Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<syntax> |
a0d0e21e LW |
3 | |
4 | perlsyn - Perl syntax | |
5 | ||
6 | =head1 DESCRIPTION | |
7 | ||
6014d0cb MS |
8 | A Perl program consists of a sequence of declarations and statements |
9 | which run from the top to the bottom. Loops, subroutines and other | |
10 | control structures allow you to jump around within the code. | |
11 | ||
12 | Perl is a B<free-form> language, you can format and indent it however | |
13 | you like. Whitespace mostly serves to separate tokens, unlike | |
14 | languages like Python where it is an important part of the syntax. | |
15 | ||
16 | Many of Perl's syntactic elements are B<optional>. Rather than | |
110b9c83 | 17 | requiring you to put parentheses around every function call and |
6014d0cb MS |
18 | declare every variable, you can often leave such explicit elements off |
19 | and Perl will figure out what you meant. This is known as B<Do What I | |
20 | Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to | |
110b9c83 | 21 | code in a style with which they are comfortable. |
6014d0cb MS |
22 | |
23 | Perl B<borrows syntax> and concepts from many languages: awk, sed, C, | |
24 | Bourne Shell, Smalltalk, Lisp and even English. Other | |
25 | languages have borrowed syntax from Perl, particularly its regular | |
26 | expression extensions. So if you have programmed in another language | |
27 | you will see familiar pieces in Perl. They often work the same, but | |
28 | see L<perltrap> for information about how they differ. | |
a0d0e21e | 29 | |
0b8d69e9 | 30 | =head2 Declarations |
d74e8afc | 31 | X<declaration> X<undef> X<undefined> X<uninitialized> |
0b8d69e9 | 32 | |
cf48932e SF |
33 | The only things you need to declare in Perl are report formats and |
34 | subroutines (and sometimes not even subroutines). A variable holds | |
35 | the undefined value (C<undef>) until it has been assigned a defined | |
36 | value, which is anything other than C<undef>. When used as a number, | |
37 | C<undef> is treated as C<0>; when used as a string, it is treated as | |
38 | the empty string, C<"">; and when used as a reference that isn't being | |
39 | assigned to, it is treated as an error. If you enable warnings, | |
40 | you'll be notified of an uninitialized value whenever you treat | |
41 | C<undef> as a string or a number. Well, usually. Boolean contexts, | |
42 | such as: | |
7bd1983c EM |
43 | |
44 | my $a; | |
45 | if ($a) {} | |
46 | ||
a6b1f6d8 RGS |
47 | are exempt from warnings (because they care about truth rather than |
48 | definedness). Operators such as C<++>, C<-->, C<+=>, | |
7bd1983c EM |
49 | C<-=>, and C<.=>, that operate on undefined left values such as: |
50 | ||
51 | my $a; | |
52 | $a++; | |
53 | ||
54 | are also always exempt from such warnings. | |
0b8d69e9 | 55 | |
a0d0e21e LW |
56 | A declaration can be put anywhere a statement can, but has no effect on |
57 | the execution of the primary sequence of statements--declarations all | |
58 | take effect at compile time. Typically all the declarations are put at | |
54310121 | 59 | the beginning or the end of the script. However, if you're using |
0b8d69e9 GS |
60 | lexically-scoped private variables created with C<my()>, you'll |
61 | have to make sure | |
4633a7c4 | 62 | your format or subroutine definition is within the same block scope |
5f05dabc | 63 | as the my if you expect to be able to access those private variables. |
a0d0e21e | 64 | |
4633a7c4 LW |
65 | Declaring a subroutine allows a subroutine name to be used as if it were a |
66 | list operator from that point forward in the program. You can declare a | |
54310121 | 67 | subroutine without defining it by saying C<sub name>, thus: |
d74e8afc | 68 | X<subroutine, declaration> |
a0d0e21e | 69 | |
54310121 | 70 | sub myname; |
a0d0e21e LW |
71 | $me = myname $0 or die "can't get myname"; |
72 | ||
1f950eb4 JB |
73 | Note that myname() functions as a list operator, not as a unary operator; |
74 | so be careful to use C<or> instead of C<||> in this case. However, if | |
54310121 | 75 | you were to declare the subroutine as C<sub myname ($)>, then |
02c45c47 | 76 | C<myname> would function as a unary operator, so either C<or> or |
54310121 | 77 | C<||> would work. |
a0d0e21e | 78 | |
4633a7c4 LW |
79 | Subroutines declarations can also be loaded up with the C<require> statement |
80 | or both loaded and imported into your namespace with a C<use> statement. | |
81 | See L<perlmod> for details on this. | |
a0d0e21e | 82 | |
4633a7c4 LW |
83 | A statement sequence may contain declarations of lexically-scoped |
84 | variables, but apart from declaring a variable name, the declaration acts | |
85 | like an ordinary statement, and is elaborated within the sequence of | |
86 | statements as if it were an ordinary statement. That means it actually | |
87 | has both compile-time and run-time effects. | |
a0d0e21e | 88 | |
6014d0cb | 89 | =head2 Comments |
d74e8afc | 90 | X<comment> X<#> |
6014d0cb MS |
91 | |
92 | Text from a C<"#"> character until the end of the line is a comment, | |
93 | and is ignored. Exceptions include C<"#"> inside a string or regular | |
94 | expression. | |
95 | ||
6ec4bd10 | 96 | =head2 Simple Statements |
d74e8afc | 97 | X<statement> X<semicolon> X<expression> X<;> |
a0d0e21e LW |
98 | |
99 | The only kind of simple statement is an expression evaluated for its | |
100 | side effects. Every simple statement must be terminated with a | |
101 | semicolon, unless it is the final statement in a block, in which case | |
f386e492 AMS |
102 | the semicolon is optional. (A semicolon is still encouraged if the |
103 | block takes up more than one line, because you may eventually add | |
cf48932e SF |
104 | another line.) Note that there are some operators like C<eval {}> and |
105 | C<do {}> that look like compound statements, but aren't (they're just | |
106 | TERMs in an expression), and thus need an explicit termination if used | |
107 | as the last item in a statement. | |
108 | ||
109 | =head2 Truth and Falsehood | |
d74e8afc | 110 | X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0> |
cf48932e | 111 | |
f92061c1 AMS |
112 | The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and |
113 | C<undef> are all false in a boolean context. All other values are true. | |
52ea55c9 SP |
114 | Negation of a true value by C<!> or C<not> returns a special false value. |
115 | When evaluated as a string it is treated as C<''>, but as a number, it | |
116 | is treated as 0. | |
cf48932e | 117 | |
cf48932e | 118 | =head2 Statement Modifiers |
d74e8afc ITB |
119 | X<statement modifier> X<modifier> X<if> X<unless> X<while> |
120 | X<until> X<foreach> X<for> | |
a0d0e21e LW |
121 | |
122 | Any simple statement may optionally be followed by a I<SINGLE> modifier, | |
123 | just before the terminating semicolon (or block ending). The possible | |
124 | modifiers are: | |
125 | ||
126 | if EXPR | |
127 | unless EXPR | |
128 | while EXPR | |
129 | until EXPR | |
cf48932e SF |
130 | foreach LIST |
131 | ||
132 | The C<EXPR> following the modifier is referred to as the "condition". | |
133 | Its truth or falsehood determines how the modifier will behave. | |
134 | ||
135 | C<if> executes the statement once I<if> and only if the condition is | |
136 | true. C<unless> is the opposite, it executes the statement I<unless> | |
137 | the condition is true (i.e., if the condition is false). | |
138 | ||
139 | print "Basset hounds got long ears" if length $ear >= 10; | |
140 | go_outside() and play() unless $is_raining; | |
141 | ||
142 | The C<foreach> modifier is an iterator: it executes the statement once | |
143 | for each item in the LIST (with C<$_> aliased to each item in turn). | |
144 | ||
145 | print "Hello $_!\n" foreach qw(world Dolly nurse); | |
146 | ||
147 | C<while> repeats the statement I<while> the condition is true. | |
148 | C<until> does the opposite, it repeats the statement I<until> the | |
149 | condition is true (or while the condition is false): | |
150 | ||
151 | # Both of these count from 0 to 10. | |
152 | print $i++ while $i <= 10; | |
153 | print $j++ until $j > 10; | |
154 | ||
155 | The C<while> and C<until> modifiers have the usual "C<while> loop" | |
156 | semantics (conditional evaluated first), except when applied to a | |
157 | C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in | |
158 | which case the block executes once before the conditional is | |
159 | evaluated. This is so that you can write loops like: | |
a0d0e21e LW |
160 | |
161 | do { | |
4633a7c4 | 162 | $line = <STDIN>; |
a0d0e21e | 163 | ... |
4633a7c4 | 164 | } until $line eq ".\n"; |
a0d0e21e | 165 | |
5a964f20 TC |
166 | See L<perlfunc/do>. Note also that the loop control statements described |
167 | later will I<NOT> work in this construct, because modifiers don't take | |
168 | loop labels. Sorry. You can always put another block inside of it | |
169 | (for C<next>) or around it (for C<last>) to do that sort of thing. | |
f86cebdf | 170 | For C<next>, just double the braces: |
d74e8afc | 171 | X<next> X<last> X<redo> |
5a964f20 TC |
172 | |
173 | do {{ | |
174 | next if $x == $y; | |
175 | # do something here | |
176 | }} until $x++ > $z; | |
177 | ||
f86cebdf | 178 | For C<last>, you have to be more elaborate: |
d74e8afc | 179 | X<last> |
5a964f20 TC |
180 | |
181 | LOOP: { | |
182 | do { | |
183 | last if $x = $y**2; | |
184 | # do something here | |
185 | } while $x++ <= $z; | |
186 | } | |
a0d0e21e | 187 | |
457b36cb MV |
188 | B<NOTE:> The behaviour of a C<my> statement modified with a statement |
189 | modifier conditional or loop construct (e.g. C<my $x if ...>) is | |
190 | B<undefined>. The value of the C<my> variable may be C<undef>, any | |
191 | previously assigned value, or possibly anything else. Don't rely on | |
192 | it. Future versions of perl might do something different from the | |
193 | version of perl you try it out on. Here be dragons. | |
d74e8afc | 194 | X<my> |
457b36cb | 195 | |
6ec4bd10 | 196 | =head2 Compound Statements |
d74e8afc ITB |
197 | X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace> |
198 | X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue> | |
a0d0e21e LW |
199 | |
200 | In Perl, a sequence of statements that defines a scope is called a block. | |
201 | Sometimes a block is delimited by the file containing it (in the case | |
202 | of a required file, or the program as a whole), and sometimes a block | |
203 | is delimited by the extent of a string (in the case of an eval). | |
204 | ||
205 | But generally, a block is delimited by curly brackets, also known as braces. | |
206 | We will call this syntactic construct a BLOCK. | |
207 | ||
208 | The following compound statements may be used to control flow: | |
209 | ||
210 | if (EXPR) BLOCK | |
211 | if (EXPR) BLOCK else BLOCK | |
212 | if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK | |
213 | LABEL while (EXPR) BLOCK | |
214 | LABEL while (EXPR) BLOCK continue BLOCK | |
5ec6d87f EA |
215 | LABEL until (EXPR) BLOCK |
216 | LABEL until (EXPR) BLOCK continue BLOCK | |
a0d0e21e | 217 | LABEL for (EXPR; EXPR; EXPR) BLOCK |
748a9306 | 218 | LABEL foreach VAR (LIST) BLOCK |
b303ae78 | 219 | LABEL foreach VAR (LIST) BLOCK continue BLOCK |
a0d0e21e LW |
220 | LABEL BLOCK continue BLOCK |
221 | ||
222 | Note that, unlike C and Pascal, these are defined in terms of BLOCKs, | |
223 | not statements. This means that the curly brackets are I<required>--no | |
224 | dangling statements allowed. If you want to write conditionals without | |
225 | curly brackets there are several other ways to do it. The following | |
226 | all do the same thing: | |
227 | ||
228 | if (!open(FOO)) { die "Can't open $FOO: $!"; } | |
229 | die "Can't open $FOO: $!" unless open(FOO); | |
230 | open(FOO) or die "Can't open $FOO: $!"; # FOO or bust! | |
231 | open(FOO) ? 'hi mom' : die "Can't open $FOO: $!"; | |
232 | # a bit exotic, that last one | |
233 | ||
5f05dabc | 234 | The C<if> statement is straightforward. Because BLOCKs are always |
a0d0e21e LW |
235 | bounded by curly brackets, there is never any ambiguity about which |
236 | C<if> an C<else> goes with. If you use C<unless> in place of C<if>, | |
237 | the sense of the test is reversed. | |
238 | ||
239 | The C<while> statement executes the block as long as the expression is | |
0eb389d5 | 240 | true (does not evaluate to the null string C<""> or C<0> or C<"0">). |
1d5653dd RGS |
241 | The C<until> statement executes the block as long as the expression is |
242 | false. | |
b78218b7 GS |
243 | The LABEL is optional, and if present, consists of an identifier followed |
244 | by a colon. The LABEL identifies the loop for the loop control | |
245 | statements C<next>, C<last>, and C<redo>. | |
246 | If the LABEL is omitted, the loop control statement | |
4633a7c4 LW |
247 | refers to the innermost enclosing loop. This may include dynamically |
248 | looking back your call-stack at run time to find the LABEL. Such | |
9f1b1f2d | 249 | desperate behavior triggers a warning if you use the C<use warnings> |
a2293a43 | 250 | pragma or the B<-w> flag. |
4633a7c4 LW |
251 | |
252 | If there is a C<continue> BLOCK, it is always executed just before the | |
6ec4bd10 MS |
253 | conditional is about to be evaluated again. Thus it can be used to |
254 | increment a loop variable, even when the loop has been continued via | |
255 | the C<next> statement. | |
4633a7c4 LW |
256 | |
257 | =head2 Loop Control | |
d74e8afc | 258 | X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue> |
4633a7c4 | 259 | |
6ec4bd10 | 260 | The C<next> command starts the next iteration of the loop: |
4633a7c4 LW |
261 | |
262 | LINE: while (<STDIN>) { | |
263 | next LINE if /^#/; # discard comments | |
264 | ... | |
265 | } | |
266 | ||
6ec4bd10 | 267 | The C<last> command immediately exits the loop in question. The |
4633a7c4 LW |
268 | C<continue> block, if any, is not executed: |
269 | ||
270 | LINE: while (<STDIN>) { | |
271 | last LINE if /^$/; # exit when done with header | |
272 | ... | |
273 | } | |
274 | ||
275 | The C<redo> command restarts the loop block without evaluating the | |
276 | conditional again. The C<continue> block, if any, is I<not> executed. | |
277 | This command is normally used by programs that want to lie to themselves | |
278 | about what was just input. | |
279 | ||
280 | For example, when processing a file like F</etc/termcap>. | |
281 | If your input lines might end in backslashes to indicate continuation, you | |
282 | want to skip ahead and get the next record. | |
283 | ||
284 | while (<>) { | |
285 | chomp; | |
54310121 | 286 | if (s/\\$//) { |
287 | $_ .= <>; | |
4633a7c4 LW |
288 | redo unless eof(); |
289 | } | |
290 | # now process $_ | |
54310121 | 291 | } |
4633a7c4 LW |
292 | |
293 | which is Perl short-hand for the more explicitly written version: | |
294 | ||
54310121 | 295 | LINE: while (defined($line = <ARGV>)) { |
4633a7c4 | 296 | chomp($line); |
54310121 | 297 | if ($line =~ s/\\$//) { |
298 | $line .= <ARGV>; | |
4633a7c4 LW |
299 | redo LINE unless eof(); # not eof(ARGV)! |
300 | } | |
301 | # now process $line | |
54310121 | 302 | } |
4633a7c4 | 303 | |
36e7a065 AMS |
304 | Note that if there were a C<continue> block on the above code, it would |
305 | get executed only on lines discarded by the regex (since redo skips the | |
306 | continue block). A continue block is often used to reset line counters | |
307 | or C<?pat?> one-time matches: | |
4633a7c4 | 308 | |
5a964f20 TC |
309 | # inspired by :1,$g/fred/s//WILMA/ |
310 | while (<>) { | |
311 | ?(fred)? && s//WILMA $1 WILMA/; | |
312 | ?(barney)? && s//BETTY $1 BETTY/; | |
313 | ?(homer)? && s//MARGE $1 MARGE/; | |
314 | } continue { | |
315 | print "$ARGV $.: $_"; | |
316 | close ARGV if eof(); # reset $. | |
317 | reset if eof(); # reset ?pat? | |
4633a7c4 LW |
318 | } |
319 | ||
a0d0e21e LW |
320 | If the word C<while> is replaced by the word C<until>, the sense of the |
321 | test is reversed, but the conditional is still tested before the first | |
322 | iteration. | |
323 | ||
5a964f20 TC |
324 | The loop control statements don't work in an C<if> or C<unless>, since |
325 | they aren't loops. You can double the braces to make them such, though. | |
326 | ||
327 | if (/pattern/) {{ | |
7bd1983c EM |
328 | last if /fred/; |
329 | next if /barney/; # same effect as "last", but doesn't document as well | |
330 | # do something here | |
5a964f20 TC |
331 | }} |
332 | ||
7bd1983c | 333 | This is caused by the fact that a block by itself acts as a loop that |
27cec4bd | 334 | executes once, see L<"Basic BLOCKs">. |
7bd1983c | 335 | |
5b23ba8b MG |
336 | The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer |
337 | available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>. | |
4633a7c4 | 338 | |
cb1a09d0 | 339 | =head2 For Loops |
d74e8afc | 340 | X<for> X<foreach> |
a0d0e21e | 341 | |
b78df5de | 342 | Perl's C-style C<for> loop works like the corresponding C<while> loop; |
cb1a09d0 | 343 | that means that this: |
a0d0e21e LW |
344 | |
345 | for ($i = 1; $i < 10; $i++) { | |
346 | ... | |
347 | } | |
348 | ||
cb1a09d0 | 349 | is the same as this: |
a0d0e21e LW |
350 | |
351 | $i = 1; | |
352 | while ($i < 10) { | |
353 | ... | |
354 | } continue { | |
355 | $i++; | |
356 | } | |
357 | ||
b78df5de JA |
358 | There is one minor difference: if variables are declared with C<my> |
359 | in the initialization section of the C<for>, the lexical scope of | |
360 | those variables is exactly the C<for> loop (the body of the loop | |
361 | and the control sections). | |
d74e8afc | 362 | X<my> |
55497cff | 363 | |
cb1a09d0 AD |
364 | Besides the normal array index looping, C<for> can lend itself |
365 | to many other interesting applications. Here's one that avoids the | |
54310121 | 366 | problem you get into if you explicitly test for end-of-file on |
367 | an interactive file descriptor causing your program to appear to | |
cb1a09d0 | 368 | hang. |
d74e8afc | 369 | X<eof> X<end-of-file> X<end of file> |
cb1a09d0 AD |
370 | |
371 | $on_a_tty = -t STDIN && -t STDOUT; | |
372 | sub prompt { print "yes? " if $on_a_tty } | |
373 | for ( prompt(); <STDIN>; prompt() ) { | |
374 | # do something | |
54310121 | 375 | } |
cb1a09d0 | 376 | |
00cb5da1 CW |
377 | Using C<readline> (or the operator form, C<< <EXPR> >>) as the |
378 | conditional of a C<for> loop is shorthand for the following. This | |
379 | behaviour is the same as a C<while> loop conditional. | |
d74e8afc | 380 | X<readline> X<< <> >> |
00cb5da1 CW |
381 | |
382 | for ( prompt(); defined( $_ = <STDIN> ); prompt() ) { | |
383 | # do something | |
384 | } | |
385 | ||
cb1a09d0 | 386 | =head2 Foreach Loops |
d74e8afc | 387 | X<for> X<foreach> |
cb1a09d0 | 388 | |
4633a7c4 | 389 | The C<foreach> loop iterates over a normal list value and sets the |
55497cff | 390 | variable VAR to be each element of the list in turn. If the variable |
391 | is preceded with the keyword C<my>, then it is lexically scoped, and | |
392 | is therefore visible only within the loop. Otherwise, the variable is | |
393 | implicitly local to the loop and regains its former value upon exiting | |
394 | the loop. If the variable was previously declared with C<my>, it uses | |
395 | that variable instead of the global one, but it's still localized to | |
5c502d37 MV |
396 | the loop. This implicit localisation occurs I<only> in a C<foreach> |
397 | loop. | |
d74e8afc | 398 | X<my> X<local> |
4633a7c4 LW |
399 | |
400 | The C<foreach> keyword is actually a synonym for the C<for> keyword, so | |
5a964f20 TC |
401 | you can use C<foreach> for readability or C<for> for brevity. (Or because |
402 | the Bourne shell is more familiar to you than I<csh>, so writing C<for> | |
f86cebdf | 403 | comes more naturally.) If VAR is omitted, C<$_> is set to each value. |
d74e8afc | 404 | X<$_> |
c5674021 |
405 | |
406 | If any element of LIST is an lvalue, you can modify it by modifying | |
407 | VAR inside the loop. Conversely, if any element of LIST is NOT an | |
408 | lvalue, any attempt to modify that element will fail. In other words, | |
409 | the C<foreach> loop index variable is an implicit alias for each item | |
410 | in the list that you're looping over. | |
d74e8afc | 411 | X<alias> |
302617ea MG |
412 | |
413 | If any part of LIST is an array, C<foreach> will get very confused if | |
414 | you add or remove elements within the loop body, for example with | |
415 | C<splice>. So don't do that. | |
d74e8afc | 416 | X<splice> |
302617ea MG |
417 | |
418 | C<foreach> probably won't do what you expect if VAR is a tied or other | |
419 | special variable. Don't do that either. | |
4633a7c4 | 420 | |
748a9306 | 421 | Examples: |
a0d0e21e | 422 | |
4633a7c4 | 423 | for (@ary) { s/foo/bar/ } |
a0d0e21e | 424 | |
96f2dc66 | 425 | for my $elem (@elements) { |
a0d0e21e LW |
426 | $elem *= 2; |
427 | } | |
428 | ||
4633a7c4 LW |
429 | for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') { |
430 | print $count, "\n"; sleep(1); | |
a0d0e21e LW |
431 | } |
432 | ||
433 | for (1..15) { print "Merry Christmas\n"; } | |
434 | ||
4633a7c4 | 435 | foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) { |
a0d0e21e LW |
436 | print "Item: $item\n"; |
437 | } | |
438 | ||
4633a7c4 LW |
439 | Here's how a C programmer might code up a particular algorithm in Perl: |
440 | ||
55497cff | 441 | for (my $i = 0; $i < @ary1; $i++) { |
442 | for (my $j = 0; $j < @ary2; $j++) { | |
4633a7c4 LW |
443 | if ($ary1[$i] > $ary2[$j]) { |
444 | last; # can't go to outer :-( | |
445 | } | |
446 | $ary1[$i] += $ary2[$j]; | |
447 | } | |
cb1a09d0 | 448 | # this is where that last takes me |
4633a7c4 LW |
449 | } |
450 | ||
184e9718 | 451 | Whereas here's how a Perl programmer more comfortable with the idiom might |
cb1a09d0 | 452 | do it: |
4633a7c4 | 453 | |
96f2dc66 GS |
454 | OUTER: for my $wid (@ary1) { |
455 | INNER: for my $jet (@ary2) { | |
cb1a09d0 AD |
456 | next OUTER if $wid > $jet; |
457 | $wid += $jet; | |
54310121 | 458 | } |
459 | } | |
4633a7c4 | 460 | |
cb1a09d0 AD |
461 | See how much easier this is? It's cleaner, safer, and faster. It's |
462 | cleaner because it's less noisy. It's safer because if code gets added | |
c07a80fd | 463 | between the inner and outer loops later on, the new code won't be |
5f05dabc | 464 | accidentally executed. The C<next> explicitly iterates the other loop |
c07a80fd | 465 | rather than merely terminating the inner one. And it's faster because |
466 | Perl executes a C<foreach> statement more rapidly than it would the | |
467 | equivalent C<for> loop. | |
4633a7c4 | 468 | |
0d863452 RH |
469 | =head2 Basic BLOCKs |
470 | X<block> | |
4633a7c4 | 471 | |
55497cff | 472 | A BLOCK by itself (labeled or not) is semantically equivalent to a |
473 | loop that executes once. Thus you can use any of the loop control | |
474 | statements in it to leave or restart the block. (Note that this is | |
475 | I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief | |
476 | C<do{}> blocks, which do I<NOT> count as loops.) The C<continue> | |
477 | block is optional. | |
4633a7c4 | 478 | |
27cec4bd | 479 | The BLOCK construct can be used to emulate case structures. |
a0d0e21e LW |
480 | |
481 | SWITCH: { | |
482 | if (/^abc/) { $abc = 1; last SWITCH; } | |
483 | if (/^def/) { $def = 1; last SWITCH; } | |
484 | if (/^xyz/) { $xyz = 1; last SWITCH; } | |
485 | $nothing = 1; | |
486 | } | |
487 | ||
0d863452 RH |
488 | Such constructs are quite frequently used, because older versions |
489 | of Perl had no official C<switch> statement. | |
83df6a1d | 490 | |
0d863452 RH |
491 | =head2 Switch statements |
492 | X<switch> X<case> X<given> X<when> X<default> | |
83df6a1d | 493 | |
27cec4bd | 494 | Starting from Perl 5.10, you can say |
83df6a1d | 495 | |
27cec4bd | 496 | use feature "switch"; |
a0d0e21e | 497 | |
0d863452 RH |
498 | which enables a switch feature that is closely based on the |
499 | Perl 6 proposal. | |
500 | ||
501 | The keywords C<given> and C<when> are analogous | |
502 | to C<switch> and C<case> in other languages, so the code | |
503 | above could be written as | |
504 | ||
27cec4bd RGS |
505 | given($_) { |
506 | when (/^abc/) { $abc = 1; } | |
507 | when (/^def/) { $def = 1; } | |
508 | when (/^xyz/) { $xyz = 1; } | |
509 | default { $nothing = 1; } | |
a0d0e21e LW |
510 | } |
511 | ||
0d863452 | 512 | This construct is very flexible and powerful. For example: |
a0d0e21e | 513 | |
4b7b0ae4 RH |
514 | use feature ":5.10"; |
515 | given($foo) { | |
516 | when (undef) { | |
517 | say '$foo is undefined'; | |
518 | } | |
9f435386 | 519 | |
4b7b0ae4 RH |
520 | when ("foo") { |
521 | say '$foo is the string "foo"'; | |
522 | } | |
523 | ||
524 | when ([1,3,5,7,9]) { | |
525 | say '$foo is an odd digit'; | |
526 | continue; # Fall through | |
9f435386 RGS |
527 | } |
528 | ||
4b7b0ae4 RH |
529 | when ($_ < 100) { |
530 | say '$foo is numerically less than 100'; | |
531 | } | |
532 | ||
533 | when (\&complicated_check) { | |
534 | say 'complicated_check($foo) is true'; | |
535 | } | |
9f435386 | 536 | |
4b7b0ae4 RH |
537 | default { |
538 | die q(I don't know what to do with $foo); | |
539 | } | |
540 | } | |
541 | ||
542 | C<given(EXPR)> will assign the value of EXPR to C<$_> | |
543 | within the lexical scope of the block, so it's similar to | |
544 | ||
545 | do { my $_ = EXPR; ... } | |
546 | ||
547 | except that the block is automatically broken out of by a | |
548 | successful C<when> or an explicit C<break>. | |
549 | ||
550 | Most of the power comes from implicit smart matching: | |
a0d0e21e | 551 | |
4b7b0ae4 | 552 | when($foo) |
a0d0e21e | 553 | |
0d863452 | 554 | is exactly equivalent to |
a0d0e21e | 555 | |
4b7b0ae4 | 556 | when($_ ~~ $foo) |
a0d0e21e | 557 | |
0d863452 RH |
558 | (though you need to enable the "~~" feature before you |
559 | can use the C<~~> operator directly). In fact C<when(EXPR)> | |
560 | is treated as an implicit smart match most of the time. The | |
561 | exceptions are that when EXPR is: | |
562 | ||
563 | =over 4 | |
564 | ||
565 | =item o | |
566 | ||
567 | a subroutine or method call | |
568 | ||
569 | =item o | |
570 | ||
571 | a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>, | |
572 | or a negated regular expression match C<$foo !~ /REGEX/>. | |
573 | ||
574 | =item o | |
575 | ||
4b7b0ae4 RH |
576 | a comparison such as C<$_ E<lt> 10> or C<$x eq "abc"> |
577 | (or of course C<$_ ~~ $c>) | |
0d863452 RH |
578 | |
579 | =item o | |
580 | ||
581 | C<defined(...)>, C<exists(...)>, or C<eof(...)> | |
582 | ||
583 | =item o | |
4633a7c4 | 584 | |
0d863452 RH |
585 | A negated expression C<!(...)> or C<not (...)>, or a logical |
586 | exclusive-or C<(...) xor (...)>. | |
cb1a09d0 | 587 | |
0d863452 RH |
588 | =back |
589 | ||
590 | then the value of EXPR is used directly as a boolean. | |
591 | Furthermore: | |
592 | ||
593 | =over 4 | |
594 | ||
595 | =item o | |
596 | ||
597 | If EXPR is C<... && ...> or C<... and ...>, the test | |
598 | is applied recursively to both arguments. If I<both> | |
599 | arguments pass the test, then the argument is treated | |
600 | as boolean. | |
601 | ||
602 | =item o | |
603 | ||
604 | If EXPR is C<... || ...> or C<... or ...>, the test | |
605 | is applied recursively to the first argument. | |
606 | ||
607 | =back | |
608 | ||
609 | These rules look complicated, but usually they will do what | |
610 | you want. For example you could write: | |
611 | ||
27cec4bd | 612 | when (/^\d$/ && $_ < 75) { ... } |
0d863452 | 613 | |
4b7b0ae4 RH |
614 | Another useful shortcut is that, if you use a literal array |
615 | or hash as the argument to C<when>, it is turned into a | |
616 | reference. So C<given(@foo)> is the same as C<given(\@foo)>, | |
617 | for example. | |
618 | ||
0d863452 RH |
619 | C<default> behaves exactly like C<when(1 == 1)>, which is |
620 | to say that it always matches. | |
621 | ||
622 | See L</"Smart matching in detail"> for more information | |
623 | on smart matching. | |
624 | ||
4b7b0ae4 RH |
625 | =head3 Breaking out |
626 | ||
627 | You can use the C<break> keyword to break out of the enclosing | |
628 | C<given> block. Every C<when> block is implicitly ended with | |
629 | a C<break>. | |
630 | ||
0d863452 RH |
631 | =head3 Fall-through |
632 | ||
633 | You can use the C<continue> keyword to fall through from one | |
634 | case to the next: | |
635 | ||
27cec4bd | 636 | given($foo) { |
4b7b0ae4 RH |
637 | when (/x/) { say '$foo contains an x'; continue } |
638 | when (/y/) { say '$foo contains a y' } | |
639 | default { say '$foo contains neither an x nor a y' } | |
27cec4bd | 640 | } |
0d863452 RH |
641 | |
642 | =head3 Switching in a loop | |
643 | ||
644 | Instead of using C<given()>, you can use a C<foreach()> loop. | |
645 | For example, here's one way to count how many times a particular | |
646 | string occurs in an array: | |
647 | ||
27cec4bd RGS |
648 | my $count = 0; |
649 | for (@array) { | |
650 | when ("foo") { ++$count } | |
5a964f20 | 651 | } |
27cec4bd | 652 | print "\@array contains $count copies of 'foo'\n"; |
0d863452 RH |
653 | |
654 | On exit from the C<when> block, there is an implicit C<next>. | |
655 | You can override that with an explicit C<last> if you're only | |
656 | interested in the first match. | |
657 | ||
658 | This doesn't work if you explicitly specify a loop variable, | |
659 | as in C<for $item (@array)>. You have to use the default | |
660 | variable C<$_>. (You can use C<for my $_ (@array)>.) | |
661 | ||
662 | =head3 Smart matching in detail | |
663 | ||
4b7b0ae4 RH |
664 | The behaviour of a smart match depends on what type of thing |
665 | its arguments are. It is always commutative, i.e. C<$a ~~ $b> | |
666 | behaves the same as C<$b ~~ $a>. The behaviour is determined | |
667 | by the following table: the first row that applies, in either | |
668 | order, determines the match behaviour. | |
669 | ||
670 | ||
671 | $a $b Type of Match Implied Matching Code | |
672 | ====== ===== ===================== ============= | |
673 | (overloading trumps everything) | |
674 | ||
675 | Code[+] Code[+] referential equality $a == $b | |
676 | Any Code[+] scalar sub truth $b->($a) | |
677 | ||
678 | Hash Hash hash keys identical [sort keys %$a]~~[sort keys %$b] | |
679 | Hash Array hash value slice truth grep $_, @$a{@$b} | |
680 | Hash Regex hash key grep grep /$b/, keys %$a | |
681 | Hash Any hash entry existence exists $a->{$b} | |
682 | ||
683 | Array Array arrays are identical[*] | |
684 | Array Regex array grep grep /$b/, @$a | |
685 | Array Num array contains number grep $_ == $b, @$a | |
686 | Array Any array contains string grep $_ eq $b, @$a | |
687 | ||
688 | Any undef undefined !defined $a | |
689 | Any Regex pattern match $a =~ /$b/ | |
690 | Code() Code() results are equal $a->() eq $b->() | |
691 | Any Code() simple closure truth $b->() # ignoring $a | |
692 | Num numish[!] numeric equality $a == $b | |
693 | Any Str string equality $a eq $b | |
694 | Any Num numeric equality $a == $b | |
695 | ||
696 | Any Any string equality $a eq $b | |
697 | ||
698 | ||
699 | + - this must be a code reference whose prototype (if present) is not "" | |
700 | (subs with a "" prototype are dealt with by the 'Code()' entry lower down) | |
701 | * - if a circular reference is found, we fall back to referential equality | |
702 | ! - either a real number, or a string that looks like a number | |
0d863452 | 703 | |
4b7b0ae4 RH |
704 | The "matching code" doesn't represent the I<real> matching code, |
705 | of course: it's just there to explain the intended meaning. Unlike | |
706 | C<grep>, the smart match operator will short-circuit whenever it can. | |
5a964f20 | 707 | |
0d863452 | 708 | =head3 Custom matching via overloading |
5a964f20 | 709 | |
0d863452 | 710 | You can change the way that an object is matched by overloading |
4b7b0ae4 RH |
711 | the C<~~> operator. This trumps the usual smart match semantics. |
712 | See L<overload>. | |
5a964f20 | 713 | |
54a85b95 RH |
714 | =head3 Differences from Perl 6 |
715 | ||
716 | The Perl 5 smart match and C<given>/C<when> constructs are not | |
717 | absolutely identical to their Perl 6 analogues. The most visible | |
718 | difference is that, in Perl 5, parentheses are required around | |
719 | the argument to C<given()> and C<when()>. Parentheses in Perl 6 | |
720 | are always optional in a control construct such as C<if()>, | |
721 | C<while()>, or C<when()>; they can't be made optional in Perl | |
722 | 5 without a great deal of potential confusion, because Perl 5 | |
723 | would parse the expression | |
724 | ||
725 | given $foo { | |
726 | ... | |
727 | } | |
728 | ||
729 | as though the argument to C<given> were an element of the hash | |
730 | C<%foo>, interpreting the braces as hash-element syntax. | |
731 | ||
732 | The table of smart matches is not identical to that proposed | |
733 | by the Perl 6 specification Synopsis 4. Some of the differences | |
734 | are simply a consequence of Perl 5's different data model, while | |
735 | other changes have been made to address problems with the Perl 6 | |
736 | proposal. For example, the Perl 6 specification implies that | |
737 | C<$string ~~ qr/regex/> would test string equality, rather than | |
738 | doing a regular expression match. On the other hand, informal | |
739 | examples elsewhere make it clear that a regular expression | |
740 | match is the intended behaviour. Thus the Synopsis 4 smart | |
741 | match specification cannot yet be regarded as definitive. | |
742 | ||
743 | In Perl 6, C<when()> will always do an implicit smart match | |
744 | with its argument, whilst it is convenient in Perl 5 to | |
745 | suppress this implicit smart match in certain situations, | |
746 | as documented above. (The difference is largely because Perl 5 | |
747 | does not, even internally, have a boolean type.) | |
748 | ||
4633a7c4 | 749 | =head2 Goto |
d74e8afc | 750 | X<goto> |
4633a7c4 | 751 | |
19799a22 GS |
752 | Although not for the faint of heart, Perl does support a C<goto> |
753 | statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and | |
754 | C<goto>-&NAME. A loop's LABEL is not actually a valid target for | |
755 | a C<goto>; it's just the name of the loop. | |
4633a7c4 | 756 | |
f86cebdf | 757 | The C<goto>-LABEL form finds the statement labeled with LABEL and resumes |
4633a7c4 | 758 | execution there. It may not be used to go into any construct that |
f86cebdf | 759 | requires initialization, such as a subroutine or a C<foreach> loop. It |
4633a7c4 LW |
760 | also can't be used to go into a construct that is optimized away. It |
761 | can be used to go almost anywhere else within the dynamic scope, | |
762 | including out of subroutines, but it's usually better to use some other | |
f86cebdf GS |
763 | construct such as C<last> or C<die>. The author of Perl has never felt the |
764 | need to use this form of C<goto> (in Perl, that is--C is another matter). | |
4633a7c4 | 765 | |
f86cebdf GS |
766 | The C<goto>-EXPR form expects a label name, whose scope will be resolved |
767 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't | |
4633a7c4 LW |
768 | necessarily recommended if you're optimizing for maintainability: |
769 | ||
96f2dc66 | 770 | goto(("FOO", "BAR", "GLARCH")[$i]); |
4633a7c4 | 771 | |
f86cebdf | 772 | The C<goto>-&NAME form is highly magical, and substitutes a call to the |
4633a7c4 | 773 | named subroutine for the currently running subroutine. This is used by |
f86cebdf | 774 | C<AUTOLOAD()> subroutines that wish to load another subroutine and then |
4633a7c4 | 775 | pretend that the other subroutine had been called in the first place |
f86cebdf GS |
776 | (except that any modifications to C<@_> in the current subroutine are |
777 | propagated to the other subroutine.) After the C<goto>, not even C<caller()> | |
4633a7c4 LW |
778 | will be able to tell that this routine was called first. |
779 | ||
c07a80fd | 780 | In almost all cases like this, it's usually a far, far better idea to use the |
781 | structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of | |
4633a7c4 LW |
782 | resorting to a C<goto>. For certain applications, the catch and throw pair of |
783 | C<eval{}> and die() for exception processing can also be a prudent approach. | |
cb1a09d0 AD |
784 | |
785 | =head2 PODs: Embedded Documentation | |
d74e8afc | 786 | X<POD> X<documentation> |
cb1a09d0 AD |
787 | |
788 | Perl has a mechanism for intermixing documentation with source code. | |
c07a80fd | 789 | While it's expecting the beginning of a new statement, if the compiler |
cb1a09d0 AD |
790 | encounters a line that begins with an equal sign and a word, like this |
791 | ||
792 | =head1 Here There Be Pods! | |
793 | ||
794 | Then that text and all remaining text up through and including a line | |
795 | beginning with C<=cut> will be ignored. The format of the intervening | |
54310121 | 796 | text is described in L<perlpod>. |
cb1a09d0 AD |
797 | |
798 | This allows you to intermix your source code | |
799 | and your documentation text freely, as in | |
800 | ||
801 | =item snazzle($) | |
802 | ||
54310121 | 803 | The snazzle() function will behave in the most spectacular |
cb1a09d0 AD |
804 | form that you can possibly imagine, not even excepting |
805 | cybernetic pyrotechnics. | |
806 | ||
807 | =cut back to the compiler, nuff of this pod stuff! | |
808 | ||
809 | sub snazzle($) { | |
810 | my $thingie = shift; | |
811 | ......... | |
54310121 | 812 | } |
cb1a09d0 | 813 | |
54310121 | 814 | Note that pod translators should look at only paragraphs beginning |
184e9718 | 815 | with a pod directive (it makes parsing easier), whereas the compiler |
54310121 | 816 | actually knows to look for pod escapes even in the middle of a |
cb1a09d0 AD |
817 | paragraph. This means that the following secret stuff will be |
818 | ignored by both the compiler and the translators. | |
819 | ||
820 | $a=3; | |
821 | =secret stuff | |
822 | warn "Neither POD nor CODE!?" | |
823 | =cut back | |
824 | print "got $a\n"; | |
825 | ||
f86cebdf | 826 | You probably shouldn't rely upon the C<warn()> being podded out forever. |
cb1a09d0 AD |
827 | Not all pod translators are well-behaved in this regard, and perhaps |
828 | the compiler will become pickier. | |
774d564b | 829 | |
830 | One may also use pod directives to quickly comment out a section | |
831 | of code. | |
832 | ||
833 | =head2 Plain Old Comments (Not!) | |
d74e8afc | 834 | X<comment> X<line> X<#> X<preprocessor> X<eval> |
774d564b | 835 | |
6ec4bd10 | 836 | Perl can process line directives, much like the C preprocessor. Using |
5a964f20 | 837 | this, one can control Perl's idea of filenames and line numbers in |
774d564b | 838 | error or warning messages (especially for strings that are processed |
f86cebdf | 839 | with C<eval()>). The syntax for this mechanism is the same as for most |
774d564b | 840 | C preprocessors: it matches the regular expression |
6ec4bd10 MS |
841 | |
842 | # example: '# line 42 "new_filename.plx"' | |
82d4537c | 843 | /^\# \s* |
6ec4bd10 | 844 | line \s+ (\d+) \s* |
7b6e93a8 | 845 | (?:\s("?)([^"]+)\2)? \s* |
6ec4bd10 MS |
846 | $/x |
847 | ||
7b6e93a8 CW |
848 | with C<$1> being the line number for the next line, and C<$3> being |
849 | the optional filename (specified with or without quotes). | |
774d564b | 850 | |
003183f2 GS |
851 | There is a fairly obvious gotcha included with the line directive: |
852 | Debuggers and profilers will only show the last source line to appear | |
853 | at a particular line number in a given file. Care should be taken not | |
854 | to cause line number collisions in code you'd like to debug later. | |
855 | ||
774d564b | 856 | Here are some examples that you should be able to type into your command |
857 | shell: | |
858 | ||
859 | % perl | |
860 | # line 200 "bzzzt" | |
861 | # the `#' on the previous line must be the first char on line | |
862 | die 'foo'; | |
863 | __END__ | |
864 | foo at bzzzt line 201. | |
54310121 | 865 | |
774d564b | 866 | % perl |
867 | # line 200 "bzzzt" | |
868 | eval qq[\n#line 2001 ""\ndie 'foo']; print $@; | |
869 | __END__ | |
870 | foo at - line 2001. | |
54310121 | 871 | |
774d564b | 872 | % perl |
873 | eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@; | |
874 | __END__ | |
875 | foo at foo bar line 200. | |
54310121 | 876 | |
774d564b | 877 | % perl |
878 | # line 345 "goop" | |
879 | eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'"; | |
880 | print $@; | |
881 | __END__ | |
882 | foo at goop line 345. | |
883 | ||
884 | =cut |