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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlsub - Perl subroutines | |
4 | ||
5 | =head1 SYNOPSIS | |
6 | ||
7 | To declare subroutines: | |
8 | ||
09bef843 SB |
9 | sub NAME; # A "forward" declaration. |
10 | sub NAME(PROTO); # ditto, but with prototypes | |
11 | sub NAME : ATTRS; # with attributes | |
12 | sub NAME(PROTO) : ATTRS; # with attributes and prototypes | |
cb1a09d0 | 13 | |
09bef843 SB |
14 | sub NAME BLOCK # A declaration and a definition. |
15 | sub NAME(PROTO) BLOCK # ditto, but with prototypes | |
16 | sub NAME : ATTRS BLOCK # with attributes | |
17 | sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes | |
a0d0e21e | 18 | |
748a9306 LW |
19 | To define an anonymous subroutine at runtime: |
20 | ||
09bef843 SB |
21 | $subref = sub BLOCK; # no proto |
22 | $subref = sub (PROTO) BLOCK; # with proto | |
23 | $subref = sub : ATTRS BLOCK; # with attributes | |
24 | $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes | |
748a9306 | 25 | |
a0d0e21e LW |
26 | To import subroutines: |
27 | ||
19799a22 | 28 | use MODULE qw(NAME1 NAME2 NAME3); |
a0d0e21e LW |
29 | |
30 | To call subroutines: | |
31 | ||
5f05dabc | 32 | NAME(LIST); # & is optional with parentheses. |
54310121 | 33 | NAME LIST; # Parentheses optional if predeclared/imported. |
19799a22 | 34 | &NAME(LIST); # Circumvent prototypes. |
5a964f20 | 35 | &NAME; # Makes current @_ visible to called subroutine. |
a0d0e21e LW |
36 | |
37 | =head1 DESCRIPTION | |
38 | ||
19799a22 GS |
39 | Like many languages, Perl provides for user-defined subroutines. |
40 | These may be located anywhere in the main program, loaded in from | |
41 | other files via the C<do>, C<require>, or C<use> keywords, or | |
be3174d2 | 42 | generated on the fly using C<eval> or anonymous subroutines. |
19799a22 GS |
43 | You can even call a function indirectly using a variable containing |
44 | its name or a CODE reference. | |
cb1a09d0 AD |
45 | |
46 | The Perl model for function call and return values is simple: all | |
47 | functions are passed as parameters one single flat list of scalars, and | |
48 | all functions likewise return to their caller one single flat list of | |
49 | scalars. Any arrays or hashes in these call and return lists will | |
50 | collapse, losing their identities--but you may always use | |
51 | pass-by-reference instead to avoid this. Both call and return lists may | |
52 | contain as many or as few scalar elements as you'd like. (Often a | |
53 | function without an explicit return statement is called a subroutine, but | |
19799a22 GS |
54 | there's really no difference from Perl's perspective.) |
55 | ||
56 | Any arguments passed in show up in the array C<@_>. Therefore, if | |
57 | you called a function with two arguments, those would be stored in | |
58 | C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its | |
59 | elements are aliases for the actual scalar parameters. In particular, | |
60 | if an element C<$_[0]> is updated, the corresponding argument is | |
61 | updated (or an error occurs if it is not updatable). If an argument | |
62 | is an array or hash element which did not exist when the function | |
63 | was called, that element is created only when (and if) it is modified | |
64 | or a reference to it is taken. (Some earlier versions of Perl | |
65 | created the element whether or not the element was assigned to.) | |
66 | Assigning to the whole array C<@_> removes that aliasing, and does | |
67 | not update any arguments. | |
68 | ||
69 | The return value of a subroutine is the value of the last expression | |
70 | evaluated. More explicitly, a C<return> statement may be used to exit the | |
54310121 | 71 | subroutine, optionally specifying the returned value, which will be |
72 | evaluated in the appropriate context (list, scalar, or void) depending | |
73 | on the context of the subroutine call. If you specify no return value, | |
19799a22 GS |
74 | the subroutine returns an empty list in list context, the undefined |
75 | value in scalar context, or nothing in void context. If you return | |
76 | one or more aggregates (arrays and hashes), these will be flattened | |
77 | together into one large indistinguishable list. | |
78 | ||
79 | Perl does not have named formal parameters. In practice all you | |
80 | do is assign to a C<my()> list of these. Variables that aren't | |
81 | declared to be private are global variables. For gory details | |
82 | on creating private variables, see L<"Private Variables via my()"> | |
83 | and L<"Temporary Values via local()">. To create protected | |
84 | environments for a set of functions in a separate package (and | |
85 | probably a separate file), see L<perlmod/"Packages">. | |
a0d0e21e LW |
86 | |
87 | Example: | |
88 | ||
cb1a09d0 AD |
89 | sub max { |
90 | my $max = shift(@_); | |
a0d0e21e LW |
91 | foreach $foo (@_) { |
92 | $max = $foo if $max < $foo; | |
93 | } | |
cb1a09d0 | 94 | return $max; |
a0d0e21e | 95 | } |
cb1a09d0 | 96 | $bestday = max($mon,$tue,$wed,$thu,$fri); |
a0d0e21e LW |
97 | |
98 | Example: | |
99 | ||
100 | # get a line, combining continuation lines | |
101 | # that start with whitespace | |
102 | ||
103 | sub get_line { | |
19799a22 | 104 | $thisline = $lookahead; # global variables! |
54310121 | 105 | LINE: while (defined($lookahead = <STDIN>)) { |
a0d0e21e LW |
106 | if ($lookahead =~ /^[ \t]/) { |
107 | $thisline .= $lookahead; | |
108 | } | |
109 | else { | |
110 | last LINE; | |
111 | } | |
112 | } | |
19799a22 | 113 | return $thisline; |
a0d0e21e LW |
114 | } |
115 | ||
116 | $lookahead = <STDIN>; # get first line | |
19799a22 | 117 | while (defined($line = get_line())) { |
a0d0e21e LW |
118 | ... |
119 | } | |
120 | ||
09bef843 | 121 | Assigning to a list of private variables to name your arguments: |
a0d0e21e LW |
122 | |
123 | sub maybeset { | |
124 | my($key, $value) = @_; | |
cb1a09d0 | 125 | $Foo{$key} = $value unless $Foo{$key}; |
a0d0e21e LW |
126 | } |
127 | ||
19799a22 GS |
128 | Because the assignment copies the values, this also has the effect |
129 | of turning call-by-reference into call-by-value. Otherwise a | |
130 | function is free to do in-place modifications of C<@_> and change | |
131 | its caller's values. | |
cb1a09d0 AD |
132 | |
133 | upcase_in($v1, $v2); # this changes $v1 and $v2 | |
134 | sub upcase_in { | |
54310121 | 135 | for (@_) { tr/a-z/A-Z/ } |
136 | } | |
cb1a09d0 AD |
137 | |
138 | You aren't allowed to modify constants in this way, of course. If an | |
139 | argument were actually literal and you tried to change it, you'd take a | |
140 | (presumably fatal) exception. For example, this won't work: | |
141 | ||
142 | upcase_in("frederick"); | |
143 | ||
f86cebdf | 144 | It would be much safer if the C<upcase_in()> function |
cb1a09d0 AD |
145 | were written to return a copy of its parameters instead |
146 | of changing them in place: | |
147 | ||
19799a22 | 148 | ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2 |
cb1a09d0 | 149 | sub upcase { |
54310121 | 150 | return unless defined wantarray; # void context, do nothing |
cb1a09d0 | 151 | my @parms = @_; |
54310121 | 152 | for (@parms) { tr/a-z/A-Z/ } |
c07a80fd | 153 | return wantarray ? @parms : $parms[0]; |
54310121 | 154 | } |
cb1a09d0 | 155 | |
19799a22 | 156 | Notice how this (unprototyped) function doesn't care whether it was |
a2293a43 | 157 | passed real scalars or arrays. Perl sees all arguments as one big, |
19799a22 GS |
158 | long, flat parameter list in C<@_>. This is one area where |
159 | Perl's simple argument-passing style shines. The C<upcase()> | |
160 | function would work perfectly well without changing the C<upcase()> | |
161 | definition even if we fed it things like this: | |
cb1a09d0 AD |
162 | |
163 | @newlist = upcase(@list1, @list2); | |
164 | @newlist = upcase( split /:/, $var ); | |
165 | ||
166 | Do not, however, be tempted to do this: | |
167 | ||
168 | (@a, @b) = upcase(@list1, @list2); | |
169 | ||
19799a22 GS |
170 | Like the flattened incoming parameter list, the return list is also |
171 | flattened on return. So all you have managed to do here is stored | |
17b63f68 | 172 | everything in C<@a> and made C<@b> empty. See |
13a2d996 | 173 | L<Pass by Reference> for alternatives. |
19799a22 GS |
174 | |
175 | A subroutine may be called using an explicit C<&> prefix. The | |
176 | C<&> is optional in modern Perl, as are parentheses if the | |
177 | subroutine has been predeclared. The C<&> is I<not> optional | |
178 | when just naming the subroutine, such as when it's used as | |
179 | an argument to defined() or undef(). Nor is it optional when you | |
180 | want to do an indirect subroutine call with a subroutine name or | |
181 | reference using the C<&$subref()> or C<&{$subref}()> constructs, | |
c47ff5f1 | 182 | although the C<< $subref->() >> notation solves that problem. |
19799a22 GS |
183 | See L<perlref> for more about all that. |
184 | ||
185 | Subroutines may be called recursively. If a subroutine is called | |
186 | using the C<&> form, the argument list is optional, and if omitted, | |
187 | no C<@_> array is set up for the subroutine: the C<@_> array at the | |
188 | time of the call is visible to subroutine instead. This is an | |
189 | efficiency mechanism that new users may wish to avoid. | |
a0d0e21e LW |
190 | |
191 | &foo(1,2,3); # pass three arguments | |
192 | foo(1,2,3); # the same | |
193 | ||
194 | foo(); # pass a null list | |
195 | &foo(); # the same | |
a0d0e21e | 196 | |
cb1a09d0 | 197 | &foo; # foo() get current args, like foo(@_) !! |
54310121 | 198 | foo; # like foo() IFF sub foo predeclared, else "foo" |
cb1a09d0 | 199 | |
19799a22 GS |
200 | Not only does the C<&> form make the argument list optional, it also |
201 | disables any prototype checking on arguments you do provide. This | |
c07a80fd | 202 | is partly for historical reasons, and partly for having a convenient way |
19799a22 | 203 | to cheat if you know what you're doing. See L<Prototypes> below. |
c07a80fd | 204 | |
ac90fb77 EM |
205 | Subroutines whose names are in all upper case are reserved to the Perl |
206 | core, as are modules whose names are in all lower case. A subroutine in | |
207 | all capitals is a loosely-held convention meaning it will be called | |
208 | indirectly by the run-time system itself, usually due to a triggered event. | |
209 | Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>, | |
210 | C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>. | |
211 | ||
212 | The C<BEGIN>, C<CHECK>, C<INIT> and C<END> subroutines are not so much | |
213 | subroutines as named special code blocks, of which you can have more | |
214 | than one in a package, and which you can B<not> call explicitely. See | |
215 | L<perlmod/"BEGIN, CHECK, INIT and END"> | |
5a964f20 | 216 | |
b687b08b | 217 | =head2 Private Variables via my() |
cb1a09d0 AD |
218 | |
219 | Synopsis: | |
220 | ||
221 | my $foo; # declare $foo lexically local | |
222 | my (@wid, %get); # declare list of variables local | |
223 | my $foo = "flurp"; # declare $foo lexical, and init it | |
224 | my @oof = @bar; # declare @oof lexical, and init it | |
09bef843 SB |
225 | my $x : Foo = $y; # similar, with an attribute applied |
226 | ||
a0ae32d3 JH |
227 | B<WARNING>: The use of attribute lists on C<my> declarations is still |
228 | evolving. The current semantics and interface are subject to change. | |
229 | See L<attributes> and L<Attribute::Handlers>. | |
cb1a09d0 | 230 | |
19799a22 GS |
231 | The C<my> operator declares the listed variables to be lexically |
232 | confined to the enclosing block, conditional (C<if/unless/elsif/else>), | |
233 | loop (C<for/foreach/while/until/continue>), subroutine, C<eval>, | |
234 | or C<do/require/use>'d file. If more than one value is listed, the | |
235 | list must be placed in parentheses. All listed elements must be | |
236 | legal lvalues. Only alphanumeric identifiers may be lexically | |
325192b1 | 237 | scoped--magical built-ins like C<$/> must currently be C<local>ized |
19799a22 GS |
238 | with C<local> instead. |
239 | ||
240 | Unlike dynamic variables created by the C<local> operator, lexical | |
241 | variables declared with C<my> are totally hidden from the outside | |
242 | world, including any called subroutines. This is true if it's the | |
243 | same subroutine called from itself or elsewhere--every call gets | |
244 | its own copy. | |
245 | ||
246 | This doesn't mean that a C<my> variable declared in a statically | |
247 | enclosing lexical scope would be invisible. Only dynamic scopes | |
248 | are cut off. For example, the C<bumpx()> function below has access | |
249 | to the lexical $x variable because both the C<my> and the C<sub> | |
250 | occurred at the same scope, presumably file scope. | |
5a964f20 TC |
251 | |
252 | my $x = 10; | |
253 | sub bumpx { $x++ } | |
254 | ||
19799a22 GS |
255 | An C<eval()>, however, can see lexical variables of the scope it is |
256 | being evaluated in, so long as the names aren't hidden by declarations within | |
257 | the C<eval()> itself. See L<perlref>. | |
cb1a09d0 | 258 | |
19799a22 | 259 | The parameter list to my() may be assigned to if desired, which allows you |
cb1a09d0 AD |
260 | to initialize your variables. (If no initializer is given for a |
261 | particular variable, it is created with the undefined value.) Commonly | |
19799a22 | 262 | this is used to name input parameters to a subroutine. Examples: |
cb1a09d0 AD |
263 | |
264 | $arg = "fred"; # "global" variable | |
265 | $n = cube_root(27); | |
266 | print "$arg thinks the root is $n\n"; | |
267 | fred thinks the root is 3 | |
268 | ||
269 | sub cube_root { | |
270 | my $arg = shift; # name doesn't matter | |
271 | $arg **= 1/3; | |
272 | return $arg; | |
54310121 | 273 | } |
cb1a09d0 | 274 | |
19799a22 GS |
275 | The C<my> is simply a modifier on something you might assign to. So when |
276 | you do assign to variables in its argument list, C<my> doesn't | |
6cc33c6d | 277 | change whether those variables are viewed as a scalar or an array. So |
cb1a09d0 | 278 | |
5a964f20 | 279 | my ($foo) = <STDIN>; # WRONG? |
cb1a09d0 AD |
280 | my @FOO = <STDIN>; |
281 | ||
5f05dabc | 282 | both supply a list context to the right-hand side, while |
cb1a09d0 AD |
283 | |
284 | my $foo = <STDIN>; | |
285 | ||
5f05dabc | 286 | supplies a scalar context. But the following declares only one variable: |
748a9306 | 287 | |
5a964f20 | 288 | my $foo, $bar = 1; # WRONG |
748a9306 | 289 | |
cb1a09d0 | 290 | That has the same effect as |
748a9306 | 291 | |
cb1a09d0 AD |
292 | my $foo; |
293 | $bar = 1; | |
a0d0e21e | 294 | |
cb1a09d0 AD |
295 | The declared variable is not introduced (is not visible) until after |
296 | the current statement. Thus, | |
297 | ||
298 | my $x = $x; | |
299 | ||
19799a22 | 300 | can be used to initialize a new $x with the value of the old $x, and |
cb1a09d0 AD |
301 | the expression |
302 | ||
303 | my $x = 123 and $x == 123 | |
304 | ||
19799a22 | 305 | is false unless the old $x happened to have the value C<123>. |
cb1a09d0 | 306 | |
55497cff | 307 | Lexical scopes of control structures are not bounded precisely by the |
308 | braces that delimit their controlled blocks; control expressions are | |
19799a22 | 309 | part of that scope, too. Thus in the loop |
55497cff | 310 | |
19799a22 | 311 | while (my $line = <>) { |
55497cff | 312 | $line = lc $line; |
313 | } continue { | |
314 | print $line; | |
315 | } | |
316 | ||
19799a22 | 317 | the scope of $line extends from its declaration throughout the rest of |
55497cff | 318 | the loop construct (including the C<continue> clause), but not beyond |
319 | it. Similarly, in the conditional | |
320 | ||
321 | if ((my $answer = <STDIN>) =~ /^yes$/i) { | |
322 | user_agrees(); | |
323 | } elsif ($answer =~ /^no$/i) { | |
324 | user_disagrees(); | |
325 | } else { | |
326 | chomp $answer; | |
327 | die "'$answer' is neither 'yes' nor 'no'"; | |
328 | } | |
329 | ||
19799a22 GS |
330 | the scope of $answer extends from its declaration through the rest |
331 | of that conditional, including any C<elsif> and C<else> clauses, | |
457b36cb MV |
332 | but not beyond it. See L<perlsyn/"Simple statements"> for information |
333 | on the scope of variables in statements with modifiers. | |
55497cff | 334 | |
5f05dabc | 335 | The C<foreach> loop defaults to scoping its index variable dynamically |
19799a22 GS |
336 | in the manner of C<local>. However, if the index variable is |
337 | prefixed with the keyword C<my>, or if there is already a lexical | |
338 | by that name in scope, then a new lexical is created instead. Thus | |
339 | in the loop | |
55497cff | 340 | |
341 | for my $i (1, 2, 3) { | |
342 | some_function(); | |
343 | } | |
344 | ||
19799a22 GS |
345 | the scope of $i extends to the end of the loop, but not beyond it, |
346 | rendering the value of $i inaccessible within C<some_function()>. | |
55497cff | 347 | |
cb1a09d0 | 348 | Some users may wish to encourage the use of lexically scoped variables. |
19799a22 GS |
349 | As an aid to catching implicit uses to package variables, |
350 | which are always global, if you say | |
cb1a09d0 AD |
351 | |
352 | use strict 'vars'; | |
353 | ||
19799a22 GS |
354 | then any variable mentioned from there to the end of the enclosing |
355 | block must either refer to a lexical variable, be predeclared via | |
77ca0c92 | 356 | C<our> or C<use vars>, or else must be fully qualified with the package name. |
19799a22 GS |
357 | A compilation error results otherwise. An inner block may countermand |
358 | this with C<no strict 'vars'>. | |
359 | ||
360 | A C<my> has both a compile-time and a run-time effect. At compile | |
8593bda5 | 361 | time, the compiler takes notice of it. The principal usefulness |
19799a22 GS |
362 | of this is to quiet C<use strict 'vars'>, but it is also essential |
363 | for generation of closures as detailed in L<perlref>. Actual | |
364 | initialization is delayed until run time, though, so it gets executed | |
365 | at the appropriate time, such as each time through a loop, for | |
366 | example. | |
367 | ||
368 | Variables declared with C<my> are not part of any package and are therefore | |
cb1a09d0 AD |
369 | never fully qualified with the package name. In particular, you're not |
370 | allowed to try to make a package variable (or other global) lexical: | |
371 | ||
372 | my $pack::var; # ERROR! Illegal syntax | |
373 | my $_; # also illegal (currently) | |
374 | ||
375 | In fact, a dynamic variable (also known as package or global variables) | |
f86cebdf | 376 | are still accessible using the fully qualified C<::> notation even while a |
cb1a09d0 AD |
377 | lexical of the same name is also visible: |
378 | ||
379 | package main; | |
380 | local $x = 10; | |
381 | my $x = 20; | |
382 | print "$x and $::x\n"; | |
383 | ||
f86cebdf | 384 | That will print out C<20> and C<10>. |
cb1a09d0 | 385 | |
19799a22 GS |
386 | You may declare C<my> variables at the outermost scope of a file |
387 | to hide any such identifiers from the world outside that file. This | |
388 | is similar in spirit to C's static variables when they are used at | |
389 | the file level. To do this with a subroutine requires the use of | |
390 | a closure (an anonymous function that accesses enclosing lexicals). | |
391 | If you want to create a private subroutine that cannot be called | |
392 | from outside that block, it can declare a lexical variable containing | |
393 | an anonymous sub reference: | |
cb1a09d0 AD |
394 | |
395 | my $secret_version = '1.001-beta'; | |
396 | my $secret_sub = sub { print $secret_version }; | |
397 | &$secret_sub(); | |
398 | ||
399 | As long as the reference is never returned by any function within the | |
5f05dabc | 400 | module, no outside module can see the subroutine, because its name is not in |
cb1a09d0 | 401 | any package's symbol table. Remember that it's not I<REALLY> called |
19799a22 | 402 | C<$some_pack::secret_version> or anything; it's just $secret_version, |
cb1a09d0 AD |
403 | unqualified and unqualifiable. |
404 | ||
19799a22 GS |
405 | This does not work with object methods, however; all object methods |
406 | have to be in the symbol table of some package to be found. See | |
407 | L<perlref/"Function Templates"> for something of a work-around to | |
408 | this. | |
cb1a09d0 | 409 | |
c2611fb3 | 410 | =head2 Persistent Private Variables |
5a964f20 TC |
411 | |
412 | Just because a lexical variable is lexically (also called statically) | |
f86cebdf | 413 | scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that |
5a964f20 TC |
414 | within a function it works like a C static. It normally works more |
415 | like a C auto, but with implicit garbage collection. | |
416 | ||
417 | Unlike local variables in C or C++, Perl's lexical variables don't | |
418 | necessarily get recycled just because their scope has exited. | |
419 | If something more permanent is still aware of the lexical, it will | |
420 | stick around. So long as something else references a lexical, that | |
421 | lexical won't be freed--which is as it should be. You wouldn't want | |
422 | memory being free until you were done using it, or kept around once you | |
423 | were done. Automatic garbage collection takes care of this for you. | |
424 | ||
425 | This means that you can pass back or save away references to lexical | |
426 | variables, whereas to return a pointer to a C auto is a grave error. | |
427 | It also gives us a way to simulate C's function statics. Here's a | |
428 | mechanism for giving a function private variables with both lexical | |
429 | scoping and a static lifetime. If you do want to create something like | |
430 | C's static variables, just enclose the whole function in an extra block, | |
431 | and put the static variable outside the function but in the block. | |
cb1a09d0 AD |
432 | |
433 | { | |
54310121 | 434 | my $secret_val = 0; |
cb1a09d0 AD |
435 | sub gimme_another { |
436 | return ++$secret_val; | |
54310121 | 437 | } |
438 | } | |
cb1a09d0 AD |
439 | # $secret_val now becomes unreachable by the outside |
440 | # world, but retains its value between calls to gimme_another | |
441 | ||
54310121 | 442 | If this function is being sourced in from a separate file |
cb1a09d0 | 443 | via C<require> or C<use>, then this is probably just fine. If it's |
19799a22 | 444 | all in the main program, you'll need to arrange for the C<my> |
cb1a09d0 | 445 | to be executed early, either by putting the whole block above |
f86cebdf | 446 | your main program, or more likely, placing merely a C<BEGIN> |
ac90fb77 | 447 | code block around it to make sure it gets executed before your program |
cb1a09d0 AD |
448 | starts to run: |
449 | ||
ac90fb77 | 450 | BEGIN { |
54310121 | 451 | my $secret_val = 0; |
cb1a09d0 AD |
452 | sub gimme_another { |
453 | return ++$secret_val; | |
54310121 | 454 | } |
455 | } | |
cb1a09d0 | 456 | |
ac90fb77 EM |
457 | See L<perlmod/"BEGIN, CHECK, INIT and END"> about the |
458 | special triggered code blocks, C<BEGIN>, C<CHECK>, C<INIT> and C<END>. | |
cb1a09d0 | 459 | |
19799a22 GS |
460 | If declared at the outermost scope (the file scope), then lexicals |
461 | work somewhat like C's file statics. They are available to all | |
462 | functions in that same file declared below them, but are inaccessible | |
463 | from outside that file. This strategy is sometimes used in modules | |
464 | to create private variables that the whole module can see. | |
5a964f20 | 465 | |
cb1a09d0 AD |
466 | =head2 Temporary Values via local() |
467 | ||
19799a22 | 468 | B<WARNING>: In general, you should be using C<my> instead of C<local>, because |
6d28dffb | 469 | it's faster and safer. Exceptions to this include the global punctuation |
325192b1 RGS |
470 | variables, global filehandles and formats, and direct manipulation of the |
471 | Perl symbol table itself. C<local> is mostly used when the current value | |
472 | of a variable must be visible to called subroutines. | |
cb1a09d0 AD |
473 | |
474 | Synopsis: | |
475 | ||
325192b1 RGS |
476 | # localization of values |
477 | ||
478 | local $foo; # make $foo dynamically local | |
479 | local (@wid, %get); # make list of variables local | |
480 | local $foo = "flurp"; # make $foo dynamic, and init it | |
481 | local @oof = @bar; # make @oof dynamic, and init it | |
482 | ||
483 | local $hash{key} = "val"; # sets a local value for this hash entry | |
484 | local ($cond ? $v1 : $v2); # several types of lvalues support | |
485 | # localization | |
486 | ||
487 | # localization of symbols | |
cb1a09d0 AD |
488 | |
489 | local *FH; # localize $FH, @FH, %FH, &FH ... | |
490 | local *merlyn = *randal; # now $merlyn is really $randal, plus | |
491 | # @merlyn is really @randal, etc | |
492 | local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal | |
54310121 | 493 | local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc |
cb1a09d0 | 494 | |
19799a22 GS |
495 | A C<local> modifies its listed variables to be "local" to the |
496 | enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine | |
497 | called from within that block>. A C<local> just gives temporary | |
498 | values to global (meaning package) variables. It does I<not> create | |
499 | a local variable. This is known as dynamic scoping. Lexical scoping | |
500 | is done with C<my>, which works more like C's auto declarations. | |
cb1a09d0 | 501 | |
325192b1 RGS |
502 | Some types of lvalues can be localized as well : hash and array elements |
503 | and slices, conditionals (provided that their result is always | |
504 | localizable), and symbolic references. As for simple variables, this | |
505 | creates new, dynamically scoped values. | |
506 | ||
507 | If more than one variable or expression is given to C<local>, they must be | |
508 | placed in parentheses. This operator works | |
cb1a09d0 | 509 | by saving the current values of those variables in its argument list on a |
5f05dabc | 510 | hidden stack and restoring them upon exiting the block, subroutine, or |
cb1a09d0 AD |
511 | eval. This means that called subroutines can also reference the local |
512 | variable, but not the global one. The argument list may be assigned to if | |
513 | desired, which allows you to initialize your local variables. (If no | |
514 | initializer is given for a particular variable, it is created with an | |
325192b1 | 515 | undefined value.) |
cb1a09d0 | 516 | |
19799a22 | 517 | Because C<local> is a run-time operator, it gets executed each time |
325192b1 RGS |
518 | through a loop. Consequently, it's more efficient to localize your |
519 | variables outside the loop. | |
520 | ||
521 | =head3 Grammatical note on local() | |
cb1a09d0 | 522 | |
f86cebdf GS |
523 | A C<local> is simply a modifier on an lvalue expression. When you assign to |
524 | a C<local>ized variable, the C<local> doesn't change whether its list is viewed | |
cb1a09d0 AD |
525 | as a scalar or an array. So |
526 | ||
527 | local($foo) = <STDIN>; | |
528 | local @FOO = <STDIN>; | |
529 | ||
5f05dabc | 530 | both supply a list context to the right-hand side, while |
cb1a09d0 AD |
531 | |
532 | local $foo = <STDIN>; | |
533 | ||
534 | supplies a scalar context. | |
535 | ||
325192b1 | 536 | =head3 Localization of special variables |
3e3baf6d | 537 | |
325192b1 RGS |
538 | If you localize a special variable, you'll be giving a new value to it, |
539 | but its magic won't go away. That means that all side-effects related | |
540 | to this magic still work with the localized value. | |
3e3baf6d | 541 | |
325192b1 RGS |
542 | This feature allows code like this to work : |
543 | ||
544 | # Read the whole contents of FILE in $slurp | |
545 | { local $/ = undef; $slurp = <FILE>; } | |
546 | ||
547 | Note, however, that this restricts localization of some values ; for | |
548 | example, the following statement dies, as of perl 5.9.0, with an error | |
549 | I<Modification of a read-only value attempted>, because the $1 variable is | |
550 | magical and read-only : | |
551 | ||
552 | local $1 = 2; | |
553 | ||
554 | Similarly, but in a way more difficult to spot, the following snippet will | |
555 | die in perl 5.9.0 : | |
556 | ||
557 | sub f { local $_ = "foo"; print } | |
558 | for ($1) { | |
559 | # now $_ is aliased to $1, thus is magic and readonly | |
560 | f(); | |
3e3baf6d | 561 | } |
3e3baf6d | 562 | |
325192b1 RGS |
563 | See next section for an alternative to this situation. |
564 | ||
565 | B<WARNING>: Localization of tied arrays and hashes does not currently | |
566 | work as described. | |
fd5a896a DM |
567 | This will be fixed in a future release of Perl; in the meantime, avoid |
568 | code that relies on any particular behaviour of localising tied arrays | |
569 | or hashes (localising individual elements is still okay). | |
325192b1 | 570 | See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more |
fd5a896a DM |
571 | details. |
572 | ||
325192b1 | 573 | =head3 Localization of globs |
3e3baf6d | 574 | |
325192b1 RGS |
575 | The construct |
576 | ||
577 | local *name; | |
578 | ||
579 | creates a whole new symbol table entry for the glob C<name> in the | |
580 | current package. That means that all variables in its glob slot ($name, | |
581 | @name, %name, &name, and the C<name> filehandle) are dynamically reset. | |
582 | ||
583 | This implies, among other things, that any magic eventually carried by | |
584 | those variables is locally lost. In other words, saying C<local */> | |
585 | will not have any effect on the internal value of the input record | |
586 | separator. | |
587 | ||
588 | Notably, if you want to work with a brand new value of the default scalar | |
589 | $_, and avoid the potential problem listed above about $_ previously | |
590 | carrying a magic value, you should use C<local *_> instead of C<local $_>. | |
591 | ||
592 | =head3 Localization of elements of composite types | |
3e3baf6d | 593 | |
6ee623d5 | 594 | It's also worth taking a moment to explain what happens when you |
f86cebdf GS |
595 | C<local>ize a member of a composite type (i.e. an array or hash element). |
596 | In this case, the element is C<local>ized I<by name>. This means that | |
6ee623d5 GS |
597 | when the scope of the C<local()> ends, the saved value will be |
598 | restored to the hash element whose key was named in the C<local()>, or | |
599 | the array element whose index was named in the C<local()>. If that | |
600 | element was deleted while the C<local()> was in effect (e.g. by a | |
601 | C<delete()> from a hash or a C<shift()> of an array), it will spring | |
602 | back into existence, possibly extending an array and filling in the | |
603 | skipped elements with C<undef>. For instance, if you say | |
604 | ||
605 | %hash = ( 'This' => 'is', 'a' => 'test' ); | |
606 | @ary = ( 0..5 ); | |
607 | { | |
608 | local($ary[5]) = 6; | |
609 | local($hash{'a'}) = 'drill'; | |
610 | while (my $e = pop(@ary)) { | |
611 | print "$e . . .\n"; | |
612 | last unless $e > 3; | |
613 | } | |
614 | if (@ary) { | |
615 | $hash{'only a'} = 'test'; | |
616 | delete $hash{'a'}; | |
617 | } | |
618 | } | |
619 | print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n"; | |
620 | print "The array has ",scalar(@ary)," elements: ", | |
621 | join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n"; | |
622 | ||
623 | Perl will print | |
624 | ||
625 | 6 . . . | |
626 | 4 . . . | |
627 | 3 . . . | |
628 | This is a test only a test. | |
629 | The array has 6 elements: 0, 1, 2, undef, undef, 5 | |
630 | ||
19799a22 | 631 | The behavior of local() on non-existent members of composite |
7185e5cc GS |
632 | types is subject to change in future. |
633 | ||
cd06dffe GS |
634 | =head2 Lvalue subroutines |
635 | ||
e6a32221 JC |
636 | B<WARNING>: Lvalue subroutines are still experimental and the |
637 | implementation may change in future versions of Perl. | |
cd06dffe GS |
638 | |
639 | It is possible to return a modifiable value from a subroutine. | |
640 | To do this, you have to declare the subroutine to return an lvalue. | |
641 | ||
642 | my $val; | |
643 | sub canmod : lvalue { | |
e6a32221 | 644 | # return $val; this doesn't work, don't say "return" |
cd06dffe GS |
645 | $val; |
646 | } | |
647 | sub nomod { | |
648 | $val; | |
649 | } | |
650 | ||
651 | canmod() = 5; # assigns to $val | |
652 | nomod() = 5; # ERROR | |
653 | ||
654 | The scalar/list context for the subroutine and for the right-hand | |
655 | side of assignment is determined as if the subroutine call is replaced | |
656 | by a scalar. For example, consider: | |
657 | ||
658 | data(2,3) = get_data(3,4); | |
659 | ||
660 | Both subroutines here are called in a scalar context, while in: | |
661 | ||
662 | (data(2,3)) = get_data(3,4); | |
663 | ||
664 | and in: | |
665 | ||
666 | (data(2),data(3)) = get_data(3,4); | |
667 | ||
668 | all the subroutines are called in a list context. | |
669 | ||
e6a32221 JC |
670 | =over 4 |
671 | ||
672 | =item Lvalue subroutines are EXPERIMENTAL | |
673 | ||
674 | They appear to be convenient, but there are several reasons to be | |
675 | circumspect. | |
676 | ||
677 | You can't use the return keyword, you must pass out the value before | |
678 | falling out of subroutine scope. (see comment in example above). This | |
679 | is usually not a problem, but it disallows an explicit return out of a | |
680 | deeply nested loop, which is sometimes a nice way out. | |
681 | ||
682 | They violate encapsulation. A normal mutator can check the supplied | |
683 | argument before setting the attribute it is protecting, an lvalue | |
684 | subroutine never gets that chance. Consider; | |
685 | ||
686 | my $some_array_ref = []; # protected by mutators ?? | |
687 | ||
688 | sub set_arr { # normal mutator | |
689 | my $val = shift; | |
690 | die("expected array, you supplied ", ref $val) | |
691 | unless ref $val eq 'ARRAY'; | |
692 | $some_array_ref = $val; | |
693 | } | |
694 | sub set_arr_lv : lvalue { # lvalue mutator | |
695 | $some_array_ref; | |
696 | } | |
697 | ||
698 | # set_arr_lv cannot stop this ! | |
699 | set_arr_lv() = { a => 1 }; | |
818c4caa | 700 | |
e6a32221 JC |
701 | =back |
702 | ||
cb1a09d0 AD |
703 | =head2 Passing Symbol Table Entries (typeglobs) |
704 | ||
19799a22 GS |
705 | B<WARNING>: The mechanism described in this section was originally |
706 | the only way to simulate pass-by-reference in older versions of | |
707 | Perl. While it still works fine in modern versions, the new reference | |
708 | mechanism is generally easier to work with. See below. | |
a0d0e21e LW |
709 | |
710 | Sometimes you don't want to pass the value of an array to a subroutine | |
711 | but rather the name of it, so that the subroutine can modify the global | |
712 | copy of it rather than working with a local copy. In perl you can | |
cb1a09d0 | 713 | refer to all objects of a particular name by prefixing the name |
5f05dabc | 714 | with a star: C<*foo>. This is often known as a "typeglob", because the |
a0d0e21e LW |
715 | star on the front can be thought of as a wildcard match for all the |
716 | funny prefix characters on variables and subroutines and such. | |
717 | ||
55497cff | 718 | When evaluated, the typeglob produces a scalar value that represents |
5f05dabc | 719 | all the objects of that name, including any filehandle, format, or |
a0d0e21e | 720 | subroutine. When assigned to, it causes the name mentioned to refer to |
19799a22 | 721 | whatever C<*> value was assigned to it. Example: |
a0d0e21e LW |
722 | |
723 | sub doubleary { | |
724 | local(*someary) = @_; | |
725 | foreach $elem (@someary) { | |
726 | $elem *= 2; | |
727 | } | |
728 | } | |
729 | doubleary(*foo); | |
730 | doubleary(*bar); | |
731 | ||
19799a22 | 732 | Scalars are already passed by reference, so you can modify |
a0d0e21e | 733 | scalar arguments without using this mechanism by referring explicitly |
1fef88e7 | 734 | to C<$_[0]> etc. You can modify all the elements of an array by passing |
f86cebdf GS |
735 | all the elements as scalars, but you have to use the C<*> mechanism (or |
736 | the equivalent reference mechanism) to C<push>, C<pop>, or change the size of | |
a0d0e21e LW |
737 | an array. It will certainly be faster to pass the typeglob (or reference). |
738 | ||
739 | Even if you don't want to modify an array, this mechanism is useful for | |
5f05dabc | 740 | passing multiple arrays in a single LIST, because normally the LIST |
a0d0e21e | 741 | mechanism will merge all the array values so that you can't extract out |
55497cff | 742 | the individual arrays. For more on typeglobs, see |
2ae324a7 | 743 | L<perldata/"Typeglobs and Filehandles">. |
cb1a09d0 | 744 | |
5a964f20 TC |
745 | =head2 When to Still Use local() |
746 | ||
19799a22 GS |
747 | Despite the existence of C<my>, there are still three places where the |
748 | C<local> operator still shines. In fact, in these three places, you | |
5a964f20 TC |
749 | I<must> use C<local> instead of C<my>. |
750 | ||
13a2d996 | 751 | =over 4 |
5a964f20 | 752 | |
551e1d92 RB |
753 | =item 1. |
754 | ||
755 | You need to give a global variable a temporary value, especially $_. | |
5a964f20 | 756 | |
f86cebdf GS |
757 | The global variables, like C<@ARGV> or the punctuation variables, must be |
758 | C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits | |
5a964f20 | 759 | it up into chunks separated by lines of equal signs, which are placed |
f86cebdf | 760 | in C<@Fields>. |
5a964f20 TC |
761 | |
762 | { | |
763 | local @ARGV = ("/etc/motd"); | |
764 | local $/ = undef; | |
765 | local $_ = <>; | |
766 | @Fields = split /^\s*=+\s*$/; | |
767 | } | |
768 | ||
19799a22 | 769 | It particular, it's important to C<local>ize $_ in any routine that assigns |
5a964f20 TC |
770 | to it. Look out for implicit assignments in C<while> conditionals. |
771 | ||
551e1d92 RB |
772 | =item 2. |
773 | ||
774 | You need to create a local file or directory handle or a local function. | |
5a964f20 | 775 | |
09bef843 SB |
776 | A function that needs a filehandle of its own must use |
777 | C<local()> on a complete typeglob. This can be used to create new symbol | |
5a964f20 TC |
778 | table entries: |
779 | ||
780 | sub ioqueue { | |
781 | local (*READER, *WRITER); # not my! | |
17b63f68 | 782 | pipe (READER, WRITER) or die "pipe: $!"; |
5a964f20 TC |
783 | return (*READER, *WRITER); |
784 | } | |
785 | ($head, $tail) = ioqueue(); | |
786 | ||
787 | See the Symbol module for a way to create anonymous symbol table | |
788 | entries. | |
789 | ||
790 | Because assignment of a reference to a typeglob creates an alias, this | |
791 | can be used to create what is effectively a local function, or at least, | |
792 | a local alias. | |
793 | ||
794 | { | |
f86cebdf GS |
795 | local *grow = \&shrink; # only until this block exists |
796 | grow(); # really calls shrink() | |
797 | move(); # if move() grow()s, it shrink()s too | |
5a964f20 | 798 | } |
f86cebdf | 799 | grow(); # get the real grow() again |
5a964f20 TC |
800 | |
801 | See L<perlref/"Function Templates"> for more about manipulating | |
802 | functions by name in this way. | |
803 | ||
551e1d92 RB |
804 | =item 3. |
805 | ||
806 | You want to temporarily change just one element of an array or hash. | |
5a964f20 | 807 | |
f86cebdf | 808 | You can C<local>ize just one element of an aggregate. Usually this |
5a964f20 TC |
809 | is done on dynamics: |
810 | ||
811 | { | |
812 | local $SIG{INT} = 'IGNORE'; | |
813 | funct(); # uninterruptible | |
814 | } | |
815 | # interruptibility automatically restored here | |
816 | ||
817 | But it also works on lexically declared aggregates. Prior to 5.005, | |
818 | this operation could on occasion misbehave. | |
819 | ||
820 | =back | |
821 | ||
cb1a09d0 AD |
822 | =head2 Pass by Reference |
823 | ||
55497cff | 824 | If you want to pass more than one array or hash into a function--or |
825 | return them from it--and have them maintain their integrity, then | |
826 | you're going to have to use an explicit pass-by-reference. Before you | |
827 | do that, you need to understand references as detailed in L<perlref>. | |
c07a80fd | 828 | This section may not make much sense to you otherwise. |
cb1a09d0 | 829 | |
19799a22 GS |
830 | Here are a few simple examples. First, let's pass in several arrays |
831 | to a function and have it C<pop> all of then, returning a new list | |
832 | of all their former last elements: | |
cb1a09d0 AD |
833 | |
834 | @tailings = popmany ( \@a, \@b, \@c, \@d ); | |
835 | ||
836 | sub popmany { | |
837 | my $aref; | |
838 | my @retlist = (); | |
839 | foreach $aref ( @_ ) { | |
840 | push @retlist, pop @$aref; | |
54310121 | 841 | } |
cb1a09d0 | 842 | return @retlist; |
54310121 | 843 | } |
cb1a09d0 | 844 | |
54310121 | 845 | Here's how you might write a function that returns a |
cb1a09d0 AD |
846 | list of keys occurring in all the hashes passed to it: |
847 | ||
54310121 | 848 | @common = inter( \%foo, \%bar, \%joe ); |
cb1a09d0 AD |
849 | sub inter { |
850 | my ($k, $href, %seen); # locals | |
851 | foreach $href (@_) { | |
852 | while ( $k = each %$href ) { | |
853 | $seen{$k}++; | |
54310121 | 854 | } |
855 | } | |
cb1a09d0 | 856 | return grep { $seen{$_} == @_ } keys %seen; |
54310121 | 857 | } |
cb1a09d0 | 858 | |
5f05dabc | 859 | So far, we're using just the normal list return mechanism. |
54310121 | 860 | What happens if you want to pass or return a hash? Well, |
861 | if you're using only one of them, or you don't mind them | |
cb1a09d0 | 862 | concatenating, then the normal calling convention is ok, although |
54310121 | 863 | a little expensive. |
cb1a09d0 AD |
864 | |
865 | Where people get into trouble is here: | |
866 | ||
867 | (@a, @b) = func(@c, @d); | |
868 | or | |
869 | (%a, %b) = func(%c, %d); | |
870 | ||
19799a22 GS |
871 | That syntax simply won't work. It sets just C<@a> or C<%a> and |
872 | clears the C<@b> or C<%b>. Plus the function didn't get passed | |
873 | into two separate arrays or hashes: it got one long list in C<@_>, | |
874 | as always. | |
cb1a09d0 AD |
875 | |
876 | If you can arrange for everyone to deal with this through references, it's | |
877 | cleaner code, although not so nice to look at. Here's a function that | |
878 | takes two array references as arguments, returning the two array elements | |
879 | in order of how many elements they have in them: | |
880 | ||
881 | ($aref, $bref) = func(\@c, \@d); | |
882 | print "@$aref has more than @$bref\n"; | |
883 | sub func { | |
884 | my ($cref, $dref) = @_; | |
885 | if (@$cref > @$dref) { | |
886 | return ($cref, $dref); | |
887 | } else { | |
c07a80fd | 888 | return ($dref, $cref); |
54310121 | 889 | } |
890 | } | |
cb1a09d0 AD |
891 | |
892 | It turns out that you can actually do this also: | |
893 | ||
894 | (*a, *b) = func(\@c, \@d); | |
895 | print "@a has more than @b\n"; | |
896 | sub func { | |
897 | local (*c, *d) = @_; | |
898 | if (@c > @d) { | |
899 | return (\@c, \@d); | |
900 | } else { | |
901 | return (\@d, \@c); | |
54310121 | 902 | } |
903 | } | |
cb1a09d0 AD |
904 | |
905 | Here we're using the typeglobs to do symbol table aliasing. It's | |
19799a22 | 906 | a tad subtle, though, and also won't work if you're using C<my> |
09bef843 | 907 | variables, because only globals (even in disguise as C<local>s) |
19799a22 | 908 | are in the symbol table. |
5f05dabc | 909 | |
910 | If you're passing around filehandles, you could usually just use the bare | |
19799a22 GS |
911 | typeglob, like C<*STDOUT>, but typeglobs references work, too. |
912 | For example: | |
5f05dabc | 913 | |
914 | splutter(\*STDOUT); | |
915 | sub splutter { | |
916 | my $fh = shift; | |
917 | print $fh "her um well a hmmm\n"; | |
918 | } | |
919 | ||
920 | $rec = get_rec(\*STDIN); | |
921 | sub get_rec { | |
922 | my $fh = shift; | |
923 | return scalar <$fh>; | |
924 | } | |
925 | ||
19799a22 GS |
926 | If you're planning on generating new filehandles, you could do this. |
927 | Notice to pass back just the bare *FH, not its reference. | |
5f05dabc | 928 | |
929 | sub openit { | |
19799a22 | 930 | my $path = shift; |
5f05dabc | 931 | local *FH; |
e05a3a1e | 932 | return open (FH, $path) ? *FH : undef; |
54310121 | 933 | } |
5f05dabc | 934 | |
cb1a09d0 AD |
935 | =head2 Prototypes |
936 | ||
19799a22 GS |
937 | Perl supports a very limited kind of compile-time argument checking |
938 | using function prototyping. If you declare | |
cb1a09d0 AD |
939 | |
940 | sub mypush (\@@) | |
941 | ||
19799a22 GS |
942 | then C<mypush()> takes arguments exactly like C<push()> does. The |
943 | function declaration must be visible at compile time. The prototype | |
944 | affects only interpretation of new-style calls to the function, | |
945 | where new-style is defined as not using the C<&> character. In | |
946 | other words, if you call it like a built-in function, then it behaves | |
947 | like a built-in function. If you call it like an old-fashioned | |
948 | subroutine, then it behaves like an old-fashioned subroutine. It | |
949 | naturally falls out from this rule that prototypes have no influence | |
950 | on subroutine references like C<\&foo> or on indirect subroutine | |
c47ff5f1 | 951 | calls like C<&{$subref}> or C<< $subref->() >>. |
c07a80fd | 952 | |
953 | Method calls are not influenced by prototypes either, because the | |
19799a22 GS |
954 | function to be called is indeterminate at compile time, since |
955 | the exact code called depends on inheritance. | |
cb1a09d0 | 956 | |
19799a22 GS |
957 | Because the intent of this feature is primarily to let you define |
958 | subroutines that work like built-in functions, here are prototypes | |
959 | for some other functions that parse almost exactly like the | |
960 | corresponding built-in. | |
cb1a09d0 AD |
961 | |
962 | Declared as Called as | |
963 | ||
f86cebdf GS |
964 | sub mylink ($$) mylink $old, $new |
965 | sub myvec ($$$) myvec $var, $offset, 1 | |
966 | sub myindex ($$;$) myindex &getstring, "substr" | |
967 | sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off | |
968 | sub myreverse (@) myreverse $a, $b, $c | |
969 | sub myjoin ($@) myjoin ":", $a, $b, $c | |
970 | sub mypop (\@) mypop @array | |
971 | sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme | |
972 | sub mykeys (\%) mykeys %{$hashref} | |
973 | sub myopen (*;$) myopen HANDLE, $name | |
974 | sub mypipe (**) mypipe READHANDLE, WRITEHANDLE | |
975 | sub mygrep (&@) mygrep { /foo/ } $a, $b, $c | |
976 | sub myrand ($) myrand 42 | |
977 | sub mytime () mytime | |
cb1a09d0 | 978 | |
c07a80fd | 979 | Any backslashed prototype character represents an actual argument |
6e47f808 | 980 | that absolutely must start with that character. The value passed |
19799a22 GS |
981 | as part of C<@_> will be a reference to the actual argument given |
982 | in the subroutine call, obtained by applying C<\> to that argument. | |
c07a80fd | 983 | |
5b794e05 JH |
984 | You can also backslash several argument types simultaneously by using |
985 | the C<\[]> notation: | |
986 | ||
987 | sub myref (\[$@%&*]) | |
988 | ||
989 | will allow calling myref() as | |
990 | ||
991 | myref $var | |
992 | myref @array | |
993 | myref %hash | |
994 | myref &sub | |
995 | myref *glob | |
996 | ||
997 | and the first argument of myref() will be a reference to | |
998 | a scalar, an array, a hash, a code, or a glob. | |
999 | ||
c07a80fd | 1000 | Unbackslashed prototype characters have special meanings. Any |
19799a22 | 1001 | unbackslashed C<@> or C<%> eats all remaining arguments, and forces |
f86cebdf GS |
1002 | list context. An argument represented by C<$> forces scalar context. An |
1003 | C<&> requires an anonymous subroutine, which, if passed as the first | |
0df79f0c GS |
1004 | argument, does not require the C<sub> keyword or a subsequent comma. |
1005 | ||
1006 | A C<*> allows the subroutine to accept a bareword, constant, scalar expression, | |
648ca4f7 GS |
1007 | typeglob, or a reference to a typeglob in that slot. The value will be |
1008 | available to the subroutine either as a simple scalar, or (in the latter | |
0df79f0c GS |
1009 | two cases) as a reference to the typeglob. If you wish to always convert |
1010 | such arguments to a typeglob reference, use Symbol::qualify_to_ref() as | |
1011 | follows: | |
1012 | ||
1013 | use Symbol 'qualify_to_ref'; | |
1014 | ||
1015 | sub foo (*) { | |
1016 | my $fh = qualify_to_ref(shift, caller); | |
1017 | ... | |
1018 | } | |
c07a80fd | 1019 | |
1020 | A semicolon separates mandatory arguments from optional arguments. | |
19799a22 | 1021 | It is redundant before C<@> or C<%>, which gobble up everything else. |
cb1a09d0 | 1022 | |
19799a22 GS |
1023 | Note how the last three examples in the table above are treated |
1024 | specially by the parser. C<mygrep()> is parsed as a true list | |
1025 | operator, C<myrand()> is parsed as a true unary operator with unary | |
1026 | precedence the same as C<rand()>, and C<mytime()> is truly without | |
1027 | arguments, just like C<time()>. That is, if you say | |
cb1a09d0 AD |
1028 | |
1029 | mytime +2; | |
1030 | ||
f86cebdf | 1031 | you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed |
19799a22 | 1032 | without a prototype. |
cb1a09d0 | 1033 | |
19799a22 GS |
1034 | The interesting thing about C<&> is that you can generate new syntax with it, |
1035 | provided it's in the initial position: | |
cb1a09d0 | 1036 | |
6d28dffb | 1037 | sub try (&@) { |
cb1a09d0 AD |
1038 | my($try,$catch) = @_; |
1039 | eval { &$try }; | |
1040 | if ($@) { | |
1041 | local $_ = $@; | |
1042 | &$catch; | |
1043 | } | |
1044 | } | |
55497cff | 1045 | sub catch (&) { $_[0] } |
cb1a09d0 AD |
1046 | |
1047 | try { | |
1048 | die "phooey"; | |
1049 | } catch { | |
1050 | /phooey/ and print "unphooey\n"; | |
1051 | }; | |
1052 | ||
f86cebdf | 1053 | That prints C<"unphooey">. (Yes, there are still unresolved |
19799a22 | 1054 | issues having to do with visibility of C<@_>. I'm ignoring that |
f86cebdf | 1055 | question for the moment. (But note that if we make C<@_> lexically |
cb1a09d0 | 1056 | scoped, those anonymous subroutines can act like closures... (Gee, |
5f05dabc | 1057 | is this sounding a little Lispish? (Never mind.)))) |
cb1a09d0 | 1058 | |
19799a22 | 1059 | And here's a reimplementation of the Perl C<grep> operator: |
cb1a09d0 AD |
1060 | |
1061 | sub mygrep (&@) { | |
1062 | my $code = shift; | |
1063 | my @result; | |
1064 | foreach $_ (@_) { | |
6e47f808 | 1065 | push(@result, $_) if &$code; |
cb1a09d0 AD |
1066 | } |
1067 | @result; | |
1068 | } | |
a0d0e21e | 1069 | |
cb1a09d0 AD |
1070 | Some folks would prefer full alphanumeric prototypes. Alphanumerics have |
1071 | been intentionally left out of prototypes for the express purpose of | |
1072 | someday in the future adding named, formal parameters. The current | |
1073 | mechanism's main goal is to let module writers provide better diagnostics | |
1074 | for module users. Larry feels the notation quite understandable to Perl | |
1075 | programmers, and that it will not intrude greatly upon the meat of the | |
1076 | module, nor make it harder to read. The line noise is visually | |
1077 | encapsulated into a small pill that's easy to swallow. | |
1078 | ||
420cdfc1 ST |
1079 | If you try to use an alphanumeric sequence in a prototype you will |
1080 | generate an optional warning - "Illegal character in prototype...". | |
1081 | Unfortunately earlier versions of Perl allowed the prototype to be | |
1082 | used as long as its prefix was a valid prototype. The warning may be | |
1083 | upgraded to a fatal error in a future version of Perl once the | |
1084 | majority of offending code is fixed. | |
1085 | ||
cb1a09d0 AD |
1086 | It's probably best to prototype new functions, not retrofit prototyping |
1087 | into older ones. That's because you must be especially careful about | |
1088 | silent impositions of differing list versus scalar contexts. For example, | |
1089 | if you decide that a function should take just one parameter, like this: | |
1090 | ||
1091 | sub func ($) { | |
1092 | my $n = shift; | |
1093 | print "you gave me $n\n"; | |
54310121 | 1094 | } |
cb1a09d0 AD |
1095 | |
1096 | and someone has been calling it with an array or expression | |
1097 | returning a list: | |
1098 | ||
1099 | func(@foo); | |
1100 | func( split /:/ ); | |
1101 | ||
19799a22 | 1102 | Then you've just supplied an automatic C<scalar> in front of their |
f86cebdf | 1103 | argument, which can be more than a bit surprising. The old C<@foo> |
cb1a09d0 | 1104 | which used to hold one thing doesn't get passed in. Instead, |
19799a22 GS |
1105 | C<func()> now gets passed in a C<1>; that is, the number of elements |
1106 | in C<@foo>. And the C<split> gets called in scalar context so it | |
1107 | starts scribbling on your C<@_> parameter list. Ouch! | |
cb1a09d0 | 1108 | |
5f05dabc | 1109 | This is all very powerful, of course, and should be used only in moderation |
54310121 | 1110 | to make the world a better place. |
44a8e56a | 1111 | |
1112 | =head2 Constant Functions | |
1113 | ||
1114 | Functions with a prototype of C<()> are potential candidates for | |
19799a22 GS |
1115 | inlining. If the result after optimization and constant folding |
1116 | is either a constant or a lexically-scoped scalar which has no other | |
54310121 | 1117 | references, then it will be used in place of function calls made |
19799a22 GS |
1118 | without C<&>. Calls made using C<&> are never inlined. (See |
1119 | F<constant.pm> for an easy way to declare most constants.) | |
44a8e56a | 1120 | |
5a964f20 | 1121 | The following functions would all be inlined: |
44a8e56a | 1122 | |
699e6cd4 TP |
1123 | sub pi () { 3.14159 } # Not exact, but close. |
1124 | sub PI () { 4 * atan2 1, 1 } # As good as it gets, | |
1125 | # and it's inlined, too! | |
44a8e56a | 1126 | sub ST_DEV () { 0 } |
1127 | sub ST_INO () { 1 } | |
1128 | ||
1129 | sub FLAG_FOO () { 1 << 8 } | |
1130 | sub FLAG_BAR () { 1 << 9 } | |
1131 | sub FLAG_MASK () { FLAG_FOO | FLAG_BAR } | |
54310121 | 1132 | |
1133 | sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) } | |
44a8e56a | 1134 | sub BAZ_VAL () { |
1135 | if (OPT_BAZ) { | |
1136 | return 23; | |
1137 | } | |
1138 | else { | |
1139 | return 42; | |
1140 | } | |
1141 | } | |
cb1a09d0 | 1142 | |
54310121 | 1143 | sub N () { int(BAZ_VAL) / 3 } |
1144 | BEGIN { | |
1145 | my $prod = 1; | |
1146 | for (1..N) { $prod *= $_ } | |
1147 | sub N_FACTORIAL () { $prod } | |
1148 | } | |
1149 | ||
5a964f20 | 1150 | If you redefine a subroutine that was eligible for inlining, you'll get |
4cee8e80 CS |
1151 | a mandatory warning. (You can use this warning to tell whether or not a |
1152 | particular subroutine is considered constant.) The warning is | |
1153 | considered severe enough not to be optional because previously compiled | |
1154 | invocations of the function will still be using the old value of the | |
19799a22 | 1155 | function. If you need to be able to redefine the subroutine, you need to |
4cee8e80 | 1156 | ensure that it isn't inlined, either by dropping the C<()> prototype |
19799a22 | 1157 | (which changes calling semantics, so beware) or by thwarting the |
4cee8e80 CS |
1158 | inlining mechanism in some other way, such as |
1159 | ||
4cee8e80 | 1160 | sub not_inlined () { |
54310121 | 1161 | 23 if $]; |
4cee8e80 CS |
1162 | } |
1163 | ||
19799a22 | 1164 | =head2 Overriding Built-in Functions |
a0d0e21e | 1165 | |
19799a22 | 1166 | Many built-in functions may be overridden, though this should be tried |
5f05dabc | 1167 | only occasionally and for good reason. Typically this might be |
19799a22 | 1168 | done by a package attempting to emulate missing built-in functionality |
a0d0e21e LW |
1169 | on a non-Unix system. |
1170 | ||
163e3a99 JP |
1171 | Overriding may be done only by importing the name from a module at |
1172 | compile time--ordinary predeclaration isn't good enough. However, the | |
19799a22 GS |
1173 | C<use subs> pragma lets you, in effect, predeclare subs |
1174 | via the import syntax, and these names may then override built-in ones: | |
a0d0e21e LW |
1175 | |
1176 | use subs 'chdir', 'chroot', 'chmod', 'chown'; | |
1177 | chdir $somewhere; | |
1178 | sub chdir { ... } | |
1179 | ||
19799a22 GS |
1180 | To unambiguously refer to the built-in form, precede the |
1181 | built-in name with the special package qualifier C<CORE::>. For example, | |
1182 | saying C<CORE::open()> always refers to the built-in C<open()>, even | |
fb73857a | 1183 | if the current package has imported some other subroutine called |
19799a22 | 1184 | C<&open()> from elsewhere. Even though it looks like a regular |
09bef843 | 1185 | function call, it isn't: you can't take a reference to it, such as |
19799a22 | 1186 | the incorrect C<\&CORE::open> might appear to produce. |
fb73857a | 1187 | |
19799a22 GS |
1188 | Library modules should not in general export built-in names like C<open> |
1189 | or C<chdir> as part of their default C<@EXPORT> list, because these may | |
a0d0e21e | 1190 | sneak into someone else's namespace and change the semantics unexpectedly. |
19799a22 | 1191 | Instead, if the module adds that name to C<@EXPORT_OK>, then it's |
a0d0e21e LW |
1192 | possible for a user to import the name explicitly, but not implicitly. |
1193 | That is, they could say | |
1194 | ||
1195 | use Module 'open'; | |
1196 | ||
19799a22 | 1197 | and it would import the C<open> override. But if they said |
a0d0e21e LW |
1198 | |
1199 | use Module; | |
1200 | ||
19799a22 | 1201 | they would get the default imports without overrides. |
a0d0e21e | 1202 | |
19799a22 | 1203 | The foregoing mechanism for overriding built-in is restricted, quite |
95d94a4f | 1204 | deliberately, to the package that requests the import. There is a second |
19799a22 | 1205 | method that is sometimes applicable when you wish to override a built-in |
95d94a4f GS |
1206 | everywhere, without regard to namespace boundaries. This is achieved by |
1207 | importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an | |
1208 | example that quite brazenly replaces the C<glob> operator with something | |
1209 | that understands regular expressions. | |
1210 | ||
1211 | package REGlob; | |
1212 | require Exporter; | |
1213 | @ISA = 'Exporter'; | |
1214 | @EXPORT_OK = 'glob'; | |
1215 | ||
1216 | sub import { | |
1217 | my $pkg = shift; | |
1218 | return unless @_; | |
1219 | my $sym = shift; | |
1220 | my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0)); | |
1221 | $pkg->export($where, $sym, @_); | |
1222 | } | |
1223 | ||
1224 | sub glob { | |
1225 | my $pat = shift; | |
1226 | my @got; | |
19799a22 GS |
1227 | local *D; |
1228 | if (opendir D, '.') { | |
1229 | @got = grep /$pat/, readdir D; | |
1230 | closedir D; | |
1231 | } | |
1232 | return @got; | |
95d94a4f GS |
1233 | } |
1234 | 1; | |
1235 | ||
1236 | And here's how it could be (ab)used: | |
1237 | ||
1238 | #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces | |
1239 | package Foo; | |
1240 | use REGlob 'glob'; # override glob() in Foo:: only | |
1241 | print for <^[a-z_]+\.pm\$>; # show all pragmatic modules | |
1242 | ||
19799a22 | 1243 | The initial comment shows a contrived, even dangerous example. |
95d94a4f | 1244 | By overriding C<glob> globally, you would be forcing the new (and |
19799a22 | 1245 | subversive) behavior for the C<glob> operator for I<every> namespace, |
95d94a4f GS |
1246 | without the complete cognizance or cooperation of the modules that own |
1247 | those namespaces. Naturally, this should be done with extreme caution--if | |
1248 | it must be done at all. | |
1249 | ||
1250 | The C<REGlob> example above does not implement all the support needed to | |
19799a22 | 1251 | cleanly override perl's C<glob> operator. The built-in C<glob> has |
95d94a4f | 1252 | different behaviors depending on whether it appears in a scalar or list |
19799a22 | 1253 | context, but our C<REGlob> doesn't. Indeed, many perl built-in have such |
95d94a4f GS |
1254 | context sensitive behaviors, and these must be adequately supported by |
1255 | a properly written override. For a fully functional example of overriding | |
1256 | C<glob>, study the implementation of C<File::DosGlob> in the standard | |
1257 | library. | |
1258 | ||
77bc9082 RGS |
1259 | When you override a built-in, your replacement should be consistent (if |
1260 | possible) with the built-in native syntax. You can achieve this by using | |
1261 | a suitable prototype. To get the prototype of an overridable built-in, | |
1262 | use the C<prototype> function with an argument of C<"CORE::builtin_name"> | |
1263 | (see L<perlfunc/prototype>). | |
1264 | ||
1265 | Note however that some built-ins can't have their syntax expressed by a | |
1266 | prototype (such as C<system> or C<chomp>). If you override them you won't | |
1267 | be able to fully mimic their original syntax. | |
1268 | ||
fe854a6f | 1269 | The built-ins C<do>, C<require> and C<glob> can also be overridden, but due |
77bc9082 RGS |
1270 | to special magic, their original syntax is preserved, and you don't have |
1271 | to define a prototype for their replacements. (You can't override the | |
1272 | C<do BLOCK> syntax, though). | |
1273 | ||
1274 | C<require> has special additional dark magic: if you invoke your | |
1275 | C<require> replacement as C<require Foo::Bar>, it will actually receive | |
1276 | the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>. | |
1277 | ||
1278 | And, as you'll have noticed from the previous example, if you override | |
593b9c14 | 1279 | C<glob>, the C<< <*> >> glob operator is overridden as well. |
77bc9082 | 1280 | |
9b3023bc RGS |
1281 | In a similar fashion, overriding the C<readline> function also overrides |
1282 | the equivalent I/O operator C<< <FILEHANDLE> >>. | |
1283 | ||
fe854a6f | 1284 | Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden. |
77bc9082 | 1285 | |
a0d0e21e LW |
1286 | =head2 Autoloading |
1287 | ||
19799a22 GS |
1288 | If you call a subroutine that is undefined, you would ordinarily |
1289 | get an immediate, fatal error complaining that the subroutine doesn't | |
1290 | exist. (Likewise for subroutines being used as methods, when the | |
1291 | method doesn't exist in any base class of the class's package.) | |
1292 | However, if an C<AUTOLOAD> subroutine is defined in the package or | |
1293 | packages used to locate the original subroutine, then that | |
1294 | C<AUTOLOAD> subroutine is called with the arguments that would have | |
1295 | been passed to the original subroutine. The fully qualified name | |
1296 | of the original subroutine magically appears in the global $AUTOLOAD | |
1297 | variable of the same package as the C<AUTOLOAD> routine. The name | |
1298 | is not passed as an ordinary argument because, er, well, just | |
593b9c14 YST |
1299 | because, that's why. (As an exception, a method call to a nonexistent |
1300 | C<import> or C<unimport> method is just skipped instead.) | |
19799a22 GS |
1301 | |
1302 | Many C<AUTOLOAD> routines load in a definition for the requested | |
1303 | subroutine using eval(), then execute that subroutine using a special | |
1304 | form of goto() that erases the stack frame of the C<AUTOLOAD> routine | |
1305 | without a trace. (See the source to the standard module documented | |
1306 | in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can | |
1307 | also just emulate the routine and never define it. For example, | |
1308 | let's pretend that a function that wasn't defined should just invoke | |
1309 | C<system> with those arguments. All you'd do is: | |
cb1a09d0 AD |
1310 | |
1311 | sub AUTOLOAD { | |
1312 | my $program = $AUTOLOAD; | |
1313 | $program =~ s/.*:://; | |
1314 | system($program, @_); | |
54310121 | 1315 | } |
cb1a09d0 | 1316 | date(); |
6d28dffb | 1317 | who('am', 'i'); |
cb1a09d0 AD |
1318 | ls('-l'); |
1319 | ||
19799a22 GS |
1320 | In fact, if you predeclare functions you want to call that way, you don't |
1321 | even need parentheses: | |
cb1a09d0 AD |
1322 | |
1323 | use subs qw(date who ls); | |
1324 | date; | |
1325 | who "am", "i"; | |
593b9c14 | 1326 | ls '-l'; |
cb1a09d0 AD |
1327 | |
1328 | A more complete example of this is the standard Shell module, which | |
19799a22 | 1329 | can treat undefined subroutine calls as calls to external programs. |
a0d0e21e | 1330 | |
19799a22 GS |
1331 | Mechanisms are available to help modules writers split their modules |
1332 | into autoloadable files. See the standard AutoLoader module | |
6d28dffb | 1333 | described in L<AutoLoader> and in L<AutoSplit>, the standard |
1334 | SelfLoader modules in L<SelfLoader>, and the document on adding C | |
19799a22 | 1335 | functions to Perl code in L<perlxs>. |
cb1a09d0 | 1336 | |
09bef843 SB |
1337 | =head2 Subroutine Attributes |
1338 | ||
1339 | A subroutine declaration or definition may have a list of attributes | |
1340 | associated with it. If such an attribute list is present, it is | |
0120eecf | 1341 | broken up at space or colon boundaries and treated as though a |
09bef843 SB |
1342 | C<use attributes> had been seen. See L<attributes> for details |
1343 | about what attributes are currently supported. | |
1344 | Unlike the limitation with the obsolescent C<use attrs>, the | |
1345 | C<sub : ATTRLIST> syntax works to associate the attributes with | |
1346 | a pre-declaration, and not just with a subroutine definition. | |
1347 | ||
1348 | The attributes must be valid as simple identifier names (without any | |
1349 | punctuation other than the '_' character). They may have a parameter | |
1350 | list appended, which is only checked for whether its parentheses ('(',')') | |
1351 | nest properly. | |
1352 | ||
1353 | Examples of valid syntax (even though the attributes are unknown): | |
1354 | ||
0120eecf GS |
1355 | sub fnord (&\%) : switch(10,foo(7,3)) : expensive ; |
1356 | sub plugh () : Ugly('\(") :Bad ; | |
09bef843 SB |
1357 | sub xyzzy : _5x5 { ... } |
1358 | ||
1359 | Examples of invalid syntax: | |
1360 | ||
1361 | sub fnord : switch(10,foo() ; # ()-string not balanced | |
1362 | sub snoid : Ugly('(') ; # ()-string not balanced | |
1363 | sub xyzzy : 5x5 ; # "5x5" not a valid identifier | |
1364 | sub plugh : Y2::north ; # "Y2::north" not a simple identifier | |
0120eecf | 1365 | sub snurt : foo + bar ; # "+" not a colon or space |
09bef843 SB |
1366 | |
1367 | The attribute list is passed as a list of constant strings to the code | |
1368 | which associates them with the subroutine. In particular, the second example | |
1369 | of valid syntax above currently looks like this in terms of how it's | |
1370 | parsed and invoked: | |
1371 | ||
1372 | use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad'; | |
1373 | ||
1374 | For further details on attribute lists and their manipulation, | |
a0ae32d3 | 1375 | see L<attributes> and L<Attribute::Handlers>. |
09bef843 | 1376 | |
cb1a09d0 | 1377 | =head1 SEE ALSO |
a0d0e21e | 1378 | |
19799a22 GS |
1379 | See L<perlref/"Function Templates"> for more about references and closures. |
1380 | See L<perlxs> if you'd like to learn about calling C subroutines from Perl. | |
a2293a43 | 1381 | See L<perlembed> if you'd like to learn about calling Perl subroutines from C. |
19799a22 GS |
1382 | See L<perlmod> to learn about bundling up your functions in separate files. |
1383 | See L<perlmodlib> to learn what library modules come standard on your system. | |
1384 | See L<perltoot> to learn how to make object method calls. |