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a0d0e21e 1=head1 NAME
d74e8afc 2X<subroutine> X<function>
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3
4perlsub - Perl subroutines
5
6=head1 SYNOPSIS
7
8To declare subroutines:
d74e8afc 9X<subroutine, declaration> X<sub>
a0d0e21e 10
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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
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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 21To define an anonymous subroutine at runtime:
d74e8afc 22X<subroutine, anonymous>
748a9306 23
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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 29To import subroutines:
d74e8afc 30X<import>
a0d0e21e 31
19799a22 32 use MODULE qw(NAME1 NAME2 NAME3);
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33
34To call subroutines:
d74e8afc 35X<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.
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41
42=head1 DESCRIPTION
43
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44Like many languages, Perl provides for user-defined subroutines.
45These may be located anywhere in the main program, loaded in from
46other files via the C<do>, C<require>, or C<use> keywords, or
be3174d2 47generated on the fly using C<eval> or anonymous subroutines.
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48You can even call a function indirectly using a variable containing
49its name or a CODE reference.
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50
51The Perl model for function call and return values is simple: all
52functions are passed as parameters one single flat list of scalars, and
53all functions likewise return to their caller one single flat list of
54scalars. Any arrays or hashes in these call and return lists will
55collapse, losing their identities--but you may always use
56pass-by-reference instead to avoid this. Both call and return lists may
57contain as many or as few scalar elements as you'd like. (Often a
58function without an explicit return statement is called a subroutine, but
19799a22 59there's really no difference from Perl's perspective.)
d74e8afc 60X<subroutine, parameter> X<parameter>
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61
62Any arguments passed in show up in the array C<@_>. Therefore, if
63you called a function with two arguments, those would be stored in
64C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its
65elements are aliases for the actual scalar parameters. In particular,
66if an element C<$_[0]> is updated, the corresponding argument is
67updated (or an error occurs if it is not updatable). If an argument
68is an array or hash element which did not exist when the function
69was called, that element is created only when (and if) it is modified
70or a reference to it is taken. (Some earlier versions of Perl
71created the element whether or not the element was assigned to.)
72Assigning to the whole array C<@_> removes that aliasing, and does
73not update any arguments.
d74e8afc 74X<subroutine, argument> X<argument> X<@_>
19799a22 75
dbb128be
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76A C<return> statement may be used to exit a subroutine, optionally
77specifying the returned value, which will be evaluated in the
78appropriate context (list, scalar, or void) depending on the context of
79the subroutine call. If you specify no return value, the subroutine
80returns an empty list in list context, the undefined value in scalar
81context, or nothing in void context. If you return one or more
82aggregates (arrays and hashes), these will be flattened together into
83one large indistinguishable list.
84
85If no C<return> is found and if the last statement is an expression, its
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RGS
86value is returned. If the last statement is a loop control structure
87like a C<foreach> or a C<while>, the returned value is unspecified. The
88empty sub returns the empty list.
d74e8afc 89X<subroutine, return value> X<return value> X<return>
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90
91Perl does not have named formal parameters. In practice all you
92do is assign to a C<my()> list of these. Variables that aren't
93declared to be private are global variables. For gory details
94on creating private variables, see L<"Private Variables via my()">
95and L<"Temporary Values via local()">. To create protected
96environments for a set of functions in a separate package (and
97probably a separate file), see L<perlmod/"Packages">.
d74e8afc 98X<formal parameter> X<parameter, formal>
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99
100Example:
101
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102 sub max {
103 my $max = shift(@_);
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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);
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110
111Example:
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>)) {
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119 if ($lookahead =~ /^[ \t]/) {
120 $thisline .= $lookahead;
121 }
122 else {
123 last LINE;
124 }
125 }
19799a22 126 return $thisline;
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127 }
128
129 $lookahead = <STDIN>; # get first line
19799a22 130 while (defined($line = get_line())) {
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131 ...
132 }
133
09bef843 134Assigning to a list of private variables to name your arguments:
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135
136 sub maybeset {
137 my($key, $value) = @_;
cb1a09d0 138 $Foo{$key} = $value unless $Foo{$key};
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139 }
140
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141Because the assignment copies the values, this also has the effect
142of turning call-by-reference into call-by-value. Otherwise a
143function is free to do in-place modifications of C<@_> and change
144its caller's values.
d74e8afc 145X<call-by-reference> X<call-by-value>
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146
147 upcase_in($v1, $v2); # this changes $v1 and $v2
148 sub upcase_in {
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149 for (@_) { tr/a-z/A-Z/ }
150 }
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151
152You aren't allowed to modify constants in this way, of course. If an
153argument were actually literal and you tried to change it, you'd take a
154(presumably fatal) exception. For example, this won't work:
d74e8afc 155X<call-by-reference> X<call-by-value>
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156
157 upcase_in("frederick");
158
f86cebdf 159It would be much safer if the C<upcase_in()> function
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160were written to return a copy of its parameters instead
161of changing them in place:
162
19799a22 163 ($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 171Notice how this (unprototyped) function doesn't care whether it was
a2293a43 172passed real scalars or arrays. Perl sees all arguments as one big,
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173long, flat parameter list in C<@_>. This is one area where
174Perl's simple argument-passing style shines. The C<upcase()>
175function would work perfectly well without changing the C<upcase()>
176definition even if we fed it things like this:
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177
178 @newlist = upcase(@list1, @list2);
179 @newlist = upcase( split /:/, $var );
180
181Do not, however, be tempted to do this:
182
183 (@a, @b) = upcase(@list1, @list2);
184
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185Like the flattened incoming parameter list, the return list is also
186flattened on return. So all you have managed to do here is stored
17b63f68 187everything in C<@a> and made C<@b> empty. See
13a2d996 188L<Pass by Reference> for alternatives.
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189
190A subroutine may be called using an explicit C<&> prefix. The
191C<&> is optional in modern Perl, as are parentheses if the
192subroutine has been predeclared. The C<&> is I<not> optional
193when just naming the subroutine, such as when it's used as
194an argument to defined() or undef(). Nor is it optional when you
195want to do an indirect subroutine call with a subroutine name or
196reference using the C<&$subref()> or C<&{$subref}()> constructs,
c47ff5f1 197although the C<< $subref->() >> notation solves that problem.
19799a22 198See L<perlref> for more about all that.
d74e8afc 199X<&>
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200
201Subroutines may be called recursively. If a subroutine is called
202using the C<&> form, the argument list is optional, and if omitted,
203no C<@_> array is set up for the subroutine: the C<@_> array at the
204time of the call is visible to subroutine instead. This is an
205efficiency mechanism that new users may wish to avoid.
d74e8afc 206X<recursion>
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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
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217Not only does the C<&> form make the argument list optional, it also
218disables any prototype checking on arguments you do provide. This
c07a80fd 219is partly for historical reasons, and partly for having a convenient way
9688be67 220to cheat if you know what you're doing. See L</Prototypes> below.
d74e8afc 221X<&>
c07a80fd 222
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223Subroutines whose names are in all upper case are reserved to the Perl
224core, as are modules whose names are in all lower case. A subroutine in
225all capitals is a loosely-held convention meaning it will be called
226indirectly by the run-time system itself, usually due to a triggered event.
227Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>,
228C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>.
229
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230The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines
231are not so much subroutines as named special code blocks, of which you
232can have more than one in a package, and which you can B<not> call
233explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">
5a964f20 234
b687b08b 235=head2 Private Variables via my()
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236X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical>
237X<lexical scope> X<attributes, my>
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238
239Synopsis:
240
241 my $foo; # declare $foo lexically local
242 my (@wid, %get); # declare list of variables local
243 my $foo = "flurp"; # declare $foo lexical, and init it
244 my @oof = @bar; # declare @oof lexical, and init it
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245 my $x : Foo = $y; # similar, with an attribute applied
246
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247B<WARNING>: The use of attribute lists on C<my> declarations is still
248evolving. The current semantics and interface are subject to change.
249See L<attributes> and L<Attribute::Handlers>.
cb1a09d0 250
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251The C<my> operator declares the listed variables to be lexically
252confined to the enclosing block, conditional (C<if/unless/elsif/else>),
253loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
254or C<do/require/use>'d file. If more than one value is listed, the
255list must be placed in parentheses. All listed elements must be
256legal lvalues. Only alphanumeric identifiers may be lexically
325192b1 257scoped--magical built-ins like C<$/> must currently be C<local>ized
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258with C<local> instead.
259
260Unlike dynamic variables created by the C<local> operator, lexical
261variables declared with C<my> are totally hidden from the outside
262world, including any called subroutines. This is true if it's the
263same subroutine called from itself or elsewhere--every call gets
264its own copy.
d74e8afc 265X<local>
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266
267This doesn't mean that a C<my> variable declared in a statically
268enclosing lexical scope would be invisible. Only dynamic scopes
269are cut off. For example, the C<bumpx()> function below has access
270to the lexical $x variable because both the C<my> and the C<sub>
271occurred at the same scope, presumably file scope.
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272
273 my $x = 10;
274 sub bumpx { $x++ }
275
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276An C<eval()>, however, can see lexical variables of the scope it is
277being evaluated in, so long as the names aren't hidden by declarations within
278the C<eval()> itself. See L<perlref>.
d74e8afc 279X<eval, scope of>
cb1a09d0 280
19799a22 281The parameter list to my() may be assigned to if desired, which allows you
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282to initialize your variables. (If no initializer is given for a
283particular variable, it is created with the undefined value.) Commonly
19799a22 284this is used to name input parameters to a subroutine. Examples:
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285
286 $arg = "fred"; # "global" variable
287 $n = cube_root(27);
288 print "$arg thinks the root is $n\n";
289 fred thinks the root is 3
290
291 sub cube_root {
292 my $arg = shift; # name doesn't matter
293 $arg **= 1/3;
294 return $arg;
54310121 295 }
cb1a09d0 296
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297The C<my> is simply a modifier on something you might assign to. So when
298you do assign to variables in its argument list, C<my> doesn't
6cc33c6d 299change whether those variables are viewed as a scalar or an array. So
cb1a09d0 300
5a964f20 301 my ($foo) = <STDIN>; # WRONG?
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302 my @FOO = <STDIN>;
303
5f05dabc 304both supply a list context to the right-hand side, while
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305
306 my $foo = <STDIN>;
307
5f05dabc 308supplies a scalar context. But the following declares only one variable:
748a9306 309
5a964f20 310 my $foo, $bar = 1; # WRONG
748a9306 311
cb1a09d0 312That has the same effect as
748a9306 313
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314 my $foo;
315 $bar = 1;
a0d0e21e 316
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317The declared variable is not introduced (is not visible) until after
318the current statement. Thus,
319
320 my $x = $x;
321
19799a22 322can be used to initialize a new $x with the value of the old $x, and
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323the expression
324
325 my $x = 123 and $x == 123
326
19799a22 327is false unless the old $x happened to have the value C<123>.
cb1a09d0 328
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329Lexical scopes of control structures are not bounded precisely by the
330braces that delimit their controlled blocks; control expressions are
19799a22 331part of that scope, too. Thus in the loop
55497cff 332
19799a22 333 while (my $line = <>) {
55497cff
PP
334 $line = lc $line;
335 } continue {
336 print $line;
337 }
338
19799a22 339the scope of $line extends from its declaration throughout the rest of
55497cff
PP
340the loop construct (including the C<continue> clause), but not beyond
341it. Similarly, in the conditional
342
343 if ((my $answer = <STDIN>) =~ /^yes$/i) {
344 user_agrees();
345 } elsif ($answer =~ /^no$/i) {
346 user_disagrees();
347 } else {
348 chomp $answer;
349 die "'$answer' is neither 'yes' nor 'no'";
350 }
351
19799a22
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352the scope of $answer extends from its declaration through the rest
353of that conditional, including any C<elsif> and C<else> clauses,
96090e4f 354but not beyond it. See L<perlsyn/"Simple Statements"> for information
457b36cb 355on the scope of variables in statements with modifiers.
55497cff 356
5f05dabc 357The C<foreach> loop defaults to scoping its index variable dynamically
19799a22
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358in the manner of C<local>. However, if the index variable is
359prefixed with the keyword C<my>, or if there is already a lexical
360by that name in scope, then a new lexical is created instead. Thus
361in the loop
d74e8afc 362X<foreach> X<for>
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363
364 for my $i (1, 2, 3) {
365 some_function();
366 }
367
19799a22
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368the scope of $i extends to the end of the loop, but not beyond it,
369rendering the value of $i inaccessible within C<some_function()>.
d74e8afc 370X<foreach> X<for>
55497cff 371
cb1a09d0 372Some users may wish to encourage the use of lexically scoped variables.
19799a22
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373As an aid to catching implicit uses to package variables,
374which are always global, if you say
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375
376 use strict 'vars';
377
19799a22
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378then any variable mentioned from there to the end of the enclosing
379block must either refer to a lexical variable, be predeclared via
77ca0c92 380C<our> or C<use vars>, or else must be fully qualified with the package name.
19799a22
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381A compilation error results otherwise. An inner block may countermand
382this with C<no strict 'vars'>.
383
384A C<my> has both a compile-time and a run-time effect. At compile
8593bda5 385time, the compiler takes notice of it. The principal usefulness
19799a22
GS
386of this is to quiet C<use strict 'vars'>, but it is also essential
387for generation of closures as detailed in L<perlref>. Actual
388initialization is delayed until run time, though, so it gets executed
389at the appropriate time, such as each time through a loop, for
390example.
391
392Variables declared with C<my> are not part of any package and are therefore
cb1a09d0
AD
393never fully qualified with the package name. In particular, you're not
394allowed to try to make a package variable (or other global) lexical:
395
396 my $pack::var; # ERROR! Illegal syntax
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397
398In fact, a dynamic variable (also known as package or global variables)
f86cebdf 399are still accessible using the fully qualified C<::> notation even while a
cb1a09d0
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400lexical of the same name is also visible:
401
402 package main;
403 local $x = 10;
404 my $x = 20;
405 print "$x and $::x\n";
406
f86cebdf 407That will print out C<20> and C<10>.
cb1a09d0 408
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409You may declare C<my> variables at the outermost scope of a file
410to hide any such identifiers from the world outside that file. This
411is similar in spirit to C's static variables when they are used at
412the file level. To do this with a subroutine requires the use of
413a closure (an anonymous function that accesses enclosing lexicals).
414If you want to create a private subroutine that cannot be called
415from outside that block, it can declare a lexical variable containing
416an anonymous sub reference:
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417
418 my $secret_version = '1.001-beta';
419 my $secret_sub = sub { print $secret_version };
420 &$secret_sub();
421
422As long as the reference is never returned by any function within the
5f05dabc 423module, no outside module can see the subroutine, because its name is not in
cb1a09d0 424any package's symbol table. Remember that it's not I<REALLY> called
19799a22 425C<$some_pack::secret_version> or anything; it's just $secret_version,
cb1a09d0
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426unqualified and unqualifiable.
427
19799a22
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428This does not work with object methods, however; all object methods
429have to be in the symbol table of some package to be found. See
430L<perlref/"Function Templates"> for something of a work-around to
431this.
cb1a09d0 432
c2611fb3 433=head2 Persistent Private Variables
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434X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure>
435
436There are two ways to build persistent private variables in Perl 5.10.
437First, you can simply use the C<state> feature. Or, you can use closures,
438if you want to stay compatible with releases older than 5.10.
439
440=head3 Persistent variables via state()
441
442Beginning with perl 5.9.4, you can declare variables with the C<state>
443keyword in place of C<my>. For that to work, though, you must have
444enabled that feature beforehand, either by using the C<feature> pragma, or
445by using C<-E> on one-liners. (see L<feature>)
446
447For example, the following code maintains a private counter, incremented
448each time the gimme_another() function is called:
449
450 use feature 'state';
451 sub gimme_another { state $x; return ++$x }
452
453Also, since C<$x> is lexical, it can't be reached or modified by any Perl
454code outside.
455
f292fc7a
RS
456When combined with variable declaration, simple scalar assignment to C<state>
457variables (as in C<state $x = 42>) is executed only the first time. When such
458statements are evaluated subsequent times, the assignment is ignored. The
459behavior of this sort of assignment to non-scalar variables is undefined.
ba1f8e91
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460
461=head3 Persistent variables with closures
5a964f20
TC
462
463Just because a lexical variable is lexically (also called statically)
f86cebdf 464scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
5a964f20
TC
465within a function it works like a C static. It normally works more
466like a C auto, but with implicit garbage collection.
467
468Unlike local variables in C or C++, Perl's lexical variables don't
469necessarily get recycled just because their scope has exited.
470If something more permanent is still aware of the lexical, it will
471stick around. So long as something else references a lexical, that
472lexical won't be freed--which is as it should be. You wouldn't want
473memory being free until you were done using it, or kept around once you
474were done. Automatic garbage collection takes care of this for you.
475
476This means that you can pass back or save away references to lexical
477variables, whereas to return a pointer to a C auto is a grave error.
478It also gives us a way to simulate C's function statics. Here's a
479mechanism for giving a function private variables with both lexical
480scoping and a static lifetime. If you do want to create something like
481C's static variables, just enclose the whole function in an extra block,
482and put the static variable outside the function but in the block.
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483
484 {
54310121 485 my $secret_val = 0;
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486 sub gimme_another {
487 return ++$secret_val;
54310121
PP
488 }
489 }
cb1a09d0
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490 # $secret_val now becomes unreachable by the outside
491 # world, but retains its value between calls to gimme_another
492
54310121 493If this function is being sourced in from a separate file
cb1a09d0 494via C<require> or C<use>, then this is probably just fine. If it's
19799a22 495all in the main program, you'll need to arrange for the C<my>
cb1a09d0 496to be executed early, either by putting the whole block above
f86cebdf 497your main program, or more likely, placing merely a C<BEGIN>
ac90fb77 498code block around it to make sure it gets executed before your program
cb1a09d0
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499starts to run:
500
ac90fb77 501 BEGIN {
54310121 502 my $secret_val = 0;
cb1a09d0
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503 sub gimme_another {
504 return ++$secret_val;
54310121
PP
505 }
506 }
cb1a09d0 507
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508See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the
509special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>,
510C<INIT> and C<END>.
cb1a09d0 511
19799a22
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512If declared at the outermost scope (the file scope), then lexicals
513work somewhat like C's file statics. They are available to all
514functions in that same file declared below them, but are inaccessible
515from outside that file. This strategy is sometimes used in modules
516to create private variables that the whole module can see.
5a964f20 517
cb1a09d0 518=head2 Temporary Values via local()
d74e8afc
ITB
519X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
520X<variable, temporary>
cb1a09d0 521
19799a22 522B<WARNING>: In general, you should be using C<my> instead of C<local>, because
6d28dffb 523it's faster and safer. Exceptions to this include the global punctuation
325192b1
RGS
524variables, global filehandles and formats, and direct manipulation of the
525Perl symbol table itself. C<local> is mostly used when the current value
526of a variable must be visible to called subroutines.
cb1a09d0
AD
527
528Synopsis:
529
325192b1
RGS
530 # localization of values
531
532 local $foo; # make $foo dynamically local
533 local (@wid, %get); # make list of variables local
534 local $foo = "flurp"; # make $foo dynamic, and init it
535 local @oof = @bar; # make @oof dynamic, and init it
536
537 local $hash{key} = "val"; # sets a local value for this hash entry
d361fafa 538 delete local $hash{key}; # delete this entry for the current block
325192b1
RGS
539 local ($cond ? $v1 : $v2); # several types of lvalues support
540 # localization
541
542 # localization of symbols
cb1a09d0
AD
543
544 local *FH; # localize $FH, @FH, %FH, &FH ...
545 local *merlyn = *randal; # now $merlyn is really $randal, plus
546 # @merlyn is really @randal, etc
547 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
54310121 548 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
cb1a09d0 549
19799a22
GS
550A C<local> modifies its listed variables to be "local" to the
551enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
552called from within that block>. A C<local> just gives temporary
553values to global (meaning package) variables. It does I<not> create
554a local variable. This is known as dynamic scoping. Lexical scoping
555is done with C<my>, which works more like C's auto declarations.
cb1a09d0 556
ceb12f1f 557Some types of lvalues can be localized as well: hash and array elements
325192b1
RGS
558and slices, conditionals (provided that their result is always
559localizable), and symbolic references. As for simple variables, this
560creates new, dynamically scoped values.
561
562If more than one variable or expression is given to C<local>, they must be
563placed in parentheses. This operator works
cb1a09d0 564by saving the current values of those variables in its argument list on a
5f05dabc 565hidden stack and restoring them upon exiting the block, subroutine, or
cb1a09d0
AD
566eval. This means that called subroutines can also reference the local
567variable, but not the global one. The argument list may be assigned to if
568desired, which allows you to initialize your local variables. (If no
569initializer is given for a particular variable, it is created with an
325192b1 570undefined value.)
cb1a09d0 571
19799a22 572Because C<local> is a run-time operator, it gets executed each time
325192b1
RGS
573through a loop. Consequently, it's more efficient to localize your
574variables outside the loop.
575
576=head3 Grammatical note on local()
d74e8afc 577X<local, context>
cb1a09d0 578
f86cebdf
GS
579A C<local> is simply a modifier on an lvalue expression. When you assign to
580a C<local>ized variable, the C<local> doesn't change whether its list is viewed
cb1a09d0
AD
581as a scalar or an array. So
582
583 local($foo) = <STDIN>;
584 local @FOO = <STDIN>;
585
5f05dabc 586both supply a list context to the right-hand side, while
cb1a09d0
AD
587
588 local $foo = <STDIN>;
589
590supplies a scalar context.
591
325192b1 592=head3 Localization of special variables
d74e8afc 593X<local, special variable>
3e3baf6d 594
325192b1
RGS
595If you localize a special variable, you'll be giving a new value to it,
596but its magic won't go away. That means that all side-effects related
597to this magic still work with the localized value.
3e3baf6d 598
325192b1
RGS
599This feature allows code like this to work :
600
601 # Read the whole contents of FILE in $slurp
602 { local $/ = undef; $slurp = <FILE>; }
603
604Note, however, that this restricts localization of some values ; for
605example, the following statement dies, as of perl 5.9.0, with an error
606I<Modification of a read-only value attempted>, because the $1 variable is
607magical and read-only :
608
609 local $1 = 2;
610
658a9f31
JD
611One exception is the default scalar variable: starting with perl 5.14
612C<local($_)> will always strip all magic from $_, to make it possible
613to safely reuse $_ in a subroutine.
325192b1
RGS
614
615B<WARNING>: Localization of tied arrays and hashes does not currently
616work as described.
fd5a896a
DM
617This will be fixed in a future release of Perl; in the meantime, avoid
618code that relies on any particular behaviour of localising tied arrays
619or hashes (localising individual elements is still okay).
325192b1 620See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
fd5a896a 621details.
d74e8afc 622X<local, tie>
fd5a896a 623
325192b1 624=head3 Localization of globs
d74e8afc 625X<local, glob> X<glob>
3e3baf6d 626
325192b1
RGS
627The construct
628
629 local *name;
630
631creates a whole new symbol table entry for the glob C<name> in the
632current package. That means that all variables in its glob slot ($name,
633@name, %name, &name, and the C<name> filehandle) are dynamically reset.
634
635This implies, among other things, that any magic eventually carried by
636those variables is locally lost. In other words, saying C<local */>
637will not have any effect on the internal value of the input record
638separator.
639
325192b1 640=head3 Localization of elements of composite types
d74e8afc 641X<local, composite type element> X<local, array element> X<local, hash element>
3e3baf6d 642
6ee623d5 643It's also worth taking a moment to explain what happens when you
f86cebdf
GS
644C<local>ize a member of a composite type (i.e. an array or hash element).
645In this case, the element is C<local>ized I<by name>. This means that
6ee623d5
GS
646when the scope of the C<local()> ends, the saved value will be
647restored to the hash element whose key was named in the C<local()>, or
648the array element whose index was named in the C<local()>. If that
649element was deleted while the C<local()> was in effect (e.g. by a
650C<delete()> from a hash or a C<shift()> of an array), it will spring
651back into existence, possibly extending an array and filling in the
652skipped elements with C<undef>. For instance, if you say
653
654 %hash = ( 'This' => 'is', 'a' => 'test' );
655 @ary = ( 0..5 );
656 {
657 local($ary[5]) = 6;
658 local($hash{'a'}) = 'drill';
659 while (my $e = pop(@ary)) {
660 print "$e . . .\n";
661 last unless $e > 3;
662 }
663 if (@ary) {
664 $hash{'only a'} = 'test';
665 delete $hash{'a'};
666 }
667 }
668 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
669 print "The array has ",scalar(@ary)," elements: ",
670 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
671
672Perl will print
673
674 6 . . .
675 4 . . .
676 3 . . .
677 This is a test only a test.
678 The array has 6 elements: 0, 1, 2, undef, undef, 5
679
19799a22 680The behavior of local() on non-existent members of composite
7185e5cc
GS
681types is subject to change in future.
682
d361fafa
VP
683=head3 Localized deletion of elements of composite types
684X<delete> X<local, composite type element> X<local, array element> X<local, hash element>
685
686You can use the C<delete local $array[$idx]> and C<delete local $hash{key}>
687constructs to delete a composite type entry for the current block and restore
688it when it ends. They return the array/hash value before the localization,
689which means that they are respectively equivalent to
690
691 do {
692 my $val = $array[$idx];
693 local $array[$idx];
694 delete $array[$idx];
695 $val
696 }
697
698and
699
700 do {
701 my $val = $hash{key};
702 local $hash{key};
703 delete $hash{key};
704 $val
705 }
706
707except that for those the C<local> is scoped to the C<do> block. Slices are
708also accepted.
709
710 my %hash = (
711 a => [ 7, 8, 9 ],
712 b => 1,
713 )
714
715 {
716 my $a = delete local $hash{a};
717 # $a is [ 7, 8, 9 ]
718 # %hash is (b => 1)
719
720 {
721 my @nums = delete local @$a[0, 2]
722 # @nums is (7, 9)
723 # $a is [ undef, 8 ]
724
725 $a[0] = 999; # will be erased when the scope ends
726 }
727 # $a is back to [ 7, 8, 9 ]
728
729 }
730 # %hash is back to its original state
731
cd06dffe 732=head2 Lvalue subroutines
d74e8afc 733X<lvalue> X<subroutine, lvalue>
cd06dffe 734
cd06dffe
GS
735It is possible to return a modifiable value from a subroutine.
736To do this, you have to declare the subroutine to return an lvalue.
737
738 my $val;
739 sub canmod : lvalue {
c72c0c0b 740 $val; # or "return $val;"
cd06dffe
GS
741 }
742 sub nomod {
743 $val;
744 }
745
746 canmod() = 5; # assigns to $val
747 nomod() = 5; # ERROR
748
749The scalar/list context for the subroutine and for the right-hand
750side of assignment is determined as if the subroutine call is replaced
751by a scalar. For example, consider:
752
753 data(2,3) = get_data(3,4);
754
755Both subroutines here are called in a scalar context, while in:
756
757 (data(2,3)) = get_data(3,4);
758
759and in:
760
761 (data(2),data(3)) = get_data(3,4);
762
763all the subroutines are called in a list context.
764
c72c0c0b
JV
765Lvalue subroutines are convenient, but there are some things to keep
766in mind.
e6a32221 767
c72c0c0b
JV
768When used with objects, they violate encapsulation. A normal mutator
769can check the supplied argument before setting the attribute it is
770protecting, an lvalue subroutine cannot.
818c4caa 771
c72c0c0b
JV
772Lvalue subroutines are most valuable to contruct simple data
773structures that do not require any special processing when storing and
774retrieving the values.
e6a32221 775
cb1a09d0 776=head2 Passing Symbol Table Entries (typeglobs)
d74e8afc 777X<typeglob> X<*>
cb1a09d0 778
19799a22
GS
779B<WARNING>: The mechanism described in this section was originally
780the only way to simulate pass-by-reference in older versions of
781Perl. While it still works fine in modern versions, the new reference
782mechanism is generally easier to work with. See below.
a0d0e21e
LW
783
784Sometimes you don't want to pass the value of an array to a subroutine
785but rather the name of it, so that the subroutine can modify the global
786copy of it rather than working with a local copy. In perl you can
cb1a09d0 787refer to all objects of a particular name by prefixing the name
5f05dabc 788with a star: C<*foo>. This is often known as a "typeglob", because the
a0d0e21e
LW
789star on the front can be thought of as a wildcard match for all the
790funny prefix characters on variables and subroutines and such.
791
55497cff 792When evaluated, the typeglob produces a scalar value that represents
5f05dabc 793all the objects of that name, including any filehandle, format, or
a0d0e21e 794subroutine. When assigned to, it causes the name mentioned to refer to
19799a22 795whatever C<*> value was assigned to it. Example:
a0d0e21e
LW
796
797 sub doubleary {
798 local(*someary) = @_;
799 foreach $elem (@someary) {
800 $elem *= 2;
801 }
802 }
803 doubleary(*foo);
804 doubleary(*bar);
805
19799a22 806Scalars are already passed by reference, so you can modify
a0d0e21e 807scalar arguments without using this mechanism by referring explicitly
1fef88e7 808to C<$_[0]> etc. You can modify all the elements of an array by passing
f86cebdf
GS
809all the elements as scalars, but you have to use the C<*> mechanism (or
810the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
a0d0e21e
LW
811an array. It will certainly be faster to pass the typeglob (or reference).
812
813Even if you don't want to modify an array, this mechanism is useful for
5f05dabc 814passing multiple arrays in a single LIST, because normally the LIST
a0d0e21e 815mechanism will merge all the array values so that you can't extract out
55497cff 816the individual arrays. For more on typeglobs, see
2ae324a7 817L<perldata/"Typeglobs and Filehandles">.
cb1a09d0 818
5a964f20 819=head2 When to Still Use local()
d74e8afc 820X<local> X<variable, local>
5a964f20 821
19799a22
GS
822Despite the existence of C<my>, there are still three places where the
823C<local> operator still shines. In fact, in these three places, you
5a964f20
TC
824I<must> use C<local> instead of C<my>.
825
13a2d996 826=over 4
5a964f20 827
551e1d92
RB
828=item 1.
829
830You need to give a global variable a temporary value, especially $_.
5a964f20 831
f86cebdf
GS
832The global variables, like C<@ARGV> or the punctuation variables, must be
833C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
5a964f20 834it up into chunks separated by lines of equal signs, which are placed
f86cebdf 835in C<@Fields>.
5a964f20
TC
836
837 {
838 local @ARGV = ("/etc/motd");
839 local $/ = undef;
840 local $_ = <>;
841 @Fields = split /^\s*=+\s*$/;
842 }
843
19799a22 844It particular, it's important to C<local>ize $_ in any routine that assigns
5a964f20
TC
845to it. Look out for implicit assignments in C<while> conditionals.
846
551e1d92
RB
847=item 2.
848
849You need to create a local file or directory handle or a local function.
5a964f20 850
09bef843
SB
851A function that needs a filehandle of its own must use
852C<local()> on a complete typeglob. This can be used to create new symbol
5a964f20
TC
853table entries:
854
855 sub ioqueue {
856 local (*READER, *WRITER); # not my!
17b63f68 857 pipe (READER, WRITER) or die "pipe: $!";
5a964f20
TC
858 return (*READER, *WRITER);
859 }
860 ($head, $tail) = ioqueue();
861
862See the Symbol module for a way to create anonymous symbol table
863entries.
864
865Because assignment of a reference to a typeglob creates an alias, this
866can be used to create what is effectively a local function, or at least,
867a local alias.
868
869 {
f86cebdf
GS
870 local *grow = \&shrink; # only until this block exists
871 grow(); # really calls shrink()
872 move(); # if move() grow()s, it shrink()s too
5a964f20 873 }
f86cebdf 874 grow(); # get the real grow() again
5a964f20
TC
875
876See L<perlref/"Function Templates"> for more about manipulating
877functions by name in this way.
878
551e1d92
RB
879=item 3.
880
881You want to temporarily change just one element of an array or hash.
5a964f20 882
f86cebdf 883You can C<local>ize just one element of an aggregate. Usually this
5a964f20
TC
884is done on dynamics:
885
886 {
887 local $SIG{INT} = 'IGNORE';
888 funct(); # uninterruptible
889 }
890 # interruptibility automatically restored here
891
892But it also works on lexically declared aggregates. Prior to 5.005,
893this operation could on occasion misbehave.
894
895=back
896
cb1a09d0 897=head2 Pass by Reference
d74e8afc 898X<pass by reference> X<pass-by-reference> X<reference>
cb1a09d0 899
55497cff
PP
900If you want to pass more than one array or hash into a function--or
901return them from it--and have them maintain their integrity, then
902you're going to have to use an explicit pass-by-reference. Before you
903do that, you need to understand references as detailed in L<perlref>.
c07a80fd 904This section may not make much sense to you otherwise.
cb1a09d0 905
19799a22
GS
906Here are a few simple examples. First, let's pass in several arrays
907to a function and have it C<pop> all of then, returning a new list
908of all their former last elements:
cb1a09d0
AD
909
910 @tailings = popmany ( \@a, \@b, \@c, \@d );
911
912 sub popmany {
913 my $aref;
914 my @retlist = ();
915 foreach $aref ( @_ ) {
916 push @retlist, pop @$aref;
54310121 917 }
cb1a09d0 918 return @retlist;
54310121 919 }
cb1a09d0 920
54310121 921Here's how you might write a function that returns a
cb1a09d0
AD
922list of keys occurring in all the hashes passed to it:
923
54310121 924 @common = inter( \%foo, \%bar, \%joe );
cb1a09d0
AD
925 sub inter {
926 my ($k, $href, %seen); # locals
927 foreach $href (@_) {
928 while ( $k = each %$href ) {
929 $seen{$k}++;
54310121
PP
930 }
931 }
cb1a09d0 932 return grep { $seen{$_} == @_ } keys %seen;
54310121 933 }
cb1a09d0 934
5f05dabc 935So far, we're using just the normal list return mechanism.
54310121
PP
936What happens if you want to pass or return a hash? Well,
937if you're using only one of them, or you don't mind them
cb1a09d0 938concatenating, then the normal calling convention is ok, although
54310121 939a little expensive.
cb1a09d0
AD
940
941Where people get into trouble is here:
942
943 (@a, @b) = func(@c, @d);
944or
945 (%a, %b) = func(%c, %d);
946
19799a22
GS
947That syntax simply won't work. It sets just C<@a> or C<%a> and
948clears the C<@b> or C<%b>. Plus the function didn't get passed
949into two separate arrays or hashes: it got one long list in C<@_>,
950as always.
cb1a09d0
AD
951
952If you can arrange for everyone to deal with this through references, it's
953cleaner code, although not so nice to look at. Here's a function that
954takes two array references as arguments, returning the two array elements
955in order of how many elements they have in them:
956
957 ($aref, $bref) = func(\@c, \@d);
958 print "@$aref has more than @$bref\n";
959 sub func {
960 my ($cref, $dref) = @_;
961 if (@$cref > @$dref) {
962 return ($cref, $dref);
963 } else {
c07a80fd 964 return ($dref, $cref);
54310121
PP
965 }
966 }
cb1a09d0
AD
967
968It turns out that you can actually do this also:
969
970 (*a, *b) = func(\@c, \@d);
971 print "@a has more than @b\n";
972 sub func {
973 local (*c, *d) = @_;
974 if (@c > @d) {
975 return (\@c, \@d);
976 } else {
977 return (\@d, \@c);
54310121
PP
978 }
979 }
cb1a09d0
AD
980
981Here we're using the typeglobs to do symbol table aliasing. It's
19799a22 982a tad subtle, though, and also won't work if you're using C<my>
09bef843 983variables, because only globals (even in disguise as C<local>s)
19799a22 984are in the symbol table.
5f05dabc
PP
985
986If you're passing around filehandles, you could usually just use the bare
19799a22
GS
987typeglob, like C<*STDOUT>, but typeglobs references work, too.
988For example:
5f05dabc
PP
989
990 splutter(\*STDOUT);
991 sub splutter {
992 my $fh = shift;
993 print $fh "her um well a hmmm\n";
994 }
995
996 $rec = get_rec(\*STDIN);
997 sub get_rec {
998 my $fh = shift;
999 return scalar <$fh>;
1000 }
1001
19799a22
GS
1002If you're planning on generating new filehandles, you could do this.
1003Notice to pass back just the bare *FH, not its reference.
5f05dabc
PP
1004
1005 sub openit {
19799a22 1006 my $path = shift;
5f05dabc 1007 local *FH;
e05a3a1e 1008 return open (FH, $path) ? *FH : undef;
54310121 1009 }
5f05dabc 1010
cb1a09d0 1011=head2 Prototypes
d74e8afc 1012X<prototype> X<subroutine, prototype>
cb1a09d0 1013
19799a22
GS
1014Perl supports a very limited kind of compile-time argument checking
1015using function prototyping. If you declare
cb1a09d0 1016
cba5a3b0 1017 sub mypush (+@)
cb1a09d0 1018
19799a22
GS
1019then C<mypush()> takes arguments exactly like C<push()> does. The
1020function declaration must be visible at compile time. The prototype
1021affects only interpretation of new-style calls to the function,
1022where new-style is defined as not using the C<&> character. In
1023other words, if you call it like a built-in function, then it behaves
1024like a built-in function. If you call it like an old-fashioned
1025subroutine, then it behaves like an old-fashioned subroutine. It
1026naturally falls out from this rule that prototypes have no influence
1027on subroutine references like C<\&foo> or on indirect subroutine
c47ff5f1 1028calls like C<&{$subref}> or C<< $subref->() >>.
c07a80fd
PP
1029
1030Method calls are not influenced by prototypes either, because the
19799a22
GS
1031function to be called is indeterminate at compile time, since
1032the exact code called depends on inheritance.
cb1a09d0 1033
19799a22
GS
1034Because the intent of this feature is primarily to let you define
1035subroutines that work like built-in functions, here are prototypes
1036for some other functions that parse almost exactly like the
1037corresponding built-in.
cb1a09d0
AD
1038
1039 Declared as Called as
1040
f86cebdf
GS
1041 sub mylink ($$) mylink $old, $new
1042 sub myvec ($$$) myvec $var, $offset, 1
1043 sub myindex ($$;$) myindex &getstring, "substr"
1044 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
1045 sub myreverse (@) myreverse $a, $b, $c
1046 sub myjoin ($@) myjoin ":", $a, $b, $c
cba5a3b0
DG
1047 sub mypop (+) mypop @array
1048 sub mysplice (+$$@) mysplice @array, 0, 2, @pushme
1049 sub mykeys (+) mykeys %{$hashref}
f86cebdf
GS
1050 sub myopen (*;$) myopen HANDLE, $name
1051 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
1052 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
d822fdf9 1053 sub myrand (;$) myrand 42
f86cebdf 1054 sub mytime () mytime
cb1a09d0 1055
c07a80fd 1056Any backslashed prototype character represents an actual argument
ae7a3cfa
FC
1057that must start with that character (optionally preceded by C<my>,
1058C<our> or C<local>), with the exception of C<$>, which will accept a
1059hash or array element even without a dollar sign, such as
74083ec6 1060C<< my_function()->[0] >>. The value passed as part of C<@_> will be a
ae7a3cfa
FC
1061reference to the actual argument given in the subroutine call,
1062obtained by applying C<\> to that argument.
c07a80fd 1063
c035a075
DG
1064You can use the C<\[]> backslash group notation to specify more than one
1065allowed argument type. For example:
5b794e05
JH
1066
1067 sub myref (\[$@%&*])
1068
1069will allow calling myref() as
1070
1071 myref $var
1072 myref @array
1073 myref %hash
1074 myref &sub
1075 myref *glob
1076
1077and the first argument of myref() will be a reference to
1078a scalar, an array, a hash, a code, or a glob.
1079
c07a80fd 1080Unbackslashed prototype characters have special meanings. Any
19799a22 1081unbackslashed C<@> or C<%> eats all remaining arguments, and forces
f86cebdf
GS
1082list context. An argument represented by C<$> forces scalar context. An
1083C<&> requires an anonymous subroutine, which, if passed as the first
0df79f0c
GS
1084argument, does not require the C<sub> keyword or a subsequent comma.
1085
1086A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
648ca4f7
GS
1087typeglob, or a reference to a typeglob in that slot. The value will be
1088available to the subroutine either as a simple scalar, or (in the latter
0df79f0c
GS
1089two cases) as a reference to the typeglob. If you wish to always convert
1090such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1091follows:
1092
1093 use Symbol 'qualify_to_ref';
1094
1095 sub foo (*) {
1096 my $fh = qualify_to_ref(shift, caller);
1097 ...
1098 }
c07a80fd 1099
c035a075
DG
1100The C<+> prototype is a special alternative to C<$> that will act like
1101C<\[@%]> when given a literal array or hash variable, but will otherwise
1102force scalar context on the argument. This is useful for functions which
1103should accept either a literal array or an array reference as the argument:
1104
cba5a3b0 1105 sub mypush (+@) {
c035a075
DG
1106 my $aref = shift;
1107 die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
1108 push @$aref, @_;
1109 }
1110
1111When using the C<+> prototype, your function must check that the argument
1112is of an acceptable type.
1113
859a4967 1114A semicolon (C<;>) separates mandatory arguments from optional arguments.
19799a22 1115It is redundant before C<@> or C<%>, which gobble up everything else.
cb1a09d0 1116
7adf2bcd
RGS
1117As the last character of a prototype, or just before a semicolon, you can
1118use C<_> in place of C<$>: if this argument is not provided, C<$_> will be
1119used instead.
859a4967 1120
19799a22
GS
1121Note how the last three examples in the table above are treated
1122specially by the parser. C<mygrep()> is parsed as a true list
1123operator, C<myrand()> is parsed as a true unary operator with unary
1124precedence the same as C<rand()>, and C<mytime()> is truly without
1125arguments, just like C<time()>. That is, if you say
cb1a09d0
AD
1126
1127 mytime +2;
1128
f86cebdf 1129you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
19799a22 1130without a prototype.
cb1a09d0 1131
19799a22
GS
1132The interesting thing about C<&> is that you can generate new syntax with it,
1133provided it's in the initial position:
d74e8afc 1134X<&>
cb1a09d0 1135
6d28dffb 1136 sub try (&@) {
cb1a09d0
AD
1137 my($try,$catch) = @_;
1138 eval { &$try };
1139 if ($@) {
1140 local $_ = $@;
1141 &$catch;
1142 }
1143 }
55497cff 1144 sub catch (&) { $_[0] }
cb1a09d0
AD
1145
1146 try {
1147 die "phooey";
1148 } catch {
1149 /phooey/ and print "unphooey\n";
1150 };
1151
f86cebdf 1152That prints C<"unphooey">. (Yes, there are still unresolved
19799a22 1153issues having to do with visibility of C<@_>. I'm ignoring that
f86cebdf 1154question for the moment. (But note that if we make C<@_> lexically
cb1a09d0 1155scoped, those anonymous subroutines can act like closures... (Gee,
5f05dabc 1156is this sounding a little Lispish? (Never mind.))))
cb1a09d0 1157
19799a22 1158And here's a reimplementation of the Perl C<grep> operator:
d74e8afc 1159X<grep>
cb1a09d0
AD
1160
1161 sub mygrep (&@) {
1162 my $code = shift;
1163 my @result;
1164 foreach $_ (@_) {
6e47f808 1165 push(@result, $_) if &$code;
cb1a09d0
AD
1166 }
1167 @result;
1168 }
a0d0e21e 1169
cb1a09d0
AD
1170Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1171been intentionally left out of prototypes for the express purpose of
1172someday in the future adding named, formal parameters. The current
1173mechanism's main goal is to let module writers provide better diagnostics
1174for module users. Larry feels the notation quite understandable to Perl
1175programmers, and that it will not intrude greatly upon the meat of the
1176module, nor make it harder to read. The line noise is visually
1177encapsulated into a small pill that's easy to swallow.
1178
420cdfc1
ST
1179If you try to use an alphanumeric sequence in a prototype you will
1180generate an optional warning - "Illegal character in prototype...".
1181Unfortunately earlier versions of Perl allowed the prototype to be
1182used as long as its prefix was a valid prototype. The warning may be
1183upgraded to a fatal error in a future version of Perl once the
1184majority of offending code is fixed.
1185
cb1a09d0
AD
1186It's probably best to prototype new functions, not retrofit prototyping
1187into older ones. That's because you must be especially careful about
1188silent impositions of differing list versus scalar contexts. For example,
1189if you decide that a function should take just one parameter, like this:
1190
1191 sub func ($) {
1192 my $n = shift;
1193 print "you gave me $n\n";
54310121 1194 }
cb1a09d0
AD
1195
1196and someone has been calling it with an array or expression
1197returning a list:
1198
1199 func(@foo);
1200 func( split /:/ );
1201
19799a22 1202Then you've just supplied an automatic C<scalar> in front of their
f86cebdf 1203argument, which can be more than a bit surprising. The old C<@foo>
cb1a09d0 1204which used to hold one thing doesn't get passed in. Instead,
19799a22
GS
1205C<func()> now gets passed in a C<1>; that is, the number of elements
1206in C<@foo>. And the C<split> gets called in scalar context so it
1207starts scribbling on your C<@_> parameter list. Ouch!
cb1a09d0 1208
5f05dabc 1209This is all very powerful, of course, and should be used only in moderation
54310121 1210to make the world a better place.
44a8e56a
PP
1211
1212=head2 Constant Functions
d74e8afc 1213X<constant>
44a8e56a
PP
1214
1215Functions with a prototype of C<()> are potential candidates for
19799a22
GS
1216inlining. If the result after optimization and constant folding
1217is either a constant or a lexically-scoped scalar which has no other
54310121 1218references, then it will be used in place of function calls made
19799a22
GS
1219without C<&>. Calls made using C<&> are never inlined. (See
1220F<constant.pm> for an easy way to declare most constants.)
44a8e56a 1221
5a964f20 1222The following functions would all be inlined:
44a8e56a 1223
699e6cd4
TP
1224 sub pi () { 3.14159 } # Not exact, but close.
1225 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1226 # and it's inlined, too!
44a8e56a
PP
1227 sub ST_DEV () { 0 }
1228 sub ST_INO () { 1 }
1229
1230 sub FLAG_FOO () { 1 << 8 }
1231 sub FLAG_BAR () { 1 << 9 }
1232 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
54310121
PP
1233
1234 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
88267271 1235
1236 sub N () { int(OPT_BAZ) / 3 }
1237
1238 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1239
1240Be aware that these will not be inlined; as they contain inner scopes,
1241the constant folding doesn't reduce them to a single constant:
1242
1243 sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1244
1245 sub baz_val () {
44a8e56a
PP
1246 if (OPT_BAZ) {
1247 return 23;
1248 }
1249 else {
1250 return 42;
1251 }
1252 }
cb1a09d0 1253
5a964f20 1254If you redefine a subroutine that was eligible for inlining, you'll get
4cee8e80
CS
1255a mandatory warning. (You can use this warning to tell whether or not a
1256particular subroutine is considered constant.) The warning is
1257considered severe enough not to be optional because previously compiled
1258invocations of the function will still be using the old value of the
19799a22 1259function. If you need to be able to redefine the subroutine, you need to
4cee8e80 1260ensure that it isn't inlined, either by dropping the C<()> prototype
19799a22 1261(which changes calling semantics, so beware) or by thwarting the
4cee8e80
CS
1262inlining mechanism in some other way, such as
1263
4cee8e80 1264 sub not_inlined () {
54310121 1265 23 if $];
4cee8e80
CS
1266 }
1267
19799a22 1268=head2 Overriding Built-in Functions
d74e8afc 1269X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
a0d0e21e 1270
19799a22 1271Many built-in functions may be overridden, though this should be tried
5f05dabc 1272only occasionally and for good reason. Typically this might be
19799a22 1273done by a package attempting to emulate missing built-in functionality
a0d0e21e
LW
1274on a non-Unix system.
1275
163e3a99
JP
1276Overriding may be done only by importing the name from a module at
1277compile time--ordinary predeclaration isn't good enough. However, the
19799a22
GS
1278C<use subs> pragma lets you, in effect, predeclare subs
1279via the import syntax, and these names may then override built-in ones:
a0d0e21e
LW
1280
1281 use subs 'chdir', 'chroot', 'chmod', 'chown';
1282 chdir $somewhere;
1283 sub chdir { ... }
1284
19799a22
GS
1285To unambiguously refer to the built-in form, precede the
1286built-in name with the special package qualifier C<CORE::>. For example,
1287saying C<CORE::open()> always refers to the built-in C<open()>, even
fb73857a 1288if the current package has imported some other subroutine called
19799a22 1289C<&open()> from elsewhere. Even though it looks like a regular
09bef843 1290function call, it isn't: you can't take a reference to it, such as
19799a22 1291the incorrect C<\&CORE::open> might appear to produce.
fb73857a 1292
19799a22
GS
1293Library modules should not in general export built-in names like C<open>
1294or C<chdir> as part of their default C<@EXPORT> list, because these may
a0d0e21e 1295sneak into someone else's namespace and change the semantics unexpectedly.
19799a22 1296Instead, if the module adds that name to C<@EXPORT_OK>, then it's
a0d0e21e
LW
1297possible for a user to import the name explicitly, but not implicitly.
1298That is, they could say
1299
1300 use Module 'open';
1301
19799a22 1302and it would import the C<open> override. But if they said
a0d0e21e
LW
1303
1304 use Module;
1305
19799a22 1306they would get the default imports without overrides.
a0d0e21e 1307
19799a22 1308The foregoing mechanism for overriding built-in is restricted, quite
95d94a4f 1309deliberately, to the package that requests the import. There is a second
19799a22 1310method that is sometimes applicable when you wish to override a built-in
95d94a4f
GS
1311everywhere, without regard to namespace boundaries. This is achieved by
1312importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1313example that quite brazenly replaces the C<glob> operator with something
1314that understands regular expressions.
1315
1316 package REGlob;
1317 require Exporter;
1318 @ISA = 'Exporter';
1319 @EXPORT_OK = 'glob';
1320
1321 sub import {
1322 my $pkg = shift;
1323 return unless @_;
1324 my $sym = shift;
1325 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1326 $pkg->export($where, $sym, @_);
1327 }
1328
1329 sub glob {
1330 my $pat = shift;
1331 my @got;
7b815c67
RGS
1332 if (opendir my $d, '.') {
1333 @got = grep /$pat/, readdir $d;
1334 closedir $d;
19799a22
GS
1335 }
1336 return @got;
95d94a4f
GS
1337 }
1338 1;
1339
1340And here's how it could be (ab)used:
1341
1342 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1343 package Foo;
1344 use REGlob 'glob'; # override glob() in Foo:: only
1345 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1346
19799a22 1347The initial comment shows a contrived, even dangerous example.
95d94a4f 1348By overriding C<glob> globally, you would be forcing the new (and
19799a22 1349subversive) behavior for the C<glob> operator for I<every> namespace,
95d94a4f
GS
1350without the complete cognizance or cooperation of the modules that own
1351those namespaces. Naturally, this should be done with extreme caution--if
1352it must be done at all.
1353
1354The C<REGlob> example above does not implement all the support needed to
19799a22 1355cleanly override perl's C<glob> operator. The built-in C<glob> has
95d94a4f 1356different behaviors depending on whether it appears in a scalar or list
19799a22 1357context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
95d94a4f
GS
1358context sensitive behaviors, and these must be adequately supported by
1359a properly written override. For a fully functional example of overriding
1360C<glob>, study the implementation of C<File::DosGlob> in the standard
1361library.
1362
77bc9082
RGS
1363When you override a built-in, your replacement should be consistent (if
1364possible) with the built-in native syntax. You can achieve this by using
1365a suitable prototype. To get the prototype of an overridable built-in,
1366use the C<prototype> function with an argument of C<"CORE::builtin_name">
1367(see L<perlfunc/prototype>).
1368
1369Note however that some built-ins can't have their syntax expressed by a
1370prototype (such as C<system> or C<chomp>). If you override them you won't
1371be able to fully mimic their original syntax.
1372
fe854a6f 1373The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
77bc9082
RGS
1374to special magic, their original syntax is preserved, and you don't have
1375to define a prototype for their replacements. (You can't override the
1376C<do BLOCK> syntax, though).
1377
1378C<require> has special additional dark magic: if you invoke your
1379C<require> replacement as C<require Foo::Bar>, it will actually receive
1380the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1381
1382And, as you'll have noticed from the previous example, if you override
593b9c14 1383C<glob>, the C<< <*> >> glob operator is overridden as well.
77bc9082 1384
9b3023bc 1385In a similar fashion, overriding the C<readline> function also overrides
e3f73d4e
RGS
1386the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding
1387C<readpipe> also overrides the operators C<``> and C<qx//>.
9b3023bc 1388
fe854a6f 1389Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
77bc9082 1390
a0d0e21e 1391=head2 Autoloading
d74e8afc 1392X<autoloading> X<AUTOLOAD>
a0d0e21e 1393
19799a22
GS
1394If you call a subroutine that is undefined, you would ordinarily
1395get an immediate, fatal error complaining that the subroutine doesn't
1396exist. (Likewise for subroutines being used as methods, when the
1397method doesn't exist in any base class of the class's package.)
1398However, if an C<AUTOLOAD> subroutine is defined in the package or
1399packages used to locate the original subroutine, then that
1400C<AUTOLOAD> subroutine is called with the arguments that would have
1401been passed to the original subroutine. The fully qualified name
1402of the original subroutine magically appears in the global $AUTOLOAD
1403variable of the same package as the C<AUTOLOAD> routine. The name
1404is not passed as an ordinary argument because, er, well, just
593b9c14 1405because, that's why. (As an exception, a method call to a nonexistent
80ee23cd
RH
1406C<import> or C<unimport> method is just skipped instead. Also, if
1407the AUTOLOAD subroutine is an XSUB, C<$AUTOLOAD> is not populated;
1408instead, you should call L<< C<SvPVX>E<sol>C<SvCUR>|perlapi >> on the
1409C<CV> for C<AUTOLOAD> to retrieve the method name.)
1410
19799a22
GS
1411
1412Many C<AUTOLOAD> routines load in a definition for the requested
1413subroutine using eval(), then execute that subroutine using a special
1414form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1415without a trace. (See the source to the standard module documented
1416in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1417also just emulate the routine and never define it. For example,
1418let's pretend that a function that wasn't defined should just invoke
1419C<system> with those arguments. All you'd do is:
cb1a09d0
AD
1420
1421 sub AUTOLOAD {
1422 my $program = $AUTOLOAD;
1423 $program =~ s/.*:://;
1424 system($program, @_);
54310121 1425 }
cb1a09d0 1426 date();
6d28dffb 1427 who('am', 'i');
cb1a09d0
AD
1428 ls('-l');
1429
19799a22
GS
1430In fact, if you predeclare functions you want to call that way, you don't
1431even need parentheses:
cb1a09d0
AD
1432
1433 use subs qw(date who ls);
1434 date;
1435 who "am", "i";
593b9c14 1436 ls '-l';
cb1a09d0
AD
1437
1438A more complete example of this is the standard Shell module, which
19799a22 1439can treat undefined subroutine calls as calls to external programs.
a0d0e21e 1440
19799a22
GS
1441Mechanisms are available to help modules writers split their modules
1442into autoloadable files. See the standard AutoLoader module
6d28dffb
PP
1443described in L<AutoLoader> and in L<AutoSplit>, the standard
1444SelfLoader modules in L<SelfLoader>, and the document on adding C
19799a22 1445functions to Perl code in L<perlxs>.
cb1a09d0 1446
09bef843 1447=head2 Subroutine Attributes
d74e8afc 1448X<attribute> X<subroutine, attribute> X<attrs>
09bef843
SB
1449
1450A subroutine declaration or definition may have a list of attributes
1451associated with it. If such an attribute list is present, it is
0120eecf 1452broken up at space or colon boundaries and treated as though a
09bef843
SB
1453C<use attributes> had been seen. See L<attributes> for details
1454about what attributes are currently supported.
1455Unlike the limitation with the obsolescent C<use attrs>, the
1456C<sub : ATTRLIST> syntax works to associate the attributes with
1457a pre-declaration, and not just with a subroutine definition.
1458
1459The attributes must be valid as simple identifier names (without any
1460punctuation other than the '_' character). They may have a parameter
1461list appended, which is only checked for whether its parentheses ('(',')')
1462nest properly.
1463
1464Examples of valid syntax (even though the attributes are unknown):
1465
4358a253
SS
1466 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1467 sub plugh () : Ugly('\(") :Bad;
09bef843
SB
1468 sub xyzzy : _5x5 { ... }
1469
1470Examples of invalid syntax:
1471
4358a253
SS
1472 sub fnord : switch(10,foo(); # ()-string not balanced
1473 sub snoid : Ugly('('); # ()-string not balanced
1474 sub xyzzy : 5x5; # "5x5" not a valid identifier
1475 sub plugh : Y2::north; # "Y2::north" not a simple identifier
1476 sub snurt : foo + bar; # "+" not a colon or space
09bef843
SB
1477
1478The attribute list is passed as a list of constant strings to the code
1479which associates them with the subroutine. In particular, the second example
1480of valid syntax above currently looks like this in terms of how it's
1481parsed and invoked:
1482
1483 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1484
1485For further details on attribute lists and their manipulation,
a0ae32d3 1486see L<attributes> and L<Attribute::Handlers>.
09bef843 1487
cb1a09d0 1488=head1 SEE ALSO
a0d0e21e 1489
19799a22
GS
1490See L<perlref/"Function Templates"> for more about references and closures.
1491See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
a2293a43 1492See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
19799a22
GS
1493See L<perlmod> to learn about bundling up your functions in separate files.
1494See L<perlmodlib> to learn what library modules come standard on your system.
1495See L<perltoot> to learn how to make object method calls.