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 | |
18 | sub NAME : ATTRS BLOCK # with attributes | |
19 | sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes | |
a0d0e21e | 20 | |
748a9306 | 21 | To define an anonymous subroutine at runtime: |
d74e8afc | 22 | X<subroutine, anonymous> |
748a9306 | 23 | |
09bef843 SB |
24 | $subref = sub BLOCK; # no proto |
25 | $subref = sub (PROTO) BLOCK; # with proto | |
26 | $subref = sub : ATTRS BLOCK; # with attributes | |
27 | $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes | |
748a9306 | 28 | |
a0d0e21e | 29 | To import subroutines: |
d74e8afc | 30 | X<import> |
a0d0e21e | 31 | |
19799a22 | 32 | use MODULE qw(NAME1 NAME2 NAME3); |
a0d0e21e LW |
33 | |
34 | To call subroutines: | |
d74e8afc | 35 | X<subroutine, call> X<call> |
a0d0e21e | 36 | |
5f05dabc | 37 | NAME(LIST); # & is optional with parentheses. |
54310121 | 38 | NAME LIST; # Parentheses optional if predeclared/imported. |
19799a22 | 39 | &NAME(LIST); # Circumvent prototypes. |
5a964f20 | 40 | &NAME; # Makes current @_ visible to called subroutine. |
a0d0e21e LW |
41 | |
42 | =head1 DESCRIPTION | |
43 | ||
19799a22 GS |
44 | Like many languages, Perl provides for user-defined subroutines. |
45 | These may be located anywhere in the main program, loaded in from | |
46 | other files via the C<do>, C<require>, or C<use> keywords, or | |
be3174d2 | 47 | generated on the fly using C<eval> or anonymous subroutines. |
19799a22 GS |
48 | You can even call a function indirectly using a variable containing |
49 | its name or a CODE reference. | |
cb1a09d0 AD |
50 | |
51 | The Perl model for function call and return values is simple: all | |
52 | functions are passed as parameters one single flat list of scalars, and | |
53 | all functions likewise return to their caller one single flat list of | |
54 | scalars. Any arrays or hashes in these call and return lists will | |
55 | collapse, losing their identities--but you may always use | |
56 | pass-by-reference instead to avoid this. Both call and return lists may | |
57 | contain as many or as few scalar elements as you'd like. (Often a | |
58 | function without an explicit return statement is called a subroutine, but | |
19799a22 | 59 | there's really no difference from Perl's perspective.) |
d74e8afc | 60 | X<subroutine, parameter> X<parameter> |
19799a22 GS |
61 | |
62 | Any arguments passed in show up in the array C<@_>. Therefore, if | |
63 | you called a function with two arguments, those would be stored in | |
64 | C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its | |
65 | elements are aliases for the actual scalar parameters. In particular, | |
66 | if an element C<$_[0]> is updated, the corresponding argument is | |
67 | updated (or an error occurs if it is not updatable). If an argument | |
68 | is an array or hash element which did not exist when the function | |
69 | was called, that element is created only when (and if) it is modified | |
70 | or a reference to it is taken. (Some earlier versions of Perl | |
71 | created the element whether or not the element was assigned to.) | |
72 | Assigning to the whole array C<@_> removes that aliasing, and does | |
73 | not update any arguments. | |
d74e8afc | 74 | X<subroutine, argument> X<argument> X<@_> |
19799a22 | 75 | |
dbb128be XN |
76 | A C<return> statement may be used to exit a subroutine, optionally |
77 | specifying the returned value, which will be evaluated in the | |
78 | appropriate context (list, scalar, or void) depending on the context of | |
79 | the subroutine call. If you specify no return value, the subroutine | |
80 | returns an empty list in list context, the undefined value in scalar | |
81 | context, or nothing in void context. If you return one or more | |
82 | aggregates (arrays and hashes), these will be flattened together into | |
83 | one large indistinguishable list. | |
84 | ||
85 | If no C<return> is found and if the last statement is an expression, its | |
9a989771 RGS |
86 | value is returned. If the last statement is a loop control structure |
87 | like a C<foreach> or a C<while>, the returned value is unspecified. The | |
88 | empty sub returns the empty list. | |
d74e8afc | 89 | X<subroutine, return value> X<return value> X<return> |
19799a22 GS |
90 | |
91 | Perl does not have named formal parameters. In practice all you | |
92 | do is assign to a C<my()> list of these. Variables that aren't | |
93 | declared to be private are global variables. For gory details | |
94 | on creating private variables, see L<"Private Variables via my()"> | |
95 | and L<"Temporary Values via local()">. To create protected | |
96 | environments for a set of functions in a separate package (and | |
97 | probably a separate file), see L<perlmod/"Packages">. | |
d74e8afc | 98 | X<formal parameter> X<parameter, formal> |
a0d0e21e LW |
99 | |
100 | Example: | |
101 | ||
cb1a09d0 AD |
102 | sub max { |
103 | my $max = shift(@_); | |
a0d0e21e LW |
104 | foreach $foo (@_) { |
105 | $max = $foo if $max < $foo; | |
106 | } | |
cb1a09d0 | 107 | return $max; |
a0d0e21e | 108 | } |
cb1a09d0 | 109 | $bestday = max($mon,$tue,$wed,$thu,$fri); |
a0d0e21e LW |
110 | |
111 | Example: | |
112 | ||
113 | # get a line, combining continuation lines | |
114 | # that start with whitespace | |
115 | ||
116 | sub get_line { | |
19799a22 | 117 | $thisline = $lookahead; # global variables! |
54310121 | 118 | LINE: while (defined($lookahead = <STDIN>)) { |
a0d0e21e LW |
119 | if ($lookahead =~ /^[ \t]/) { |
120 | $thisline .= $lookahead; | |
121 | } | |
122 | else { | |
123 | last LINE; | |
124 | } | |
125 | } | |
19799a22 | 126 | return $thisline; |
a0d0e21e LW |
127 | } |
128 | ||
129 | $lookahead = <STDIN>; # get first line | |
19799a22 | 130 | while (defined($line = get_line())) { |
a0d0e21e LW |
131 | ... |
132 | } | |
133 | ||
09bef843 | 134 | Assigning to a list of private variables to name your arguments: |
a0d0e21e LW |
135 | |
136 | sub maybeset { | |
137 | my($key, $value) = @_; | |
cb1a09d0 | 138 | $Foo{$key} = $value unless $Foo{$key}; |
a0d0e21e LW |
139 | } |
140 | ||
19799a22 GS |
141 | Because the assignment copies the values, this also has the effect |
142 | of turning call-by-reference into call-by-value. Otherwise a | |
143 | function is free to do in-place modifications of C<@_> and change | |
144 | its caller's values. | |
d74e8afc | 145 | X<call-by-reference> X<call-by-value> |
cb1a09d0 AD |
146 | |
147 | upcase_in($v1, $v2); # this changes $v1 and $v2 | |
148 | sub upcase_in { | |
54310121 | 149 | for (@_) { tr/a-z/A-Z/ } |
150 | } | |
cb1a09d0 AD |
151 | |
152 | You aren't allowed to modify constants in this way, of course. If an | |
153 | argument were actually literal and you tried to change it, you'd take a | |
154 | (presumably fatal) exception. For example, this won't work: | |
d74e8afc | 155 | X<call-by-reference> X<call-by-value> |
cb1a09d0 AD |
156 | |
157 | upcase_in("frederick"); | |
158 | ||
f86cebdf | 159 | It would be much safer if the C<upcase_in()> function |
cb1a09d0 AD |
160 | were written to return a copy of its parameters instead |
161 | of changing them in place: | |
162 | ||
19799a22 | 163 | ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2 |
cb1a09d0 | 164 | sub upcase { |
54310121 | 165 | return unless defined wantarray; # void context, do nothing |
cb1a09d0 | 166 | my @parms = @_; |
54310121 | 167 | for (@parms) { tr/a-z/A-Z/ } |
c07a80fd | 168 | return wantarray ? @parms : $parms[0]; |
54310121 | 169 | } |
cb1a09d0 | 170 | |
19799a22 | 171 | Notice how this (unprototyped) function doesn't care whether it was |
a2293a43 | 172 | passed real scalars or arrays. Perl sees all arguments as one big, |
19799a22 GS |
173 | long, flat parameter list in C<@_>. This is one area where |
174 | Perl's simple argument-passing style shines. The C<upcase()> | |
175 | function would work perfectly well without changing the C<upcase()> | |
176 | definition even if we fed it things like this: | |
cb1a09d0 AD |
177 | |
178 | @newlist = upcase(@list1, @list2); | |
179 | @newlist = upcase( split /:/, $var ); | |
180 | ||
181 | Do not, however, be tempted to do this: | |
182 | ||
183 | (@a, @b) = upcase(@list1, @list2); | |
184 | ||
19799a22 GS |
185 | Like the flattened incoming parameter list, the return list is also |
186 | flattened on return. So all you have managed to do here is stored | |
17b63f68 | 187 | everything in C<@a> and made C<@b> empty. See |
13a2d996 | 188 | L<Pass by Reference> for alternatives. |
19799a22 GS |
189 | |
190 | A subroutine may be called using an explicit C<&> prefix. The | |
191 | C<&> is optional in modern Perl, as are parentheses if the | |
192 | subroutine has been predeclared. The C<&> is I<not> optional | |
193 | when just naming the subroutine, such as when it's used as | |
194 | an argument to defined() or undef(). Nor is it optional when you | |
195 | want to do an indirect subroutine call with a subroutine name or | |
196 | reference using the C<&$subref()> or C<&{$subref}()> constructs, | |
c47ff5f1 | 197 | although the C<< $subref->() >> notation solves that problem. |
19799a22 | 198 | See L<perlref> for more about all that. |
d74e8afc | 199 | X<&> |
19799a22 GS |
200 | |
201 | Subroutines may be called recursively. If a subroutine is called | |
202 | using the C<&> form, the argument list is optional, and if omitted, | |
203 | no C<@_> array is set up for the subroutine: the C<@_> array at the | |
204 | time of the call is visible to subroutine instead. This is an | |
205 | efficiency mechanism that new users may wish to avoid. | |
d74e8afc | 206 | X<recursion> |
a0d0e21e LW |
207 | |
208 | &foo(1,2,3); # pass three arguments | |
209 | foo(1,2,3); # the same | |
210 | ||
211 | foo(); # pass a null list | |
212 | &foo(); # the same | |
a0d0e21e | 213 | |
cb1a09d0 | 214 | &foo; # foo() get current args, like foo(@_) !! |
54310121 | 215 | foo; # like foo() IFF sub foo predeclared, else "foo" |
cb1a09d0 | 216 | |
19799a22 GS |
217 | Not only does the C<&> form make the argument list optional, it also |
218 | disables any prototype checking on arguments you do provide. This | |
c07a80fd | 219 | is partly for historical reasons, and partly for having a convenient way |
9688be67 | 220 | to cheat if you know what you're doing. See L</Prototypes> below. |
d74e8afc | 221 | X<&> |
c07a80fd | 222 | |
977616ef RS |
223 | Since Perl 5.16.0, the C<__SUB__> token is available under C<use feature |
224 | 'current_sub'> and C<use 5.16.0>. It will evaluate to a reference to the | |
906024c7 FC |
225 | currently-running sub, which allows for recursive calls without knowing |
226 | your subroutine's name. | |
977616ef RS |
227 | |
228 | use 5.16.0; | |
229 | my $factorial = sub { | |
230 | my ($x) = @_; | |
231 | return 1 if $x == 1; | |
232 | return($x * __SUB__->( $x - 1 ) ); | |
233 | }; | |
234 | ||
a453e28a DM |
235 | The behaviour of C<__SUB__> within a regex code block (such as C</(?{...})/>) |
236 | is subject to change. | |
237 | ||
ac90fb77 EM |
238 | Subroutines whose names are in all upper case are reserved to the Perl |
239 | core, as are modules whose names are in all lower case. A subroutine in | |
240 | all capitals is a loosely-held convention meaning it will be called | |
241 | indirectly by the run-time system itself, usually due to a triggered event. | |
242 | Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>, | |
243 | C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>. | |
244 | ||
3c10abe3 AG |
245 | The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines |
246 | are not so much subroutines as named special code blocks, of which you | |
247 | can have more than one in a package, and which you can B<not> call | |
248 | explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> | |
5a964f20 | 249 | |
b687b08b | 250 | =head2 Private Variables via my() |
d74e8afc ITB |
251 | X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical> |
252 | X<lexical scope> X<attributes, my> | |
cb1a09d0 AD |
253 | |
254 | Synopsis: | |
255 | ||
256 | my $foo; # declare $foo lexically local | |
257 | my (@wid, %get); # declare list of variables local | |
258 | my $foo = "flurp"; # declare $foo lexical, and init it | |
259 | my @oof = @bar; # declare @oof lexical, and init it | |
09bef843 SB |
260 | my $x : Foo = $y; # similar, with an attribute applied |
261 | ||
a0ae32d3 JH |
262 | B<WARNING>: The use of attribute lists on C<my> declarations is still |
263 | evolving. The current semantics and interface are subject to change. | |
264 | See L<attributes> and L<Attribute::Handlers>. | |
cb1a09d0 | 265 | |
19799a22 GS |
266 | The C<my> operator declares the listed variables to be lexically |
267 | confined to the enclosing block, conditional (C<if/unless/elsif/else>), | |
268 | loop (C<for/foreach/while/until/continue>), subroutine, C<eval>, | |
269 | or C<do/require/use>'d file. If more than one value is listed, the | |
270 | list must be placed in parentheses. All listed elements must be | |
271 | legal lvalues. Only alphanumeric identifiers may be lexically | |
325192b1 | 272 | scoped--magical built-ins like C<$/> must currently be C<local>ized |
19799a22 GS |
273 | with C<local> instead. |
274 | ||
275 | Unlike dynamic variables created by the C<local> operator, lexical | |
276 | variables declared with C<my> are totally hidden from the outside | |
277 | world, including any called subroutines. This is true if it's the | |
278 | same subroutine called from itself or elsewhere--every call gets | |
279 | its own copy. | |
d74e8afc | 280 | X<local> |
19799a22 GS |
281 | |
282 | This doesn't mean that a C<my> variable declared in a statically | |
283 | enclosing lexical scope would be invisible. Only dynamic scopes | |
284 | are cut off. For example, the C<bumpx()> function below has access | |
285 | to the lexical $x variable because both the C<my> and the C<sub> | |
286 | occurred at the same scope, presumably file scope. | |
5a964f20 TC |
287 | |
288 | my $x = 10; | |
289 | sub bumpx { $x++ } | |
290 | ||
19799a22 GS |
291 | An C<eval()>, however, can see lexical variables of the scope it is |
292 | being evaluated in, so long as the names aren't hidden by declarations within | |
293 | the C<eval()> itself. See L<perlref>. | |
d74e8afc | 294 | X<eval, scope of> |
cb1a09d0 | 295 | |
19799a22 | 296 | The parameter list to my() may be assigned to if desired, which allows you |
cb1a09d0 AD |
297 | to initialize your variables. (If no initializer is given for a |
298 | particular variable, it is created with the undefined value.) Commonly | |
19799a22 | 299 | this is used to name input parameters to a subroutine. Examples: |
cb1a09d0 AD |
300 | |
301 | $arg = "fred"; # "global" variable | |
302 | $n = cube_root(27); | |
303 | print "$arg thinks the root is $n\n"; | |
304 | fred thinks the root is 3 | |
305 | ||
306 | sub cube_root { | |
307 | my $arg = shift; # name doesn't matter | |
308 | $arg **= 1/3; | |
309 | return $arg; | |
54310121 | 310 | } |
cb1a09d0 | 311 | |
19799a22 GS |
312 | The C<my> is simply a modifier on something you might assign to. So when |
313 | you do assign to variables in its argument list, C<my> doesn't | |
6cc33c6d | 314 | change whether those variables are viewed as a scalar or an array. So |
cb1a09d0 | 315 | |
5a964f20 | 316 | my ($foo) = <STDIN>; # WRONG? |
cb1a09d0 AD |
317 | my @FOO = <STDIN>; |
318 | ||
5f05dabc | 319 | both supply a list context to the right-hand side, while |
cb1a09d0 AD |
320 | |
321 | my $foo = <STDIN>; | |
322 | ||
5f05dabc | 323 | supplies a scalar context. But the following declares only one variable: |
748a9306 | 324 | |
5a964f20 | 325 | my $foo, $bar = 1; # WRONG |
748a9306 | 326 | |
cb1a09d0 | 327 | That has the same effect as |
748a9306 | 328 | |
cb1a09d0 AD |
329 | my $foo; |
330 | $bar = 1; | |
a0d0e21e | 331 | |
cb1a09d0 AD |
332 | The declared variable is not introduced (is not visible) until after |
333 | the current statement. Thus, | |
334 | ||
335 | my $x = $x; | |
336 | ||
19799a22 | 337 | can be used to initialize a new $x with the value of the old $x, and |
cb1a09d0 AD |
338 | the expression |
339 | ||
340 | my $x = 123 and $x == 123 | |
341 | ||
19799a22 | 342 | is false unless the old $x happened to have the value C<123>. |
cb1a09d0 | 343 | |
55497cff | 344 | Lexical scopes of control structures are not bounded precisely by the |
345 | braces that delimit their controlled blocks; control expressions are | |
19799a22 | 346 | part of that scope, too. Thus in the loop |
55497cff | 347 | |
19799a22 | 348 | while (my $line = <>) { |
55497cff | 349 | $line = lc $line; |
350 | } continue { | |
351 | print $line; | |
352 | } | |
353 | ||
19799a22 | 354 | the scope of $line extends from its declaration throughout the rest of |
55497cff | 355 | the loop construct (including the C<continue> clause), but not beyond |
356 | it. Similarly, in the conditional | |
357 | ||
358 | if ((my $answer = <STDIN>) =~ /^yes$/i) { | |
359 | user_agrees(); | |
360 | } elsif ($answer =~ /^no$/i) { | |
361 | user_disagrees(); | |
362 | } else { | |
363 | chomp $answer; | |
364 | die "'$answer' is neither 'yes' nor 'no'"; | |
365 | } | |
366 | ||
19799a22 GS |
367 | the scope of $answer extends from its declaration through the rest |
368 | of that conditional, including any C<elsif> and C<else> clauses, | |
96090e4f | 369 | but not beyond it. See L<perlsyn/"Simple Statements"> for information |
457b36cb | 370 | on the scope of variables in statements with modifiers. |
55497cff | 371 | |
5f05dabc | 372 | The C<foreach> loop defaults to scoping its index variable dynamically |
19799a22 GS |
373 | in the manner of C<local>. However, if the index variable is |
374 | prefixed with the keyword C<my>, or if there is already a lexical | |
375 | by that name in scope, then a new lexical is created instead. Thus | |
376 | in the loop | |
d74e8afc | 377 | X<foreach> X<for> |
55497cff | 378 | |
379 | for my $i (1, 2, 3) { | |
380 | some_function(); | |
381 | } | |
382 | ||
19799a22 GS |
383 | the scope of $i extends to the end of the loop, but not beyond it, |
384 | rendering the value of $i inaccessible within C<some_function()>. | |
d74e8afc | 385 | X<foreach> X<for> |
55497cff | 386 | |
cb1a09d0 | 387 | Some users may wish to encourage the use of lexically scoped variables. |
19799a22 GS |
388 | As an aid to catching implicit uses to package variables, |
389 | which are always global, if you say | |
cb1a09d0 AD |
390 | |
391 | use strict 'vars'; | |
392 | ||
19799a22 GS |
393 | then any variable mentioned from there to the end of the enclosing |
394 | block must either refer to a lexical variable, be predeclared via | |
77ca0c92 | 395 | C<our> or C<use vars>, or else must be fully qualified with the package name. |
19799a22 GS |
396 | A compilation error results otherwise. An inner block may countermand |
397 | this with C<no strict 'vars'>. | |
398 | ||
399 | A C<my> has both a compile-time and a run-time effect. At compile | |
8593bda5 | 400 | time, the compiler takes notice of it. The principal usefulness |
19799a22 GS |
401 | of this is to quiet C<use strict 'vars'>, but it is also essential |
402 | for generation of closures as detailed in L<perlref>. Actual | |
403 | initialization is delayed until run time, though, so it gets executed | |
404 | at the appropriate time, such as each time through a loop, for | |
405 | example. | |
406 | ||
407 | Variables declared with C<my> are not part of any package and are therefore | |
cb1a09d0 AD |
408 | never fully qualified with the package name. In particular, you're not |
409 | allowed to try to make a package variable (or other global) lexical: | |
410 | ||
411 | my $pack::var; # ERROR! Illegal syntax | |
cb1a09d0 AD |
412 | |
413 | In fact, a dynamic variable (also known as package or global variables) | |
f86cebdf | 414 | are still accessible using the fully qualified C<::> notation even while a |
cb1a09d0 AD |
415 | lexical of the same name is also visible: |
416 | ||
417 | package main; | |
418 | local $x = 10; | |
419 | my $x = 20; | |
420 | print "$x and $::x\n"; | |
421 | ||
f86cebdf | 422 | That will print out C<20> and C<10>. |
cb1a09d0 | 423 | |
19799a22 GS |
424 | You may declare C<my> variables at the outermost scope of a file |
425 | to hide any such identifiers from the world outside that file. This | |
426 | is similar in spirit to C's static variables when they are used at | |
427 | the file level. To do this with a subroutine requires the use of | |
428 | a closure (an anonymous function that accesses enclosing lexicals). | |
429 | If you want to create a private subroutine that cannot be called | |
430 | from outside that block, it can declare a lexical variable containing | |
431 | an anonymous sub reference: | |
cb1a09d0 AD |
432 | |
433 | my $secret_version = '1.001-beta'; | |
434 | my $secret_sub = sub { print $secret_version }; | |
435 | &$secret_sub(); | |
436 | ||
437 | As long as the reference is never returned by any function within the | |
5f05dabc | 438 | module, no outside module can see the subroutine, because its name is not in |
cb1a09d0 | 439 | any package's symbol table. Remember that it's not I<REALLY> called |
19799a22 | 440 | C<$some_pack::secret_version> or anything; it's just $secret_version, |
cb1a09d0 AD |
441 | unqualified and unqualifiable. |
442 | ||
19799a22 GS |
443 | This does not work with object methods, however; all object methods |
444 | have to be in the symbol table of some package to be found. See | |
445 | L<perlref/"Function Templates"> for something of a work-around to | |
446 | this. | |
cb1a09d0 | 447 | |
c2611fb3 | 448 | =head2 Persistent Private Variables |
ba1f8e91 RGS |
449 | X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure> |
450 | ||
451 | There are two ways to build persistent private variables in Perl 5.10. | |
452 | First, you can simply use the C<state> feature. Or, you can use closures, | |
453 | if you want to stay compatible with releases older than 5.10. | |
454 | ||
455 | =head3 Persistent variables via state() | |
456 | ||
9d42615f | 457 | Beginning with Perl 5.10.0, you can declare variables with the C<state> |
4a904372 | 458 | keyword in place of C<my>. For that to work, though, you must have |
ba1f8e91 | 459 | enabled that feature beforehand, either by using the C<feature> pragma, or |
4a904372 | 460 | by using C<-E> on one-liners (see L<feature>). Beginning with Perl 5.16, |
47d235f1 | 461 | the C<CORE::state> form does not require the |
4a904372 | 462 | C<feature> pragma. |
ba1f8e91 | 463 | |
ad0cc46c FC |
464 | The C<state> keyword creates a lexical variable (following the same scoping |
465 | rules as C<my>) that persists from one subroutine call to the next. If a | |
466 | state variable resides inside an anonymous subroutine, then each copy of | |
467 | the subroutine has its own copy of the state variable. However, the value | |
468 | of the state variable will still persist between calls to the same copy of | |
469 | the anonymous subroutine. (Don't forget that C<sub { ... }> creates a new | |
470 | subroutine each time it is executed.) | |
471 | ||
ba1f8e91 RGS |
472 | For example, the following code maintains a private counter, incremented |
473 | each time the gimme_another() function is called: | |
474 | ||
475 | use feature 'state'; | |
476 | sub gimme_another { state $x; return ++$x } | |
477 | ||
ad0cc46c FC |
478 | And this example uses anonymous subroutines to create separate counters: |
479 | ||
480 | use feature 'state'; | |
481 | sub create_counter { | |
482 | return sub { state $x; return ++$x } | |
483 | } | |
484 | ||
ba1f8e91 RGS |
485 | Also, since C<$x> is lexical, it can't be reached or modified by any Perl |
486 | code outside. | |
487 | ||
f292fc7a RS |
488 | When combined with variable declaration, simple scalar assignment to C<state> |
489 | variables (as in C<state $x = 42>) is executed only the first time. When such | |
490 | statements are evaluated subsequent times, the assignment is ignored. The | |
491 | behavior of this sort of assignment to non-scalar variables is undefined. | |
ba1f8e91 RGS |
492 | |
493 | =head3 Persistent variables with closures | |
5a964f20 TC |
494 | |
495 | Just because a lexical variable is lexically (also called statically) | |
f86cebdf | 496 | scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that |
5a964f20 TC |
497 | within a function it works like a C static. It normally works more |
498 | like a C auto, but with implicit garbage collection. | |
499 | ||
500 | Unlike local variables in C or C++, Perl's lexical variables don't | |
501 | necessarily get recycled just because their scope has exited. | |
502 | If something more permanent is still aware of the lexical, it will | |
503 | stick around. So long as something else references a lexical, that | |
504 | lexical won't be freed--which is as it should be. You wouldn't want | |
505 | memory being free until you were done using it, or kept around once you | |
506 | were done. Automatic garbage collection takes care of this for you. | |
507 | ||
508 | This means that you can pass back or save away references to lexical | |
509 | variables, whereas to return a pointer to a C auto is a grave error. | |
510 | It also gives us a way to simulate C's function statics. Here's a | |
511 | mechanism for giving a function private variables with both lexical | |
512 | scoping and a static lifetime. If you do want to create something like | |
513 | C's static variables, just enclose the whole function in an extra block, | |
514 | and put the static variable outside the function but in the block. | |
cb1a09d0 AD |
515 | |
516 | { | |
54310121 | 517 | my $secret_val = 0; |
cb1a09d0 AD |
518 | sub gimme_another { |
519 | return ++$secret_val; | |
54310121 | 520 | } |
521 | } | |
cb1a09d0 AD |
522 | # $secret_val now becomes unreachable by the outside |
523 | # world, but retains its value between calls to gimme_another | |
524 | ||
54310121 | 525 | If this function is being sourced in from a separate file |
cb1a09d0 | 526 | via C<require> or C<use>, then this is probably just fine. If it's |
19799a22 | 527 | all in the main program, you'll need to arrange for the C<my> |
cb1a09d0 | 528 | to be executed early, either by putting the whole block above |
f86cebdf | 529 | your main program, or more likely, placing merely a C<BEGIN> |
ac90fb77 | 530 | code block around it to make sure it gets executed before your program |
cb1a09d0 AD |
531 | starts to run: |
532 | ||
ac90fb77 | 533 | BEGIN { |
54310121 | 534 | my $secret_val = 0; |
cb1a09d0 AD |
535 | sub gimme_another { |
536 | return ++$secret_val; | |
54310121 | 537 | } |
538 | } | |
cb1a09d0 | 539 | |
3c10abe3 AG |
540 | See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the |
541 | special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>, | |
542 | C<INIT> and C<END>. | |
cb1a09d0 | 543 | |
19799a22 GS |
544 | If declared at the outermost scope (the file scope), then lexicals |
545 | work somewhat like C's file statics. They are available to all | |
546 | functions in that same file declared below them, but are inaccessible | |
547 | from outside that file. This strategy is sometimes used in modules | |
548 | to create private variables that the whole module can see. | |
5a964f20 | 549 | |
cb1a09d0 | 550 | =head2 Temporary Values via local() |
d74e8afc ITB |
551 | X<local> X<scope, dynamic> X<dynamic scope> X<variable, local> |
552 | X<variable, temporary> | |
cb1a09d0 | 553 | |
19799a22 | 554 | B<WARNING>: In general, you should be using C<my> instead of C<local>, because |
6d28dffb | 555 | it's faster and safer. Exceptions to this include the global punctuation |
325192b1 RGS |
556 | variables, global filehandles and formats, and direct manipulation of the |
557 | Perl symbol table itself. C<local> is mostly used when the current value | |
558 | of a variable must be visible to called subroutines. | |
cb1a09d0 AD |
559 | |
560 | Synopsis: | |
561 | ||
325192b1 RGS |
562 | # localization of values |
563 | ||
555bd962 BG |
564 | local $foo; # make $foo dynamically local |
565 | local (@wid, %get); # make list of variables local | |
566 | local $foo = "flurp"; # make $foo dynamic, and init it | |
567 | local @oof = @bar; # make @oof dynamic, and init it | |
325192b1 | 568 | |
555bd962 BG |
569 | local $hash{key} = "val"; # sets a local value for this hash entry |
570 | delete local $hash{key}; # delete this entry for the current block | |
571 | local ($cond ? $v1 : $v2); # several types of lvalues support | |
572 | # localization | |
325192b1 RGS |
573 | |
574 | # localization of symbols | |
cb1a09d0 | 575 | |
555bd962 BG |
576 | local *FH; # localize $FH, @FH, %FH, &FH ... |
577 | local *merlyn = *randal; # now $merlyn is really $randal, plus | |
578 | # @merlyn is really @randal, etc | |
579 | local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal | |
580 | local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc | |
cb1a09d0 | 581 | |
19799a22 GS |
582 | A C<local> modifies its listed variables to be "local" to the |
583 | enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine | |
584 | called from within that block>. A C<local> just gives temporary | |
585 | values to global (meaning package) variables. It does I<not> create | |
586 | a local variable. This is known as dynamic scoping. Lexical scoping | |
587 | is done with C<my>, which works more like C's auto declarations. | |
cb1a09d0 | 588 | |
ceb12f1f | 589 | Some types of lvalues can be localized as well: hash and array elements |
325192b1 RGS |
590 | and slices, conditionals (provided that their result is always |
591 | localizable), and symbolic references. As for simple variables, this | |
592 | creates new, dynamically scoped values. | |
593 | ||
594 | If more than one variable or expression is given to C<local>, they must be | |
595 | placed in parentheses. This operator works | |
cb1a09d0 | 596 | by saving the current values of those variables in its argument list on a |
5f05dabc | 597 | hidden stack and restoring them upon exiting the block, subroutine, or |
cb1a09d0 AD |
598 | eval. This means that called subroutines can also reference the local |
599 | variable, but not the global one. The argument list may be assigned to if | |
600 | desired, which allows you to initialize your local variables. (If no | |
601 | initializer is given for a particular variable, it is created with an | |
325192b1 | 602 | undefined value.) |
cb1a09d0 | 603 | |
19799a22 | 604 | Because C<local> is a run-time operator, it gets executed each time |
325192b1 RGS |
605 | through a loop. Consequently, it's more efficient to localize your |
606 | variables outside the loop. | |
607 | ||
608 | =head3 Grammatical note on local() | |
d74e8afc | 609 | X<local, context> |
cb1a09d0 | 610 | |
f86cebdf GS |
611 | A C<local> is simply a modifier on an lvalue expression. When you assign to |
612 | a C<local>ized variable, the C<local> doesn't change whether its list is viewed | |
cb1a09d0 AD |
613 | as a scalar or an array. So |
614 | ||
615 | local($foo) = <STDIN>; | |
616 | local @FOO = <STDIN>; | |
617 | ||
5f05dabc | 618 | both supply a list context to the right-hand side, while |
cb1a09d0 AD |
619 | |
620 | local $foo = <STDIN>; | |
621 | ||
622 | supplies a scalar context. | |
623 | ||
325192b1 | 624 | =head3 Localization of special variables |
d74e8afc | 625 | X<local, special variable> |
3e3baf6d | 626 | |
325192b1 RGS |
627 | If you localize a special variable, you'll be giving a new value to it, |
628 | but its magic won't go away. That means that all side-effects related | |
629 | to this magic still work with the localized value. | |
3e3baf6d | 630 | |
325192b1 RGS |
631 | This feature allows code like this to work : |
632 | ||
633 | # Read the whole contents of FILE in $slurp | |
634 | { local $/ = undef; $slurp = <FILE>; } | |
635 | ||
636 | Note, however, that this restricts localization of some values ; for | |
9d42615f | 637 | example, the following statement dies, as of perl 5.10.0, with an error |
325192b1 RGS |
638 | I<Modification of a read-only value attempted>, because the $1 variable is |
639 | magical and read-only : | |
640 | ||
641 | local $1 = 2; | |
642 | ||
658a9f31 JD |
643 | One exception is the default scalar variable: starting with perl 5.14 |
644 | C<local($_)> will always strip all magic from $_, to make it possible | |
645 | to safely reuse $_ in a subroutine. | |
325192b1 RGS |
646 | |
647 | B<WARNING>: Localization of tied arrays and hashes does not currently | |
648 | work as described. | |
fd5a896a DM |
649 | This will be fixed in a future release of Perl; in the meantime, avoid |
650 | code that relies on any particular behaviour of localising tied arrays | |
651 | or hashes (localising individual elements is still okay). | |
325192b1 | 652 | See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more |
fd5a896a | 653 | details. |
d74e8afc | 654 | X<local, tie> |
fd5a896a | 655 | |
325192b1 | 656 | =head3 Localization of globs |
d74e8afc | 657 | X<local, glob> X<glob> |
3e3baf6d | 658 | |
325192b1 RGS |
659 | The construct |
660 | ||
661 | local *name; | |
662 | ||
663 | creates a whole new symbol table entry for the glob C<name> in the | |
664 | current package. That means that all variables in its glob slot ($name, | |
665 | @name, %name, &name, and the C<name> filehandle) are dynamically reset. | |
666 | ||
667 | This implies, among other things, that any magic eventually carried by | |
668 | those variables is locally lost. In other words, saying C<local */> | |
669 | will not have any effect on the internal value of the input record | |
670 | separator. | |
671 | ||
325192b1 | 672 | =head3 Localization of elements of composite types |
d74e8afc | 673 | X<local, composite type element> X<local, array element> X<local, hash element> |
3e3baf6d | 674 | |
6ee623d5 | 675 | It's also worth taking a moment to explain what happens when you |
f86cebdf GS |
676 | C<local>ize a member of a composite type (i.e. an array or hash element). |
677 | In this case, the element is C<local>ized I<by name>. This means that | |
6ee623d5 GS |
678 | when the scope of the C<local()> ends, the saved value will be |
679 | restored to the hash element whose key was named in the C<local()>, or | |
680 | the array element whose index was named in the C<local()>. If that | |
681 | element was deleted while the C<local()> was in effect (e.g. by a | |
682 | C<delete()> from a hash or a C<shift()> of an array), it will spring | |
683 | back into existence, possibly extending an array and filling in the | |
684 | skipped elements with C<undef>. For instance, if you say | |
685 | ||
686 | %hash = ( 'This' => 'is', 'a' => 'test' ); | |
687 | @ary = ( 0..5 ); | |
688 | { | |
689 | local($ary[5]) = 6; | |
690 | local($hash{'a'}) = 'drill'; | |
691 | while (my $e = pop(@ary)) { | |
692 | print "$e . . .\n"; | |
693 | last unless $e > 3; | |
694 | } | |
695 | if (@ary) { | |
696 | $hash{'only a'} = 'test'; | |
697 | delete $hash{'a'}; | |
698 | } | |
699 | } | |
700 | print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n"; | |
701 | print "The array has ",scalar(@ary)," elements: ", | |
702 | join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n"; | |
703 | ||
704 | Perl will print | |
705 | ||
706 | 6 . . . | |
707 | 4 . . . | |
708 | 3 . . . | |
709 | This is a test only a test. | |
710 | The array has 6 elements: 0, 1, 2, undef, undef, 5 | |
711 | ||
19799a22 | 712 | The behavior of local() on non-existent members of composite |
7185e5cc GS |
713 | types is subject to change in future. |
714 | ||
d361fafa VP |
715 | =head3 Localized deletion of elements of composite types |
716 | X<delete> X<local, composite type element> X<local, array element> X<local, hash element> | |
717 | ||
718 | You can use the C<delete local $array[$idx]> and C<delete local $hash{key}> | |
719 | constructs to delete a composite type entry for the current block and restore | |
720 | it when it ends. They return the array/hash value before the localization, | |
721 | which means that they are respectively equivalent to | |
722 | ||
723 | do { | |
724 | my $val = $array[$idx]; | |
725 | local $array[$idx]; | |
726 | delete $array[$idx]; | |
727 | $val | |
728 | } | |
729 | ||
730 | and | |
731 | ||
732 | do { | |
733 | my $val = $hash{key}; | |
734 | local $hash{key}; | |
735 | delete $hash{key}; | |
736 | $val | |
737 | } | |
738 | ||
739 | except that for those the C<local> is scoped to the C<do> block. Slices are | |
740 | also accepted. | |
741 | ||
742 | my %hash = ( | |
743 | a => [ 7, 8, 9 ], | |
744 | b => 1, | |
745 | ) | |
746 | ||
747 | { | |
748 | my $a = delete local $hash{a}; | |
749 | # $a is [ 7, 8, 9 ] | |
750 | # %hash is (b => 1) | |
751 | ||
752 | { | |
753 | my @nums = delete local @$a[0, 2] | |
754 | # @nums is (7, 9) | |
755 | # $a is [ undef, 8 ] | |
756 | ||
757 | $a[0] = 999; # will be erased when the scope ends | |
758 | } | |
759 | # $a is back to [ 7, 8, 9 ] | |
760 | ||
761 | } | |
762 | # %hash is back to its original state | |
763 | ||
cd06dffe | 764 | =head2 Lvalue subroutines |
d74e8afc | 765 | X<lvalue> X<subroutine, lvalue> |
cd06dffe | 766 | |
cd06dffe GS |
767 | It is possible to return a modifiable value from a subroutine. |
768 | To do this, you have to declare the subroutine to return an lvalue. | |
769 | ||
770 | my $val; | |
771 | sub canmod : lvalue { | |
4a904372 | 772 | $val; # or: return $val; |
cd06dffe GS |
773 | } |
774 | sub nomod { | |
775 | $val; | |
776 | } | |
777 | ||
778 | canmod() = 5; # assigns to $val | |
779 | nomod() = 5; # ERROR | |
780 | ||
781 | The scalar/list context for the subroutine and for the right-hand | |
782 | side of assignment is determined as if the subroutine call is replaced | |
783 | by a scalar. For example, consider: | |
784 | ||
785 | data(2,3) = get_data(3,4); | |
786 | ||
787 | Both subroutines here are called in a scalar context, while in: | |
788 | ||
789 | (data(2,3)) = get_data(3,4); | |
790 | ||
791 | and in: | |
792 | ||
793 | (data(2),data(3)) = get_data(3,4); | |
794 | ||
795 | all the subroutines are called in a list context. | |
796 | ||
771cc755 JV |
797 | Lvalue subroutines are convenient, but you have to keep in mind that, |
798 | when used with objects, they may violate encapsulation. A normal | |
799 | mutator can check the supplied argument before setting the attribute | |
800 | it is protecting, an lvalue subroutine cannot. If you require any | |
801 | special processing when storing and retrieving the values, consider | |
802 | using the CPAN module Sentinel or something similar. | |
e6a32221 | 803 | |
ca40957e FC |
804 | =head2 Lexical Subroutines |
805 | X<my sub> X<state sub> X<our sub> X<subroutine, lexical> | |
806 | ||
441078c2 FC |
807 | B<WARNING>: Lexical subroutines are still experimental. The feature may be |
808 | modified or removed in future versions of Perl. | |
ca40957e FC |
809 | |
810 | Lexical subroutines are only available under the C<use feature | |
811 | 'lexical_subs'> pragma, which produces a warning unless the | |
f1d34ca8 | 812 | "experimental::lexical_subs" warnings category is disabled. |
ca40957e FC |
813 | |
814 | Beginning with Perl 5.18, you can declare a private subroutine with C<my> | |
815 | or C<state>. As with state variables, the C<state> keyword is only | |
816 | available under C<use feature 'state'> or C<use 5.010> or higher. | |
817 | ||
818 | These subroutines are only visible within the block in which they are | |
819 | declared, and only after that declaration: | |
820 | ||
f1d34ca8 | 821 | no warnings "experimental::lexical_subs"; |
ca40957e FC |
822 | use feature 'lexical_subs'; |
823 | ||
824 | foo(); # calls the package/global subroutine | |
825 | state sub foo { | |
826 | foo(); # also calls the package subroutine | |
827 | } | |
828 | foo(); # calls "state" sub | |
829 | my $ref = \&foo; # take a reference to "state" sub | |
830 | ||
831 | my sub bar { ... } | |
832 | bar(); # calls "my" sub | |
833 | ||
834 | To use a lexical subroutine from inside the subroutine itself, you must | |
835 | predeclare it. The C<sub foo {...}> subroutine definition syntax respects | |
836 | any previous C<my sub;> or C<state sub;> declaration. | |
837 | ||
838 | my sub baz; # predeclaration | |
839 | sub baz { # define the "my" sub | |
840 | baz(); # recursive call | |
841 | } | |
842 | ||
843 | =head3 C<state sub> vs C<my sub> | |
844 | ||
845 | What is the difference between "state" subs and "my" subs? Each time that | |
846 | execution enters a block when "my" subs are declared, a new copy of each | |
847 | sub is created. "State" subroutines persist from one execution of the | |
848 | containing block to the next. | |
849 | ||
850 | So, in general, "state" subroutines are faster. But "my" subs are | |
851 | necessary if you want to create closures: | |
852 | ||
f1d34ca8 | 853 | no warnings "experimental::lexical_subs"; |
ca40957e FC |
854 | use feature 'lexical_subs'; |
855 | ||
856 | sub whatever { | |
857 | my $x = shift; | |
858 | my sub inner { | |
859 | ... do something with $x ... | |
860 | } | |
861 | inner(); | |
862 | } | |
863 | ||
864 | In this example, a new C<$x> is created when C<whatever> is called, and | |
865 | also a new C<inner>, which can see the new C<$x>. A "state" sub will only | |
866 | see the C<$x> from the first call to C<whatever>. | |
867 | ||
868 | =head3 C<our> subroutines | |
869 | ||
870 | Like C<our $variable>, C<our sub> creates a lexical alias to the package | |
871 | subroutine of the same name. | |
872 | ||
873 | The two main uses for this are to switch back to using the package sub | |
874 | inside an inner scope: | |
875 | ||
f1d34ca8 | 876 | no warnings "experimental::lexical_subs"; |
ca40957e FC |
877 | use feature 'lexical_subs'; |
878 | ||
879 | sub foo { ... } | |
880 | ||
881 | sub bar { | |
882 | my sub foo { ... } | |
883 | { | |
884 | # need to use the outer foo here | |
885 | our sub foo; | |
886 | foo(); | |
887 | } | |
888 | } | |
889 | ||
890 | and to make a subroutine visible to other packages in the same scope: | |
891 | ||
892 | package MySneakyModule; | |
893 | ||
f1d34ca8 | 894 | no warnings "experimental::lexical_subs"; |
ca40957e FC |
895 | use feature 'lexical_subs'; |
896 | ||
897 | our sub do_something { ... } | |
898 | ||
899 | sub do_something_with_caller { | |
900 | package DB; | |
901 | () = caller 1; # sets @DB::args | |
902 | do_something(@args); # uses MySneakyModule::do_something | |
903 | } | |
904 | ||
cb1a09d0 | 905 | =head2 Passing Symbol Table Entries (typeglobs) |
d74e8afc | 906 | X<typeglob> X<*> |
cb1a09d0 | 907 | |
19799a22 GS |
908 | B<WARNING>: The mechanism described in this section was originally |
909 | the only way to simulate pass-by-reference in older versions of | |
910 | Perl. While it still works fine in modern versions, the new reference | |
911 | mechanism is generally easier to work with. See below. | |
a0d0e21e LW |
912 | |
913 | Sometimes you don't want to pass the value of an array to a subroutine | |
914 | but rather the name of it, so that the subroutine can modify the global | |
915 | copy of it rather than working with a local copy. In perl you can | |
cb1a09d0 | 916 | refer to all objects of a particular name by prefixing the name |
5f05dabc | 917 | with a star: C<*foo>. This is often known as a "typeglob", because the |
a0d0e21e LW |
918 | star on the front can be thought of as a wildcard match for all the |
919 | funny prefix characters on variables and subroutines and such. | |
920 | ||
55497cff | 921 | When evaluated, the typeglob produces a scalar value that represents |
5f05dabc | 922 | all the objects of that name, including any filehandle, format, or |
a0d0e21e | 923 | subroutine. When assigned to, it causes the name mentioned to refer to |
19799a22 | 924 | whatever C<*> value was assigned to it. Example: |
a0d0e21e LW |
925 | |
926 | sub doubleary { | |
927 | local(*someary) = @_; | |
928 | foreach $elem (@someary) { | |
929 | $elem *= 2; | |
930 | } | |
931 | } | |
932 | doubleary(*foo); | |
933 | doubleary(*bar); | |
934 | ||
19799a22 | 935 | Scalars are already passed by reference, so you can modify |
a0d0e21e | 936 | scalar arguments without using this mechanism by referring explicitly |
1fef88e7 | 937 | to C<$_[0]> etc. You can modify all the elements of an array by passing |
f86cebdf GS |
938 | all the elements as scalars, but you have to use the C<*> mechanism (or |
939 | the equivalent reference mechanism) to C<push>, C<pop>, or change the size of | |
a0d0e21e LW |
940 | an array. It will certainly be faster to pass the typeglob (or reference). |
941 | ||
942 | Even if you don't want to modify an array, this mechanism is useful for | |
5f05dabc | 943 | passing multiple arrays in a single LIST, because normally the LIST |
a0d0e21e | 944 | mechanism will merge all the array values so that you can't extract out |
55497cff | 945 | the individual arrays. For more on typeglobs, see |
2ae324a7 | 946 | L<perldata/"Typeglobs and Filehandles">. |
cb1a09d0 | 947 | |
5a964f20 | 948 | =head2 When to Still Use local() |
d74e8afc | 949 | X<local> X<variable, local> |
5a964f20 | 950 | |
19799a22 GS |
951 | Despite the existence of C<my>, there are still three places where the |
952 | C<local> operator still shines. In fact, in these three places, you | |
5a964f20 TC |
953 | I<must> use C<local> instead of C<my>. |
954 | ||
13a2d996 | 955 | =over 4 |
5a964f20 | 956 | |
551e1d92 RB |
957 | =item 1. |
958 | ||
959 | You need to give a global variable a temporary value, especially $_. | |
5a964f20 | 960 | |
f86cebdf GS |
961 | The global variables, like C<@ARGV> or the punctuation variables, must be |
962 | C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits | |
5a964f20 | 963 | it up into chunks separated by lines of equal signs, which are placed |
f86cebdf | 964 | in C<@Fields>. |
5a964f20 TC |
965 | |
966 | { | |
967 | local @ARGV = ("/etc/motd"); | |
968 | local $/ = undef; | |
969 | local $_ = <>; | |
970 | @Fields = split /^\s*=+\s*$/; | |
971 | } | |
972 | ||
19799a22 | 973 | It particular, it's important to C<local>ize $_ in any routine that assigns |
5a964f20 TC |
974 | to it. Look out for implicit assignments in C<while> conditionals. |
975 | ||
551e1d92 RB |
976 | =item 2. |
977 | ||
978 | You need to create a local file or directory handle or a local function. | |
5a964f20 | 979 | |
09bef843 SB |
980 | A function that needs a filehandle of its own must use |
981 | C<local()> on a complete typeglob. This can be used to create new symbol | |
5a964f20 TC |
982 | table entries: |
983 | ||
984 | sub ioqueue { | |
985 | local (*READER, *WRITER); # not my! | |
17b63f68 | 986 | pipe (READER, WRITER) or die "pipe: $!"; |
5a964f20 TC |
987 | return (*READER, *WRITER); |
988 | } | |
989 | ($head, $tail) = ioqueue(); | |
990 | ||
991 | See the Symbol module for a way to create anonymous symbol table | |
992 | entries. | |
993 | ||
994 | Because assignment of a reference to a typeglob creates an alias, this | |
995 | can be used to create what is effectively a local function, or at least, | |
996 | a local alias. | |
997 | ||
998 | { | |
4a46e268 | 999 | local *grow = \&shrink; # only until this block exits |
555bd962 BG |
1000 | grow(); # really calls shrink() |
1001 | move(); # if move() grow()s, it shrink()s too | |
5a964f20 | 1002 | } |
555bd962 | 1003 | grow(); # get the real grow() again |
5a964f20 TC |
1004 | |
1005 | See L<perlref/"Function Templates"> for more about manipulating | |
1006 | functions by name in this way. | |
1007 | ||
551e1d92 RB |
1008 | =item 3. |
1009 | ||
1010 | You want to temporarily change just one element of an array or hash. | |
5a964f20 | 1011 | |
f86cebdf | 1012 | You can C<local>ize just one element of an aggregate. Usually this |
5a964f20 TC |
1013 | is done on dynamics: |
1014 | ||
1015 | { | |
1016 | local $SIG{INT} = 'IGNORE'; | |
1017 | funct(); # uninterruptible | |
1018 | } | |
1019 | # interruptibility automatically restored here | |
1020 | ||
9d42615f | 1021 | But it also works on lexically declared aggregates. |
5a964f20 TC |
1022 | |
1023 | =back | |
1024 | ||
cb1a09d0 | 1025 | =head2 Pass by Reference |
d74e8afc | 1026 | X<pass by reference> X<pass-by-reference> X<reference> |
cb1a09d0 | 1027 | |
55497cff | 1028 | If you want to pass more than one array or hash into a function--or |
1029 | return them from it--and have them maintain their integrity, then | |
1030 | you're going to have to use an explicit pass-by-reference. Before you | |
1031 | do that, you need to understand references as detailed in L<perlref>. | |
c07a80fd | 1032 | This section may not make much sense to you otherwise. |
cb1a09d0 | 1033 | |
19799a22 GS |
1034 | Here are a few simple examples. First, let's pass in several arrays |
1035 | to a function and have it C<pop> all of then, returning a new list | |
1036 | of all their former last elements: | |
cb1a09d0 AD |
1037 | |
1038 | @tailings = popmany ( \@a, \@b, \@c, \@d ); | |
1039 | ||
1040 | sub popmany { | |
1041 | my $aref; | |
1042 | my @retlist = (); | |
1043 | foreach $aref ( @_ ) { | |
1044 | push @retlist, pop @$aref; | |
54310121 | 1045 | } |
cb1a09d0 | 1046 | return @retlist; |
54310121 | 1047 | } |
cb1a09d0 | 1048 | |
54310121 | 1049 | Here's how you might write a function that returns a |
cb1a09d0 AD |
1050 | list of keys occurring in all the hashes passed to it: |
1051 | ||
54310121 | 1052 | @common = inter( \%foo, \%bar, \%joe ); |
cb1a09d0 AD |
1053 | sub inter { |
1054 | my ($k, $href, %seen); # locals | |
1055 | foreach $href (@_) { | |
1056 | while ( $k = each %$href ) { | |
1057 | $seen{$k}++; | |
54310121 | 1058 | } |
1059 | } | |
cb1a09d0 | 1060 | return grep { $seen{$_} == @_ } keys %seen; |
54310121 | 1061 | } |
cb1a09d0 | 1062 | |
5f05dabc | 1063 | So far, we're using just the normal list return mechanism. |
54310121 | 1064 | What happens if you want to pass or return a hash? Well, |
1065 | if you're using only one of them, or you don't mind them | |
cb1a09d0 | 1066 | concatenating, then the normal calling convention is ok, although |
54310121 | 1067 | a little expensive. |
cb1a09d0 AD |
1068 | |
1069 | Where people get into trouble is here: | |
1070 | ||
1071 | (@a, @b) = func(@c, @d); | |
1072 | or | |
1073 | (%a, %b) = func(%c, %d); | |
1074 | ||
19799a22 GS |
1075 | That syntax simply won't work. It sets just C<@a> or C<%a> and |
1076 | clears the C<@b> or C<%b>. Plus the function didn't get passed | |
1077 | into two separate arrays or hashes: it got one long list in C<@_>, | |
1078 | as always. | |
cb1a09d0 AD |
1079 | |
1080 | If you can arrange for everyone to deal with this through references, it's | |
1081 | cleaner code, although not so nice to look at. Here's a function that | |
1082 | takes two array references as arguments, returning the two array elements | |
1083 | in order of how many elements they have in them: | |
1084 | ||
1085 | ($aref, $bref) = func(\@c, \@d); | |
1086 | print "@$aref has more than @$bref\n"; | |
1087 | sub func { | |
1088 | my ($cref, $dref) = @_; | |
1089 | if (@$cref > @$dref) { | |
1090 | return ($cref, $dref); | |
1091 | } else { | |
c07a80fd | 1092 | return ($dref, $cref); |
54310121 | 1093 | } |
1094 | } | |
cb1a09d0 AD |
1095 | |
1096 | It turns out that you can actually do this also: | |
1097 | ||
1098 | (*a, *b) = func(\@c, \@d); | |
1099 | print "@a has more than @b\n"; | |
1100 | sub func { | |
1101 | local (*c, *d) = @_; | |
1102 | if (@c > @d) { | |
1103 | return (\@c, \@d); | |
1104 | } else { | |
1105 | return (\@d, \@c); | |
54310121 | 1106 | } |
1107 | } | |
cb1a09d0 AD |
1108 | |
1109 | Here we're using the typeglobs to do symbol table aliasing. It's | |
19799a22 | 1110 | a tad subtle, though, and also won't work if you're using C<my> |
09bef843 | 1111 | variables, because only globals (even in disguise as C<local>s) |
19799a22 | 1112 | are in the symbol table. |
5f05dabc | 1113 | |
1114 | If you're passing around filehandles, you could usually just use the bare | |
19799a22 GS |
1115 | typeglob, like C<*STDOUT>, but typeglobs references work, too. |
1116 | For example: | |
5f05dabc | 1117 | |
1118 | splutter(\*STDOUT); | |
1119 | sub splutter { | |
1120 | my $fh = shift; | |
1121 | print $fh "her um well a hmmm\n"; | |
1122 | } | |
1123 | ||
1124 | $rec = get_rec(\*STDIN); | |
1125 | sub get_rec { | |
1126 | my $fh = shift; | |
1127 | return scalar <$fh>; | |
1128 | } | |
1129 | ||
19799a22 GS |
1130 | If you're planning on generating new filehandles, you could do this. |
1131 | Notice to pass back just the bare *FH, not its reference. | |
5f05dabc | 1132 | |
1133 | sub openit { | |
19799a22 | 1134 | my $path = shift; |
5f05dabc | 1135 | local *FH; |
e05a3a1e | 1136 | return open (FH, $path) ? *FH : undef; |
54310121 | 1137 | } |
5f05dabc | 1138 | |
cb1a09d0 | 1139 | =head2 Prototypes |
d74e8afc | 1140 | X<prototype> X<subroutine, prototype> |
cb1a09d0 | 1141 | |
19799a22 GS |
1142 | Perl supports a very limited kind of compile-time argument checking |
1143 | using function prototyping. If you declare | |
cb1a09d0 | 1144 | |
cba5a3b0 | 1145 | sub mypush (+@) |
cb1a09d0 | 1146 | |
19799a22 GS |
1147 | then C<mypush()> takes arguments exactly like C<push()> does. The |
1148 | function declaration must be visible at compile time. The prototype | |
1149 | affects only interpretation of new-style calls to the function, | |
1150 | where new-style is defined as not using the C<&> character. In | |
1151 | other words, if you call it like a built-in function, then it behaves | |
1152 | like a built-in function. If you call it like an old-fashioned | |
1153 | subroutine, then it behaves like an old-fashioned subroutine. It | |
1154 | naturally falls out from this rule that prototypes have no influence | |
1155 | on subroutine references like C<\&foo> or on indirect subroutine | |
c47ff5f1 | 1156 | calls like C<&{$subref}> or C<< $subref->() >>. |
c07a80fd | 1157 | |
1158 | Method calls are not influenced by prototypes either, because the | |
19799a22 GS |
1159 | function to be called is indeterminate at compile time, since |
1160 | the exact code called depends on inheritance. | |
cb1a09d0 | 1161 | |
19799a22 GS |
1162 | Because the intent of this feature is primarily to let you define |
1163 | subroutines that work like built-in functions, here are prototypes | |
1164 | for some other functions that parse almost exactly like the | |
1165 | corresponding built-in. | |
cb1a09d0 | 1166 | |
555bd962 BG |
1167 | Declared as Called as |
1168 | ||
1169 | sub mylink ($$) mylink $old, $new | |
1170 | sub myvec ($$$) myvec $var, $offset, 1 | |
1171 | sub myindex ($$;$) myindex &getstring, "substr" | |
1172 | sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off | |
1173 | sub myreverse (@) myreverse $a, $b, $c | |
1174 | sub myjoin ($@) myjoin ":", $a, $b, $c | |
1175 | sub mypop (+) mypop @array | |
1176 | sub mysplice (+$$@) mysplice @array, 0, 2, @pushme | |
1177 | sub mykeys (+) mykeys %{$hashref} | |
1178 | sub myopen (*;$) myopen HANDLE, $name | |
1179 | sub mypipe (**) mypipe READHANDLE, WRITEHANDLE | |
1180 | sub mygrep (&@) mygrep { /foo/ } $a, $b, $c | |
1181 | sub myrand (;$) myrand 42 | |
1182 | sub mytime () mytime | |
cb1a09d0 | 1183 | |
c07a80fd | 1184 | Any backslashed prototype character represents an actual argument |
ae7a3cfa | 1185 | that must start with that character (optionally preceded by C<my>, |
b91b7d1a FC |
1186 | C<our> or C<local>), with the exception of C<$>, which will |
1187 | accept any scalar lvalue expression, such as C<$foo = 7> or | |
74083ec6 | 1188 | C<< my_function()->[0] >>. The value passed as part of C<@_> will be a |
ae7a3cfa FC |
1189 | reference to the actual argument given in the subroutine call, |
1190 | obtained by applying C<\> to that argument. | |
c07a80fd | 1191 | |
c035a075 DG |
1192 | You can use the C<\[]> backslash group notation to specify more than one |
1193 | allowed argument type. For example: | |
5b794e05 JH |
1194 | |
1195 | sub myref (\[$@%&*]) | |
1196 | ||
1197 | will allow calling myref() as | |
1198 | ||
1199 | myref $var | |
1200 | myref @array | |
1201 | myref %hash | |
1202 | myref &sub | |
1203 | myref *glob | |
1204 | ||
1205 | and the first argument of myref() will be a reference to | |
1206 | a scalar, an array, a hash, a code, or a glob. | |
1207 | ||
c07a80fd | 1208 | Unbackslashed prototype characters have special meanings. Any |
19799a22 | 1209 | unbackslashed C<@> or C<%> eats all remaining arguments, and forces |
f86cebdf GS |
1210 | list context. An argument represented by C<$> forces scalar context. An |
1211 | C<&> requires an anonymous subroutine, which, if passed as the first | |
0df79f0c GS |
1212 | argument, does not require the C<sub> keyword or a subsequent comma. |
1213 | ||
1214 | A C<*> allows the subroutine to accept a bareword, constant, scalar expression, | |
648ca4f7 GS |
1215 | typeglob, or a reference to a typeglob in that slot. The value will be |
1216 | available to the subroutine either as a simple scalar, or (in the latter | |
0df79f0c GS |
1217 | two cases) as a reference to the typeglob. If you wish to always convert |
1218 | such arguments to a typeglob reference, use Symbol::qualify_to_ref() as | |
1219 | follows: | |
1220 | ||
1221 | use Symbol 'qualify_to_ref'; | |
1222 | ||
1223 | sub foo (*) { | |
1224 | my $fh = qualify_to_ref(shift, caller); | |
1225 | ... | |
1226 | } | |
c07a80fd | 1227 | |
c035a075 DG |
1228 | The C<+> prototype is a special alternative to C<$> that will act like |
1229 | C<\[@%]> when given a literal array or hash variable, but will otherwise | |
1230 | force scalar context on the argument. This is useful for functions which | |
1231 | should accept either a literal array or an array reference as the argument: | |
1232 | ||
cba5a3b0 | 1233 | sub mypush (+@) { |
c035a075 DG |
1234 | my $aref = shift; |
1235 | die "Not an array or arrayref" unless ref $aref eq 'ARRAY'; | |
1236 | push @$aref, @_; | |
1237 | } | |
1238 | ||
1239 | When using the C<+> prototype, your function must check that the argument | |
1240 | is of an acceptable type. | |
1241 | ||
859a4967 | 1242 | A semicolon (C<;>) separates mandatory arguments from optional arguments. |
19799a22 | 1243 | It is redundant before C<@> or C<%>, which gobble up everything else. |
cb1a09d0 | 1244 | |
34daab0f RGS |
1245 | As the last character of a prototype, or just before a semicolon, a C<@> |
1246 | or a C<%>, you can use C<_> in place of C<$>: if this argument is not | |
1247 | provided, C<$_> will be used instead. | |
859a4967 | 1248 | |
19799a22 GS |
1249 | Note how the last three examples in the table above are treated |
1250 | specially by the parser. C<mygrep()> is parsed as a true list | |
1251 | operator, C<myrand()> is parsed as a true unary operator with unary | |
1252 | precedence the same as C<rand()>, and C<mytime()> is truly without | |
1253 | arguments, just like C<time()>. That is, if you say | |
cb1a09d0 AD |
1254 | |
1255 | mytime +2; | |
1256 | ||
f86cebdf | 1257 | you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed |
3a8944db FC |
1258 | without a prototype. If you want to force a unary function to have the |
1259 | same precedence as a list operator, add C<;> to the end of the prototype: | |
1260 | ||
1261 | sub mygetprotobynumber($;); | |
1262 | mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b) | |
cb1a09d0 | 1263 | |
19799a22 GS |
1264 | The interesting thing about C<&> is that you can generate new syntax with it, |
1265 | provided it's in the initial position: | |
d74e8afc | 1266 | X<&> |
cb1a09d0 | 1267 | |
6d28dffb | 1268 | sub try (&@) { |
cb1a09d0 AD |
1269 | my($try,$catch) = @_; |
1270 | eval { &$try }; | |
1271 | if ($@) { | |
1272 | local $_ = $@; | |
1273 | &$catch; | |
1274 | } | |
1275 | } | |
55497cff | 1276 | sub catch (&) { $_[0] } |
cb1a09d0 AD |
1277 | |
1278 | try { | |
1279 | die "phooey"; | |
1280 | } catch { | |
1281 | /phooey/ and print "unphooey\n"; | |
1282 | }; | |
1283 | ||
f86cebdf | 1284 | That prints C<"unphooey">. (Yes, there are still unresolved |
19799a22 | 1285 | issues having to do with visibility of C<@_>. I'm ignoring that |
f86cebdf | 1286 | question for the moment. (But note that if we make C<@_> lexically |
cb1a09d0 | 1287 | scoped, those anonymous subroutines can act like closures... (Gee, |
5f05dabc | 1288 | is this sounding a little Lispish? (Never mind.)))) |
cb1a09d0 | 1289 | |
19799a22 | 1290 | And here's a reimplementation of the Perl C<grep> operator: |
d74e8afc | 1291 | X<grep> |
cb1a09d0 AD |
1292 | |
1293 | sub mygrep (&@) { | |
1294 | my $code = shift; | |
1295 | my @result; | |
1296 | foreach $_ (@_) { | |
6e47f808 | 1297 | push(@result, $_) if &$code; |
cb1a09d0 AD |
1298 | } |
1299 | @result; | |
1300 | } | |
a0d0e21e | 1301 | |
cb1a09d0 AD |
1302 | Some folks would prefer full alphanumeric prototypes. Alphanumerics have |
1303 | been intentionally left out of prototypes for the express purpose of | |
1304 | someday in the future adding named, formal parameters. The current | |
1305 | mechanism's main goal is to let module writers provide better diagnostics | |
1306 | for module users. Larry feels the notation quite understandable to Perl | |
1307 | programmers, and that it will not intrude greatly upon the meat of the | |
1308 | module, nor make it harder to read. The line noise is visually | |
1309 | encapsulated into a small pill that's easy to swallow. | |
1310 | ||
420cdfc1 ST |
1311 | If you try to use an alphanumeric sequence in a prototype you will |
1312 | generate an optional warning - "Illegal character in prototype...". | |
1313 | Unfortunately earlier versions of Perl allowed the prototype to be | |
1314 | used as long as its prefix was a valid prototype. The warning may be | |
1315 | upgraded to a fatal error in a future version of Perl once the | |
1316 | majority of offending code is fixed. | |
1317 | ||
cb1a09d0 AD |
1318 | It's probably best to prototype new functions, not retrofit prototyping |
1319 | into older ones. That's because you must be especially careful about | |
1320 | silent impositions of differing list versus scalar contexts. For example, | |
1321 | if you decide that a function should take just one parameter, like this: | |
1322 | ||
1323 | sub func ($) { | |
1324 | my $n = shift; | |
1325 | print "you gave me $n\n"; | |
54310121 | 1326 | } |
cb1a09d0 AD |
1327 | |
1328 | and someone has been calling it with an array or expression | |
1329 | returning a list: | |
1330 | ||
1331 | func(@foo); | |
1332 | func( split /:/ ); | |
1333 | ||
19799a22 | 1334 | Then you've just supplied an automatic C<scalar> in front of their |
f86cebdf | 1335 | argument, which can be more than a bit surprising. The old C<@foo> |
cb1a09d0 | 1336 | which used to hold one thing doesn't get passed in. Instead, |
19799a22 GS |
1337 | C<func()> now gets passed in a C<1>; that is, the number of elements |
1338 | in C<@foo>. And the C<split> gets called in scalar context so it | |
1339 | starts scribbling on your C<@_> parameter list. Ouch! | |
cb1a09d0 | 1340 | |
5f05dabc | 1341 | This is all very powerful, of course, and should be used only in moderation |
54310121 | 1342 | to make the world a better place. |
44a8e56a | 1343 | |
1344 | =head2 Constant Functions | |
d74e8afc | 1345 | X<constant> |
44a8e56a | 1346 | |
1347 | Functions with a prototype of C<()> are potential candidates for | |
19799a22 GS |
1348 | inlining. If the result after optimization and constant folding |
1349 | is either a constant or a lexically-scoped scalar which has no other | |
54310121 | 1350 | references, then it will be used in place of function calls made |
19799a22 GS |
1351 | without C<&>. Calls made using C<&> are never inlined. (See |
1352 | F<constant.pm> for an easy way to declare most constants.) | |
44a8e56a | 1353 | |
5a964f20 | 1354 | The following functions would all be inlined: |
44a8e56a | 1355 | |
699e6cd4 TP |
1356 | sub pi () { 3.14159 } # Not exact, but close. |
1357 | sub PI () { 4 * atan2 1, 1 } # As good as it gets, | |
1358 | # and it's inlined, too! | |
44a8e56a | 1359 | sub ST_DEV () { 0 } |
1360 | sub ST_INO () { 1 } | |
1361 | ||
1362 | sub FLAG_FOO () { 1 << 8 } | |
1363 | sub FLAG_BAR () { 1 << 9 } | |
1364 | sub FLAG_MASK () { FLAG_FOO | FLAG_BAR } | |
54310121 | 1365 | |
1366 | sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) } | |
88267271 PZ |
1367 | |
1368 | sub N () { int(OPT_BAZ) / 3 } | |
1369 | ||
1370 | sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO } | |
1371 | ||
1372 | Be aware that these will not be inlined; as they contain inner scopes, | |
1373 | the constant folding doesn't reduce them to a single constant: | |
1374 | ||
1375 | sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } } | |
1376 | ||
1377 | sub baz_val () { | |
44a8e56a | 1378 | if (OPT_BAZ) { |
1379 | return 23; | |
1380 | } | |
1381 | else { | |
1382 | return 42; | |
1383 | } | |
1384 | } | |
cb1a09d0 | 1385 | |
5a964f20 | 1386 | If you redefine a subroutine that was eligible for inlining, you'll get |
2dc1f7e5 | 1387 | a warning by default. (You can use this warning to tell whether or not a |
4cee8e80 | 1388 | particular subroutine is considered constant.) The warning is |
2dc1f7e5 FC |
1389 | considered severe enough not to be affected by the B<-w> |
1390 | switch (or its absence) because previously compiled | |
4cee8e80 | 1391 | invocations of the function will still be using the old value of the |
19799a22 | 1392 | function. If you need to be able to redefine the subroutine, you need to |
4cee8e80 | 1393 | ensure that it isn't inlined, either by dropping the C<()> prototype |
19799a22 | 1394 | (which changes calling semantics, so beware) or by thwarting the |
4cee8e80 CS |
1395 | inlining mechanism in some other way, such as |
1396 | ||
4cee8e80 | 1397 | sub not_inlined () { |
54310121 | 1398 | 23 if $]; |
4cee8e80 CS |
1399 | } |
1400 | ||
19799a22 | 1401 | =head2 Overriding Built-in Functions |
d74e8afc | 1402 | X<built-in> X<override> X<CORE> X<CORE::GLOBAL> |
a0d0e21e | 1403 | |
19799a22 | 1404 | Many built-in functions may be overridden, though this should be tried |
5f05dabc | 1405 | only occasionally and for good reason. Typically this might be |
19799a22 | 1406 | done by a package attempting to emulate missing built-in functionality |
a0d0e21e LW |
1407 | on a non-Unix system. |
1408 | ||
163e3a99 JP |
1409 | Overriding may be done only by importing the name from a module at |
1410 | compile time--ordinary predeclaration isn't good enough. However, the | |
19799a22 GS |
1411 | C<use subs> pragma lets you, in effect, predeclare subs |
1412 | via the import syntax, and these names may then override built-in ones: | |
a0d0e21e LW |
1413 | |
1414 | use subs 'chdir', 'chroot', 'chmod', 'chown'; | |
1415 | chdir $somewhere; | |
1416 | sub chdir { ... } | |
1417 | ||
19799a22 GS |
1418 | To unambiguously refer to the built-in form, precede the |
1419 | built-in name with the special package qualifier C<CORE::>. For example, | |
1420 | saying C<CORE::open()> always refers to the built-in C<open()>, even | |
fb73857a | 1421 | if the current package has imported some other subroutine called |
19799a22 | 1422 | C<&open()> from elsewhere. Even though it looks like a regular |
4aaa4757 FC |
1423 | function call, it isn't: the CORE:: prefix in that case is part of Perl's |
1424 | syntax, and works for any keyword, regardless of what is in the CORE | |
1425 | package. Taking a reference to it, that is, C<\&CORE::open>, only works | |
1426 | for some keywords. See L<CORE>. | |
fb73857a | 1427 | |
19799a22 GS |
1428 | Library modules should not in general export built-in names like C<open> |
1429 | or C<chdir> as part of their default C<@EXPORT> list, because these may | |
a0d0e21e | 1430 | sneak into someone else's namespace and change the semantics unexpectedly. |
19799a22 | 1431 | Instead, if the module adds that name to C<@EXPORT_OK>, then it's |
a0d0e21e LW |
1432 | possible for a user to import the name explicitly, but not implicitly. |
1433 | That is, they could say | |
1434 | ||
1435 | use Module 'open'; | |
1436 | ||
19799a22 | 1437 | and it would import the C<open> override. But if they said |
a0d0e21e LW |
1438 | |
1439 | use Module; | |
1440 | ||
19799a22 | 1441 | they would get the default imports without overrides. |
a0d0e21e | 1442 | |
19799a22 | 1443 | The foregoing mechanism for overriding built-in is restricted, quite |
95d94a4f | 1444 | deliberately, to the package that requests the import. There is a second |
19799a22 | 1445 | method that is sometimes applicable when you wish to override a built-in |
95d94a4f GS |
1446 | everywhere, without regard to namespace boundaries. This is achieved by |
1447 | importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an | |
1448 | example that quite brazenly replaces the C<glob> operator with something | |
1449 | that understands regular expressions. | |
1450 | ||
1451 | package REGlob; | |
1452 | require Exporter; | |
1453 | @ISA = 'Exporter'; | |
1454 | @EXPORT_OK = 'glob'; | |
1455 | ||
1456 | sub import { | |
1457 | my $pkg = shift; | |
1458 | return unless @_; | |
1459 | my $sym = shift; | |
1460 | my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0)); | |
1461 | $pkg->export($where, $sym, @_); | |
1462 | } | |
1463 | ||
1464 | sub glob { | |
1465 | my $pat = shift; | |
1466 | my @got; | |
7b815c67 RGS |
1467 | if (opendir my $d, '.') { |
1468 | @got = grep /$pat/, readdir $d; | |
1469 | closedir $d; | |
19799a22 GS |
1470 | } |
1471 | return @got; | |
95d94a4f GS |
1472 | } |
1473 | 1; | |
1474 | ||
1475 | And here's how it could be (ab)used: | |
1476 | ||
1477 | #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces | |
1478 | package Foo; | |
1479 | use REGlob 'glob'; # override glob() in Foo:: only | |
1480 | print for <^[a-z_]+\.pm\$>; # show all pragmatic modules | |
1481 | ||
19799a22 | 1482 | The initial comment shows a contrived, even dangerous example. |
95d94a4f | 1483 | By overriding C<glob> globally, you would be forcing the new (and |
19799a22 | 1484 | subversive) behavior for the C<glob> operator for I<every> namespace, |
95d94a4f GS |
1485 | without the complete cognizance or cooperation of the modules that own |
1486 | those namespaces. Naturally, this should be done with extreme caution--if | |
1487 | it must be done at all. | |
1488 | ||
1489 | The C<REGlob> example above does not implement all the support needed to | |
19799a22 | 1490 | cleanly override perl's C<glob> operator. The built-in C<glob> has |
95d94a4f | 1491 | different behaviors depending on whether it appears in a scalar or list |
19799a22 | 1492 | context, but our C<REGlob> doesn't. Indeed, many perl built-in have such |
95d94a4f GS |
1493 | context sensitive behaviors, and these must be adequately supported by |
1494 | a properly written override. For a fully functional example of overriding | |
1495 | C<glob>, study the implementation of C<File::DosGlob> in the standard | |
1496 | library. | |
1497 | ||
77bc9082 RGS |
1498 | When you override a built-in, your replacement should be consistent (if |
1499 | possible) with the built-in native syntax. You can achieve this by using | |
1500 | a suitable prototype. To get the prototype of an overridable built-in, | |
1501 | use the C<prototype> function with an argument of C<"CORE::builtin_name"> | |
1502 | (see L<perlfunc/prototype>). | |
1503 | ||
1504 | Note however that some built-ins can't have their syntax expressed by a | |
1505 | prototype (such as C<system> or C<chomp>). If you override them you won't | |
1506 | be able to fully mimic their original syntax. | |
1507 | ||
fe854a6f | 1508 | The built-ins C<do>, C<require> and C<glob> can also be overridden, but due |
77bc9082 RGS |
1509 | to special magic, their original syntax is preserved, and you don't have |
1510 | to define a prototype for their replacements. (You can't override the | |
1511 | C<do BLOCK> syntax, though). | |
1512 | ||
1513 | C<require> has special additional dark magic: if you invoke your | |
1514 | C<require> replacement as C<require Foo::Bar>, it will actually receive | |
1515 | the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>. | |
1516 | ||
1517 | And, as you'll have noticed from the previous example, if you override | |
593b9c14 | 1518 | C<glob>, the C<< <*> >> glob operator is overridden as well. |
77bc9082 | 1519 | |
9b3023bc | 1520 | In a similar fashion, overriding the C<readline> function also overrides |
e3f73d4e RGS |
1521 | the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding |
1522 | C<readpipe> also overrides the operators C<``> and C<qx//>. | |
9b3023bc | 1523 | |
fe854a6f | 1524 | Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden. |
77bc9082 | 1525 | |
a0d0e21e | 1526 | =head2 Autoloading |
d74e8afc | 1527 | X<autoloading> X<AUTOLOAD> |
a0d0e21e | 1528 | |
19799a22 GS |
1529 | If you call a subroutine that is undefined, you would ordinarily |
1530 | get an immediate, fatal error complaining that the subroutine doesn't | |
1531 | exist. (Likewise for subroutines being used as methods, when the | |
1532 | method doesn't exist in any base class of the class's package.) | |
1533 | However, if an C<AUTOLOAD> subroutine is defined in the package or | |
1534 | packages used to locate the original subroutine, then that | |
1535 | C<AUTOLOAD> subroutine is called with the arguments that would have | |
1536 | been passed to the original subroutine. The fully qualified name | |
1537 | of the original subroutine magically appears in the global $AUTOLOAD | |
1538 | variable of the same package as the C<AUTOLOAD> routine. The name | |
1539 | is not passed as an ordinary argument because, er, well, just | |
593b9c14 | 1540 | because, that's why. (As an exception, a method call to a nonexistent |
80ee23cd | 1541 | C<import> or C<unimport> method is just skipped instead. Also, if |
5b36e945 FC |
1542 | the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the |
1543 | subroutine name. See L<perlguts/Autoloading with XSUBs> for details.) | |
80ee23cd | 1544 | |
19799a22 GS |
1545 | |
1546 | Many C<AUTOLOAD> routines load in a definition for the requested | |
1547 | subroutine using eval(), then execute that subroutine using a special | |
1548 | form of goto() that erases the stack frame of the C<AUTOLOAD> routine | |
1549 | without a trace. (See the source to the standard module documented | |
1550 | in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can | |
1551 | also just emulate the routine and never define it. For example, | |
1552 | let's pretend that a function that wasn't defined should just invoke | |
1553 | C<system> with those arguments. All you'd do is: | |
cb1a09d0 AD |
1554 | |
1555 | sub AUTOLOAD { | |
1556 | my $program = $AUTOLOAD; | |
1557 | $program =~ s/.*:://; | |
1558 | system($program, @_); | |
54310121 | 1559 | } |
cb1a09d0 | 1560 | date(); |
6d28dffb | 1561 | who('am', 'i'); |
cb1a09d0 AD |
1562 | ls('-l'); |
1563 | ||
19799a22 GS |
1564 | In fact, if you predeclare functions you want to call that way, you don't |
1565 | even need parentheses: | |
cb1a09d0 AD |
1566 | |
1567 | use subs qw(date who ls); | |
1568 | date; | |
1569 | who "am", "i"; | |
593b9c14 | 1570 | ls '-l'; |
cb1a09d0 | 1571 | |
13058d67 | 1572 | A more complete example of this is the Shell module on CPAN, which |
19799a22 | 1573 | can treat undefined subroutine calls as calls to external programs. |
a0d0e21e | 1574 | |
19799a22 GS |
1575 | Mechanisms are available to help modules writers split their modules |
1576 | into autoloadable files. See the standard AutoLoader module | |
6d28dffb | 1577 | described in L<AutoLoader> and in L<AutoSplit>, the standard |
1578 | SelfLoader modules in L<SelfLoader>, and the document on adding C | |
19799a22 | 1579 | functions to Perl code in L<perlxs>. |
cb1a09d0 | 1580 | |
09bef843 | 1581 | =head2 Subroutine Attributes |
d74e8afc | 1582 | X<attribute> X<subroutine, attribute> X<attrs> |
09bef843 SB |
1583 | |
1584 | A subroutine declaration or definition may have a list of attributes | |
1585 | associated with it. If such an attribute list is present, it is | |
0120eecf | 1586 | broken up at space or colon boundaries and treated as though a |
09bef843 SB |
1587 | C<use attributes> had been seen. See L<attributes> for details |
1588 | about what attributes are currently supported. | |
1589 | Unlike the limitation with the obsolescent C<use attrs>, the | |
1590 | C<sub : ATTRLIST> syntax works to associate the attributes with | |
1591 | a pre-declaration, and not just with a subroutine definition. | |
1592 | ||
1593 | The attributes must be valid as simple identifier names (without any | |
1594 | punctuation other than the '_' character). They may have a parameter | |
1595 | list appended, which is only checked for whether its parentheses ('(',')') | |
1596 | nest properly. | |
1597 | ||
1598 | Examples of valid syntax (even though the attributes are unknown): | |
1599 | ||
4358a253 SS |
1600 | sub fnord (&\%) : switch(10,foo(7,3)) : expensive; |
1601 | sub plugh () : Ugly('\(") :Bad; | |
09bef843 SB |
1602 | sub xyzzy : _5x5 { ... } |
1603 | ||
1604 | Examples of invalid syntax: | |
1605 | ||
4358a253 SS |
1606 | sub fnord : switch(10,foo(); # ()-string not balanced |
1607 | sub snoid : Ugly('('); # ()-string not balanced | |
1608 | sub xyzzy : 5x5; # "5x5" not a valid identifier | |
1609 | sub plugh : Y2::north; # "Y2::north" not a simple identifier | |
1610 | sub snurt : foo + bar; # "+" not a colon or space | |
09bef843 SB |
1611 | |
1612 | The attribute list is passed as a list of constant strings to the code | |
1613 | which associates them with the subroutine. In particular, the second example | |
1614 | of valid syntax above currently looks like this in terms of how it's | |
1615 | parsed and invoked: | |
1616 | ||
1617 | use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad'; | |
1618 | ||
1619 | For further details on attribute lists and their manipulation, | |
a0ae32d3 | 1620 | see L<attributes> and L<Attribute::Handlers>. |
09bef843 | 1621 | |
cb1a09d0 | 1622 | =head1 SEE ALSO |
a0d0e21e | 1623 | |
19799a22 GS |
1624 | See L<perlref/"Function Templates"> for more about references and closures. |
1625 | See L<perlxs> if you'd like to learn about calling C subroutines from Perl. | |
a2293a43 | 1626 | See L<perlembed> if you'd like to learn about calling Perl subroutines from C. |
19799a22 GS |
1627 | See L<perlmod> to learn about bundling up your functions in separate files. |
1628 | See L<perlmodlib> to learn what library modules come standard on your system. | |
82e1c0d9 | 1629 | See L<perlootut> to learn how to make object method calls. |