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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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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
19799a22 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
230The C<BEGIN>, C<CHECK>, C<INIT> and C<END> subroutines are not so much
231subroutines as named special code blocks, of which you can have more
fa11829f 232than one in a package, and which you can B<not> call explicitly. See
ac90fb77 233L<perlmod/"BEGIN, 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 = <>) {
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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
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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,
457b36cb
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354but not beyond it. See L<perlsyn/"Simple statements"> for information
355on the scope of variables in statements with modifiers.
55497cff 356
5f05dabc 357The C<foreach> loop defaults to scoping its index variable dynamically
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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>
55497cff
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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
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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
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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,
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426unqualified and unqualifiable.
427
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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
456You can initialize state variables, and the assigment will be executed
457only once:
458
459 sub starts_from_42 { state $x = 42; return ++$x }
460
461You can also, as a syntactic shortcut, initialize more than one if they're
462all declared within the same state() clause:
463
464 state ($a, $b, $c) = ( 'one', 'two', 'three' );
465
466However, be warned that state variables declared as part of a list will
467get assigned each time the statement will be executed, since it will be
468considered as a regular list assigment, not one to be executed only once:
469
470 (state $x, my $y) = (1, 2); # $x gets reinitialized every time !
471
472B<Caveat>: the code at the right side of the assignment to a state
473variable will be executed every time; only the assignment is disabled. So,
474avoid code that has side-effects, or that is slow to execute. This might
475be optimized out in a future version of Perl.
476
477=head3 Persistent variables with closures
5a964f20
TC
478
479Just because a lexical variable is lexically (also called statically)
f86cebdf 480scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
5a964f20
TC
481within a function it works like a C static. It normally works more
482like a C auto, but with implicit garbage collection.
483
484Unlike local variables in C or C++, Perl's lexical variables don't
485necessarily get recycled just because their scope has exited.
486If something more permanent is still aware of the lexical, it will
487stick around. So long as something else references a lexical, that
488lexical won't be freed--which is as it should be. You wouldn't want
489memory being free until you were done using it, or kept around once you
490were done. Automatic garbage collection takes care of this for you.
491
492This means that you can pass back or save away references to lexical
493variables, whereas to return a pointer to a C auto is a grave error.
494It also gives us a way to simulate C's function statics. Here's a
495mechanism for giving a function private variables with both lexical
496scoping and a static lifetime. If you do want to create something like
497C's static variables, just enclose the whole function in an extra block,
498and put the static variable outside the function but in the block.
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499
500 {
54310121 501 my $secret_val = 0;
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502 sub gimme_another {
503 return ++$secret_val;
54310121
PP
504 }
505 }
cb1a09d0
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506 # $secret_val now becomes unreachable by the outside
507 # world, but retains its value between calls to gimme_another
508
54310121 509If this function is being sourced in from a separate file
cb1a09d0 510via C<require> or C<use>, then this is probably just fine. If it's
19799a22 511all in the main program, you'll need to arrange for the C<my>
cb1a09d0 512to be executed early, either by putting the whole block above
f86cebdf 513your main program, or more likely, placing merely a C<BEGIN>
ac90fb77 514code block around it to make sure it gets executed before your program
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515starts to run:
516
ac90fb77 517 BEGIN {
54310121 518 my $secret_val = 0;
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519 sub gimme_another {
520 return ++$secret_val;
54310121
PP
521 }
522 }
cb1a09d0 523
ac90fb77
EM
524See L<perlmod/"BEGIN, CHECK, INIT and END"> about the
525special triggered code blocks, C<BEGIN>, C<CHECK>, C<INIT> and C<END>.
cb1a09d0 526
19799a22
GS
527If declared at the outermost scope (the file scope), then lexicals
528work somewhat like C's file statics. They are available to all
529functions in that same file declared below them, but are inaccessible
530from outside that file. This strategy is sometimes used in modules
531to create private variables that the whole module can see.
5a964f20 532
cb1a09d0 533=head2 Temporary Values via local()
d74e8afc
ITB
534X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
535X<variable, temporary>
cb1a09d0 536
19799a22 537B<WARNING>: In general, you should be using C<my> instead of C<local>, because
6d28dffb 538it's faster and safer. Exceptions to this include the global punctuation
325192b1
RGS
539variables, global filehandles and formats, and direct manipulation of the
540Perl symbol table itself. C<local> is mostly used when the current value
541of a variable must be visible to called subroutines.
cb1a09d0
AD
542
543Synopsis:
544
325192b1
RGS
545 # localization of values
546
547 local $foo; # make $foo dynamically local
548 local (@wid, %get); # make list of variables local
549 local $foo = "flurp"; # make $foo dynamic, and init it
550 local @oof = @bar; # make @oof dynamic, and init it
551
552 local $hash{key} = "val"; # sets a local value for this hash entry
553 local ($cond ? $v1 : $v2); # several types of lvalues support
554 # localization
555
556 # localization of symbols
cb1a09d0
AD
557
558 local *FH; # localize $FH, @FH, %FH, &FH ...
559 local *merlyn = *randal; # now $merlyn is really $randal, plus
560 # @merlyn is really @randal, etc
561 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
54310121 562 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
cb1a09d0 563
19799a22
GS
564A C<local> modifies its listed variables to be "local" to the
565enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
566called from within that block>. A C<local> just gives temporary
567values to global (meaning package) variables. It does I<not> create
568a local variable. This is known as dynamic scoping. Lexical scoping
569is done with C<my>, which works more like C's auto declarations.
cb1a09d0 570
325192b1
RGS
571Some types of lvalues can be localized as well : hash and array elements
572and slices, conditionals (provided that their result is always
573localizable), and symbolic references. As for simple variables, this
574creates new, dynamically scoped values.
575
576If more than one variable or expression is given to C<local>, they must be
577placed in parentheses. This operator works
cb1a09d0 578by saving the current values of those variables in its argument list on a
5f05dabc 579hidden stack and restoring them upon exiting the block, subroutine, or
cb1a09d0
AD
580eval. This means that called subroutines can also reference the local
581variable, but not the global one. The argument list may be assigned to if
582desired, which allows you to initialize your local variables. (If no
583initializer is given for a particular variable, it is created with an
325192b1 584undefined value.)
cb1a09d0 585
19799a22 586Because C<local> is a run-time operator, it gets executed each time
325192b1
RGS
587through a loop. Consequently, it's more efficient to localize your
588variables outside the loop.
589
590=head3 Grammatical note on local()
d74e8afc 591X<local, context>
cb1a09d0 592
f86cebdf
GS
593A C<local> is simply a modifier on an lvalue expression. When you assign to
594a C<local>ized variable, the C<local> doesn't change whether its list is viewed
cb1a09d0
AD
595as a scalar or an array. So
596
597 local($foo) = <STDIN>;
598 local @FOO = <STDIN>;
599
5f05dabc 600both supply a list context to the right-hand side, while
cb1a09d0
AD
601
602 local $foo = <STDIN>;
603
604supplies a scalar context.
605
325192b1 606=head3 Localization of special variables
d74e8afc 607X<local, special variable>
3e3baf6d 608
325192b1
RGS
609If you localize a special variable, you'll be giving a new value to it,
610but its magic won't go away. That means that all side-effects related
611to this magic still work with the localized value.
3e3baf6d 612
325192b1
RGS
613This feature allows code like this to work :
614
615 # Read the whole contents of FILE in $slurp
616 { local $/ = undef; $slurp = <FILE>; }
617
618Note, however, that this restricts localization of some values ; for
619example, the following statement dies, as of perl 5.9.0, with an error
620I<Modification of a read-only value attempted>, because the $1 variable is
621magical and read-only :
622
623 local $1 = 2;
624
625Similarly, but in a way more difficult to spot, the following snippet will
626die in perl 5.9.0 :
627
628 sub f { local $_ = "foo"; print }
629 for ($1) {
630 # now $_ is aliased to $1, thus is magic and readonly
631 f();
3e3baf6d 632 }
3e3baf6d 633
325192b1
RGS
634See next section for an alternative to this situation.
635
636B<WARNING>: Localization of tied arrays and hashes does not currently
637work as described.
fd5a896a
DM
638This will be fixed in a future release of Perl; in the meantime, avoid
639code that relies on any particular behaviour of localising tied arrays
640or hashes (localising individual elements is still okay).
325192b1 641See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
fd5a896a 642details.
d74e8afc 643X<local, tie>
fd5a896a 644
325192b1 645=head3 Localization of globs
d74e8afc 646X<local, glob> X<glob>
3e3baf6d 647
325192b1
RGS
648The construct
649
650 local *name;
651
652creates a whole new symbol table entry for the glob C<name> in the
653current package. That means that all variables in its glob slot ($name,
654@name, %name, &name, and the C<name> filehandle) are dynamically reset.
655
656This implies, among other things, that any magic eventually carried by
657those variables is locally lost. In other words, saying C<local */>
658will not have any effect on the internal value of the input record
659separator.
660
661Notably, if you want to work with a brand new value of the default scalar
662$_, and avoid the potential problem listed above about $_ previously
663carrying a magic value, you should use C<local *_> instead of C<local $_>.
a4fb8298
RGS
664As of perl 5.9.1, you can also use the lexical form of C<$_> (declaring it
665with C<my $_>), which avoids completely this problem.
325192b1
RGS
666
667=head3 Localization of elements of composite types
d74e8afc 668X<local, composite type element> X<local, array element> X<local, hash element>
3e3baf6d 669
6ee623d5 670It's also worth taking a moment to explain what happens when you
f86cebdf
GS
671C<local>ize a member of a composite type (i.e. an array or hash element).
672In this case, the element is C<local>ized I<by name>. This means that
6ee623d5
GS
673when the scope of the C<local()> ends, the saved value will be
674restored to the hash element whose key was named in the C<local()>, or
675the array element whose index was named in the C<local()>. If that
676element was deleted while the C<local()> was in effect (e.g. by a
677C<delete()> from a hash or a C<shift()> of an array), it will spring
678back into existence, possibly extending an array and filling in the
679skipped elements with C<undef>. For instance, if you say
680
681 %hash = ( 'This' => 'is', 'a' => 'test' );
682 @ary = ( 0..5 );
683 {
684 local($ary[5]) = 6;
685 local($hash{'a'}) = 'drill';
686 while (my $e = pop(@ary)) {
687 print "$e . . .\n";
688 last unless $e > 3;
689 }
690 if (@ary) {
691 $hash{'only a'} = 'test';
692 delete $hash{'a'};
693 }
694 }
695 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
696 print "The array has ",scalar(@ary)," elements: ",
697 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
698
699Perl will print
700
701 6 . . .
702 4 . . .
703 3 . . .
704 This is a test only a test.
705 The array has 6 elements: 0, 1, 2, undef, undef, 5
706
19799a22 707The behavior of local() on non-existent members of composite
7185e5cc
GS
708types is subject to change in future.
709
cd06dffe 710=head2 Lvalue subroutines
d74e8afc 711X<lvalue> X<subroutine, lvalue>
cd06dffe 712
e6a32221
JC
713B<WARNING>: Lvalue subroutines are still experimental and the
714implementation may change in future versions of Perl.
cd06dffe
GS
715
716It is possible to return a modifiable value from a subroutine.
717To do this, you have to declare the subroutine to return an lvalue.
718
719 my $val;
720 sub canmod : lvalue {
e6a32221 721 # return $val; this doesn't work, don't say "return"
cd06dffe
GS
722 $val;
723 }
724 sub nomod {
725 $val;
726 }
727
728 canmod() = 5; # assigns to $val
729 nomod() = 5; # ERROR
730
731The scalar/list context for the subroutine and for the right-hand
732side of assignment is determined as if the subroutine call is replaced
733by a scalar. For example, consider:
734
735 data(2,3) = get_data(3,4);
736
737Both subroutines here are called in a scalar context, while in:
738
739 (data(2,3)) = get_data(3,4);
740
741and in:
742
743 (data(2),data(3)) = get_data(3,4);
744
745all the subroutines are called in a list context.
746
e6a32221
JC
747=over 4
748
749=item Lvalue subroutines are EXPERIMENTAL
750
751They appear to be convenient, but there are several reasons to be
752circumspect.
753
754You can't use the return keyword, you must pass out the value before
755falling out of subroutine scope. (see comment in example above). This
756is usually not a problem, but it disallows an explicit return out of a
757deeply nested loop, which is sometimes a nice way out.
758
759They violate encapsulation. A normal mutator can check the supplied
760argument before setting the attribute it is protecting, an lvalue
761subroutine never gets that chance. Consider;
762
763 my $some_array_ref = []; # protected by mutators ??
764
765 sub set_arr { # normal mutator
766 my $val = shift;
767 die("expected array, you supplied ", ref $val)
768 unless ref $val eq 'ARRAY';
769 $some_array_ref = $val;
770 }
771 sub set_arr_lv : lvalue { # lvalue mutator
772 $some_array_ref;
773 }
774
775 # set_arr_lv cannot stop this !
776 set_arr_lv() = { a => 1 };
818c4caa 777
e6a32221
JC
778=back
779
cb1a09d0 780=head2 Passing Symbol Table Entries (typeglobs)
d74e8afc 781X<typeglob> X<*>
cb1a09d0 782
19799a22
GS
783B<WARNING>: The mechanism described in this section was originally
784the only way to simulate pass-by-reference in older versions of
785Perl. While it still works fine in modern versions, the new reference
786mechanism is generally easier to work with. See below.
a0d0e21e
LW
787
788Sometimes you don't want to pass the value of an array to a subroutine
789but rather the name of it, so that the subroutine can modify the global
790copy of it rather than working with a local copy. In perl you can
cb1a09d0 791refer to all objects of a particular name by prefixing the name
5f05dabc 792with a star: C<*foo>. This is often known as a "typeglob", because the
a0d0e21e
LW
793star on the front can be thought of as a wildcard match for all the
794funny prefix characters on variables and subroutines and such.
795
55497cff 796When evaluated, the typeglob produces a scalar value that represents
5f05dabc 797all the objects of that name, including any filehandle, format, or
a0d0e21e 798subroutine. When assigned to, it causes the name mentioned to refer to
19799a22 799whatever C<*> value was assigned to it. Example:
a0d0e21e
LW
800
801 sub doubleary {
802 local(*someary) = @_;
803 foreach $elem (@someary) {
804 $elem *= 2;
805 }
806 }
807 doubleary(*foo);
808 doubleary(*bar);
809
19799a22 810Scalars are already passed by reference, so you can modify
a0d0e21e 811scalar arguments without using this mechanism by referring explicitly
1fef88e7 812to C<$_[0]> etc. You can modify all the elements of an array by passing
f86cebdf
GS
813all the elements as scalars, but you have to use the C<*> mechanism (or
814the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
a0d0e21e
LW
815an array. It will certainly be faster to pass the typeglob (or reference).
816
817Even if you don't want to modify an array, this mechanism is useful for
5f05dabc 818passing multiple arrays in a single LIST, because normally the LIST
a0d0e21e 819mechanism will merge all the array values so that you can't extract out
55497cff 820the individual arrays. For more on typeglobs, see
2ae324a7 821L<perldata/"Typeglobs and Filehandles">.
cb1a09d0 822
5a964f20 823=head2 When to Still Use local()
d74e8afc 824X<local> X<variable, local>
5a964f20 825
19799a22
GS
826Despite the existence of C<my>, there are still three places where the
827C<local> operator still shines. In fact, in these three places, you
5a964f20
TC
828I<must> use C<local> instead of C<my>.
829
13a2d996 830=over 4
5a964f20 831
551e1d92
RB
832=item 1.
833
834You need to give a global variable a temporary value, especially $_.
5a964f20 835
f86cebdf
GS
836The global variables, like C<@ARGV> or the punctuation variables, must be
837C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
5a964f20 838it up into chunks separated by lines of equal signs, which are placed
f86cebdf 839in C<@Fields>.
5a964f20
TC
840
841 {
842 local @ARGV = ("/etc/motd");
843 local $/ = undef;
844 local $_ = <>;
845 @Fields = split /^\s*=+\s*$/;
846 }
847
19799a22 848It particular, it's important to C<local>ize $_ in any routine that assigns
5a964f20
TC
849to it. Look out for implicit assignments in C<while> conditionals.
850
551e1d92
RB
851=item 2.
852
853You need to create a local file or directory handle or a local function.
5a964f20 854
09bef843
SB
855A function that needs a filehandle of its own must use
856C<local()> on a complete typeglob. This can be used to create new symbol
5a964f20
TC
857table entries:
858
859 sub ioqueue {
860 local (*READER, *WRITER); # not my!
17b63f68 861 pipe (READER, WRITER) or die "pipe: $!";
5a964f20
TC
862 return (*READER, *WRITER);
863 }
864 ($head, $tail) = ioqueue();
865
866See the Symbol module for a way to create anonymous symbol table
867entries.
868
869Because assignment of a reference to a typeglob creates an alias, this
870can be used to create what is effectively a local function, or at least,
871a local alias.
872
873 {
f86cebdf
GS
874 local *grow = \&shrink; # only until this block exists
875 grow(); # really calls shrink()
876 move(); # if move() grow()s, it shrink()s too
5a964f20 877 }
f86cebdf 878 grow(); # get the real grow() again
5a964f20
TC
879
880See L<perlref/"Function Templates"> for more about manipulating
881functions by name in this way.
882
551e1d92
RB
883=item 3.
884
885You want to temporarily change just one element of an array or hash.
5a964f20 886
f86cebdf 887You can C<local>ize just one element of an aggregate. Usually this
5a964f20
TC
888is done on dynamics:
889
890 {
891 local $SIG{INT} = 'IGNORE';
892 funct(); # uninterruptible
893 }
894 # interruptibility automatically restored here
895
896But it also works on lexically declared aggregates. Prior to 5.005,
897this operation could on occasion misbehave.
898
899=back
900
cb1a09d0 901=head2 Pass by Reference
d74e8afc 902X<pass by reference> X<pass-by-reference> X<reference>
cb1a09d0 903
55497cff
PP
904If you want to pass more than one array or hash into a function--or
905return them from it--and have them maintain their integrity, then
906you're going to have to use an explicit pass-by-reference. Before you
907do that, you need to understand references as detailed in L<perlref>.
c07a80fd 908This section may not make much sense to you otherwise.
cb1a09d0 909
19799a22
GS
910Here are a few simple examples. First, let's pass in several arrays
911to a function and have it C<pop> all of then, returning a new list
912of all their former last elements:
cb1a09d0
AD
913
914 @tailings = popmany ( \@a, \@b, \@c, \@d );
915
916 sub popmany {
917 my $aref;
918 my @retlist = ();
919 foreach $aref ( @_ ) {
920 push @retlist, pop @$aref;
54310121 921 }
cb1a09d0 922 return @retlist;
54310121 923 }
cb1a09d0 924
54310121 925Here's how you might write a function that returns a
cb1a09d0
AD
926list of keys occurring in all the hashes passed to it:
927
54310121 928 @common = inter( \%foo, \%bar, \%joe );
cb1a09d0
AD
929 sub inter {
930 my ($k, $href, %seen); # locals
931 foreach $href (@_) {
932 while ( $k = each %$href ) {
933 $seen{$k}++;
54310121
PP
934 }
935 }
cb1a09d0 936 return grep { $seen{$_} == @_ } keys %seen;
54310121 937 }
cb1a09d0 938
5f05dabc 939So far, we're using just the normal list return mechanism.
54310121
PP
940What happens if you want to pass or return a hash? Well,
941if you're using only one of them, or you don't mind them
cb1a09d0 942concatenating, then the normal calling convention is ok, although
54310121 943a little expensive.
cb1a09d0
AD
944
945Where people get into trouble is here:
946
947 (@a, @b) = func(@c, @d);
948or
949 (%a, %b) = func(%c, %d);
950
19799a22
GS
951That syntax simply won't work. It sets just C<@a> or C<%a> and
952clears the C<@b> or C<%b>. Plus the function didn't get passed
953into two separate arrays or hashes: it got one long list in C<@_>,
954as always.
cb1a09d0
AD
955
956If you can arrange for everyone to deal with this through references, it's
957cleaner code, although not so nice to look at. Here's a function that
958takes two array references as arguments, returning the two array elements
959in order of how many elements they have in them:
960
961 ($aref, $bref) = func(\@c, \@d);
962 print "@$aref has more than @$bref\n";
963 sub func {
964 my ($cref, $dref) = @_;
965 if (@$cref > @$dref) {
966 return ($cref, $dref);
967 } else {
c07a80fd 968 return ($dref, $cref);
54310121
PP
969 }
970 }
cb1a09d0
AD
971
972It turns out that you can actually do this also:
973
974 (*a, *b) = func(\@c, \@d);
975 print "@a has more than @b\n";
976 sub func {
977 local (*c, *d) = @_;
978 if (@c > @d) {
979 return (\@c, \@d);
980 } else {
981 return (\@d, \@c);
54310121
PP
982 }
983 }
cb1a09d0
AD
984
985Here we're using the typeglobs to do symbol table aliasing. It's
19799a22 986a tad subtle, though, and also won't work if you're using C<my>
09bef843 987variables, because only globals (even in disguise as C<local>s)
19799a22 988are in the symbol table.
5f05dabc
PP
989
990If you're passing around filehandles, you could usually just use the bare
19799a22
GS
991typeglob, like C<*STDOUT>, but typeglobs references work, too.
992For example:
5f05dabc
PP
993
994 splutter(\*STDOUT);
995 sub splutter {
996 my $fh = shift;
997 print $fh "her um well a hmmm\n";
998 }
999
1000 $rec = get_rec(\*STDIN);
1001 sub get_rec {
1002 my $fh = shift;
1003 return scalar <$fh>;
1004 }
1005
19799a22
GS
1006If you're planning on generating new filehandles, you could do this.
1007Notice to pass back just the bare *FH, not its reference.
5f05dabc
PP
1008
1009 sub openit {
19799a22 1010 my $path = shift;
5f05dabc 1011 local *FH;
e05a3a1e 1012 return open (FH, $path) ? *FH : undef;
54310121 1013 }
5f05dabc 1014
cb1a09d0 1015=head2 Prototypes
d74e8afc 1016X<prototype> X<subroutine, prototype>
cb1a09d0 1017
19799a22
GS
1018Perl supports a very limited kind of compile-time argument checking
1019using function prototyping. If you declare
cb1a09d0
AD
1020
1021 sub mypush (\@@)
1022
19799a22
GS
1023then C<mypush()> takes arguments exactly like C<push()> does. The
1024function declaration must be visible at compile time. The prototype
1025affects only interpretation of new-style calls to the function,
1026where new-style is defined as not using the C<&> character. In
1027other words, if you call it like a built-in function, then it behaves
1028like a built-in function. If you call it like an old-fashioned
1029subroutine, then it behaves like an old-fashioned subroutine. It
1030naturally falls out from this rule that prototypes have no influence
1031on subroutine references like C<\&foo> or on indirect subroutine
c47ff5f1 1032calls like C<&{$subref}> or C<< $subref->() >>.
c07a80fd
PP
1033
1034Method calls are not influenced by prototypes either, because the
19799a22
GS
1035function to be called is indeterminate at compile time, since
1036the exact code called depends on inheritance.
cb1a09d0 1037
19799a22
GS
1038Because the intent of this feature is primarily to let you define
1039subroutines that work like built-in functions, here are prototypes
1040for some other functions that parse almost exactly like the
1041corresponding built-in.
cb1a09d0
AD
1042
1043 Declared as Called as
1044
f86cebdf
GS
1045 sub mylink ($$) mylink $old, $new
1046 sub myvec ($$$) myvec $var, $offset, 1
1047 sub myindex ($$;$) myindex &getstring, "substr"
1048 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
1049 sub myreverse (@) myreverse $a, $b, $c
1050 sub myjoin ($@) myjoin ":", $a, $b, $c
1051 sub mypop (\@) mypop @array
1052 sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme
1053 sub mykeys (\%) mykeys %{$hashref}
1054 sub myopen (*;$) myopen HANDLE, $name
1055 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
1056 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
d822fdf9 1057 sub myrand (;$) myrand 42
f86cebdf 1058 sub mytime () mytime
cb1a09d0 1059
c07a80fd 1060Any backslashed prototype character represents an actual argument
6e47f808 1061that absolutely must start with that character. The value passed
19799a22
GS
1062as part of C<@_> will be a reference to the actual argument given
1063in the subroutine call, obtained by applying C<\> to that argument.
c07a80fd 1064
5b794e05
JH
1065You can also backslash several argument types simultaneously by using
1066the C<\[]> notation:
1067
1068 sub myref (\[$@%&*])
1069
1070will allow calling myref() as
1071
1072 myref $var
1073 myref @array
1074 myref %hash
1075 myref &sub
1076 myref *glob
1077
1078and the first argument of myref() will be a reference to
1079a scalar, an array, a hash, a code, or a glob.
1080
c07a80fd 1081Unbackslashed prototype characters have special meanings. Any
19799a22 1082unbackslashed C<@> or C<%> eats all remaining arguments, and forces
f86cebdf
GS
1083list context. An argument represented by C<$> forces scalar context. An
1084C<&> requires an anonymous subroutine, which, if passed as the first
0df79f0c
GS
1085argument, does not require the C<sub> keyword or a subsequent comma.
1086
1087A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
648ca4f7
GS
1088typeglob, or a reference to a typeglob in that slot. The value will be
1089available to the subroutine either as a simple scalar, or (in the latter
0df79f0c
GS
1090two cases) as a reference to the typeglob. If you wish to always convert
1091such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1092follows:
1093
1094 use Symbol 'qualify_to_ref';
1095
1096 sub foo (*) {
1097 my $fh = qualify_to_ref(shift, caller);
1098 ...
1099 }
c07a80fd 1100
859a4967 1101A semicolon (C<;>) separates mandatory arguments from optional arguments.
19799a22 1102It is redundant before C<@> or C<%>, which gobble up everything else.
cb1a09d0 1103
7adf2bcd
RGS
1104As the last character of a prototype, or just before a semicolon, you can
1105use C<_> in place of C<$>: if this argument is not provided, C<$_> will be
1106used instead.
859a4967 1107
19799a22
GS
1108Note how the last three examples in the table above are treated
1109specially by the parser. C<mygrep()> is parsed as a true list
1110operator, C<myrand()> is parsed as a true unary operator with unary
1111precedence the same as C<rand()>, and C<mytime()> is truly without
1112arguments, just like C<time()>. That is, if you say
cb1a09d0
AD
1113
1114 mytime +2;
1115
f86cebdf 1116you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
19799a22 1117without a prototype.
cb1a09d0 1118
19799a22
GS
1119The interesting thing about C<&> is that you can generate new syntax with it,
1120provided it's in the initial position:
d74e8afc 1121X<&>
cb1a09d0 1122
6d28dffb 1123 sub try (&@) {
cb1a09d0
AD
1124 my($try,$catch) = @_;
1125 eval { &$try };
1126 if ($@) {
1127 local $_ = $@;
1128 &$catch;
1129 }
1130 }
55497cff 1131 sub catch (&) { $_[0] }
cb1a09d0
AD
1132
1133 try {
1134 die "phooey";
1135 } catch {
1136 /phooey/ and print "unphooey\n";
1137 };
1138
f86cebdf 1139That prints C<"unphooey">. (Yes, there are still unresolved
19799a22 1140issues having to do with visibility of C<@_>. I'm ignoring that
f86cebdf 1141question for the moment. (But note that if we make C<@_> lexically
cb1a09d0 1142scoped, those anonymous subroutines can act like closures... (Gee,
5f05dabc 1143is this sounding a little Lispish? (Never mind.))))
cb1a09d0 1144
19799a22 1145And here's a reimplementation of the Perl C<grep> operator:
d74e8afc 1146X<grep>
cb1a09d0
AD
1147
1148 sub mygrep (&@) {
1149 my $code = shift;
1150 my @result;
1151 foreach $_ (@_) {
6e47f808 1152 push(@result, $_) if &$code;
cb1a09d0
AD
1153 }
1154 @result;
1155 }
a0d0e21e 1156
cb1a09d0
AD
1157Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1158been intentionally left out of prototypes for the express purpose of
1159someday in the future adding named, formal parameters. The current
1160mechanism's main goal is to let module writers provide better diagnostics
1161for module users. Larry feels the notation quite understandable to Perl
1162programmers, and that it will not intrude greatly upon the meat of the
1163module, nor make it harder to read. The line noise is visually
1164encapsulated into a small pill that's easy to swallow.
1165
420cdfc1
ST
1166If you try to use an alphanumeric sequence in a prototype you will
1167generate an optional warning - "Illegal character in prototype...".
1168Unfortunately earlier versions of Perl allowed the prototype to be
1169used as long as its prefix was a valid prototype. The warning may be
1170upgraded to a fatal error in a future version of Perl once the
1171majority of offending code is fixed.
1172
cb1a09d0
AD
1173It's probably best to prototype new functions, not retrofit prototyping
1174into older ones. That's because you must be especially careful about
1175silent impositions of differing list versus scalar contexts. For example,
1176if you decide that a function should take just one parameter, like this:
1177
1178 sub func ($) {
1179 my $n = shift;
1180 print "you gave me $n\n";
54310121 1181 }
cb1a09d0
AD
1182
1183and someone has been calling it with an array or expression
1184returning a list:
1185
1186 func(@foo);
1187 func( split /:/ );
1188
19799a22 1189Then you've just supplied an automatic C<scalar> in front of their
f86cebdf 1190argument, which can be more than a bit surprising. The old C<@foo>
cb1a09d0 1191which used to hold one thing doesn't get passed in. Instead,
19799a22
GS
1192C<func()> now gets passed in a C<1>; that is, the number of elements
1193in C<@foo>. And the C<split> gets called in scalar context so it
1194starts scribbling on your C<@_> parameter list. Ouch!
cb1a09d0 1195
5f05dabc 1196This is all very powerful, of course, and should be used only in moderation
54310121 1197to make the world a better place.
44a8e56a
PP
1198
1199=head2 Constant Functions
d74e8afc 1200X<constant>
44a8e56a
PP
1201
1202Functions with a prototype of C<()> are potential candidates for
19799a22
GS
1203inlining. If the result after optimization and constant folding
1204is either a constant or a lexically-scoped scalar which has no other
54310121 1205references, then it will be used in place of function calls made
19799a22
GS
1206without C<&>. Calls made using C<&> are never inlined. (See
1207F<constant.pm> for an easy way to declare most constants.)
44a8e56a 1208
5a964f20 1209The following functions would all be inlined:
44a8e56a 1210
699e6cd4
TP
1211 sub pi () { 3.14159 } # Not exact, but close.
1212 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1213 # and it's inlined, too!
44a8e56a
PP
1214 sub ST_DEV () { 0 }
1215 sub ST_INO () { 1 }
1216
1217 sub FLAG_FOO () { 1 << 8 }
1218 sub FLAG_BAR () { 1 << 9 }
1219 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
54310121
PP
1220
1221 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
88267271 1222
1223 sub N () { int(OPT_BAZ) / 3 }
1224
1225 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1226
1227Be aware that these will not be inlined; as they contain inner scopes,
1228the constant folding doesn't reduce them to a single constant:
1229
1230 sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1231
1232 sub baz_val () {
44a8e56a
PP
1233 if (OPT_BAZ) {
1234 return 23;
1235 }
1236 else {
1237 return 42;
1238 }
1239 }
cb1a09d0 1240
5a964f20 1241If you redefine a subroutine that was eligible for inlining, you'll get
4cee8e80
CS
1242a mandatory warning. (You can use this warning to tell whether or not a
1243particular subroutine is considered constant.) The warning is
1244considered severe enough not to be optional because previously compiled
1245invocations of the function will still be using the old value of the
19799a22 1246function. If you need to be able to redefine the subroutine, you need to
4cee8e80 1247ensure that it isn't inlined, either by dropping the C<()> prototype
19799a22 1248(which changes calling semantics, so beware) or by thwarting the
4cee8e80
CS
1249inlining mechanism in some other way, such as
1250
4cee8e80 1251 sub not_inlined () {
54310121 1252 23 if $];
4cee8e80
CS
1253 }
1254
19799a22 1255=head2 Overriding Built-in Functions
d74e8afc 1256X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
a0d0e21e 1257
19799a22 1258Many built-in functions may be overridden, though this should be tried
5f05dabc 1259only occasionally and for good reason. Typically this might be
19799a22 1260done by a package attempting to emulate missing built-in functionality
a0d0e21e
LW
1261on a non-Unix system.
1262
163e3a99
JP
1263Overriding may be done only by importing the name from a module at
1264compile time--ordinary predeclaration isn't good enough. However, the
19799a22
GS
1265C<use subs> pragma lets you, in effect, predeclare subs
1266via the import syntax, and these names may then override built-in ones:
a0d0e21e
LW
1267
1268 use subs 'chdir', 'chroot', 'chmod', 'chown';
1269 chdir $somewhere;
1270 sub chdir { ... }
1271
19799a22
GS
1272To unambiguously refer to the built-in form, precede the
1273built-in name with the special package qualifier C<CORE::>. For example,
1274saying C<CORE::open()> always refers to the built-in C<open()>, even
fb73857a 1275if the current package has imported some other subroutine called
19799a22 1276C<&open()> from elsewhere. Even though it looks like a regular
09bef843 1277function call, it isn't: you can't take a reference to it, such as
19799a22 1278the incorrect C<\&CORE::open> might appear to produce.
fb73857a 1279
19799a22
GS
1280Library modules should not in general export built-in names like C<open>
1281or C<chdir> as part of their default C<@EXPORT> list, because these may
a0d0e21e 1282sneak into someone else's namespace and change the semantics unexpectedly.
19799a22 1283Instead, if the module adds that name to C<@EXPORT_OK>, then it's
a0d0e21e
LW
1284possible for a user to import the name explicitly, but not implicitly.
1285That is, they could say
1286
1287 use Module 'open';
1288
19799a22 1289and it would import the C<open> override. But if they said
a0d0e21e
LW
1290
1291 use Module;
1292
19799a22 1293they would get the default imports without overrides.
a0d0e21e 1294
19799a22 1295The foregoing mechanism for overriding built-in is restricted, quite
95d94a4f 1296deliberately, to the package that requests the import. There is a second
19799a22 1297method that is sometimes applicable when you wish to override a built-in
95d94a4f
GS
1298everywhere, without regard to namespace boundaries. This is achieved by
1299importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1300example that quite brazenly replaces the C<glob> operator with something
1301that understands regular expressions.
1302
1303 package REGlob;
1304 require Exporter;
1305 @ISA = 'Exporter';
1306 @EXPORT_OK = 'glob';
1307
1308 sub import {
1309 my $pkg = shift;
1310 return unless @_;
1311 my $sym = shift;
1312 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1313 $pkg->export($where, $sym, @_);
1314 }
1315
1316 sub glob {
1317 my $pat = shift;
1318 my @got;
19799a22
GS
1319 local *D;
1320 if (opendir D, '.') {
1321 @got = grep /$pat/, readdir D;
1322 closedir D;
1323 }
1324 return @got;
95d94a4f
GS
1325 }
1326 1;
1327
1328And here's how it could be (ab)used:
1329
1330 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1331 package Foo;
1332 use REGlob 'glob'; # override glob() in Foo:: only
1333 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1334
19799a22 1335The initial comment shows a contrived, even dangerous example.
95d94a4f 1336By overriding C<glob> globally, you would be forcing the new (and
19799a22 1337subversive) behavior for the C<glob> operator for I<every> namespace,
95d94a4f
GS
1338without the complete cognizance or cooperation of the modules that own
1339those namespaces. Naturally, this should be done with extreme caution--if
1340it must be done at all.
1341
1342The C<REGlob> example above does not implement all the support needed to
19799a22 1343cleanly override perl's C<glob> operator. The built-in C<glob> has
95d94a4f 1344different behaviors depending on whether it appears in a scalar or list
19799a22 1345context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
95d94a4f
GS
1346context sensitive behaviors, and these must be adequately supported by
1347a properly written override. For a fully functional example of overriding
1348C<glob>, study the implementation of C<File::DosGlob> in the standard
1349library.
1350
77bc9082
RGS
1351When you override a built-in, your replacement should be consistent (if
1352possible) with the built-in native syntax. You can achieve this by using
1353a suitable prototype. To get the prototype of an overridable built-in,
1354use the C<prototype> function with an argument of C<"CORE::builtin_name">
1355(see L<perlfunc/prototype>).
1356
1357Note however that some built-ins can't have their syntax expressed by a
1358prototype (such as C<system> or C<chomp>). If you override them you won't
1359be able to fully mimic their original syntax.
1360
fe854a6f 1361The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
77bc9082
RGS
1362to special magic, their original syntax is preserved, and you don't have
1363to define a prototype for their replacements. (You can't override the
1364C<do BLOCK> syntax, though).
1365
1366C<require> has special additional dark magic: if you invoke your
1367C<require> replacement as C<require Foo::Bar>, it will actually receive
1368the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1369
1370And, as you'll have noticed from the previous example, if you override
593b9c14 1371C<glob>, the C<< <*> >> glob operator is overridden as well.
77bc9082 1372
9b3023bc
RGS
1373In a similar fashion, overriding the C<readline> function also overrides
1374the equivalent I/O operator C<< <FILEHANDLE> >>.
1375
fe854a6f 1376Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
77bc9082 1377
a0d0e21e 1378=head2 Autoloading
d74e8afc 1379X<autoloading> X<AUTOLOAD>
a0d0e21e 1380
19799a22
GS
1381If you call a subroutine that is undefined, you would ordinarily
1382get an immediate, fatal error complaining that the subroutine doesn't
1383exist. (Likewise for subroutines being used as methods, when the
1384method doesn't exist in any base class of the class's package.)
1385However, if an C<AUTOLOAD> subroutine is defined in the package or
1386packages used to locate the original subroutine, then that
1387C<AUTOLOAD> subroutine is called with the arguments that would have
1388been passed to the original subroutine. The fully qualified name
1389of the original subroutine magically appears in the global $AUTOLOAD
1390variable of the same package as the C<AUTOLOAD> routine. The name
1391is not passed as an ordinary argument because, er, well, just
593b9c14
YST
1392because, that's why. (As an exception, a method call to a nonexistent
1393C<import> or C<unimport> method is just skipped instead.)
19799a22
GS
1394
1395Many C<AUTOLOAD> routines load in a definition for the requested
1396subroutine using eval(), then execute that subroutine using a special
1397form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1398without a trace. (See the source to the standard module documented
1399in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1400also just emulate the routine and never define it. For example,
1401let's pretend that a function that wasn't defined should just invoke
1402C<system> with those arguments. All you'd do is:
cb1a09d0
AD
1403
1404 sub AUTOLOAD {
1405 my $program = $AUTOLOAD;
1406 $program =~ s/.*:://;
1407 system($program, @_);
54310121 1408 }
cb1a09d0 1409 date();
6d28dffb 1410 who('am', 'i');
cb1a09d0
AD
1411 ls('-l');
1412
19799a22
GS
1413In fact, if you predeclare functions you want to call that way, you don't
1414even need parentheses:
cb1a09d0
AD
1415
1416 use subs qw(date who ls);
1417 date;
1418 who "am", "i";
593b9c14 1419 ls '-l';
cb1a09d0
AD
1420
1421A more complete example of this is the standard Shell module, which
19799a22 1422can treat undefined subroutine calls as calls to external programs.
a0d0e21e 1423
19799a22
GS
1424Mechanisms are available to help modules writers split their modules
1425into autoloadable files. See the standard AutoLoader module
6d28dffb
PP
1426described in L<AutoLoader> and in L<AutoSplit>, the standard
1427SelfLoader modules in L<SelfLoader>, and the document on adding C
19799a22 1428functions to Perl code in L<perlxs>.
cb1a09d0 1429
09bef843 1430=head2 Subroutine Attributes
d74e8afc 1431X<attribute> X<subroutine, attribute> X<attrs>
09bef843
SB
1432
1433A subroutine declaration or definition may have a list of attributes
1434associated with it. If such an attribute list is present, it is
0120eecf 1435broken up at space or colon boundaries and treated as though a
09bef843
SB
1436C<use attributes> had been seen. See L<attributes> for details
1437about what attributes are currently supported.
1438Unlike the limitation with the obsolescent C<use attrs>, the
1439C<sub : ATTRLIST> syntax works to associate the attributes with
1440a pre-declaration, and not just with a subroutine definition.
1441
1442The attributes must be valid as simple identifier names (without any
1443punctuation other than the '_' character). They may have a parameter
1444list appended, which is only checked for whether its parentheses ('(',')')
1445nest properly.
1446
1447Examples of valid syntax (even though the attributes are unknown):
1448
4358a253
SS
1449 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1450 sub plugh () : Ugly('\(") :Bad;
09bef843
SB
1451 sub xyzzy : _5x5 { ... }
1452
1453Examples of invalid syntax:
1454
4358a253
SS
1455 sub fnord : switch(10,foo(); # ()-string not balanced
1456 sub snoid : Ugly('('); # ()-string not balanced
1457 sub xyzzy : 5x5; # "5x5" not a valid identifier
1458 sub plugh : Y2::north; # "Y2::north" not a simple identifier
1459 sub snurt : foo + bar; # "+" not a colon or space
09bef843
SB
1460
1461The attribute list is passed as a list of constant strings to the code
1462which associates them with the subroutine. In particular, the second example
1463of valid syntax above currently looks like this in terms of how it's
1464parsed and invoked:
1465
1466 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1467
1468For further details on attribute lists and their manipulation,
a0ae32d3 1469see L<attributes> and L<Attribute::Handlers>.
09bef843 1470
cb1a09d0 1471=head1 SEE ALSO
a0d0e21e 1472
19799a22
GS
1473See L<perlref/"Function Templates"> for more about references and closures.
1474See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
a2293a43 1475See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
19799a22
GS
1476See L<perlmod> to learn about bundling up your functions in separate files.
1477See L<perlmodlib> to learn what library modules come standard on your system.
1478See L<perltoot> to learn how to make object method calls.