equivalent C<for> loop. | |
4633a7c4 LW |
373 | |
374 | =head2 Basic BLOCKs and Switch Statements | |
375 | ||
55497cff | 376 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
377 | loop that executes once. Thus you can use any of the loop control | |
378 | statements in it to leave or restart the block. (Note that this is | |
379 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
380 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
381 | block is optional. | |
4633a7c4 LW |
382 | |
383 | The BLOCK construct is particularly nice for doing case | |
a0d0e21e LW |
384 | structures. |
385 | ||
386 | SWITCH: { | |
387 | if (/^abc/) { $abc = 1; last SWITCH; } | |
388 | if (/^def/) { $def = 1; last SWITCH; } | |
389 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
390 | $nothing = 1; | |
391 | } | |
392 | ||
f86cebdf | 393 | There is no official C<switch> statement in Perl, because there are |
a0d0e21e LW |
394 | already several ways to write the equivalent. In addition to the |
395 | above, you could write | |
396 | ||
397 | SWITCH: { | |
398 | $abc = 1, last SWITCH if /^abc/; | |
399 | $def = 1, last SWITCH if /^def/; | |
400 | $xyz = 1, last SWITCH if /^xyz/; | |
401 | $nothing = 1; | |
402 | } | |
403 | ||
cb1a09d0 | 404 | (That's actually not as strange as it looks once you realize that you can |
a0d0e21e LW |
405 | use loop control "operators" within an expression, That's just the normal |
406 | C comma operator.) | |
407 | ||
408 | or | |
409 | ||
410 | SWITCH: { | |
411 | /^abc/ && do { $abc = 1; last SWITCH; }; | |
412 | /^def/ && do { $def = 1; last SWITCH; }; | |
413 | /^xyz/ && do { $xyz = 1; last SWITCH; }; | |
414 | $nothing = 1; | |
415 | } | |
416 | ||
f86cebdf | 417 | or formatted so it stands out more as a "proper" C<switch> statement: |
a0d0e21e LW |
418 | |
419 | SWITCH: { | |
54310121 | 420 | /^abc/ && do { |
421 | $abc = 1; | |
422 | last SWITCH; | |
a0d0e21e LW |
423 | }; |
424 | ||
54310121 | 425 | /^def/ && do { |
426 | $def = 1; | |
427 | last SWITCH; | |
a0d0e21e LW |
428 | }; |
429 | ||
54310121 | 430 | /^xyz/ && do { |
431 | $xyz = 1; | |
432 | last SWITCH; | |
a0d0e21e LW |
433 | }; |
434 | $nothing = 1; | |
435 | } | |
436 | ||
437 | or | |
438 | ||
439 | SWITCH: { | |
440 | /^abc/ and $abc = 1, last SWITCH; | |
441 | /^def/ and $def = 1, last SWITCH; | |
442 | /^xyz/ and $xyz = 1, last SWITCH; | |
443 | $nothing = 1; | |
444 | } | |
445 | ||
446 | or even, horrors, | |
447 | ||
448 | if (/^abc/) | |
449 | { $abc = 1 } | |
450 | elsif (/^def/) | |
451 | { $def = 1 } | |
452 | elsif (/^xyz/) | |
453 | { $xyz = 1 } | |
454 | else | |
455 | { $nothing = 1 } | |
456 | ||
f86cebdf GS |
457 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make |
458 | a temporary assignment to C<$_> for convenient matching: | |
4633a7c4 LW |
459 | |
460 | SWITCH: for ($where) { | |
461 | /In Card Names/ && do { push @flags, '-e'; last; }; | |
462 | /Anywhere/ && do { push @flags, '-h'; last; }; | |
463 | /In Rulings/ && do { last; }; | |
464 | die "unknown value for form variable where: `$where'"; | |
54310121 | 465 | } |
4633a7c4 | 466 | |
cb1a09d0 AD |
467 | Another interesting approach to a switch statement is arrange |
468 | for a C<do> block to return the proper value: | |
469 | ||
470 | $amode = do { | |
5a964f20 | 471 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? |
54310121 | 472 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
cb1a09d0 AD |
473 | elsif ($flag & O_RDWR) { |
474 | if ($flag & O_CREAT) { "w+" } | |
c07a80fd | 475 | else { ($flag & O_APPEND) ? "a+" : "r+" } |
cb1a09d0 AD |
476 | } |
477 | }; | |
478 | ||
5a964f20 TC |
479 | Or |
480 | ||
481 | print do { | |
482 | ($flags & O_WRONLY) ? "write-only" : | |
483 | ($flags & O_RDWR) ? "read-write" : | |
484 | "read-only"; | |
485 | }; | |
486 | ||
487 | Or if you are certainly that all the C<&&> clauses are true, you can use | |
488 | something like this, which "switches" on the value of the | |
a2293a43 | 489 | C<HTTP_USER_AGENT> environment variable. |
5a964f20 TC |
490 | |
491 | #!/usr/bin/perl | |
492 | # pick out jargon file page based on browser | |
493 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; | |
494 | for ($ENV{HTTP_USER_AGENT}) { | |
495 | $page = /Mac/ && 'm/Macintrash.html' | |
496 | || /Win(dows )?NT/ && 'e/evilandrude.html' | |
497 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' | |
498 | || /Linux/ && 'l/Linux.html' | |
499 | || /HP-UX/ && 'h/HP-SUX.html' | |
500 | || /SunOS/ && 's/ScumOS.html' | |
501 | || 'a/AppendixB.html'; | |
502 | } | |
503 | print "Location: $dir/$page\015\012\015\012"; | |
504 | ||
505 | That kind of switch statement only works when you know the C<&&> clauses | |
506 | will be true. If you don't, the previous C<?:> example should be used. | |
507 | ||
19799a22 GS |
508 | You might also consider writing a hash of subroutine references |
509 | instead of synthesizing a C<switch> statement. | |
5a964f20 | 510 | |
4633a7c4 LW |
511 | =head2 Goto |
512 | ||
19799a22 GS |
513 | Although not for the faint of heart, Perl does support a C<goto> |
514 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
515 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
516 | a C<goto>; it's just the name of the loop. | |
4633a7c4 | 517 | |
f86cebdf | 518 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 | 519 | execution there. It may not be used to go into any construct that |
f86cebdf | 520 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 LW |
521 | also can't be used to go into a construct that is optimized away. It |
522 | can be used to go almost anywhere else within the dynamic scope, | |
523 | including out of subroutines, but it's usually better to use some other | |
f86cebdf GS |
524 | construct such as C<last> or C<die>. The author of Perl has never felt the |
525 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
4633a7c4 | 526 | |
f86cebdf GS |
527 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
528 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
4633a7c4 LW |
529 | necessarily recommended if you're optimizing for maintainability: |
530 | ||
96f2dc66 | 531 | goto(("FOO", "BAR", "GLARCH")[$i]); |
4633a7c4 | 532 | |
f86cebdf | 533 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 | 534 | named subroutine for the currently running subroutine. This is used by |
f86cebdf | 535 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 | 536 | pretend that the other subroutine had been called in the first place |
f86cebdf GS |
537 | (except that any modifications to C<@_> in the current subroutine are |
538 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
4633a7c4 LW |
539 | will be able to tell that this routine was called first. |
540 | ||
c07a80fd | 541 | In almost all cases like this, it's usually a far, far better idea to use the |
542 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
4633a7c4 LW |
543 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
544 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
cb1a09d0 AD |
545 | |
546 | =head2 PODs: Embedded Documentation | |
547 | ||
548 | Perl has a mechanism for intermixing documentation with source code. | |
c07a80fd | 549 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 AD |
550 | encounters a line that begins with an equal sign and a word, like this |
551 | ||
552 | =head1 Here There Be Pods! | |
553 | ||
554 | Then that text and all remaining text up through and including a line | |
555 | beginning with C<=cut> will be ignored. The format of the intervening | |
54310121 | 556 | text is described in L<perlpod>. |
cb1a09d0 AD |
557 | |
558 | This allows you to intermix your source code | |
559 | and your documentation text freely, as in | |
560 | ||
561 | =item snazzle($) | |
562 | ||
54310121 | 563 | The snazzle() function will behave in the most spectacular |
cb1a09d0 AD |
564 | form that you can possibly imagine, not even excepting |
565 | cybernetic pyrotechnics. | |
566 | ||
567 | =cut back to the compiler, nuff of this pod stuff! | |
568 | ||
569 | sub snazzle($) { | |
570 | my $thingie = shift; | |
571 | ......... | |
54310121 | 572 | } |
cb1a09d0 | 573 | |
54310121 | 574 | Note that pod translators should look at only paragraphs beginning |
184e9718 | 575 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 | 576 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 AD |
577 | paragraph. This means that the following secret stuff will be |
578 | ignored by both the compiler and the translators. | |
579 | ||
580 | $a=3; | |
581 | =secret stuff | |
582 | warn "Neither POD nor CODE!?" | |
583 | =cut back | |
584 | print "got $a\n"; | |
585 | ||
f86cebdf | 586 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 AD |
587 | Not all pod translators are well-behaved in this regard, and perhaps |
588 | the compiler will become pickier. | |
774d564b | 589 | |
590 | One may also use pod directives to quickly comment out a section | |
591 | of code. | |
592 | ||
593 | =head2 Plain Old Comments (Not!) | |
594 | ||
5a964f20 TC |
595 | Much like the C preprocessor, Perl can process line directives. Using |
596 | this, one can control Perl's idea of filenames and line numbers in | |
774d564b | 597 | error or warning messages (especially for strings that are processed |
f86cebdf | 598 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b | 599 | C preprocessors: it matches the regular expression |
73659bf1 | 600 | C</^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/> with C<$1> being the line |
774d564b | 601 | number for the next line, and C<$2> being the optional filename |
602 | (specified within quotes). | |
603 | ||
8e3036a1 GS |
604 | There is a fairly obvious gotcha included with the line directive: |
605 | Debuggers and profilers will only show the last source line to appear | |
606 | at a particular line number in a given file. Care should be taken not | |
607 | to cause line number collisions in code you'd like to debug later. | |
608 | ||
774d564b | 609 | Here are some examples that you should be able to type into your command |
610 | shell: | |
611 | ||
612 | % perl | |
613 | # line 200 "bzzzt" | |
614 | # the `#' on the previous line must be the first char on line | |
615 | die 'foo'; | |
616 | __END__ | |
617 | foo at bzzzt line 201. | |
54310121 | 618 | |
774d564b | 619 | % perl |
620 | # line 200 "bzzzt" | |
621 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
622 | __END__ | |
623 | foo at - line 2001. | |
54310121 | 624 | |
774d564b | 625 | % perl |
626 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
627 | __END__ | |
628 | foo at foo bar line 200. | |
54310121 | 629 | |
774d564b | 630 | % perl |
631 | # line 345 "goop" | |
632 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
633 | print $@; | |
634 | __END__ | |
635 | foo at goop line 345. | |
636 | ||
637 | =cut |