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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlsyn - Perl syntax | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
6014d0cb MS |
7 | A Perl program consists of a sequence of declarations and statements |
8 | which run from the top to the bottom. Loops, subroutines and other | |
9 | control structures allow you to jump around within the code. | |
10 | ||
11 | Perl is a B<free-form> language, you can format and indent it however | |
12 | you like. Whitespace mostly serves to separate tokens, unlike | |
13 | languages like Python where it is an important part of the syntax. | |
14 | ||
15 | Many of Perl's syntactic elements are B<optional>. Rather than | |
110b9c83 | 16 | requiring you to put parentheses around every function call and |
6014d0cb MS |
17 | declare every variable, you can often leave such explicit elements off |
18 | and Perl will figure out what you meant. This is known as B<Do What I | |
19 | Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to | |
110b9c83 | 20 | code in a style with which they are comfortable. |
6014d0cb MS |
21 | |
22 | Perl B<borrows syntax> and concepts from many languages: awk, sed, C, | |
23 | Bourne Shell, Smalltalk, Lisp and even English. Other | |
24 | languages have borrowed syntax from Perl, particularly its regular | |
25 | expression extensions. So if you have programmed in another language | |
26 | you will see familiar pieces in Perl. They often work the same, but | |
27 | see L<perltrap> for information about how they differ. | |
a0d0e21e | 28 | |
0b8d69e9 GS |
29 | =head2 Declarations |
30 | ||
31 | The only things you need to declare in Perl are report formats | |
32 | and subroutines--and even undefined subroutines can be handled | |
33 | through AUTOLOAD. A variable holds the undefined value (C<undef>) | |
34 | until it has been assigned a defined value, which is anything | |
35 | other than C<undef>. When used as a number, C<undef> is treated | |
36 | as C<0>; when used as a string, it is treated the empty string, | |
37 | C<"">; and when used as a reference that isn't being assigned | |
38 | to, it is treated as an error. If you enable warnings, you'll | |
39 | be notified of an uninitialized value whenever you treat C<undef> | |
a6b1f6d8 | 40 | as a string or a number. Well, usually. Boolean contexts, such as: |
7bd1983c EM |
41 | |
42 | my $a; | |
43 | if ($a) {} | |
44 | ||
a6b1f6d8 RGS |
45 | are exempt from warnings (because they care about truth rather than |
46 | definedness). Operators such as C<++>, C<-->, C<+=>, | |
7bd1983c EM |
47 | C<-=>, and C<.=>, that operate on undefined left values such as: |
48 | ||
49 | my $a; | |
50 | $a++; | |
51 | ||
52 | are also always exempt from such warnings. | |
0b8d69e9 | 53 | |
a0d0e21e LW |
54 | A declaration can be put anywhere a statement can, but has no effect on |
55 | the execution of the primary sequence of statements--declarations all | |
56 | take effect at compile time. Typically all the declarations are put at | |
54310121 | 57 | the beginning or the end of the script. However, if you're using |
0b8d69e9 GS |
58 | lexically-scoped private variables created with C<my()>, you'll |
59 | have to make sure | |
4633a7c4 | 60 | your format or subroutine definition is within the same block scope |
5f05dabc | 61 | as the my if you expect to be able to access those private variables. |
a0d0e21e | 62 | |
4633a7c4 LW |
63 | Declaring a subroutine allows a subroutine name to be used as if it were a |
64 | list operator from that point forward in the program. You can declare a | |
54310121 | 65 | subroutine without defining it by saying C<sub name>, thus: |
a0d0e21e | 66 | |
54310121 | 67 | sub myname; |
a0d0e21e LW |
68 | $me = myname $0 or die "can't get myname"; |
69 | ||
1f950eb4 JB |
70 | Note that myname() functions as a list operator, not as a unary operator; |
71 | so be careful to use C<or> instead of C<||> in this case. However, if | |
54310121 | 72 | you were to declare the subroutine as C<sub myname ($)>, then |
02c45c47 | 73 | C<myname> would function as a unary operator, so either C<or> or |
54310121 | 74 | C<||> would work. |
a0d0e21e | 75 | |
4633a7c4 LW |
76 | Subroutines declarations can also be loaded up with the C<require> statement |
77 | or both loaded and imported into your namespace with a C<use> statement. | |
78 | See L<perlmod> for details on this. | |
a0d0e21e | 79 | |
4633a7c4 LW |
80 | A statement sequence may contain declarations of lexically-scoped |
81 | variables, but apart from declaring a variable name, the declaration acts | |
82 | like an ordinary statement, and is elaborated within the sequence of | |
83 | statements as if it were an ordinary statement. That means it actually | |
84 | has both compile-time and run-time effects. | |
a0d0e21e | 85 | |
6014d0cb MS |
86 | =head2 Comments |
87 | ||
88 | Text from a C<"#"> character until the end of the line is a comment, | |
89 | and is ignored. Exceptions include C<"#"> inside a string or regular | |
90 | expression. | |
91 | ||
6ec4bd10 | 92 | =head2 Simple Statements |
a0d0e21e LW |
93 | |
94 | The only kind of simple statement is an expression evaluated for its | |
95 | side effects. Every simple statement must be terminated with a | |
96 | semicolon, unless it is the final statement in a block, in which case | |
97 | the semicolon is optional. (A semicolon is still encouraged there if the | |
5f05dabc | 98 | block takes up more than one line, because you may eventually add another line.) |
a0d0e21e | 99 | Note that there are some operators like C<eval {}> and C<do {}> that look |
54310121 | 100 | like compound statements, but aren't (they're just TERMs in an expression), |
4633a7c4 | 101 | and thus need an explicit termination if used as the last item in a statement. |
a0d0e21e LW |
102 | |
103 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
104 | just before the terminating semicolon (or block ending). The possible | |
105 | modifiers are: | |
106 | ||
107 | if EXPR | |
108 | unless EXPR | |
109 | while EXPR | |
110 | until EXPR | |
ecca16b0 | 111 | foreach EXPR |
a0d0e21e LW |
112 | |
113 | The C<if> and C<unless> modifiers have the expected semantics, | |
ecca16b0 | 114 | presuming you're a speaker of English. The C<foreach> modifier is an |
f86cebdf | 115 | iterator: For each value in EXPR, it aliases C<$_> to the value and |
ecca16b0 | 116 | executes the statement. The C<while> and C<until> modifiers have the |
f86cebdf | 117 | usual "C<while> loop" semantics (conditional evaluated first), except |
19799a22 | 118 | when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE |
ecca16b0 CS |
119 | statement), in which case the block executes once before the |
120 | conditional is evaluated. This is so that you can write loops like: | |
a0d0e21e LW |
121 | |
122 | do { | |
4633a7c4 | 123 | $line = <STDIN>; |
a0d0e21e | 124 | ... |
4633a7c4 | 125 | } until $line eq ".\n"; |
a0d0e21e | 126 | |
5a964f20 TC |
127 | See L<perlfunc/do>. Note also that the loop control statements described |
128 | later will I<NOT> work in this construct, because modifiers don't take | |
129 | loop labels. Sorry. You can always put another block inside of it | |
130 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
f86cebdf | 131 | For C<next>, just double the braces: |
5a964f20 TC |
132 | |
133 | do {{ | |
134 | next if $x == $y; | |
135 | # do something here | |
136 | }} until $x++ > $z; | |
137 | ||
f86cebdf | 138 | For C<last>, you have to be more elaborate: |
5a964f20 TC |
139 | |
140 | LOOP: { | |
141 | do { | |
142 | last if $x = $y**2; | |
143 | # do something here | |
144 | } while $x++ <= $z; | |
145 | } | |
a0d0e21e | 146 | |
457b36cb MV |
147 | B<NOTE:> The behaviour of a C<my> statement modified with a statement |
148 | modifier conditional or loop construct (e.g. C<my $x if ...>) is | |
149 | B<undefined>. The value of the C<my> variable may be C<undef>, any | |
150 | previously assigned value, or possibly anything else. Don't rely on | |
151 | it. Future versions of perl might do something different from the | |
152 | version of perl you try it out on. Here be dragons. | |
153 | ||
6ec4bd10 | 154 | =head2 Compound Statements |
a0d0e21e LW |
155 | |
156 | In Perl, a sequence of statements that defines a scope is called a block. | |
157 | Sometimes a block is delimited by the file containing it (in the case | |
158 | of a required file, or the program as a whole), and sometimes a block | |
159 | is delimited by the extent of a string (in the case of an eval). | |
160 | ||
161 | But generally, a block is delimited by curly brackets, also known as braces. | |
162 | We will call this syntactic construct a BLOCK. | |
163 | ||
164 | The following compound statements may be used to control flow: | |
165 | ||
166 | if (EXPR) BLOCK | |
167 | if (EXPR) BLOCK else BLOCK | |
168 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK | |
169 | LABEL while (EXPR) BLOCK | |
170 | LABEL while (EXPR) BLOCK continue BLOCK | |
171 | LABEL for (EXPR; EXPR; EXPR) BLOCK | |
748a9306 | 172 | LABEL foreach VAR (LIST) BLOCK |
b303ae78 | 173 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
a0d0e21e LW |
174 | LABEL BLOCK continue BLOCK |
175 | ||
176 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, | |
177 | not statements. This means that the curly brackets are I<required>--no | |
178 | dangling statements allowed. If you want to write conditionals without | |
179 | curly brackets there are several other ways to do it. The following | |
180 | all do the same thing: | |
181 | ||
182 | if (!open(FOO)) { die "Can't open $FOO: $!"; } | |
183 | die "Can't open $FOO: $!" unless open(FOO); | |
184 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! | |
185 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; | |
186 | # a bit exotic, that last one | |
187 | ||
5f05dabc | 188 | The C<if> statement is straightforward. Because BLOCKs are always |
a0d0e21e LW |
189 | bounded by curly brackets, there is never any ambiguity about which |
190 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
191 | the sense of the test is reversed. | |
192 | ||
193 | The C<while> statement executes the block as long as the expression is | |
0eb389d5 | 194 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). |
b78218b7 GS |
195 | The LABEL is optional, and if present, consists of an identifier followed |
196 | by a colon. The LABEL identifies the loop for the loop control | |
197 | statements C<next>, C<last>, and C<redo>. | |
198 | If the LABEL is omitted, the loop control statement | |
4633a7c4 LW |
199 | refers to the innermost enclosing loop. This may include dynamically |
200 | looking back your call-stack at run time to find the LABEL. Such | |
9f1b1f2d | 201 | desperate behavior triggers a warning if you use the C<use warnings> |
a2293a43 | 202 | pragma or the B<-w> flag. |
4633a7c4 LW |
203 | |
204 | If there is a C<continue> BLOCK, it is always executed just before the | |
6ec4bd10 MS |
205 | conditional is about to be evaluated again. Thus it can be used to |
206 | increment a loop variable, even when the loop has been continued via | |
207 | the C<next> statement. | |
4633a7c4 LW |
208 | |
209 | =head2 Loop Control | |
210 | ||
6ec4bd10 | 211 | The C<next> command starts the next iteration of the loop: |
4633a7c4 LW |
212 | |
213 | LINE: while (<STDIN>) { | |
214 | next LINE if /^#/; # discard comments | |
215 | ... | |
216 | } | |
217 | ||
6ec4bd10 | 218 | The C<last> command immediately exits the loop in question. The |
4633a7c4 LW |
219 | C<continue> block, if any, is not executed: |
220 | ||
221 | LINE: while (<STDIN>) { | |
222 | last LINE if /^$/; # exit when done with header | |
223 | ... | |
224 | } | |
225 | ||
226 | The C<redo> command restarts the loop block without evaluating the | |
227 | conditional again. The C<continue> block, if any, is I<not> executed. | |
228 | This command is normally used by programs that want to lie to themselves | |
229 | about what was just input. | |
230 | ||
231 | For example, when processing a file like F</etc/termcap>. | |
232 | If your input lines might end in backslashes to indicate continuation, you | |
233 | want to skip ahead and get the next record. | |
234 | ||
235 | while (<>) { | |
236 | chomp; | |
54310121 | 237 | if (s/\\$//) { |
238 | $_ .= <>; | |
4633a7c4 LW |
239 | redo unless eof(); |
240 | } | |
241 | # now process $_ | |
54310121 | 242 | } |
4633a7c4 LW |
243 | |
244 | which is Perl short-hand for the more explicitly written version: | |
245 | ||
54310121 | 246 | LINE: while (defined($line = <ARGV>)) { |
4633a7c4 | 247 | chomp($line); |
54310121 | 248 | if ($line =~ s/\\$//) { |
249 | $line .= <ARGV>; | |
4633a7c4 LW |
250 | redo LINE unless eof(); # not eof(ARGV)! |
251 | } | |
252 | # now process $line | |
54310121 | 253 | } |
4633a7c4 | 254 | |
36e7a065 AMS |
255 | Note that if there were a C<continue> block on the above code, it would |
256 | get executed only on lines discarded by the regex (since redo skips the | |
257 | continue block). A continue block is often used to reset line counters | |
258 | or C<?pat?> one-time matches: | |
4633a7c4 | 259 | |
5a964f20 TC |
260 | # inspired by :1,$g/fred/s//WILMA/ |
261 | while (<>) { | |
262 | ?(fred)? && s//WILMA $1 WILMA/; | |
263 | ?(barney)? && s//BETTY $1 BETTY/; | |
264 | ?(homer)? && s//MARGE $1 MARGE/; | |
265 | } continue { | |
266 | print "$ARGV $.: $_"; | |
267 | close ARGV if eof(); # reset $. | |
268 | reset if eof(); # reset ?pat? | |
4633a7c4 LW |
269 | } |
270 | ||
a0d0e21e LW |
271 | If the word C<while> is replaced by the word C<until>, the sense of the |
272 | test is reversed, but the conditional is still tested before the first | |
273 | iteration. | |
274 | ||
5a964f20 TC |
275 | The loop control statements don't work in an C<if> or C<unless>, since |
276 | they aren't loops. You can double the braces to make them such, though. | |
277 | ||
278 | if (/pattern/) {{ | |
7bd1983c EM |
279 | last if /fred/; |
280 | next if /barney/; # same effect as "last", but doesn't document as well | |
281 | # do something here | |
5a964f20 TC |
282 | }} |
283 | ||
7bd1983c EM |
284 | This is caused by the fact that a block by itself acts as a loop that |
285 | executes once, see L<"Basic BLOCKs and Switch Statements">. | |
286 | ||
5b23ba8b MG |
287 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
288 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
4633a7c4 | 289 | |
cb1a09d0 | 290 | =head2 For Loops |
a0d0e21e | 291 | |
b78df5de | 292 | Perl's C-style C<for> loop works like the corresponding C<while> loop; |
cb1a09d0 | 293 | that means that this: |
a0d0e21e LW |
294 | |
295 | for ($i = 1; $i < 10; $i++) { | |
296 | ... | |
297 | } | |
298 | ||
cb1a09d0 | 299 | is the same as this: |
a0d0e21e LW |
300 | |
301 | $i = 1; | |
302 | while ($i < 10) { | |
303 | ... | |
304 | } continue { | |
305 | $i++; | |
306 | } | |
307 | ||
b78df5de JA |
308 | There is one minor difference: if variables are declared with C<my> |
309 | in the initialization section of the C<for>, the lexical scope of | |
310 | those variables is exactly the C<for> loop (the body of the loop | |
311 | and the control sections). | |
55497cff | 312 | |
cb1a09d0 AD |
313 | Besides the normal array index looping, C<for> can lend itself |
314 | to many other interesting applications. Here's one that avoids the | |
54310121 | 315 | problem you get into if you explicitly test for end-of-file on |
316 | an interactive file descriptor causing your program to appear to | |
cb1a09d0 AD |
317 | hang. |
318 | ||
319 | $on_a_tty = -t STDIN && -t STDOUT; | |
320 | sub prompt { print "yes? " if $on_a_tty } | |
321 | for ( prompt(); <STDIN>; prompt() ) { | |
322 | # do something | |
54310121 | 323 | } |
cb1a09d0 | 324 | |
00cb5da1 CW |
325 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the |
326 | conditional of a C<for> loop is shorthand for the following. This | |
327 | behaviour is the same as a C<while> loop conditional. | |
328 | ||
329 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { | |
330 | # do something | |
331 | } | |
332 | ||
cb1a09d0 AD |
333 | =head2 Foreach Loops |
334 | ||
4633a7c4 | 335 | The C<foreach> loop iterates over a normal list value and sets the |
55497cff | 336 | variable VAR to be each element of the list in turn. If the variable |
337 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
338 | is therefore visible only within the loop. Otherwise, the variable is | |
339 | implicitly local to the loop and regains its former value upon exiting | |
340 | the loop. If the variable was previously declared with C<my>, it uses | |
341 | that variable instead of the global one, but it's still localized to | |
5c502d37 MV |
342 | the loop. This implicit localisation occurs I<only> in a C<foreach> |
343 | loop. | |
4633a7c4 LW |
344 | |
345 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
5a964f20 TC |
346 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
347 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> | |
f86cebdf | 348 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
c5674021 |
349 | |
350 | If any element of LIST is an lvalue, you can modify it by modifying | |
351 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
352 | lvalue, any attempt to modify that element will fail. In other words, | |
353 | the C<foreach> loop index variable is an implicit alias for each item | |
354 | in the list that you're looping over. | |
302617ea MG |
355 | |
356 | If any part of LIST is an array, C<foreach> will get very confused if | |
357 | you add or remove elements within the loop body, for example with | |
358 | C<splice>. So don't do that. | |
359 | ||
360 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
361 | special variable. Don't do that either. | |
4633a7c4 | 362 | |
748a9306 | 363 | Examples: |
a0d0e21e | 364 | |
4633a7c4 | 365 | for (@ary) { s/foo/bar/ } |
a0d0e21e | 366 | |
96f2dc66 | 367 | for my $elem (@elements) { |
a0d0e21e LW |
368 | $elem *= 2; |
369 | } | |
370 | ||
4633a7c4 LW |
371 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
372 | print $count, "\n"; sleep(1); | |
a0d0e21e LW |
373 | } |
374 | ||
375 | for (1..15) { print "Merry Christmas\n"; } | |
376 | ||
4633a7c4 | 377 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
a0d0e21e LW |
378 | print "Item: $item\n"; |
379 | } | |
380 | ||
4633a7c4 LW |
381 | Here's how a C programmer might code up a particular algorithm in Perl: |
382 | ||
55497cff | 383 | for (my $i = 0; $i < @ary1; $i++) { |
384 | for (my $j = 0; $j < @ary2; $j++) { | |
4633a7c4 LW |
385 | if ($ary1[$i] > $ary2[$j]) { |
386 | last; # can't go to outer :-( | |
387 | } | |
388 | $ary1[$i] += $ary2[$j]; | |
389 | } | |
cb1a09d0 | 390 | # this is where that last takes me |
4633a7c4 LW |
391 | } |
392 | ||
184e9718 | 393 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 | 394 | do it: |
4633a7c4 | 395 | |
96f2dc66 GS |
396 | OUTER: for my $wid (@ary1) { |
397 | INNER: for my $jet (@ary2) { | |
cb1a09d0 AD |
398 | next OUTER if $wid > $jet; |
399 | $wid += $jet; | |
54310121 | 400 | } |
401 | } | |
4633a7c4 | 402 | |
cb1a09d0 AD |
403 | See how much easier this is? It's cleaner, safer, and faster. It's |
404 | cleaner because it's less noisy. It's safer because if code gets added | |
c07a80fd | 405 | between the inner and outer loops later on, the new code won't be |
5f05dabc | 406 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd | 407 | rather than merely terminating the inner one. And it's faster because |
408 | Perl executes a C<foreach> statement more rapidly than it would the | |
409 | equivalent C<for> loop. | |
4633a7c4 LW |
410 | |
411 | =head2 Basic BLOCKs and Switch Statements | |
412 | ||
55497cff | 413 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
414 | loop that executes once. Thus you can use any of the loop control | |
415 | statements in it to leave or restart the block. (Note that this is | |
416 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
417 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
418 | block is optional. | |
4633a7c4 LW |
419 | |
420 | The BLOCK construct is particularly nice for doing case | |
a0d0e21e LW |
421 | structures. |
422 | ||
423 | SWITCH: { | |
424 | if (/^abc/) { $abc = 1; last SWITCH; } | |
425 | if (/^def/) { $def = 1; last SWITCH; } | |
426 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
427 | $nothing = 1; | |
428 | } | |
429 | ||
f86cebdf | 430 | There is no official C<switch> statement in Perl, because there are |
83df6a1d JH |
431 | already several ways to write the equivalent. |
432 | ||
433 | However, starting from Perl 5.8 to get switch and case one can use | |
434 | the Switch extension and say: | |
435 | ||
436 | use Switch; | |
437 | ||
438 | after which one has switch and case. It is not as fast as it could be | |
439 | because it's not really part of the language (it's done using source | |
440 | filters) but it is available, and it's very flexible. | |
441 | ||
442 | In addition to the above BLOCK construct, you could write | |
a0d0e21e LW |
443 | |
444 | SWITCH: { | |
445 | $abc = 1, last SWITCH if /^abc/; | |
446 | $def = 1, last SWITCH if /^def/; | |
447 | $xyz = 1, last SWITCH if /^xyz/; | |
448 | $nothing = 1; | |
449 | } | |
450 | ||
cb1a09d0 | 451 | (That's actually not as strange as it looks once you realize that you can |
6ec4bd10 MS |
452 | use loop control "operators" within an expression. That's just the binary |
453 | comma operator in scalar context. See L<perlop/"Comma Operator">.) | |
a0d0e21e LW |
454 | |
455 | or | |
456 | ||
457 | SWITCH: { | |
458 | /^abc/ && do { $abc = 1; last SWITCH; }; | |
459 | /^def/ && do { $def = 1; last SWITCH; }; | |
460 | /^xyz/ && do { $xyz = 1; last SWITCH; }; | |
461 | $nothing = 1; | |
462 | } | |
463 | ||
f86cebdf | 464 | or formatted so it stands out more as a "proper" C<switch> statement: |
a0d0e21e LW |
465 | |
466 | SWITCH: { | |
54310121 | 467 | /^abc/ && do { |
468 | $abc = 1; | |
469 | last SWITCH; | |
a0d0e21e LW |
470 | }; |
471 | ||
54310121 | 472 | /^def/ && do { |
473 | $def = 1; | |
474 | last SWITCH; | |
a0d0e21e LW |
475 | }; |
476 | ||
54310121 | 477 | /^xyz/ && do { |
478 | $xyz = 1; | |
479 | last SWITCH; | |
a0d0e21e LW |
480 | }; |
481 | $nothing = 1; | |
482 | } | |
483 | ||
484 | or | |
485 | ||
486 | SWITCH: { | |
487 | /^abc/ and $abc = 1, last SWITCH; | |
488 | /^def/ and $def = 1, last SWITCH; | |
489 | /^xyz/ and $xyz = 1, last SWITCH; | |
490 | $nothing = 1; | |
491 | } | |
492 | ||
493 | or even, horrors, | |
494 | ||
495 | if (/^abc/) | |
496 | { $abc = 1 } | |
497 | elsif (/^def/) | |
498 | { $def = 1 } | |
499 | elsif (/^xyz/) | |
500 | { $xyz = 1 } | |
501 | else | |
502 | { $nothing = 1 } | |
503 | ||
f86cebdf GS |
504 | A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make |
505 | a temporary assignment to C<$_> for convenient matching: | |
4633a7c4 LW |
506 | |
507 | SWITCH: for ($where) { | |
508 | /In Card Names/ && do { push @flags, '-e'; last; }; | |
509 | /Anywhere/ && do { push @flags, '-h'; last; }; | |
510 | /In Rulings/ && do { last; }; | |
511 | die "unknown value for form variable where: `$where'"; | |
54310121 | 512 | } |
4633a7c4 | 513 | |
cb1a09d0 AD |
514 | Another interesting approach to a switch statement is arrange |
515 | for a C<do> block to return the proper value: | |
516 | ||
517 | $amode = do { | |
5a964f20 | 518 | if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0? |
54310121 | 519 | elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" } |
cb1a09d0 AD |
520 | elsif ($flag & O_RDWR) { |
521 | if ($flag & O_CREAT) { "w+" } | |
c07a80fd | 522 | else { ($flag & O_APPEND) ? "a+" : "r+" } |
cb1a09d0 AD |
523 | } |
524 | }; | |
525 | ||
5a964f20 TC |
526 | Or |
527 | ||
528 | print do { | |
529 | ($flags & O_WRONLY) ? "write-only" : | |
530 | ($flags & O_RDWR) ? "read-write" : | |
531 | "read-only"; | |
532 | }; | |
533 | ||
a031eab2 | 534 | Or if you are certain that all the C<&&> clauses are true, you can use |
5a964f20 | 535 | something like this, which "switches" on the value of the |
a2293a43 | 536 | C<HTTP_USER_AGENT> environment variable. |
5a964f20 TC |
537 | |
538 | #!/usr/bin/perl | |
539 | # pick out jargon file page based on browser | |
540 | $dir = 'http://www.wins.uva.nl/~mes/jargon'; | |
541 | for ($ENV{HTTP_USER_AGENT}) { | |
542 | $page = /Mac/ && 'm/Macintrash.html' | |
543 | || /Win(dows )?NT/ && 'e/evilandrude.html' | |
544 | || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html' | |
545 | || /Linux/ && 'l/Linux.html' | |
546 | || /HP-UX/ && 'h/HP-SUX.html' | |
547 | || /SunOS/ && 's/ScumOS.html' | |
548 | || 'a/AppendixB.html'; | |
549 | } | |
550 | print "Location: $dir/$page\015\012\015\012"; | |
551 | ||
552 | That kind of switch statement only works when you know the C<&&> clauses | |
553 | will be true. If you don't, the previous C<?:> example should be used. | |
554 | ||
19799a22 GS |
555 | You might also consider writing a hash of subroutine references |
556 | instead of synthesizing a C<switch> statement. | |
5a964f20 | 557 | |
4633a7c4 LW |
558 | =head2 Goto |
559 | ||
19799a22 GS |
560 | Although not for the faint of heart, Perl does support a C<goto> |
561 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
562 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
563 | a C<goto>; it's just the name of the loop. | |
4633a7c4 | 564 | |
f86cebdf | 565 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 | 566 | execution there. It may not be used to go into any construct that |
f86cebdf | 567 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 LW |
568 | also can't be used to go into a construct that is optimized away. It |
569 | can be used to go almost anywhere else within the dynamic scope, | |
570 | including out of subroutines, but it's usually better to use some other | |
f86cebdf GS |
571 | construct such as C<last> or C<die>. The author of Perl has never felt the |
572 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
4633a7c4 | 573 | |
f86cebdf GS |
574 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
575 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
4633a7c4 LW |
576 | necessarily recommended if you're optimizing for maintainability: |
577 | ||
96f2dc66 | 578 | goto(("FOO", "BAR", "GLARCH")[$i]); |
4633a7c4 | 579 | |
f86cebdf | 580 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 | 581 | named subroutine for the currently running subroutine. This is used by |
f86cebdf | 582 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 | 583 | pretend that the other subroutine had been called in the first place |
f86cebdf GS |
584 | (except that any modifications to C<@_> in the current subroutine are |
585 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
4633a7c4 LW |
586 | will be able to tell that this routine was called first. |
587 | ||
c07a80fd | 588 | In almost all cases like this, it's usually a far, far better idea to use the |
589 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
4633a7c4 LW |
590 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
591 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
cb1a09d0 AD |
592 | |
593 | =head2 PODs: Embedded Documentation | |
594 | ||
595 | Perl has a mechanism for intermixing documentation with source code. | |
c07a80fd | 596 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 AD |
597 | encounters a line that begins with an equal sign and a word, like this |
598 | ||
599 | =head1 Here There Be Pods! | |
600 | ||
601 | Then that text and all remaining text up through and including a line | |
602 | beginning with C<=cut> will be ignored. The format of the intervening | |
54310121 | 603 | text is described in L<perlpod>. |
cb1a09d0 AD |
604 | |
605 | This allows you to intermix your source code | |
606 | and your documentation text freely, as in | |
607 | ||
608 | =item snazzle($) | |
609 | ||
54310121 | 610 | The snazzle() function will behave in the most spectacular |
cb1a09d0 AD |
611 | form that you can possibly imagine, not even excepting |
612 | cybernetic pyrotechnics. | |
613 | ||
614 | =cut back to the compiler, nuff of this pod stuff! | |
615 | ||
616 | sub snazzle($) { | |
617 | my $thingie = shift; | |
618 | ......... | |
54310121 | 619 | } |
cb1a09d0 | 620 | |
54310121 | 621 | Note that pod translators should look at only paragraphs beginning |
184e9718 | 622 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 | 623 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 AD |
624 | paragraph. This means that the following secret stuff will be |
625 | ignored by both the compiler and the translators. | |
626 | ||
627 | $a=3; | |
628 | =secret stuff | |
629 | warn "Neither POD nor CODE!?" | |
630 | =cut back | |
631 | print "got $a\n"; | |
632 | ||
f86cebdf | 633 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 AD |
634 | Not all pod translators are well-behaved in this regard, and perhaps |
635 | the compiler will become pickier. | |
774d564b | 636 | |
637 | One may also use pod directives to quickly comment out a section | |
638 | of code. | |
639 | ||
640 | =head2 Plain Old Comments (Not!) | |
641 | ||
6ec4bd10 | 642 | Perl can process line directives, much like the C preprocessor. Using |
5a964f20 | 643 | this, one can control Perl's idea of filenames and line numbers in |
774d564b | 644 | error or warning messages (especially for strings that are processed |
f86cebdf | 645 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b | 646 | C preprocessors: it matches the regular expression |
6ec4bd10 MS |
647 | |
648 | # example: '# line 42 "new_filename.plx"' | |
82d4537c | 649 | /^\# \s* |
6ec4bd10 | 650 | line \s+ (\d+) \s* |
7b6e93a8 | 651 | (?:\s("?)([^"]+)\2)? \s* |
6ec4bd10 MS |
652 | $/x |
653 | ||
7b6e93a8 CW |
654 | with C<$1> being the line number for the next line, and C<$3> being |
655 | the optional filename (specified with or without quotes). | |
774d564b | 656 | |
003183f2 GS |
657 | There is a fairly obvious gotcha included with the line directive: |
658 | Debuggers and profilers will only show the last source line to appear | |
659 | at a particular line number in a given file. Care should be taken not | |
660 | to cause line number collisions in code you'd like to debug later. | |
661 | ||
774d564b | 662 | Here are some examples that you should be able to type into your command |
663 | shell: | |
664 | ||
665 | % perl | |
666 | # line 200 "bzzzt" | |
667 | # the `#' on the previous line must be the first char on line | |
668 | die 'foo'; | |
669 | __END__ | |
670 | foo at bzzzt line 201. | |
54310121 | 671 | |
774d564b | 672 | % perl |
673 | # line 200 "bzzzt" | |
674 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
675 | __END__ | |
676 | foo at - line 2001. | |
54310121 | 677 | |
774d564b | 678 | % perl |
679 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
680 | __END__ | |
681 | foo at foo bar line 200. | |
54310121 | 682 | |
774d564b | 683 | % perl |
684 | # line 345 "goop" | |
685 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
686 | print $@; | |
687 | __END__ | |
688 | foo at goop line 345. | |
689 | ||
690 | =cut |