Commit | Line | Data |
---|---|---|
a0d0e21e | 1 | =head1 NAME |
d74e8afc | 2 | X<subroutine> X<function> |
a0d0e21e LW |
3 | |
4 | perlsub - Perl subroutines | |
5 | ||
6 | =head1 SYNOPSIS | |
7 | ||
8 | To declare subroutines: | |
d74e8afc | 9 | X<subroutine, declaration> X<sub> |
a0d0e21e | 10 | |
09bef843 SB |
11 | sub NAME; # A "forward" declaration. |
12 | sub NAME(PROTO); # ditto, but with prototypes | |
13 | sub NAME : ATTRS; # with attributes | |
14 | sub NAME(PROTO) : ATTRS; # with attributes and prototypes | |
cb1a09d0 | 15 | |
09bef843 SB |
16 | sub NAME BLOCK # A declaration and a definition. |
17 | sub NAME(PROTO) BLOCK # ditto, but with prototypes | |
63ccd0da | 18 | sub NAME(SIG) BLOCK # with a signature instead |
09bef843 SB |
19 | sub NAME : ATTRS BLOCK # with attributes |
20 | sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes | |
63ccd0da | 21 | sub NAME(SIG) : ATTRS BLOCK # with a signature and attributes |
a0d0e21e | 22 | |
748a9306 | 23 | To define an anonymous subroutine at runtime: |
d74e8afc | 24 | X<subroutine, anonymous> |
748a9306 | 25 | |
09bef843 SB |
26 | $subref = sub BLOCK; # no proto |
27 | $subref = sub (PROTO) BLOCK; # with proto | |
63ccd0da | 28 | $subref = sub (SIG) BLOCK; # with signature |
09bef843 SB |
29 | $subref = sub : ATTRS BLOCK; # with attributes |
30 | $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes | |
63ccd0da | 31 | $subref = sub (SIG) : ATTRS BLOCK; # with signature and attributes |
748a9306 | 32 | |
a0d0e21e | 33 | To import subroutines: |
d74e8afc | 34 | X<import> |
a0d0e21e | 35 | |
19799a22 | 36 | use MODULE qw(NAME1 NAME2 NAME3); |
a0d0e21e LW |
37 | |
38 | To call subroutines: | |
d74e8afc | 39 | X<subroutine, call> X<call> |
a0d0e21e | 40 | |
5f05dabc | 41 | NAME(LIST); # & is optional with parentheses. |
54310121 | 42 | NAME LIST; # Parentheses optional if predeclared/imported. |
19799a22 | 43 | &NAME(LIST); # Circumvent prototypes. |
5a964f20 | 44 | &NAME; # Makes current @_ visible to called subroutine. |
a0d0e21e LW |
45 | |
46 | =head1 DESCRIPTION | |
47 | ||
19799a22 GS |
48 | Like many languages, Perl provides for user-defined subroutines. |
49 | These may be located anywhere in the main program, loaded in from | |
50 | other files via the C<do>, C<require>, or C<use> keywords, or | |
be3174d2 | 51 | generated on the fly using C<eval> or anonymous subroutines. |
19799a22 GS |
52 | You can even call a function indirectly using a variable containing |
53 | its name or a CODE reference. | |
cb1a09d0 AD |
54 | |
55 | The Perl model for function call and return values is simple: all | |
56 | functions are passed as parameters one single flat list of scalars, and | |
57 | all functions likewise return to their caller one single flat list of | |
58 | scalars. Any arrays or hashes in these call and return lists will | |
59 | collapse, losing their identities--but you may always use | |
60 | pass-by-reference instead to avoid this. Both call and return lists may | |
61 | contain as many or as few scalar elements as you'd like. (Often a | |
62 | function without an explicit return statement is called a subroutine, but | |
19799a22 | 63 | there's really no difference from Perl's perspective.) |
d74e8afc | 64 | X<subroutine, parameter> X<parameter> |
19799a22 | 65 | |
30d9c59b Z |
66 | Any arguments passed in show up in the array C<@_>. |
67 | (They may also show up in lexical variables introduced by a signature; | |
68 | see L</Signatures> below.) Therefore, if | |
19799a22 GS |
69 | you called a function with two arguments, those would be stored in |
70 | C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its | |
71 | elements are aliases for the actual scalar parameters. In particular, | |
72 | if an element C<$_[0]> is updated, the corresponding argument is | |
73 | updated (or an error occurs if it is not updatable). If an argument | |
74 | is an array or hash element which did not exist when the function | |
75 | was called, that element is created only when (and if) it is modified | |
76 | or a reference to it is taken. (Some earlier versions of Perl | |
77 | created the element whether or not the element was assigned to.) | |
78 | Assigning to the whole array C<@_> removes that aliasing, and does | |
79 | not update any arguments. | |
d74e8afc | 80 | X<subroutine, argument> X<argument> X<@_> |
19799a22 | 81 | |
dbb128be XN |
82 | A C<return> statement may be used to exit a subroutine, optionally |
83 | specifying the returned value, which will be evaluated in the | |
84 | appropriate context (list, scalar, or void) depending on the context of | |
85 | the subroutine call. If you specify no return value, the subroutine | |
86 | returns an empty list in list context, the undefined value in scalar | |
87 | context, or nothing in void context. If you return one or more | |
88 | aggregates (arrays and hashes), these will be flattened together into | |
89 | one large indistinguishable list. | |
90 | ||
91 | If no C<return> is found and if the last statement is an expression, its | |
b77865f5 FC |
92 | value is returned. If the last statement is a loop control structure |
93 | like a C<foreach> or a C<while>, the returned value is unspecified. The | |
9a989771 | 94 | empty sub returns the empty list. |
d74e8afc | 95 | X<subroutine, return value> X<return value> X<return> |
19799a22 | 96 | |
30d9c59b | 97 | Aside from an experimental facility (see L</Signatures> below), |
19799a22 GS |
98 | Perl does not have named formal parameters. In practice all you |
99 | do is assign to a C<my()> list of these. Variables that aren't | |
100 | declared to be private are global variables. For gory details | |
5a0de581 LM |
101 | on creating private variables, see L</"Private Variables via my()"> |
102 | and L</"Temporary Values via local()">. To create protected | |
19799a22 GS |
103 | environments for a set of functions in a separate package (and |
104 | probably a separate file), see L<perlmod/"Packages">. | |
d74e8afc | 105 | X<formal parameter> X<parameter, formal> |
a0d0e21e LW |
106 | |
107 | Example: | |
108 | ||
cb1a09d0 AD |
109 | sub max { |
110 | my $max = shift(@_); | |
a0d0e21e LW |
111 | foreach $foo (@_) { |
112 | $max = $foo if $max < $foo; | |
113 | } | |
cb1a09d0 | 114 | return $max; |
a0d0e21e | 115 | } |
cb1a09d0 | 116 | $bestday = max($mon,$tue,$wed,$thu,$fri); |
a0d0e21e LW |
117 | |
118 | Example: | |
119 | ||
120 | # get a line, combining continuation lines | |
121 | # that start with whitespace | |
122 | ||
123 | sub get_line { | |
19799a22 | 124 | $thisline = $lookahead; # global variables! |
54310121 | 125 | LINE: while (defined($lookahead = <STDIN>)) { |
a0d0e21e LW |
126 | if ($lookahead =~ /^[ \t]/) { |
127 | $thisline .= $lookahead; | |
128 | } | |
129 | else { | |
130 | last LINE; | |
131 | } | |
132 | } | |
19799a22 | 133 | return $thisline; |
a0d0e21e LW |
134 | } |
135 | ||
136 | $lookahead = <STDIN>; # get first line | |
19799a22 | 137 | while (defined($line = get_line())) { |
a0d0e21e LW |
138 | ... |
139 | } | |
140 | ||
09bef843 | 141 | Assigning to a list of private variables to name your arguments: |
a0d0e21e LW |
142 | |
143 | sub maybeset { | |
144 | my($key, $value) = @_; | |
cb1a09d0 | 145 | $Foo{$key} = $value unless $Foo{$key}; |
a0d0e21e LW |
146 | } |
147 | ||
19799a22 GS |
148 | Because the assignment copies the values, this also has the effect |
149 | of turning call-by-reference into call-by-value. Otherwise a | |
150 | function is free to do in-place modifications of C<@_> and change | |
151 | its caller's values. | |
d74e8afc | 152 | X<call-by-reference> X<call-by-value> |
cb1a09d0 AD |
153 | |
154 | upcase_in($v1, $v2); # this changes $v1 and $v2 | |
155 | sub upcase_in { | |
54310121 | 156 | for (@_) { tr/a-z/A-Z/ } |
157 | } | |
cb1a09d0 AD |
158 | |
159 | You aren't allowed to modify constants in this way, of course. If an | |
160 | argument were actually literal and you tried to change it, you'd take a | |
161 | (presumably fatal) exception. For example, this won't work: | |
d74e8afc | 162 | X<call-by-reference> X<call-by-value> |
cb1a09d0 AD |
163 | |
164 | upcase_in("frederick"); | |
165 | ||
f86cebdf | 166 | It would be much safer if the C<upcase_in()> function |
cb1a09d0 AD |
167 | were written to return a copy of its parameters instead |
168 | of changing them in place: | |
169 | ||
19799a22 | 170 | ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2 |
cb1a09d0 | 171 | sub upcase { |
54310121 | 172 | return unless defined wantarray; # void context, do nothing |
cb1a09d0 | 173 | my @parms = @_; |
54310121 | 174 | for (@parms) { tr/a-z/A-Z/ } |
c07a80fd | 175 | return wantarray ? @parms : $parms[0]; |
54310121 | 176 | } |
cb1a09d0 | 177 | |
19799a22 | 178 | Notice how this (unprototyped) function doesn't care whether it was |
a2293a43 | 179 | passed real scalars or arrays. Perl sees all arguments as one big, |
19799a22 GS |
180 | long, flat parameter list in C<@_>. This is one area where |
181 | Perl's simple argument-passing style shines. The C<upcase()> | |
182 | function would work perfectly well without changing the C<upcase()> | |
183 | definition even if we fed it things like this: | |
cb1a09d0 AD |
184 | |
185 | @newlist = upcase(@list1, @list2); | |
186 | @newlist = upcase( split /:/, $var ); | |
187 | ||
188 | Do not, however, be tempted to do this: | |
189 | ||
190 | (@a, @b) = upcase(@list1, @list2); | |
191 | ||
19799a22 GS |
192 | Like the flattened incoming parameter list, the return list is also |
193 | flattened on return. So all you have managed to do here is stored | |
17b63f68 | 194 | everything in C<@a> and made C<@b> empty. See |
5a0de581 | 195 | L</Pass by Reference> for alternatives. |
19799a22 GS |
196 | |
197 | A subroutine may be called using an explicit C<&> prefix. The | |
198 | C<&> is optional in modern Perl, as are parentheses if the | |
199 | subroutine has been predeclared. The C<&> is I<not> optional | |
200 | when just naming the subroutine, such as when it's used as | |
201 | an argument to defined() or undef(). Nor is it optional when you | |
202 | want to do an indirect subroutine call with a subroutine name or | |
203 | reference using the C<&$subref()> or C<&{$subref}()> constructs, | |
c47ff5f1 | 204 | although the C<< $subref->() >> notation solves that problem. |
19799a22 | 205 | See L<perlref> for more about all that. |
d74e8afc | 206 | X<&> |
19799a22 GS |
207 | |
208 | Subroutines may be called recursively. If a subroutine is called | |
209 | using the C<&> form, the argument list is optional, and if omitted, | |
210 | no C<@_> array is set up for the subroutine: the C<@_> array at the | |
211 | time of the call is visible to subroutine instead. This is an | |
212 | efficiency mechanism that new users may wish to avoid. | |
d74e8afc | 213 | X<recursion> |
a0d0e21e LW |
214 | |
215 | &foo(1,2,3); # pass three arguments | |
216 | foo(1,2,3); # the same | |
217 | ||
218 | foo(); # pass a null list | |
219 | &foo(); # the same | |
a0d0e21e | 220 | |
cb1a09d0 | 221 | &foo; # foo() get current args, like foo(@_) !! |
54310121 | 222 | foo; # like foo() IFF sub foo predeclared, else "foo" |
cb1a09d0 | 223 | |
19799a22 GS |
224 | Not only does the C<&> form make the argument list optional, it also |
225 | disables any prototype checking on arguments you do provide. This | |
c07a80fd | 226 | is partly for historical reasons, and partly for having a convenient way |
9688be67 | 227 | to cheat if you know what you're doing. See L</Prototypes> below. |
d74e8afc | 228 | X<&> |
c07a80fd | 229 | |
977616ef RS |
230 | Since Perl 5.16.0, the C<__SUB__> token is available under C<use feature |
231 | 'current_sub'> and C<use 5.16.0>. It will evaluate to a reference to the | |
906024c7 FC |
232 | currently-running sub, which allows for recursive calls without knowing |
233 | your subroutine's name. | |
977616ef RS |
234 | |
235 | use 5.16.0; | |
236 | my $factorial = sub { | |
237 | my ($x) = @_; | |
238 | return 1 if $x == 1; | |
239 | return($x * __SUB__->( $x - 1 ) ); | |
240 | }; | |
241 | ||
89d1beed | 242 | The behavior of C<__SUB__> within a regex code block (such as C</(?{...})/>) |
a453e28a DM |
243 | is subject to change. |
244 | ||
ac90fb77 EM |
245 | Subroutines whose names are in all upper case are reserved to the Perl |
246 | core, as are modules whose names are in all lower case. A subroutine in | |
247 | all capitals is a loosely-held convention meaning it will be called | |
248 | indirectly by the run-time system itself, usually due to a triggered event. | |
bf5513e0 | 249 | Subroutines whose name start with a left parenthesis are also reserved the |
b77865f5 | 250 | same way. The following is a list of some subroutines that currently do |
bf5513e0 ZA |
251 | special, pre-defined things. |
252 | ||
253 | =over | |
254 | ||
255 | =item documented later in this document | |
256 | ||
257 | C<AUTOLOAD> | |
258 | ||
259 | =item documented in L<perlmod> | |
260 | ||
8b7906d1 | 261 | C<CLONE>, C<CLONE_SKIP> |
bf5513e0 ZA |
262 | |
263 | =item documented in L<perlobj> | |
264 | ||
8b7906d1 | 265 | C<DESTROY>, C<DOES> |
bf5513e0 ZA |
266 | |
267 | =item documented in L<perltie> | |
268 | ||
269 | C<BINMODE>, C<CLEAR>, C<CLOSE>, C<DELETE>, C<DESTROY>, C<EOF>, C<EXISTS>, | |
270 | C<EXTEND>, C<FETCH>, C<FETCHSIZE>, C<FILENO>, C<FIRSTKEY>, C<GETC>, | |
271 | C<NEXTKEY>, C<OPEN>, C<POP>, C<PRINT>, C<PRINTF>, C<PUSH>, C<READ>, | |
272 | C<READLINE>, C<SCALAR>, C<SEEK>, C<SHIFT>, C<SPLICE>, C<STORE>, | |
273 | C<STORESIZE>, C<TELL>, C<TIEARRAY>, C<TIEHANDLE>, C<TIEHASH>, | |
274 | C<TIESCALAR>, C<UNSHIFT>, C<UNTIE>, C<WRITE> | |
275 | ||
276 | =item documented in L<PerlIO::via> | |
277 | ||
278 | C<BINMODE>, C<CLEARERR>, C<CLOSE>, C<EOF>, C<ERROR>, C<FDOPEN>, C<FILENO>, | |
279 | C<FILL>, C<FLUSH>, C<OPEN>, C<POPPED>, C<PUSHED>, C<READ>, C<SEEK>, | |
280 | C<SETLINEBUF>, C<SYSOPEN>, C<TELL>, C<UNREAD>, C<UTF8>, C<WRITE> | |
281 | ||
ec2eb8a9 TC |
282 | =item documented in L<perlfunc> |
283 | ||
284 | L<< C<import> | perlfunc/use >>, L<< C<unimport> | perlfunc/use >>, | |
285 | L<< C<INC> | perlfunc/require >> | |
286 | ||
287 | =item documented in L<UNIVERSAL> | |
288 | ||
289 | C<VERSION> | |
290 | ||
291 | =item documented in L<perldebguts> | |
292 | ||
293 | C<DB::DB>, C<DB::sub>, C<DB::lsub>, C<DB::goto>, C<DB::postponed> | |
294 | ||
bf5513e0 ZA |
295 | =item undocumented, used internally by the L<overload> feature |
296 | ||
297 | any starting with C<(> | |
298 | ||
299 | =back | |
ac90fb77 | 300 | |
3c10abe3 AG |
301 | The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines |
302 | are not so much subroutines as named special code blocks, of which you | |
303 | can have more than one in a package, and which you can B<not> call | |
304 | explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> | |
5a964f20 | 305 | |
30d9c59b Z |
306 | =head2 Signatures |
307 | ||
308 | B<WARNING>: Subroutine signatures are experimental. The feature may be | |
309 | modified or removed in future versions of Perl. | |
310 | ||
311 | Perl has an experimental facility to allow a subroutine's formal | |
312 | parameters to be introduced by special syntax, separate from the | |
313 | procedural code of the subroutine body. The formal parameter list | |
314 | is known as a I<signature>. The facility must be enabled first by a | |
315 | pragmatic declaration, C<use feature 'signatures'>, and it will produce | |
316 | a warning unless the "experimental::signatures" warnings category is | |
317 | disabled. | |
318 | ||
319 | The signature is part of a subroutine's body. Normally the body of a | |
320 | subroutine is simply a braced block of code. When using a signature, | |
fa14274e | 321 | the signature is a parenthesised list that goes immediately after |
55121b6b RS |
322 | the subroutine name (or, for anonymous subroutines, immediately after |
323 | the C<sub> keyword). The signature declares lexical variables that are | |
30d9c59b Z |
324 | in scope for the block. When the subroutine is called, the signature |
325 | takes control first. It populates the signature variables from the | |
326 | list of arguments that were passed. If the argument list doesn't meet | |
327 | the requirements of the signature, then it will throw an exception. | |
328 | When the signature processing is complete, control passes to the block. | |
329 | ||
330 | Positional parameters are handled by simply naming scalar variables in | |
331 | the signature. For example, | |
332 | ||
333 | sub foo ($left, $right) { | |
334 | return $left + $right; | |
335 | } | |
336 | ||
337 | takes two positional parameters, which must be filled at runtime by | |
338 | two arguments. By default the parameters are mandatory, and it is | |
339 | not permitted to pass more arguments than expected. So the above is | |
340 | equivalent to | |
341 | ||
342 | sub foo { | |
343 | die "Too many arguments for subroutine" unless @_ <= 2; | |
344 | die "Too few arguments for subroutine" unless @_ >= 2; | |
345 | my $left = $_[0]; | |
346 | my $right = $_[1]; | |
347 | return $left + $right; | |
348 | } | |
349 | ||
350 | An argument can be ignored by omitting the main part of the name from | |
351 | a parameter declaration, leaving just a bare C<$> sigil. For example, | |
352 | ||
353 | sub foo ($first, $, $third) { | |
354 | return "first=$first, third=$third"; | |
355 | } | |
356 | ||
357 | Although the ignored argument doesn't go into a variable, it is still | |
358 | mandatory for the caller to pass it. | |
359 | ||
360 | A positional parameter is made optional by giving a default value, | |
361 | separated from the parameter name by C<=>: | |
362 | ||
363 | sub foo ($left, $right = 0) { | |
364 | return $left + $right; | |
365 | } | |
366 | ||
367 | The above subroutine may be called with either one or two arguments. | |
368 | The default value expression is evaluated when the subroutine is called, | |
369 | so it may provide different default values for different calls. It is | |
370 | only evaluated if the argument was actually omitted from the call. | |
371 | For example, | |
372 | ||
373 | my $auto_id = 0; | |
374 | sub foo ($thing, $id = $auto_id++) { | |
375 | print "$thing has ID $id"; | |
376 | } | |
377 | ||
378 | automatically assigns distinct sequential IDs to things for which no | |
379 | ID was supplied by the caller. A default value expression may also | |
380 | refer to parameters earlier in the signature, making the default for | |
381 | one parameter vary according to the earlier parameters. For example, | |
382 | ||
383 | sub foo ($first_name, $surname, $nickname = $first_name) { | |
384 | print "$first_name $surname is known as \"$nickname\""; | |
385 | } | |
386 | ||
387 | An optional parameter can be nameless just like a mandatory parameter. | |
388 | For example, | |
389 | ||
390 | sub foo ($thing, $ = 1) { | |
391 | print $thing; | |
392 | } | |
393 | ||
394 | The parameter's default value will still be evaluated if the corresponding | |
395 | argument isn't supplied, even though the value won't be stored anywhere. | |
396 | This is in case evaluating it has important side effects. However, it | |
397 | will be evaluated in void context, so if it doesn't have side effects | |
398 | and is not trivial it will generate a warning if the "void" warning | |
399 | category is enabled. If a nameless optional parameter's default value | |
400 | is not important, it may be omitted just as the parameter's name was: | |
401 | ||
402 | sub foo ($thing, $=) { | |
403 | print $thing; | |
404 | } | |
405 | ||
406 | Optional positional parameters must come after all mandatory positional | |
407 | parameters. (If there are no mandatory positional parameters then an | |
408 | optional positional parameters can be the first thing in the signature.) | |
409 | If there are multiple optional positional parameters and not enough | |
410 | arguments are supplied to fill them all, they will be filled from left | |
411 | to right. | |
412 | ||
413 | After positional parameters, additional arguments may be captured in a | |
414 | slurpy parameter. The simplest form of this is just an array variable: | |
415 | ||
416 | sub foo ($filter, @inputs) { | |
417 | print $filter->($_) foreach @inputs; | |
418 | } | |
419 | ||
420 | With a slurpy parameter in the signature, there is no upper limit on how | |
421 | many arguments may be passed. A slurpy array parameter may be nameless | |
422 | just like a positional parameter, in which case its only effect is to | |
423 | turn off the argument limit that would otherwise apply: | |
424 | ||
425 | sub foo ($thing, @) { | |
426 | print $thing; | |
427 | } | |
428 | ||
429 | A slurpy parameter may instead be a hash, in which case the arguments | |
430 | available to it are interpreted as alternating keys and values. | |
431 | There must be as many keys as values: if there is an odd argument then | |
432 | an exception will be thrown. Keys will be stringified, and if there are | |
433 | duplicates then the later instance takes precedence over the earlier, | |
434 | as with standard hash construction. | |
435 | ||
436 | sub foo ($filter, %inputs) { | |
437 | print $filter->($_, $inputs{$_}) foreach sort keys %inputs; | |
438 | } | |
439 | ||
440 | A slurpy hash parameter may be nameless just like other kinds of | |
441 | parameter. It still insists that the number of arguments available to | |
442 | it be even, even though they're not being put into a variable. | |
443 | ||
444 | sub foo ($thing, %) { | |
445 | print $thing; | |
446 | } | |
447 | ||
448 | A slurpy parameter, either array or hash, must be the last thing in the | |
449 | signature. It may follow mandatory and optional positional parameters; | |
450 | it may also be the only thing in the signature. Slurpy parameters cannot | |
451 | have default values: if no arguments are supplied for them then you get | |
452 | an empty array or empty hash. | |
453 | ||
454 | A signature may be entirely empty, in which case all it does is check | |
455 | that the caller passed no arguments: | |
456 | ||
457 | sub foo () { | |
458 | return 123; | |
459 | } | |
460 | ||
8c13e946 RS |
461 | When using a signature, the arguments are still available in the special |
462 | array variable C<@_>, in addition to the lexical variables of the | |
463 | signature. There is a difference between the two ways of accessing the | |
30d9c59b Z |
464 | arguments: C<@_> I<aliases> the arguments, but the signature variables |
465 | get I<copies> of the arguments. So writing to a signature variable | |
466 | only changes that variable, and has no effect on the caller's variables, | |
467 | but writing to an element of C<@_> modifies whatever the caller used to | |
468 | supply that argument. | |
469 | ||
470 | There is a potential syntactic ambiguity between signatures and prototypes | |
471 | (see L</Prototypes>), because both start with an opening parenthesis and | |
472 | both can appear in some of the same places, such as just after the name | |
473 | in a subroutine declaration. For historical reasons, when signatures | |
474 | are not enabled, any opening parenthesis in such a context will trigger | |
475 | very forgiving prototype parsing. Most signatures will be interpreted | |
476 | as prototypes in those circumstances, but won't be valid prototypes. | |
477 | (A valid prototype cannot contain any alphabetic character.) This will | |
478 | lead to somewhat confusing error messages. | |
479 | ||
480 | To avoid ambiguity, when signatures are enabled the special syntax | |
481 | for prototypes is disabled. There is no attempt to guess whether a | |
482 | parenthesised group was intended to be a prototype or a signature. | |
483 | To give a subroutine a prototype under these circumstances, use a | |
484 | L<prototype attribute|attributes/Built-in Attributes>. For example, | |
485 | ||
486 | sub foo :prototype($) { $_[0] } | |
487 | ||
488 | It is entirely possible for a subroutine to have both a prototype and | |
489 | a signature. They do different jobs: the prototype affects compilation | |
490 | of calls to the subroutine, and the signature puts argument values into | |
491 | lexical variables at runtime. You can therefore write | |
492 | ||
71917f62 | 493 | sub foo ($left, $right) : prototype($$) { |
30d9c59b Z |
494 | return $left + $right; |
495 | } | |
496 | ||
71917f62 PM |
497 | The prototype attribute, and any other attributes, come after |
498 | the signature. | |
30d9c59b | 499 | |
b687b08b | 500 | =head2 Private Variables via my() |
d74e8afc ITB |
501 | X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical> |
502 | X<lexical scope> X<attributes, my> | |
cb1a09d0 AD |
503 | |
504 | Synopsis: | |
505 | ||
506 | my $foo; # declare $foo lexically local | |
507 | my (@wid, %get); # declare list of variables local | |
508 | my $foo = "flurp"; # declare $foo lexical, and init it | |
509 | my @oof = @bar; # declare @oof lexical, and init it | |
09bef843 SB |
510 | my $x : Foo = $y; # similar, with an attribute applied |
511 | ||
a0ae32d3 JH |
512 | B<WARNING>: The use of attribute lists on C<my> declarations is still |
513 | evolving. The current semantics and interface are subject to change. | |
514 | See L<attributes> and L<Attribute::Handlers>. | |
cb1a09d0 | 515 | |
19799a22 | 516 | The C<my> operator declares the listed variables to be lexically |
f185f654 KW |
517 | confined to the enclosing block, conditional |
518 | (C<if>/C<unless>/C<elsif>/C<else>), loop | |
519 | (C<for>/C<foreach>/C<while>/C<until>/C<continue>), subroutine, C<eval>, | |
520 | or C<do>/C<require>/C<use>'d file. If more than one value is listed, the | |
19799a22 GS |
521 | list must be placed in parentheses. All listed elements must be |
522 | legal lvalues. Only alphanumeric identifiers may be lexically | |
325192b1 | 523 | scoped--magical built-ins like C<$/> must currently be C<local>ized |
19799a22 GS |
524 | with C<local> instead. |
525 | ||
526 | Unlike dynamic variables created by the C<local> operator, lexical | |
527 | variables declared with C<my> are totally hidden from the outside | |
528 | world, including any called subroutines. This is true if it's the | |
529 | same subroutine called from itself or elsewhere--every call gets | |
530 | its own copy. | |
d74e8afc | 531 | X<local> |
19799a22 GS |
532 | |
533 | This doesn't mean that a C<my> variable declared in a statically | |
534 | enclosing lexical scope would be invisible. Only dynamic scopes | |
535 | are cut off. For example, the C<bumpx()> function below has access | |
536 | to the lexical $x variable because both the C<my> and the C<sub> | |
537 | occurred at the same scope, presumably file scope. | |
5a964f20 TC |
538 | |
539 | my $x = 10; | |
540 | sub bumpx { $x++ } | |
541 | ||
19799a22 GS |
542 | An C<eval()>, however, can see lexical variables of the scope it is |
543 | being evaluated in, so long as the names aren't hidden by declarations within | |
544 | the C<eval()> itself. See L<perlref>. | |
d74e8afc | 545 | X<eval, scope of> |
cb1a09d0 | 546 | |
19799a22 | 547 | The parameter list to my() may be assigned to if desired, which allows you |
cb1a09d0 AD |
548 | to initialize your variables. (If no initializer is given for a |
549 | particular variable, it is created with the undefined value.) Commonly | |
19799a22 | 550 | this is used to name input parameters to a subroutine. Examples: |
cb1a09d0 AD |
551 | |
552 | $arg = "fred"; # "global" variable | |
553 | $n = cube_root(27); | |
554 | print "$arg thinks the root is $n\n"; | |
555 | fred thinks the root is 3 | |
556 | ||
557 | sub cube_root { | |
558 | my $arg = shift; # name doesn't matter | |
559 | $arg **= 1/3; | |
560 | return $arg; | |
54310121 | 561 | } |
cb1a09d0 | 562 | |
19799a22 GS |
563 | The C<my> is simply a modifier on something you might assign to. So when |
564 | you do assign to variables in its argument list, C<my> doesn't | |
6cc33c6d | 565 | change whether those variables are viewed as a scalar or an array. So |
cb1a09d0 | 566 | |
5a964f20 | 567 | my ($foo) = <STDIN>; # WRONG? |
cb1a09d0 AD |
568 | my @FOO = <STDIN>; |
569 | ||
5f05dabc | 570 | both supply a list context to the right-hand side, while |
cb1a09d0 AD |
571 | |
572 | my $foo = <STDIN>; | |
573 | ||
5f05dabc | 574 | supplies a scalar context. But the following declares only one variable: |
748a9306 | 575 | |
5a964f20 | 576 | my $foo, $bar = 1; # WRONG |
748a9306 | 577 | |
cb1a09d0 | 578 | That has the same effect as |
748a9306 | 579 | |
cb1a09d0 AD |
580 | my $foo; |
581 | $bar = 1; | |
a0d0e21e | 582 | |
cb1a09d0 AD |
583 | The declared variable is not introduced (is not visible) until after |
584 | the current statement. Thus, | |
585 | ||
586 | my $x = $x; | |
587 | ||
19799a22 | 588 | can be used to initialize a new $x with the value of the old $x, and |
cb1a09d0 AD |
589 | the expression |
590 | ||
591 | my $x = 123 and $x == 123 | |
592 | ||
19799a22 | 593 | is false unless the old $x happened to have the value C<123>. |
cb1a09d0 | 594 | |
55497cff | 595 | Lexical scopes of control structures are not bounded precisely by the |
596 | braces that delimit their controlled blocks; control expressions are | |
19799a22 | 597 | part of that scope, too. Thus in the loop |
55497cff | 598 | |
19799a22 | 599 | while (my $line = <>) { |
55497cff | 600 | $line = lc $line; |
601 | } continue { | |
602 | print $line; | |
603 | } | |
604 | ||
19799a22 | 605 | the scope of $line extends from its declaration throughout the rest of |
55497cff | 606 | the loop construct (including the C<continue> clause), but not beyond |
607 | it. Similarly, in the conditional | |
608 | ||
609 | if ((my $answer = <STDIN>) =~ /^yes$/i) { | |
610 | user_agrees(); | |
611 | } elsif ($answer =~ /^no$/i) { | |
612 | user_disagrees(); | |
613 | } else { | |
614 | chomp $answer; | |
615 | die "'$answer' is neither 'yes' nor 'no'"; | |
616 | } | |
617 | ||
19799a22 GS |
618 | the scope of $answer extends from its declaration through the rest |
619 | of that conditional, including any C<elsif> and C<else> clauses, | |
96090e4f | 620 | but not beyond it. See L<perlsyn/"Simple Statements"> for information |
457b36cb | 621 | on the scope of variables in statements with modifiers. |
55497cff | 622 | |
5f05dabc | 623 | The C<foreach> loop defaults to scoping its index variable dynamically |
19799a22 GS |
624 | in the manner of C<local>. However, if the index variable is |
625 | prefixed with the keyword C<my>, or if there is already a lexical | |
626 | by that name in scope, then a new lexical is created instead. Thus | |
627 | in the loop | |
d74e8afc | 628 | X<foreach> X<for> |
55497cff | 629 | |
630 | for my $i (1, 2, 3) { | |
631 | some_function(); | |
632 | } | |
633 | ||
19799a22 GS |
634 | the scope of $i extends to the end of the loop, but not beyond it, |
635 | rendering the value of $i inaccessible within C<some_function()>. | |
d74e8afc | 636 | X<foreach> X<for> |
55497cff | 637 | |
cb1a09d0 | 638 | Some users may wish to encourage the use of lexically scoped variables. |
19799a22 GS |
639 | As an aid to catching implicit uses to package variables, |
640 | which are always global, if you say | |
cb1a09d0 AD |
641 | |
642 | use strict 'vars'; | |
643 | ||
19799a22 GS |
644 | then any variable mentioned from there to the end of the enclosing |
645 | block must either refer to a lexical variable, be predeclared via | |
77ca0c92 | 646 | C<our> or C<use vars>, or else must be fully qualified with the package name. |
19799a22 GS |
647 | A compilation error results otherwise. An inner block may countermand |
648 | this with C<no strict 'vars'>. | |
649 | ||
650 | A C<my> has both a compile-time and a run-time effect. At compile | |
8593bda5 | 651 | time, the compiler takes notice of it. The principal usefulness |
19799a22 GS |
652 | of this is to quiet C<use strict 'vars'>, but it is also essential |
653 | for generation of closures as detailed in L<perlref>. Actual | |
654 | initialization is delayed until run time, though, so it gets executed | |
655 | at the appropriate time, such as each time through a loop, for | |
656 | example. | |
657 | ||
658 | Variables declared with C<my> are not part of any package and are therefore | |
cb1a09d0 AD |
659 | never fully qualified with the package name. In particular, you're not |
660 | allowed to try to make a package variable (or other global) lexical: | |
661 | ||
662 | my $pack::var; # ERROR! Illegal syntax | |
cb1a09d0 AD |
663 | |
664 | In fact, a dynamic variable (also known as package or global variables) | |
f86cebdf | 665 | are still accessible using the fully qualified C<::> notation even while a |
cb1a09d0 AD |
666 | lexical of the same name is also visible: |
667 | ||
668 | package main; | |
669 | local $x = 10; | |
670 | my $x = 20; | |
671 | print "$x and $::x\n"; | |
672 | ||
f86cebdf | 673 | That will print out C<20> and C<10>. |
cb1a09d0 | 674 | |
19799a22 GS |
675 | You may declare C<my> variables at the outermost scope of a file |
676 | to hide any such identifiers from the world outside that file. This | |
677 | is similar in spirit to C's static variables when they are used at | |
678 | the file level. To do this with a subroutine requires the use of | |
679 | a closure (an anonymous function that accesses enclosing lexicals). | |
680 | If you want to create a private subroutine that cannot be called | |
681 | from outside that block, it can declare a lexical variable containing | |
682 | an anonymous sub reference: | |
cb1a09d0 AD |
683 | |
684 | my $secret_version = '1.001-beta'; | |
685 | my $secret_sub = sub { print $secret_version }; | |
686 | &$secret_sub(); | |
687 | ||
688 | As long as the reference is never returned by any function within the | |
5f05dabc | 689 | module, no outside module can see the subroutine, because its name is not in |
cb1a09d0 | 690 | any package's symbol table. Remember that it's not I<REALLY> called |
19799a22 | 691 | C<$some_pack::secret_version> or anything; it's just $secret_version, |
cb1a09d0 AD |
692 | unqualified and unqualifiable. |
693 | ||
19799a22 GS |
694 | This does not work with object methods, however; all object methods |
695 | have to be in the symbol table of some package to be found. See | |
696 | L<perlref/"Function Templates"> for something of a work-around to | |
697 | this. | |
cb1a09d0 | 698 | |
c2611fb3 | 699 | =head2 Persistent Private Variables |
ba1f8e91 RGS |
700 | X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure> |
701 | ||
702 | There are two ways to build persistent private variables in Perl 5.10. | |
b77865f5 | 703 | First, you can simply use the C<state> feature. Or, you can use closures, |
ba1f8e91 RGS |
704 | if you want to stay compatible with releases older than 5.10. |
705 | ||
706 | =head3 Persistent variables via state() | |
707 | ||
9d42615f | 708 | Beginning with Perl 5.10.0, you can declare variables with the C<state> |
4a904372 | 709 | keyword in place of C<my>. For that to work, though, you must have |
ba1f8e91 | 710 | enabled that feature beforehand, either by using the C<feature> pragma, or |
4a904372 | 711 | by using C<-E> on one-liners (see L<feature>). Beginning with Perl 5.16, |
47d235f1 | 712 | the C<CORE::state> form does not require the |
4a904372 | 713 | C<feature> pragma. |
ba1f8e91 | 714 | |
ad0cc46c FC |
715 | The C<state> keyword creates a lexical variable (following the same scoping |
716 | rules as C<my>) that persists from one subroutine call to the next. If a | |
717 | state variable resides inside an anonymous subroutine, then each copy of | |
718 | the subroutine has its own copy of the state variable. However, the value | |
719 | of the state variable will still persist between calls to the same copy of | |
720 | the anonymous subroutine. (Don't forget that C<sub { ... }> creates a new | |
721 | subroutine each time it is executed.) | |
722 | ||
ba1f8e91 RGS |
723 | For example, the following code maintains a private counter, incremented |
724 | each time the gimme_another() function is called: | |
725 | ||
726 | use feature 'state'; | |
727 | sub gimme_another { state $x; return ++$x } | |
728 | ||
ad0cc46c FC |
729 | And this example uses anonymous subroutines to create separate counters: |
730 | ||
731 | use feature 'state'; | |
732 | sub create_counter { | |
733 | return sub { state $x; return ++$x } | |
734 | } | |
735 | ||
ba1f8e91 RGS |
736 | Also, since C<$x> is lexical, it can't be reached or modified by any Perl |
737 | code outside. | |
738 | ||
f292fc7a RS |
739 | When combined with variable declaration, simple scalar assignment to C<state> |
740 | variables (as in C<state $x = 42>) is executed only the first time. When such | |
741 | statements are evaluated subsequent times, the assignment is ignored. The | |
742 | behavior of this sort of assignment to non-scalar variables is undefined. | |
ba1f8e91 RGS |
743 | |
744 | =head3 Persistent variables with closures | |
5a964f20 TC |
745 | |
746 | Just because a lexical variable is lexically (also called statically) | |
f86cebdf | 747 | scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that |
5a964f20 TC |
748 | within a function it works like a C static. It normally works more |
749 | like a C auto, but with implicit garbage collection. | |
750 | ||
751 | Unlike local variables in C or C++, Perl's lexical variables don't | |
752 | necessarily get recycled just because their scope has exited. | |
753 | If something more permanent is still aware of the lexical, it will | |
754 | stick around. So long as something else references a lexical, that | |
755 | lexical won't be freed--which is as it should be. You wouldn't want | |
756 | memory being free until you were done using it, or kept around once you | |
757 | were done. Automatic garbage collection takes care of this for you. | |
758 | ||
759 | This means that you can pass back or save away references to lexical | |
760 | variables, whereas to return a pointer to a C auto is a grave error. | |
761 | It also gives us a way to simulate C's function statics. Here's a | |
762 | mechanism for giving a function private variables with both lexical | |
763 | scoping and a static lifetime. If you do want to create something like | |
764 | C's static variables, just enclose the whole function in an extra block, | |
765 | and put the static variable outside the function but in the block. | |
cb1a09d0 AD |
766 | |
767 | { | |
54310121 | 768 | my $secret_val = 0; |
cb1a09d0 AD |
769 | sub gimme_another { |
770 | return ++$secret_val; | |
54310121 | 771 | } |
772 | } | |
cb1a09d0 AD |
773 | # $secret_val now becomes unreachable by the outside |
774 | # world, but retains its value between calls to gimme_another | |
775 | ||
54310121 | 776 | If this function is being sourced in from a separate file |
cb1a09d0 | 777 | via C<require> or C<use>, then this is probably just fine. If it's |
19799a22 | 778 | all in the main program, you'll need to arrange for the C<my> |
cb1a09d0 | 779 | to be executed early, either by putting the whole block above |
f86cebdf | 780 | your main program, or more likely, placing merely a C<BEGIN> |
ac90fb77 | 781 | code block around it to make sure it gets executed before your program |
cb1a09d0 AD |
782 | starts to run: |
783 | ||
ac90fb77 | 784 | BEGIN { |
54310121 | 785 | my $secret_val = 0; |
cb1a09d0 AD |
786 | sub gimme_another { |
787 | return ++$secret_val; | |
54310121 | 788 | } |
789 | } | |
cb1a09d0 | 790 | |
3c10abe3 AG |
791 | See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the |
792 | special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>, | |
793 | C<INIT> and C<END>. | |
cb1a09d0 | 794 | |
19799a22 GS |
795 | If declared at the outermost scope (the file scope), then lexicals |
796 | work somewhat like C's file statics. They are available to all | |
797 | functions in that same file declared below them, but are inaccessible | |
798 | from outside that file. This strategy is sometimes used in modules | |
799 | to create private variables that the whole module can see. | |
5a964f20 | 800 | |
cb1a09d0 | 801 | =head2 Temporary Values via local() |
d74e8afc ITB |
802 | X<local> X<scope, dynamic> X<dynamic scope> X<variable, local> |
803 | X<variable, temporary> | |
cb1a09d0 | 804 | |
19799a22 | 805 | B<WARNING>: In general, you should be using C<my> instead of C<local>, because |
6d28dffb | 806 | it's faster and safer. Exceptions to this include the global punctuation |
325192b1 RGS |
807 | variables, global filehandles and formats, and direct manipulation of the |
808 | Perl symbol table itself. C<local> is mostly used when the current value | |
809 | of a variable must be visible to called subroutines. | |
cb1a09d0 AD |
810 | |
811 | Synopsis: | |
812 | ||
325192b1 RGS |
813 | # localization of values |
814 | ||
555bd962 BG |
815 | local $foo; # make $foo dynamically local |
816 | local (@wid, %get); # make list of variables local | |
817 | local $foo = "flurp"; # make $foo dynamic, and init it | |
818 | local @oof = @bar; # make @oof dynamic, and init it | |
325192b1 | 819 | |
555bd962 BG |
820 | local $hash{key} = "val"; # sets a local value for this hash entry |
821 | delete local $hash{key}; # delete this entry for the current block | |
822 | local ($cond ? $v1 : $v2); # several types of lvalues support | |
823 | # localization | |
325192b1 RGS |
824 | |
825 | # localization of symbols | |
cb1a09d0 | 826 | |
555bd962 BG |
827 | local *FH; # localize $FH, @FH, %FH, &FH ... |
828 | local *merlyn = *randal; # now $merlyn is really $randal, plus | |
829 | # @merlyn is really @randal, etc | |
830 | local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal | |
831 | local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc | |
cb1a09d0 | 832 | |
19799a22 GS |
833 | A C<local> modifies its listed variables to be "local" to the |
834 | enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine | |
835 | called from within that block>. A C<local> just gives temporary | |
836 | values to global (meaning package) variables. It does I<not> create | |
837 | a local variable. This is known as dynamic scoping. Lexical scoping | |
838 | is done with C<my>, which works more like C's auto declarations. | |
cb1a09d0 | 839 | |
ceb12f1f | 840 | Some types of lvalues can be localized as well: hash and array elements |
325192b1 RGS |
841 | and slices, conditionals (provided that their result is always |
842 | localizable), and symbolic references. As for simple variables, this | |
843 | creates new, dynamically scoped values. | |
844 | ||
845 | If more than one variable or expression is given to C<local>, they must be | |
846 | placed in parentheses. This operator works | |
cb1a09d0 | 847 | by saving the current values of those variables in its argument list on a |
5f05dabc | 848 | hidden stack and restoring them upon exiting the block, subroutine, or |
cb1a09d0 AD |
849 | eval. This means that called subroutines can also reference the local |
850 | variable, but not the global one. The argument list may be assigned to if | |
851 | desired, which allows you to initialize your local variables. (If no | |
852 | initializer is given for a particular variable, it is created with an | |
325192b1 | 853 | undefined value.) |
cb1a09d0 | 854 | |
19799a22 | 855 | Because C<local> is a run-time operator, it gets executed each time |
325192b1 RGS |
856 | through a loop. Consequently, it's more efficient to localize your |
857 | variables outside the loop. | |
858 | ||
859 | =head3 Grammatical note on local() | |
d74e8afc | 860 | X<local, context> |
cb1a09d0 | 861 | |
f86cebdf GS |
862 | A C<local> is simply a modifier on an lvalue expression. When you assign to |
863 | a C<local>ized variable, the C<local> doesn't change whether its list is viewed | |
cb1a09d0 AD |
864 | as a scalar or an array. So |
865 | ||
866 | local($foo) = <STDIN>; | |
867 | local @FOO = <STDIN>; | |
868 | ||
5f05dabc | 869 | both supply a list context to the right-hand side, while |
cb1a09d0 AD |
870 | |
871 | local $foo = <STDIN>; | |
872 | ||
873 | supplies a scalar context. | |
874 | ||
325192b1 | 875 | =head3 Localization of special variables |
d74e8afc | 876 | X<local, special variable> |
3e3baf6d | 877 | |
325192b1 RGS |
878 | If you localize a special variable, you'll be giving a new value to it, |
879 | but its magic won't go away. That means that all side-effects related | |
880 | to this magic still work with the localized value. | |
3e3baf6d | 881 | |
325192b1 RGS |
882 | This feature allows code like this to work : |
883 | ||
884 | # Read the whole contents of FILE in $slurp | |
885 | { local $/ = undef; $slurp = <FILE>; } | |
886 | ||
887 | Note, however, that this restricts localization of some values ; for | |
9d42615f | 888 | example, the following statement dies, as of perl 5.10.0, with an error |
325192b1 RGS |
889 | I<Modification of a read-only value attempted>, because the $1 variable is |
890 | magical and read-only : | |
891 | ||
892 | local $1 = 2; | |
893 | ||
658a9f31 JD |
894 | One exception is the default scalar variable: starting with perl 5.14 |
895 | C<local($_)> will always strip all magic from $_, to make it possible | |
896 | to safely reuse $_ in a subroutine. | |
325192b1 RGS |
897 | |
898 | B<WARNING>: Localization of tied arrays and hashes does not currently | |
899 | work as described. | |
fd5a896a | 900 | This will be fixed in a future release of Perl; in the meantime, avoid |
89d1beed | 901 | code that relies on any particular behavior of localising tied arrays |
fd5a896a | 902 | or hashes (localising individual elements is still okay). |
325192b1 | 903 | See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more |
fd5a896a | 904 | details. |
d74e8afc | 905 | X<local, tie> |
fd5a896a | 906 | |
325192b1 | 907 | =head3 Localization of globs |
d74e8afc | 908 | X<local, glob> X<glob> |
3e3baf6d | 909 | |
325192b1 RGS |
910 | The construct |
911 | ||
912 | local *name; | |
913 | ||
914 | creates a whole new symbol table entry for the glob C<name> in the | |
915 | current package. That means that all variables in its glob slot ($name, | |
916 | @name, %name, &name, and the C<name> filehandle) are dynamically reset. | |
917 | ||
918 | This implies, among other things, that any magic eventually carried by | |
919 | those variables is locally lost. In other words, saying C<local */> | |
920 | will not have any effect on the internal value of the input record | |
921 | separator. | |
922 | ||
325192b1 | 923 | =head3 Localization of elements of composite types |
d74e8afc | 924 | X<local, composite type element> X<local, array element> X<local, hash element> |
3e3baf6d | 925 | |
6ee623d5 | 926 | It's also worth taking a moment to explain what happens when you |
f86cebdf | 927 | C<local>ize a member of a composite type (i.e. an array or hash element). |
b77865f5 | 928 | In this case, the element is C<local>ized I<by name>. This means that |
6ee623d5 GS |
929 | when the scope of the C<local()> ends, the saved value will be |
930 | restored to the hash element whose key was named in the C<local()>, or | |
931 | the array element whose index was named in the C<local()>. If that | |
932 | element was deleted while the C<local()> was in effect (e.g. by a | |
933 | C<delete()> from a hash or a C<shift()> of an array), it will spring | |
934 | back into existence, possibly extending an array and filling in the | |
935 | skipped elements with C<undef>. For instance, if you say | |
936 | ||
937 | %hash = ( 'This' => 'is', 'a' => 'test' ); | |
938 | @ary = ( 0..5 ); | |
939 | { | |
940 | local($ary[5]) = 6; | |
941 | local($hash{'a'}) = 'drill'; | |
942 | while (my $e = pop(@ary)) { | |
943 | print "$e . . .\n"; | |
944 | last unless $e > 3; | |
945 | } | |
946 | if (@ary) { | |
947 | $hash{'only a'} = 'test'; | |
948 | delete $hash{'a'}; | |
949 | } | |
950 | } | |
951 | print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n"; | |
952 | print "The array has ",scalar(@ary)," elements: ", | |
953 | join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n"; | |
954 | ||
955 | Perl will print | |
956 | ||
957 | 6 . . . | |
958 | 4 . . . | |
959 | 3 . . . | |
960 | This is a test only a test. | |
961 | The array has 6 elements: 0, 1, 2, undef, undef, 5 | |
962 | ||
19799a22 | 963 | The behavior of local() on non-existent members of composite |
7185e5cc GS |
964 | types is subject to change in future. |
965 | ||
d361fafa VP |
966 | =head3 Localized deletion of elements of composite types |
967 | X<delete> X<local, composite type element> X<local, array element> X<local, hash element> | |
968 | ||
969 | You can use the C<delete local $array[$idx]> and C<delete local $hash{key}> | |
970 | constructs to delete a composite type entry for the current block and restore | |
b77865f5 | 971 | it when it ends. They return the array/hash value before the localization, |
d361fafa VP |
972 | which means that they are respectively equivalent to |
973 | ||
974 | do { | |
975 | my $val = $array[$idx]; | |
976 | local $array[$idx]; | |
977 | delete $array[$idx]; | |
978 | $val | |
979 | } | |
980 | ||
981 | and | |
982 | ||
983 | do { | |
984 | my $val = $hash{key}; | |
985 | local $hash{key}; | |
986 | delete $hash{key}; | |
987 | $val | |
988 | } | |
989 | ||
b77865f5 FC |
990 | except that for those the C<local> is |
991 | scoped to the C<do> block. Slices are | |
d361fafa VP |
992 | also accepted. |
993 | ||
994 | my %hash = ( | |
995 | a => [ 7, 8, 9 ], | |
996 | b => 1, | |
997 | ) | |
998 | ||
999 | { | |
1000 | my $a = delete local $hash{a}; | |
1001 | # $a is [ 7, 8, 9 ] | |
1002 | # %hash is (b => 1) | |
1003 | ||
1004 | { | |
1005 | my @nums = delete local @$a[0, 2] | |
1006 | # @nums is (7, 9) | |
1007 | # $a is [ undef, 8 ] | |
1008 | ||
1009 | $a[0] = 999; # will be erased when the scope ends | |
1010 | } | |
1011 | # $a is back to [ 7, 8, 9 ] | |
1012 | ||
1013 | } | |
1014 | # %hash is back to its original state | |
1015 | ||
cd06dffe | 1016 | =head2 Lvalue subroutines |
d74e8afc | 1017 | X<lvalue> X<subroutine, lvalue> |
cd06dffe | 1018 | |
cd06dffe GS |
1019 | It is possible to return a modifiable value from a subroutine. |
1020 | To do this, you have to declare the subroutine to return an lvalue. | |
1021 | ||
1022 | my $val; | |
1023 | sub canmod : lvalue { | |
4a904372 | 1024 | $val; # or: return $val; |
cd06dffe GS |
1025 | } |
1026 | sub nomod { | |
1027 | $val; | |
1028 | } | |
1029 | ||
1030 | canmod() = 5; # assigns to $val | |
1031 | nomod() = 5; # ERROR | |
1032 | ||
1033 | The scalar/list context for the subroutine and for the right-hand | |
1034 | side of assignment is determined as if the subroutine call is replaced | |
b77865f5 | 1035 | by a scalar. For example, consider: |
cd06dffe GS |
1036 | |
1037 | data(2,3) = get_data(3,4); | |
1038 | ||
1039 | Both subroutines here are called in a scalar context, while in: | |
1040 | ||
1041 | (data(2,3)) = get_data(3,4); | |
1042 | ||
1043 | and in: | |
1044 | ||
1045 | (data(2),data(3)) = get_data(3,4); | |
1046 | ||
1047 | all the subroutines are called in a list context. | |
1048 | ||
771cc755 | 1049 | Lvalue subroutines are convenient, but you have to keep in mind that, |
b77865f5 | 1050 | when used with objects, they may violate encapsulation. A normal |
771cc755 | 1051 | mutator can check the supplied argument before setting the attribute |
b77865f5 | 1052 | it is protecting, an lvalue subroutine cannot. If you require any |
771cc755 JV |
1053 | special processing when storing and retrieving the values, consider |
1054 | using the CPAN module Sentinel or something similar. | |
e6a32221 | 1055 | |
ca40957e FC |
1056 | =head2 Lexical Subroutines |
1057 | X<my sub> X<state sub> X<our sub> X<subroutine, lexical> | |
1058 | ||
ca40957e FC |
1059 | Beginning with Perl 5.18, you can declare a private subroutine with C<my> |
1060 | or C<state>. As with state variables, the C<state> keyword is only | |
1061 | available under C<use feature 'state'> or C<use 5.010> or higher. | |
1062 | ||
06c4bad0 FC |
1063 | Prior to Perl 5.26, lexical subroutines were deemed experimental and were |
1064 | available only under the C<use feature 'lexical_subs'> pragma. They also | |
1065 | produced a warning unless the "experimental::lexical_subs" warnings | |
1066 | category was disabled. | |
1067 | ||
ca40957e FC |
1068 | These subroutines are only visible within the block in which they are |
1069 | declared, and only after that declaration: | |
1070 | ||
06c4bad0 FC |
1071 | # Include these two lines if your code is intended to run under Perl |
1072 | # versions earlier than 5.26. | |
f1d34ca8 | 1073 | no warnings "experimental::lexical_subs"; |
ca40957e FC |
1074 | use feature 'lexical_subs'; |
1075 | ||
1076 | foo(); # calls the package/global subroutine | |
1077 | state sub foo { | |
1078 | foo(); # also calls the package subroutine | |
1079 | } | |
1080 | foo(); # calls "state" sub | |
1081 | my $ref = \&foo; # take a reference to "state" sub | |
1082 | ||
1083 | my sub bar { ... } | |
1084 | bar(); # calls "my" sub | |
1085 | ||
1086 | To use a lexical subroutine from inside the subroutine itself, you must | |
1087 | predeclare it. The C<sub foo {...}> subroutine definition syntax respects | |
1088 | any previous C<my sub;> or C<state sub;> declaration. | |
1089 | ||
1090 | my sub baz; # predeclaration | |
1091 | sub baz { # define the "my" sub | |
1092 | baz(); # recursive call | |
1093 | } | |
1094 | ||
1095 | =head3 C<state sub> vs C<my sub> | |
1096 | ||
1097 | What is the difference between "state" subs and "my" subs? Each time that | |
1098 | execution enters a block when "my" subs are declared, a new copy of each | |
1099 | sub is created. "State" subroutines persist from one execution of the | |
1100 | containing block to the next. | |
1101 | ||
1102 | So, in general, "state" subroutines are faster. But "my" subs are | |
1103 | necessary if you want to create closures: | |
1104 | ||
ca40957e FC |
1105 | sub whatever { |
1106 | my $x = shift; | |
1107 | my sub inner { | |
1108 | ... do something with $x ... | |
1109 | } | |
1110 | inner(); | |
1111 | } | |
1112 | ||
1113 | In this example, a new C<$x> is created when C<whatever> is called, and | |
1114 | also a new C<inner>, which can see the new C<$x>. A "state" sub will only | |
1115 | see the C<$x> from the first call to C<whatever>. | |
1116 | ||
1117 | =head3 C<our> subroutines | |
1118 | ||
1119 | Like C<our $variable>, C<our sub> creates a lexical alias to the package | |
1120 | subroutine of the same name. | |
1121 | ||
1122 | The two main uses for this are to switch back to using the package sub | |
1123 | inside an inner scope: | |
1124 | ||
ca40957e FC |
1125 | sub foo { ... } |
1126 | ||
1127 | sub bar { | |
1128 | my sub foo { ... } | |
1129 | { | |
1130 | # need to use the outer foo here | |
1131 | our sub foo; | |
1132 | foo(); | |
1133 | } | |
1134 | } | |
1135 | ||
1136 | and to make a subroutine visible to other packages in the same scope: | |
1137 | ||
1138 | package MySneakyModule; | |
1139 | ||
ca40957e FC |
1140 | our sub do_something { ... } |
1141 | ||
1142 | sub do_something_with_caller { | |
1143 | package DB; | |
1144 | () = caller 1; # sets @DB::args | |
1145 | do_something(@args); # uses MySneakyModule::do_something | |
1146 | } | |
1147 | ||
cb1a09d0 | 1148 | =head2 Passing Symbol Table Entries (typeglobs) |
d74e8afc | 1149 | X<typeglob> X<*> |
cb1a09d0 | 1150 | |
19799a22 GS |
1151 | B<WARNING>: The mechanism described in this section was originally |
1152 | the only way to simulate pass-by-reference in older versions of | |
1153 | Perl. While it still works fine in modern versions, the new reference | |
1154 | mechanism is generally easier to work with. See below. | |
a0d0e21e LW |
1155 | |
1156 | Sometimes you don't want to pass the value of an array to a subroutine | |
1157 | but rather the name of it, so that the subroutine can modify the global | |
1158 | copy of it rather than working with a local copy. In perl you can | |
cb1a09d0 | 1159 | refer to all objects of a particular name by prefixing the name |
5f05dabc | 1160 | with a star: C<*foo>. This is often known as a "typeglob", because the |
a0d0e21e LW |
1161 | star on the front can be thought of as a wildcard match for all the |
1162 | funny prefix characters on variables and subroutines and such. | |
1163 | ||
55497cff | 1164 | When evaluated, the typeglob produces a scalar value that represents |
5f05dabc | 1165 | all the objects of that name, including any filehandle, format, or |
a0d0e21e | 1166 | subroutine. When assigned to, it causes the name mentioned to refer to |
19799a22 | 1167 | whatever C<*> value was assigned to it. Example: |
a0d0e21e LW |
1168 | |
1169 | sub doubleary { | |
1170 | local(*someary) = @_; | |
1171 | foreach $elem (@someary) { | |
1172 | $elem *= 2; | |
1173 | } | |
1174 | } | |
1175 | doubleary(*foo); | |
1176 | doubleary(*bar); | |
1177 | ||
19799a22 | 1178 | Scalars are already passed by reference, so you can modify |
a0d0e21e | 1179 | scalar arguments without using this mechanism by referring explicitly |
1fef88e7 | 1180 | to C<$_[0]> etc. You can modify all the elements of an array by passing |
f86cebdf GS |
1181 | all the elements as scalars, but you have to use the C<*> mechanism (or |
1182 | the equivalent reference mechanism) to C<push>, C<pop>, or change the size of | |
a0d0e21e LW |
1183 | an array. It will certainly be faster to pass the typeglob (or reference). |
1184 | ||
1185 | Even if you don't want to modify an array, this mechanism is useful for | |
5f05dabc | 1186 | passing multiple arrays in a single LIST, because normally the LIST |
a0d0e21e | 1187 | mechanism will merge all the array values so that you can't extract out |
55497cff | 1188 | the individual arrays. For more on typeglobs, see |
2ae324a7 | 1189 | L<perldata/"Typeglobs and Filehandles">. |
cb1a09d0 | 1190 | |
5a964f20 | 1191 | =head2 When to Still Use local() |
d74e8afc | 1192 | X<local> X<variable, local> |
5a964f20 | 1193 | |
19799a22 GS |
1194 | Despite the existence of C<my>, there are still three places where the |
1195 | C<local> operator still shines. In fact, in these three places, you | |
5a964f20 TC |
1196 | I<must> use C<local> instead of C<my>. |
1197 | ||
13a2d996 | 1198 | =over 4 |
5a964f20 | 1199 | |
551e1d92 RB |
1200 | =item 1. |
1201 | ||
1202 | You need to give a global variable a temporary value, especially $_. | |
5a964f20 | 1203 | |
f86cebdf GS |
1204 | The global variables, like C<@ARGV> or the punctuation variables, must be |
1205 | C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits | |
5a964f20 | 1206 | it up into chunks separated by lines of equal signs, which are placed |
f86cebdf | 1207 | in C<@Fields>. |
5a964f20 TC |
1208 | |
1209 | { | |
1210 | local @ARGV = ("/etc/motd"); | |
1211 | local $/ = undef; | |
1212 | local $_ = <>; | |
1213 | @Fields = split /^\s*=+\s*$/; | |
1214 | } | |
1215 | ||
19799a22 | 1216 | It particular, it's important to C<local>ize $_ in any routine that assigns |
5a964f20 TC |
1217 | to it. Look out for implicit assignments in C<while> conditionals. |
1218 | ||
551e1d92 RB |
1219 | =item 2. |
1220 | ||
1221 | You need to create a local file or directory handle or a local function. | |
5a964f20 | 1222 | |
09bef843 SB |
1223 | A function that needs a filehandle of its own must use |
1224 | C<local()> on a complete typeglob. This can be used to create new symbol | |
5a964f20 TC |
1225 | table entries: |
1226 | ||
1227 | sub ioqueue { | |
1228 | local (*READER, *WRITER); # not my! | |
17b63f68 | 1229 | pipe (READER, WRITER) or die "pipe: $!"; |
5a964f20 TC |
1230 | return (*READER, *WRITER); |
1231 | } | |
1232 | ($head, $tail) = ioqueue(); | |
1233 | ||
1234 | See the Symbol module for a way to create anonymous symbol table | |
1235 | entries. | |
1236 | ||
1237 | Because assignment of a reference to a typeglob creates an alias, this | |
1238 | can be used to create what is effectively a local function, or at least, | |
1239 | a local alias. | |
1240 | ||
1241 | { | |
4a46e268 | 1242 | local *grow = \&shrink; # only until this block exits |
555bd962 BG |
1243 | grow(); # really calls shrink() |
1244 | move(); # if move() grow()s, it shrink()s too | |
5a964f20 | 1245 | } |
555bd962 | 1246 | grow(); # get the real grow() again |
5a964f20 TC |
1247 | |
1248 | See L<perlref/"Function Templates"> for more about manipulating | |
1249 | functions by name in this way. | |
1250 | ||
551e1d92 RB |
1251 | =item 3. |
1252 | ||
1253 | You want to temporarily change just one element of an array or hash. | |
5a964f20 | 1254 | |
f86cebdf | 1255 | You can C<local>ize just one element of an aggregate. Usually this |
5a964f20 TC |
1256 | is done on dynamics: |
1257 | ||
1258 | { | |
1259 | local $SIG{INT} = 'IGNORE'; | |
1260 | funct(); # uninterruptible | |
1261 | } | |
1262 | # interruptibility automatically restored here | |
1263 | ||
9d42615f | 1264 | But it also works on lexically declared aggregates. |
5a964f20 TC |
1265 | |
1266 | =back | |
1267 | ||
cb1a09d0 | 1268 | =head2 Pass by Reference |
d74e8afc | 1269 | X<pass by reference> X<pass-by-reference> X<reference> |
cb1a09d0 | 1270 | |
55497cff | 1271 | If you want to pass more than one array or hash into a function--or |
1272 | return them from it--and have them maintain their integrity, then | |
1273 | you're going to have to use an explicit pass-by-reference. Before you | |
1274 | do that, you need to understand references as detailed in L<perlref>. | |
c07a80fd | 1275 | This section may not make much sense to you otherwise. |
cb1a09d0 | 1276 | |
19799a22 GS |
1277 | Here are a few simple examples. First, let's pass in several arrays |
1278 | to a function and have it C<pop> all of then, returning a new list | |
1279 | of all their former last elements: | |
cb1a09d0 AD |
1280 | |
1281 | @tailings = popmany ( \@a, \@b, \@c, \@d ); | |
1282 | ||
1283 | sub popmany { | |
1284 | my $aref; | |
8b7906d1 | 1285 | my @retlist; |
cb1a09d0 AD |
1286 | foreach $aref ( @_ ) { |
1287 | push @retlist, pop @$aref; | |
54310121 | 1288 | } |
cb1a09d0 | 1289 | return @retlist; |
54310121 | 1290 | } |
cb1a09d0 | 1291 | |
54310121 | 1292 | Here's how you might write a function that returns a |
cb1a09d0 AD |
1293 | list of keys occurring in all the hashes passed to it: |
1294 | ||
54310121 | 1295 | @common = inter( \%foo, \%bar, \%joe ); |
cb1a09d0 AD |
1296 | sub inter { |
1297 | my ($k, $href, %seen); # locals | |
1298 | foreach $href (@_) { | |
1299 | while ( $k = each %$href ) { | |
1300 | $seen{$k}++; | |
54310121 | 1301 | } |
1302 | } | |
cb1a09d0 | 1303 | return grep { $seen{$_} == @_ } keys %seen; |
54310121 | 1304 | } |
cb1a09d0 | 1305 | |
5f05dabc | 1306 | So far, we're using just the normal list return mechanism. |
54310121 | 1307 | What happens if you want to pass or return a hash? Well, |
1308 | if you're using only one of them, or you don't mind them | |
cb1a09d0 | 1309 | concatenating, then the normal calling convention is ok, although |
54310121 | 1310 | a little expensive. |
cb1a09d0 AD |
1311 | |
1312 | Where people get into trouble is here: | |
1313 | ||
1314 | (@a, @b) = func(@c, @d); | |
1315 | or | |
1316 | (%a, %b) = func(%c, %d); | |
1317 | ||
19799a22 GS |
1318 | That syntax simply won't work. It sets just C<@a> or C<%a> and |
1319 | clears the C<@b> or C<%b>. Plus the function didn't get passed | |
1320 | into two separate arrays or hashes: it got one long list in C<@_>, | |
1321 | as always. | |
cb1a09d0 AD |
1322 | |
1323 | If you can arrange for everyone to deal with this through references, it's | |
1324 | cleaner code, although not so nice to look at. Here's a function that | |
1325 | takes two array references as arguments, returning the two array elements | |
1326 | in order of how many elements they have in them: | |
1327 | ||
1328 | ($aref, $bref) = func(\@c, \@d); | |
1329 | print "@$aref has more than @$bref\n"; | |
1330 | sub func { | |
1331 | my ($cref, $dref) = @_; | |
1332 | if (@$cref > @$dref) { | |
1333 | return ($cref, $dref); | |
1334 | } else { | |
c07a80fd | 1335 | return ($dref, $cref); |
54310121 | 1336 | } |
1337 | } | |
cb1a09d0 AD |
1338 | |
1339 | It turns out that you can actually do this also: | |
1340 | ||
1341 | (*a, *b) = func(\@c, \@d); | |
1342 | print "@a has more than @b\n"; | |
1343 | sub func { | |
1344 | local (*c, *d) = @_; | |
1345 | if (@c > @d) { | |
1346 | return (\@c, \@d); | |
1347 | } else { | |
1348 | return (\@d, \@c); | |
54310121 | 1349 | } |
1350 | } | |
cb1a09d0 AD |
1351 | |
1352 | Here we're using the typeglobs to do symbol table aliasing. It's | |
19799a22 | 1353 | a tad subtle, though, and also won't work if you're using C<my> |
09bef843 | 1354 | variables, because only globals (even in disguise as C<local>s) |
19799a22 | 1355 | are in the symbol table. |
5f05dabc | 1356 | |
1357 | If you're passing around filehandles, you could usually just use the bare | |
19799a22 GS |
1358 | typeglob, like C<*STDOUT>, but typeglobs references work, too. |
1359 | For example: | |
5f05dabc | 1360 | |
1361 | splutter(\*STDOUT); | |
1362 | sub splutter { | |
1363 | my $fh = shift; | |
1364 | print $fh "her um well a hmmm\n"; | |
1365 | } | |
1366 | ||
1367 | $rec = get_rec(\*STDIN); | |
1368 | sub get_rec { | |
1369 | my $fh = shift; | |
1370 | return scalar <$fh>; | |
1371 | } | |
1372 | ||
19799a22 GS |
1373 | If you're planning on generating new filehandles, you could do this. |
1374 | Notice to pass back just the bare *FH, not its reference. | |
5f05dabc | 1375 | |
1376 | sub openit { | |
19799a22 | 1377 | my $path = shift; |
5f05dabc | 1378 | local *FH; |
e05a3a1e | 1379 | return open (FH, $path) ? *FH : undef; |
54310121 | 1380 | } |
5f05dabc | 1381 | |
cb1a09d0 | 1382 | =head2 Prototypes |
d74e8afc | 1383 | X<prototype> X<subroutine, prototype> |
cb1a09d0 | 1384 | |
19799a22 | 1385 | Perl supports a very limited kind of compile-time argument checking |
eedb00fa PM |
1386 | using function prototyping. This can be declared in either the PROTO |
1387 | section or with a L<prototype attribute|attributes/Built-in Attributes>. | |
30d9c59b | 1388 | If you declare either of |
cb1a09d0 | 1389 | |
26230909 AC |
1390 | sub mypush (\@@) |
1391 | sub mypush :prototype(\@@) | |
30d9c59b Z |
1392 | |
1393 | then C<mypush()> takes arguments exactly like C<push()> does. | |
1394 | ||
1395 | If subroutine signatures are enabled (see L</Signatures>), then | |
1396 | the shorter PROTO syntax is unavailable, because it would clash with | |
1397 | signatures. In that case, a prototype can only be declared in the form | |
1398 | of an attribute. | |
cb1a09d0 | 1399 | |
30d9c59b | 1400 | The |
19799a22 GS |
1401 | function declaration must be visible at compile time. The prototype |
1402 | affects only interpretation of new-style calls to the function, | |
1403 | where new-style is defined as not using the C<&> character. In | |
1404 | other words, if you call it like a built-in function, then it behaves | |
1405 | like a built-in function. If you call it like an old-fashioned | |
1406 | subroutine, then it behaves like an old-fashioned subroutine. It | |
1407 | naturally falls out from this rule that prototypes have no influence | |
1408 | on subroutine references like C<\&foo> or on indirect subroutine | |
c47ff5f1 | 1409 | calls like C<&{$subref}> or C<< $subref->() >>. |
c07a80fd | 1410 | |
1411 | Method calls are not influenced by prototypes either, because the | |
19799a22 GS |
1412 | function to be called is indeterminate at compile time, since |
1413 | the exact code called depends on inheritance. | |
cb1a09d0 | 1414 | |
19799a22 GS |
1415 | Because the intent of this feature is primarily to let you define |
1416 | subroutines that work like built-in functions, here are prototypes | |
1417 | for some other functions that parse almost exactly like the | |
1418 | corresponding built-in. | |
cb1a09d0 | 1419 | |
555bd962 BG |
1420 | Declared as Called as |
1421 | ||
1422 | sub mylink ($$) mylink $old, $new | |
1423 | sub myvec ($$$) myvec $var, $offset, 1 | |
1424 | sub myindex ($$;$) myindex &getstring, "substr" | |
1425 | sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off | |
1426 | sub myreverse (@) myreverse $a, $b, $c | |
1427 | sub myjoin ($@) myjoin ":", $a, $b, $c | |
26230909 AC |
1428 | sub mypop (\@) mypop @array |
1429 | sub mysplice (\@$$@) mysplice @array, 0, 2, @pushme | |
1430 | sub mykeys (\[%@]) mykeys %{$hashref} | |
555bd962 BG |
1431 | sub myopen (*;$) myopen HANDLE, $name |
1432 | sub mypipe (**) mypipe READHANDLE, WRITEHANDLE | |
1433 | sub mygrep (&@) mygrep { /foo/ } $a, $b, $c | |
1434 | sub myrand (;$) myrand 42 | |
1435 | sub mytime () mytime | |
cb1a09d0 | 1436 | |
c07a80fd | 1437 | Any backslashed prototype character represents an actual argument |
ae7a3cfa | 1438 | that must start with that character (optionally preceded by C<my>, |
b91b7d1a FC |
1439 | C<our> or C<local>), with the exception of C<$>, which will |
1440 | accept any scalar lvalue expression, such as C<$foo = 7> or | |
b77865f5 | 1441 | C<< my_function()->[0] >>. The value passed as part of C<@_> will be a |
ae7a3cfa FC |
1442 | reference to the actual argument given in the subroutine call, |
1443 | obtained by applying C<\> to that argument. | |
c07a80fd | 1444 | |
c035a075 | 1445 | You can use the C<\[]> backslash group notation to specify more than one |
b77865f5 | 1446 | allowed argument type. For example: |
5b794e05 JH |
1447 | |
1448 | sub myref (\[$@%&*]) | |
1449 | ||
1450 | will allow calling myref() as | |
1451 | ||
1452 | myref $var | |
1453 | myref @array | |
1454 | myref %hash | |
1455 | myref &sub | |
1456 | myref *glob | |
1457 | ||
1458 | and the first argument of myref() will be a reference to | |
1459 | a scalar, an array, a hash, a code, or a glob. | |
1460 | ||
c07a80fd | 1461 | Unbackslashed prototype characters have special meanings. Any |
19799a22 | 1462 | unbackslashed C<@> or C<%> eats all remaining arguments, and forces |
f86cebdf GS |
1463 | list context. An argument represented by C<$> forces scalar context. An |
1464 | C<&> requires an anonymous subroutine, which, if passed as the first | |
0df79f0c GS |
1465 | argument, does not require the C<sub> keyword or a subsequent comma. |
1466 | ||
1467 | A C<*> allows the subroutine to accept a bareword, constant, scalar expression, | |
648ca4f7 GS |
1468 | typeglob, or a reference to a typeglob in that slot. The value will be |
1469 | available to the subroutine either as a simple scalar, or (in the latter | |
0df79f0c GS |
1470 | two cases) as a reference to the typeglob. If you wish to always convert |
1471 | such arguments to a typeglob reference, use Symbol::qualify_to_ref() as | |
1472 | follows: | |
1473 | ||
1474 | use Symbol 'qualify_to_ref'; | |
1475 | ||
1476 | sub foo (*) { | |
1477 | my $fh = qualify_to_ref(shift, caller); | |
1478 | ... | |
1479 | } | |
c07a80fd | 1480 | |
c035a075 DG |
1481 | The C<+> prototype is a special alternative to C<$> that will act like |
1482 | C<\[@%]> when given a literal array or hash variable, but will otherwise | |
1483 | force scalar context on the argument. This is useful for functions which | |
1484 | should accept either a literal array or an array reference as the argument: | |
1485 | ||
cba5a3b0 | 1486 | sub mypush (+@) { |
c035a075 DG |
1487 | my $aref = shift; |
1488 | die "Not an array or arrayref" unless ref $aref eq 'ARRAY'; | |
1489 | push @$aref, @_; | |
1490 | } | |
1491 | ||
1492 | When using the C<+> prototype, your function must check that the argument | |
1493 | is of an acceptable type. | |
1494 | ||
859a4967 | 1495 | A semicolon (C<;>) separates mandatory arguments from optional arguments. |
19799a22 | 1496 | It is redundant before C<@> or C<%>, which gobble up everything else. |
cb1a09d0 | 1497 | |
34daab0f RGS |
1498 | As the last character of a prototype, or just before a semicolon, a C<@> |
1499 | or a C<%>, you can use C<_> in place of C<$>: if this argument is not | |
1500 | provided, C<$_> will be used instead. | |
859a4967 | 1501 | |
19799a22 GS |
1502 | Note how the last three examples in the table above are treated |
1503 | specially by the parser. C<mygrep()> is parsed as a true list | |
1504 | operator, C<myrand()> is parsed as a true unary operator with unary | |
1505 | precedence the same as C<rand()>, and C<mytime()> is truly without | |
1506 | arguments, just like C<time()>. That is, if you say | |
cb1a09d0 AD |
1507 | |
1508 | mytime +2; | |
1509 | ||
f86cebdf | 1510 | you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed |
3a8944db FC |
1511 | without a prototype. If you want to force a unary function to have the |
1512 | same precedence as a list operator, add C<;> to the end of the prototype: | |
1513 | ||
1514 | sub mygetprotobynumber($;); | |
1515 | mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b) | |
cb1a09d0 | 1516 | |
19799a22 GS |
1517 | The interesting thing about C<&> is that you can generate new syntax with it, |
1518 | provided it's in the initial position: | |
d74e8afc | 1519 | X<&> |
cb1a09d0 | 1520 | |
6d28dffb | 1521 | sub try (&@) { |
cb1a09d0 AD |
1522 | my($try,$catch) = @_; |
1523 | eval { &$try }; | |
1524 | if ($@) { | |
1525 | local $_ = $@; | |
1526 | &$catch; | |
1527 | } | |
1528 | } | |
55497cff | 1529 | sub catch (&) { $_[0] } |
cb1a09d0 AD |
1530 | |
1531 | try { | |
1532 | die "phooey"; | |
1533 | } catch { | |
1534 | /phooey/ and print "unphooey\n"; | |
1535 | }; | |
1536 | ||
f86cebdf | 1537 | That prints C<"unphooey">. (Yes, there are still unresolved |
19799a22 | 1538 | issues having to do with visibility of C<@_>. I'm ignoring that |
f86cebdf | 1539 | question for the moment. (But note that if we make C<@_> lexically |
cb1a09d0 | 1540 | scoped, those anonymous subroutines can act like closures... (Gee, |
5f05dabc | 1541 | is this sounding a little Lispish? (Never mind.)))) |
cb1a09d0 | 1542 | |
19799a22 | 1543 | And here's a reimplementation of the Perl C<grep> operator: |
d74e8afc | 1544 | X<grep> |
cb1a09d0 AD |
1545 | |
1546 | sub mygrep (&@) { | |
1547 | my $code = shift; | |
1548 | my @result; | |
1549 | foreach $_ (@_) { | |
6e47f808 | 1550 | push(@result, $_) if &$code; |
cb1a09d0 AD |
1551 | } |
1552 | @result; | |
1553 | } | |
a0d0e21e | 1554 | |
cb1a09d0 AD |
1555 | Some folks would prefer full alphanumeric prototypes. Alphanumerics have |
1556 | been intentionally left out of prototypes for the express purpose of | |
1557 | someday in the future adding named, formal parameters. The current | |
1558 | mechanism's main goal is to let module writers provide better diagnostics | |
1559 | for module users. Larry feels the notation quite understandable to Perl | |
1560 | programmers, and that it will not intrude greatly upon the meat of the | |
1561 | module, nor make it harder to read. The line noise is visually | |
1562 | encapsulated into a small pill that's easy to swallow. | |
1563 | ||
420cdfc1 ST |
1564 | If you try to use an alphanumeric sequence in a prototype you will |
1565 | generate an optional warning - "Illegal character in prototype...". | |
1566 | Unfortunately earlier versions of Perl allowed the prototype to be | |
1567 | used as long as its prefix was a valid prototype. The warning may be | |
1568 | upgraded to a fatal error in a future version of Perl once the | |
1569 | majority of offending code is fixed. | |
1570 | ||
cb1a09d0 AD |
1571 | It's probably best to prototype new functions, not retrofit prototyping |
1572 | into older ones. That's because you must be especially careful about | |
1573 | silent impositions of differing list versus scalar contexts. For example, | |
1574 | if you decide that a function should take just one parameter, like this: | |
1575 | ||
1576 | sub func ($) { | |
1577 | my $n = shift; | |
1578 | print "you gave me $n\n"; | |
54310121 | 1579 | } |
cb1a09d0 AD |
1580 | |
1581 | and someone has been calling it with an array or expression | |
1582 | returning a list: | |
1583 | ||
1584 | func(@foo); | |
f2606479 | 1585 | func( $text =~ /\w+/g ); |
cb1a09d0 | 1586 | |
19799a22 | 1587 | Then you've just supplied an automatic C<scalar> in front of their |
f86cebdf | 1588 | argument, which can be more than a bit surprising. The old C<@foo> |
cb1a09d0 | 1589 | which used to hold one thing doesn't get passed in. Instead, |
19799a22 | 1590 | C<func()> now gets passed in a C<1>; that is, the number of elements |
f2606479 LM |
1591 | in C<@foo>. And the C<m//g> gets called in scalar context so instead of a |
1592 | list of words it returns a boolean result and advances C<pos($text)>. Ouch! | |
cb1a09d0 | 1593 | |
eb40d2ca PM |
1594 | If a sub has both a PROTO and a BLOCK, the prototype is not applied |
1595 | until after the BLOCK is completely defined. This means that a recursive | |
1596 | function with a prototype has to be predeclared for the prototype to take | |
1597 | effect, like so: | |
1598 | ||
1599 | sub foo($$); | |
1600 | sub foo($$) { | |
1601 | foo 1, 2; | |
1602 | } | |
1603 | ||
5f05dabc | 1604 | This is all very powerful, of course, and should be used only in moderation |
54310121 | 1605 | to make the world a better place. |
44a8e56a | 1606 | |
1607 | =head2 Constant Functions | |
d74e8afc | 1608 | X<constant> |
44a8e56a | 1609 | |
1610 | Functions with a prototype of C<()> are potential candidates for | |
19799a22 GS |
1611 | inlining. If the result after optimization and constant folding |
1612 | is either a constant or a lexically-scoped scalar which has no other | |
54310121 | 1613 | references, then it will be used in place of function calls made |
19799a22 GS |
1614 | without C<&>. Calls made using C<&> are never inlined. (See |
1615 | F<constant.pm> for an easy way to declare most constants.) | |
44a8e56a | 1616 | |
5a964f20 | 1617 | The following functions would all be inlined: |
44a8e56a | 1618 | |
699e6cd4 TP |
1619 | sub pi () { 3.14159 } # Not exact, but close. |
1620 | sub PI () { 4 * atan2 1, 1 } # As good as it gets, | |
1621 | # and it's inlined, too! | |
44a8e56a | 1622 | sub ST_DEV () { 0 } |
1623 | sub ST_INO () { 1 } | |
1624 | ||
1625 | sub FLAG_FOO () { 1 << 8 } | |
1626 | sub FLAG_BAR () { 1 << 9 } | |
1627 | sub FLAG_MASK () { FLAG_FOO | FLAG_BAR } | |
54310121 | 1628 | |
1629 | sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) } | |
88267271 PZ |
1630 | |
1631 | sub N () { int(OPT_BAZ) / 3 } | |
1632 | ||
1633 | sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO } | |
d3c633ba | 1634 | sub FOO_SET2 () { if (FLAG_MASK & FLAG_FOO) { 1 } } |
88267271 | 1635 | |
d3c633ba FC |
1636 | (Be aware that the last example was not always inlined in Perl 5.20 and |
1637 | earlier, which did not behave consistently with subroutines containing | |
1638 | inner scopes.) You can countermand inlining by using an explicit | |
1639 | C<return>: | |
88267271 PZ |
1640 | |
1641 | sub baz_val () { | |
44a8e56a | 1642 | if (OPT_BAZ) { |
1643 | return 23; | |
1644 | } | |
1645 | else { | |
1646 | return 42; | |
1647 | } | |
1648 | } | |
d3c633ba | 1649 | sub bonk_val () { return 12345 } |
cb1a09d0 | 1650 | |
fe39f0d5 AB |
1651 | As alluded to earlier you can also declare inlined subs dynamically at |
1652 | BEGIN time if their body consists of a lexically-scoped scalar which | |
b77865f5 | 1653 | has no other references. Only the first example here will be inlined: |
fe39f0d5 AB |
1654 | |
1655 | BEGIN { | |
1656 | my $var = 1; | |
1657 | no strict 'refs'; | |
1658 | *INLINED = sub () { $var }; | |
1659 | } | |
1660 | ||
1661 | BEGIN { | |
1662 | my $var = 1; | |
1663 | my $ref = \$var; | |
1664 | no strict 'refs'; | |
1665 | *NOT_INLINED = sub () { $var }; | |
1666 | } | |
1667 | ||
1668 | A not so obvious caveat with this (see [RT #79908]) is that the | |
1669 | variable will be immediately inlined, and will stop behaving like a | |
1670 | normal lexical variable, e.g. this will print C<79907>, not C<79908>: | |
1671 | ||
1672 | BEGIN { | |
1673 | my $x = 79907; | |
1674 | *RT_79908 = sub () { $x }; | |
1675 | $x++; | |
1676 | } | |
1677 | print RT_79908(); # prints 79907 | |
1678 | ||
d3c633ba FC |
1679 | As of Perl 5.22, this buggy behavior, while preserved for backward |
1680 | compatibility, is detected and emits a deprecation warning. If you want | |
1681 | the subroutine to be inlined (with no warning), make sure the variable is | |
1682 | not used in a context where it could be modified aside from where it is | |
1683 | declared. | |
1684 | ||
1685 | # Fine, no warning | |
1686 | BEGIN { | |
1687 | my $x = 54321; | |
1688 | *INLINED = sub () { $x }; | |
1689 | } | |
1690 | # Warns. Future Perl versions will stop inlining it. | |
1691 | BEGIN { | |
1692 | my $x; | |
1693 | $x = 54321; | |
1694 | *ALSO_INLINED = sub () { $x }; | |
1695 | } | |
1696 | ||
99734069 FC |
1697 | Perl 5.22 also introduces the experimental "const" attribute as an |
1698 | alternative. (Disable the "experimental::const_attr" warnings if you want | |
1699 | to use it.) When applied to an anonymous subroutine, it forces the sub to | |
1700 | be called when the C<sub> expression is evaluated. The return value is | |
1701 | captured and turned into a constant subroutine: | |
1702 | ||
1703 | my $x = 54321; | |
1704 | *INLINED = sub : const { $x }; | |
1705 | $x++; | |
1706 | ||
1707 | The return value of C<INLINED> in this example will always be 54321, | |
1708 | regardless of later modifications to $x. You can also put any arbitrary | |
1709 | code inside the sub, at it will be executed immediately and its return | |
1710 | value captured the same way. | |
1711 | ||
fe39f0d5 AB |
1712 | If you really want a subroutine with a C<()> prototype that returns a |
1713 | lexical variable you can easily force it to not be inlined by adding | |
1714 | an explicit C<return>: | |
1715 | ||
1716 | BEGIN { | |
1717 | my $x = 79907; | |
1718 | *RT_79908 = sub () { return $x }; | |
1719 | $x++; | |
1720 | } | |
1721 | print RT_79908(); # prints 79908 | |
1722 | ||
1723 | The easiest way to tell if a subroutine was inlined is by using | |
d3c633ba | 1724 | L<B::Deparse>. Consider this example of two subroutines returning |
fe39f0d5 AB |
1725 | C<1>, one with a C<()> prototype causing it to be inlined, and one |
1726 | without (with deparse output truncated for clarity): | |
1727 | ||
cb07e2f2 KW |
1728 | $ perl -MO=Deparse -le 'sub ONE { 1 } if (ONE) { print ONE if ONE }' |
1729 | sub ONE { | |
1730 | 1; | |
1731 | } | |
1732 | if (ONE ) { | |
1733 | print ONE() if ONE ; | |
1734 | } | |
1735 | $ perl -MO=Deparse -le 'sub ONE () { 1 } if (ONE) { print ONE if ONE }' | |
1736 | sub ONE () { 1 } | |
1737 | do { | |
1738 | print 1 | |
1739 | }; | |
fe39f0d5 AB |
1740 | |
1741 | If you redefine a subroutine that was eligible for inlining, you'll | |
b77865f5 | 1742 | get a warning by default. You can use this warning to tell whether or |
fe39f0d5 AB |
1743 | not a particular subroutine is considered inlinable, since it's |
1744 | different than the warning for overriding non-inlined subroutines: | |
1745 | ||
1746 | $ perl -e 'sub one () {1} sub one () {2}' | |
1747 | Constant subroutine one redefined at -e line 1. | |
1748 | $ perl -we 'sub one {1} sub one {2}' | |
1749 | Subroutine one redefined at -e line 1. | |
1750 | ||
1751 | The warning is considered severe enough not to be affected by the | |
1752 | B<-w> switch (or its absence) because previously compiled invocations | |
1753 | of the function will still be using the old value of the function. If | |
1754 | you need to be able to redefine the subroutine, you need to ensure | |
1755 | that it isn't inlined, either by dropping the C<()> prototype (which | |
1756 | changes calling semantics, so beware) or by thwarting the inlining | |
d3c633ba FC |
1757 | mechanism in some other way, e.g. by adding an explicit C<return>, as |
1758 | mentioned above: | |
fe39f0d5 AB |
1759 | |
1760 | sub not_inlined () { return 23 } | |
4cee8e80 | 1761 | |
19799a22 | 1762 | =head2 Overriding Built-in Functions |
d74e8afc | 1763 | X<built-in> X<override> X<CORE> X<CORE::GLOBAL> |
a0d0e21e | 1764 | |
19799a22 | 1765 | Many built-in functions may be overridden, though this should be tried |
5f05dabc | 1766 | only occasionally and for good reason. Typically this might be |
19799a22 | 1767 | done by a package attempting to emulate missing built-in functionality |
a0d0e21e LW |
1768 | on a non-Unix system. |
1769 | ||
163e3a99 JP |
1770 | Overriding may be done only by importing the name from a module at |
1771 | compile time--ordinary predeclaration isn't good enough. However, the | |
19799a22 GS |
1772 | C<use subs> pragma lets you, in effect, predeclare subs |
1773 | via the import syntax, and these names may then override built-in ones: | |
a0d0e21e LW |
1774 | |
1775 | use subs 'chdir', 'chroot', 'chmod', 'chown'; | |
1776 | chdir $somewhere; | |
1777 | sub chdir { ... } | |
1778 | ||
19799a22 GS |
1779 | To unambiguously refer to the built-in form, precede the |
1780 | built-in name with the special package qualifier C<CORE::>. For example, | |
1781 | saying C<CORE::open()> always refers to the built-in C<open()>, even | |
fb73857a | 1782 | if the current package has imported some other subroutine called |
19799a22 | 1783 | C<&open()> from elsewhere. Even though it looks like a regular |
4aaa4757 FC |
1784 | function call, it isn't: the CORE:: prefix in that case is part of Perl's |
1785 | syntax, and works for any keyword, regardless of what is in the CORE | |
1786 | package. Taking a reference to it, that is, C<\&CORE::open>, only works | |
1787 | for some keywords. See L<CORE>. | |
fb73857a | 1788 | |
19799a22 GS |
1789 | Library modules should not in general export built-in names like C<open> |
1790 | or C<chdir> as part of their default C<@EXPORT> list, because these may | |
a0d0e21e | 1791 | sneak into someone else's namespace and change the semantics unexpectedly. |
19799a22 | 1792 | Instead, if the module adds that name to C<@EXPORT_OK>, then it's |
a0d0e21e LW |
1793 | possible for a user to import the name explicitly, but not implicitly. |
1794 | That is, they could say | |
1795 | ||
1796 | use Module 'open'; | |
1797 | ||
19799a22 | 1798 | and it would import the C<open> override. But if they said |
a0d0e21e LW |
1799 | |
1800 | use Module; | |
1801 | ||
19799a22 | 1802 | they would get the default imports without overrides. |
a0d0e21e | 1803 | |
19799a22 | 1804 | The foregoing mechanism for overriding built-in is restricted, quite |
95d94a4f | 1805 | deliberately, to the package that requests the import. There is a second |
19799a22 | 1806 | method that is sometimes applicable when you wish to override a built-in |
95d94a4f GS |
1807 | everywhere, without regard to namespace boundaries. This is achieved by |
1808 | importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an | |
1809 | example that quite brazenly replaces the C<glob> operator with something | |
1810 | that understands regular expressions. | |
1811 | ||
1812 | package REGlob; | |
1813 | require Exporter; | |
1814 | @ISA = 'Exporter'; | |
1815 | @EXPORT_OK = 'glob'; | |
1816 | ||
1817 | sub import { | |
1818 | my $pkg = shift; | |
1819 | return unless @_; | |
1820 | my $sym = shift; | |
1821 | my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0)); | |
1822 | $pkg->export($where, $sym, @_); | |
1823 | } | |
1824 | ||
1825 | sub glob { | |
1826 | my $pat = shift; | |
1827 | my @got; | |
7b815c67 RGS |
1828 | if (opendir my $d, '.') { |
1829 | @got = grep /$pat/, readdir $d; | |
1830 | closedir $d; | |
19799a22 GS |
1831 | } |
1832 | return @got; | |
95d94a4f GS |
1833 | } |
1834 | 1; | |
1835 | ||
1836 | And here's how it could be (ab)used: | |
1837 | ||
1838 | #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces | |
1839 | package Foo; | |
1840 | use REGlob 'glob'; # override glob() in Foo:: only | |
1841 | print for <^[a-z_]+\.pm\$>; # show all pragmatic modules | |
1842 | ||
19799a22 | 1843 | The initial comment shows a contrived, even dangerous example. |
95d94a4f | 1844 | By overriding C<glob> globally, you would be forcing the new (and |
19799a22 | 1845 | subversive) behavior for the C<glob> operator for I<every> namespace, |
95d94a4f GS |
1846 | without the complete cognizance or cooperation of the modules that own |
1847 | those namespaces. Naturally, this should be done with extreme caution--if | |
1848 | it must be done at all. | |
1849 | ||
1850 | The C<REGlob> example above does not implement all the support needed to | |
19799a22 | 1851 | cleanly override perl's C<glob> operator. The built-in C<glob> has |
95d94a4f | 1852 | different behaviors depending on whether it appears in a scalar or list |
19799a22 | 1853 | context, but our C<REGlob> doesn't. Indeed, many perl built-in have such |
95d94a4f GS |
1854 | context sensitive behaviors, and these must be adequately supported by |
1855 | a properly written override. For a fully functional example of overriding | |
1856 | C<glob>, study the implementation of C<File::DosGlob> in the standard | |
1857 | library. | |
1858 | ||
77bc9082 RGS |
1859 | When you override a built-in, your replacement should be consistent (if |
1860 | possible) with the built-in native syntax. You can achieve this by using | |
1861 | a suitable prototype. To get the prototype of an overridable built-in, | |
1862 | use the C<prototype> function with an argument of C<"CORE::builtin_name"> | |
1863 | (see L<perlfunc/prototype>). | |
1864 | ||
1865 | Note however that some built-ins can't have their syntax expressed by a | |
1866 | prototype (such as C<system> or C<chomp>). If you override them you won't | |
1867 | be able to fully mimic their original syntax. | |
1868 | ||
fe854a6f | 1869 | The built-ins C<do>, C<require> and C<glob> can also be overridden, but due |
77bc9082 RGS |
1870 | to special magic, their original syntax is preserved, and you don't have |
1871 | to define a prototype for their replacements. (You can't override the | |
1872 | C<do BLOCK> syntax, though). | |
1873 | ||
1874 | C<require> has special additional dark magic: if you invoke your | |
1875 | C<require> replacement as C<require Foo::Bar>, it will actually receive | |
1876 | the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>. | |
1877 | ||
1878 | And, as you'll have noticed from the previous example, if you override | |
593b9c14 | 1879 | C<glob>, the C<< <*> >> glob operator is overridden as well. |
77bc9082 | 1880 | |
9b3023bc | 1881 | In a similar fashion, overriding the C<readline> function also overrides |
b77865f5 | 1882 | the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding |
e3f73d4e | 1883 | C<readpipe> also overrides the operators C<``> and C<qx//>. |
9b3023bc | 1884 | |
fe854a6f | 1885 | Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden. |
77bc9082 | 1886 | |
a0d0e21e | 1887 | =head2 Autoloading |
d74e8afc | 1888 | X<autoloading> X<AUTOLOAD> |
a0d0e21e | 1889 | |
19799a22 GS |
1890 | If you call a subroutine that is undefined, you would ordinarily |
1891 | get an immediate, fatal error complaining that the subroutine doesn't | |
1892 | exist. (Likewise for subroutines being used as methods, when the | |
1893 | method doesn't exist in any base class of the class's package.) | |
1894 | However, if an C<AUTOLOAD> subroutine is defined in the package or | |
1895 | packages used to locate the original subroutine, then that | |
1896 | C<AUTOLOAD> subroutine is called with the arguments that would have | |
1897 | been passed to the original subroutine. The fully qualified name | |
1898 | of the original subroutine magically appears in the global $AUTOLOAD | |
1899 | variable of the same package as the C<AUTOLOAD> routine. The name | |
1900 | is not passed as an ordinary argument because, er, well, just | |
593b9c14 | 1901 | because, that's why. (As an exception, a method call to a nonexistent |
80ee23cd | 1902 | C<import> or C<unimport> method is just skipped instead. Also, if |
5b36e945 FC |
1903 | the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the |
1904 | subroutine name. See L<perlguts/Autoloading with XSUBs> for details.) | |
80ee23cd | 1905 | |
19799a22 GS |
1906 | |
1907 | Many C<AUTOLOAD> routines load in a definition for the requested | |
1908 | subroutine using eval(), then execute that subroutine using a special | |
1909 | form of goto() that erases the stack frame of the C<AUTOLOAD> routine | |
1910 | without a trace. (See the source to the standard module documented | |
1911 | in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can | |
1912 | also just emulate the routine and never define it. For example, | |
1913 | let's pretend that a function that wasn't defined should just invoke | |
1914 | C<system> with those arguments. All you'd do is: | |
cb1a09d0 AD |
1915 | |
1916 | sub AUTOLOAD { | |
1917 | my $program = $AUTOLOAD; | |
1918 | $program =~ s/.*:://; | |
1919 | system($program, @_); | |
54310121 | 1920 | } |
cb1a09d0 | 1921 | date(); |
6d28dffb | 1922 | who('am', 'i'); |
cb1a09d0 AD |
1923 | ls('-l'); |
1924 | ||
19799a22 GS |
1925 | In fact, if you predeclare functions you want to call that way, you don't |
1926 | even need parentheses: | |
cb1a09d0 AD |
1927 | |
1928 | use subs qw(date who ls); | |
1929 | date; | |
1930 | who "am", "i"; | |
593b9c14 | 1931 | ls '-l'; |
cb1a09d0 | 1932 | |
13058d67 | 1933 | A more complete example of this is the Shell module on CPAN, which |
19799a22 | 1934 | can treat undefined subroutine calls as calls to external programs. |
a0d0e21e | 1935 | |
19799a22 GS |
1936 | Mechanisms are available to help modules writers split their modules |
1937 | into autoloadable files. See the standard AutoLoader module | |
6d28dffb | 1938 | described in L<AutoLoader> and in L<AutoSplit>, the standard |
1939 | SelfLoader modules in L<SelfLoader>, and the document on adding C | |
19799a22 | 1940 | functions to Perl code in L<perlxs>. |
cb1a09d0 | 1941 | |
09bef843 | 1942 | =head2 Subroutine Attributes |
d74e8afc | 1943 | X<attribute> X<subroutine, attribute> X<attrs> |
09bef843 SB |
1944 | |
1945 | A subroutine declaration or definition may have a list of attributes | |
1946 | associated with it. If such an attribute list is present, it is | |
0120eecf | 1947 | broken up at space or colon boundaries and treated as though a |
09bef843 SB |
1948 | C<use attributes> had been seen. See L<attributes> for details |
1949 | about what attributes are currently supported. | |
1950 | Unlike the limitation with the obsolescent C<use attrs>, the | |
1951 | C<sub : ATTRLIST> syntax works to associate the attributes with | |
1952 | a pre-declaration, and not just with a subroutine definition. | |
1953 | ||
1954 | The attributes must be valid as simple identifier names (without any | |
1955 | punctuation other than the '_' character). They may have a parameter | |
1956 | list appended, which is only checked for whether its parentheses ('(',')') | |
1957 | nest properly. | |
1958 | ||
1959 | Examples of valid syntax (even though the attributes are unknown): | |
1960 | ||
4358a253 SS |
1961 | sub fnord (&\%) : switch(10,foo(7,3)) : expensive; |
1962 | sub plugh () : Ugly('\(") :Bad; | |
09bef843 SB |
1963 | sub xyzzy : _5x5 { ... } |
1964 | ||
1965 | Examples of invalid syntax: | |
1966 | ||
4358a253 SS |
1967 | sub fnord : switch(10,foo(); # ()-string not balanced |
1968 | sub snoid : Ugly('('); # ()-string not balanced | |
1969 | sub xyzzy : 5x5; # "5x5" not a valid identifier | |
1970 | sub plugh : Y2::north; # "Y2::north" not a simple identifier | |
1971 | sub snurt : foo + bar; # "+" not a colon or space | |
09bef843 SB |
1972 | |
1973 | The attribute list is passed as a list of constant strings to the code | |
1974 | which associates them with the subroutine. In particular, the second example | |
1975 | of valid syntax above currently looks like this in terms of how it's | |
1976 | parsed and invoked: | |
1977 | ||
1978 | use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad'; | |
1979 | ||
1980 | For further details on attribute lists and their manipulation, | |
a0ae32d3 | 1981 | see L<attributes> and L<Attribute::Handlers>. |
09bef843 | 1982 | |
cb1a09d0 | 1983 | =head1 SEE ALSO |
a0d0e21e | 1984 | |
19799a22 GS |
1985 | See L<perlref/"Function Templates"> for more about references and closures. |
1986 | See L<perlxs> if you'd like to learn about calling C subroutines from Perl. | |
a2293a43 | 1987 | See L<perlembed> if you'd like to learn about calling Perl subroutines from C. |
19799a22 GS |
1988 | See L<perlmod> to learn about bundling up your functions in separate files. |
1989 | See L<perlmodlib> to learn what library modules come standard on your system. | |
82e1c0d9 | 1990 | See L<perlootut> to learn how to make object method calls. |