2 X<subroutine> X<function>
4 perlsub - Perl subroutines
8 To declare subroutines:
9 X<subroutine, declaration> X<sub>
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
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
21 To define an anonymous subroutine at runtime:
22 X<subroutine, anonymous>
24 $subref = sub BLOCK; # no proto
25 $subref = sub (PROTO) BLOCK; # with proto
26 $subref = sub : ATTRS BLOCK; # with attributes
27 $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
29 To import subroutines:
32 use MODULE qw(NAME1 NAME2 NAME3);
35 X<subroutine, call> X<call>
37 NAME(LIST); # & is optional with parentheses.
38 NAME LIST; # Parentheses optional if predeclared/imported.
39 &NAME(LIST); # Circumvent prototypes.
40 &NAME; # Makes current @_ visible to called subroutine.
44 Like many languages, Perl provides for user-defined subroutines.
45 These may be located anywhere in the main program, loaded in from
46 other files via the C<do>, C<require>, or C<use> keywords, or
47 generated on the fly using C<eval> or anonymous subroutines.
48 You can even call a function indirectly using a variable containing
49 its name or a CODE reference.
51 The Perl model for function call and return values is simple: all
52 functions are passed as parameters one single flat list of scalars, and
53 all functions likewise return to their caller one single flat list of
54 scalars. Any arrays or hashes in these call and return lists will
55 collapse, losing their identities--but you may always use
56 pass-by-reference instead to avoid this. Both call and return lists may
57 contain as many or as few scalar elements as you'd like. (Often a
58 function without an explicit return statement is called a subroutine, but
59 there's really no difference from Perl's perspective.)
60 X<subroutine, parameter> X<parameter>
62 Any arguments passed in show up in the array C<@_>. Therefore, if
63 you called a function with two arguments, those would be stored in
64 C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its
65 elements are aliases for the actual scalar parameters. In particular,
66 if an element C<$_[0]> is updated, the corresponding argument is
67 updated (or an error occurs if it is not updatable). If an argument
68 is an array or hash element which did not exist when the function
69 was called, that element is created only when (and if) it is modified
70 or a reference to it is taken. (Some earlier versions of Perl
71 created the element whether or not the element was assigned to.)
72 Assigning to the whole array C<@_> removes that aliasing, and does
73 not update any arguments.
74 X<subroutine, argument> X<argument> X<@_>
76 A C<return> statement may be used to exit a subroutine, optionally
77 specifying the returned value, which will be evaluated in the
78 appropriate context (list, scalar, or void) depending on the context of
79 the subroutine call. If you specify no return value, the subroutine
80 returns an empty list in list context, the undefined value in scalar
81 context, or nothing in void context. If you return one or more
82 aggregates (arrays and hashes), these will be flattened together into
83 one large indistinguishable list.
85 If no C<return> is found and if the last statement is an expression, its
86 value is returned. If the last statement is a loop control structure
87 like a C<foreach> or a C<while>, the returned value is unspecified. The
88 empty sub returns the empty list.
89 X<subroutine, return value> X<return value> X<return>
91 Perl does not have named formal parameters. In practice all you
92 do is assign to a C<my()> list of these. Variables that aren't
93 declared to be private are global variables. For gory details
94 on creating private variables, see L<"Private Variables via my()">
95 and L<"Temporary Values via local()">. To create protected
96 environments for a set of functions in a separate package (and
97 probably a separate file), see L<perlmod/"Packages">.
98 X<formal parameter> X<parameter, formal>
105 $max = $foo if $max < $foo;
109 $bestday = max($mon,$tue,$wed,$thu,$fri);
113 # get a line, combining continuation lines
114 # that start with whitespace
117 $thisline = $lookahead; # global variables!
118 LINE: while (defined($lookahead = <STDIN>)) {
119 if ($lookahead =~ /^[ \t]/) {
120 $thisline .= $lookahead;
129 $lookahead = <STDIN>; # get first line
130 while (defined($line = get_line())) {
134 Assigning to a list of private variables to name your arguments:
137 my($key, $value) = @_;
138 $Foo{$key} = $value unless $Foo{$key};
141 Because the assignment copies the values, this also has the effect
142 of turning call-by-reference into call-by-value. Otherwise a
143 function is free to do in-place modifications of C<@_> and change
145 X<call-by-reference> X<call-by-value>
147 upcase_in($v1, $v2); # this changes $v1 and $v2
149 for (@_) { tr/a-z/A-Z/ }
152 You aren't allowed to modify constants in this way, of course. If an
153 argument were actually literal and you tried to change it, you'd take a
154 (presumably fatal) exception. For example, this won't work:
155 X<call-by-reference> X<call-by-value>
157 upcase_in("frederick");
159 It would be much safer if the C<upcase_in()> function
160 were written to return a copy of its parameters instead
161 of changing them in place:
163 ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
165 return unless defined wantarray; # void context, do nothing
167 for (@parms) { tr/a-z/A-Z/ }
168 return wantarray ? @parms : $parms[0];
171 Notice how this (unprototyped) function doesn't care whether it was
172 passed real scalars or arrays. Perl sees all arguments as one big,
173 long, flat parameter list in C<@_>. This is one area where
174 Perl's simple argument-passing style shines. The C<upcase()>
175 function would work perfectly well without changing the C<upcase()>
176 definition even if we fed it things like this:
178 @newlist = upcase(@list1, @list2);
179 @newlist = upcase( split /:/, $var );
181 Do not, however, be tempted to do this:
183 (@a, @b) = upcase(@list1, @list2);
185 Like the flattened incoming parameter list, the return list is also
186 flattened on return. So all you have managed to do here is stored
187 everything in C<@a> and made C<@b> empty. See
188 L<Pass by Reference> for alternatives.
190 A subroutine may be called using an explicit C<&> prefix. The
191 C<&> is optional in modern Perl, as are parentheses if the
192 subroutine has been predeclared. The C<&> is I<not> optional
193 when just naming the subroutine, such as when it's used as
194 an argument to defined() or undef(). Nor is it optional when you
195 want to do an indirect subroutine call with a subroutine name or
196 reference using the C<&$subref()> or C<&{$subref}()> constructs,
197 although the C<< $subref->() >> notation solves that problem.
198 See L<perlref> for more about all that.
201 Subroutines may be called recursively. If a subroutine is called
202 using the C<&> form, the argument list is optional, and if omitted,
203 no C<@_> array is set up for the subroutine: the C<@_> array at the
204 time of the call is visible to subroutine instead. This is an
205 efficiency mechanism that new users may wish to avoid.
208 &foo(1,2,3); # pass three arguments
209 foo(1,2,3); # the same
211 foo(); # pass a null list
214 &foo; # foo() get current args, like foo(@_) !!
215 foo; # like foo() IFF sub foo predeclared, else "foo"
217 Not only does the C<&> form make the argument list optional, it also
218 disables any prototype checking on arguments you do provide. This
219 is partly for historical reasons, and partly for having a convenient way
220 to cheat if you know what you're doing. See L</Prototypes> below.
223 Since Perl 5.16.0, the C<__SUB__> token is available under C<use feature
224 'current_sub'> and C<use 5.16.0>. It will evaluate to a reference to the
225 currently-running sub, which allows for recursive calls without knowing
226 your subroutine's name.
229 my $factorial = sub {
232 return($x * __SUB__->( $x - 1 ) );
235 The behaviour of C<__SUB__> within a regex code block (such as C</(?{...})/>)
236 is subject to change.
238 Subroutines whose names are in all upper case are reserved to the Perl
239 core, as are modules whose names are in all lower case. A subroutine in
240 all capitals is a loosely-held convention meaning it will be called
241 indirectly by the run-time system itself, usually due to a triggered event.
242 Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>,
243 C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>.
245 The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines
246 are not so much subroutines as named special code blocks, of which you
247 can have more than one in a package, and which you can B<not> call
248 explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">
250 =head2 Private Variables via my()
251 X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical>
252 X<lexical scope> X<attributes, my>
256 my $foo; # declare $foo lexically local
257 my (@wid, %get); # declare list of variables local
258 my $foo = "flurp"; # declare $foo lexical, and init it
259 my @oof = @bar; # declare @oof lexical, and init it
260 my $x : Foo = $y; # similar, with an attribute applied
262 B<WARNING>: The use of attribute lists on C<my> declarations is still
263 evolving. The current semantics and interface are subject to change.
264 See L<attributes> and L<Attribute::Handlers>.
266 The C<my> operator declares the listed variables to be lexically
267 confined to the enclosing block, conditional (C<if/unless/elsif/else>),
268 loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
269 or C<do/require/use>'d file. If more than one value is listed, the
270 list must be placed in parentheses. All listed elements must be
271 legal lvalues. Only alphanumeric identifiers may be lexically
272 scoped--magical built-ins like C<$/> must currently be C<local>ized
273 with C<local> instead.
275 Unlike dynamic variables created by the C<local> operator, lexical
276 variables declared with C<my> are totally hidden from the outside
277 world, including any called subroutines. This is true if it's the
278 same subroutine called from itself or elsewhere--every call gets
282 This doesn't mean that a C<my> variable declared in a statically
283 enclosing lexical scope would be invisible. Only dynamic scopes
284 are cut off. For example, the C<bumpx()> function below has access
285 to the lexical $x variable because both the C<my> and the C<sub>
286 occurred at the same scope, presumably file scope.
291 An C<eval()>, however, can see lexical variables of the scope it is
292 being evaluated in, so long as the names aren't hidden by declarations within
293 the C<eval()> itself. See L<perlref>.
296 The parameter list to my() may be assigned to if desired, which allows you
297 to initialize your variables. (If no initializer is given for a
298 particular variable, it is created with the undefined value.) Commonly
299 this is used to name input parameters to a subroutine. Examples:
301 $arg = "fred"; # "global" variable
303 print "$arg thinks the root is $n\n";
304 fred thinks the root is 3
307 my $arg = shift; # name doesn't matter
312 The C<my> is simply a modifier on something you might assign to. So when
313 you do assign to variables in its argument list, C<my> doesn't
314 change whether those variables are viewed as a scalar or an array. So
316 my ($foo) = <STDIN>; # WRONG?
319 both supply a list context to the right-hand side, while
323 supplies a scalar context. But the following declares only one variable:
325 my $foo, $bar = 1; # WRONG
327 That has the same effect as
332 The declared variable is not introduced (is not visible) until after
333 the current statement. Thus,
337 can be used to initialize a new $x with the value of the old $x, and
340 my $x = 123 and $x == 123
342 is false unless the old $x happened to have the value C<123>.
344 Lexical scopes of control structures are not bounded precisely by the
345 braces that delimit their controlled blocks; control expressions are
346 part of that scope, too. Thus in the loop
348 while (my $line = <>) {
354 the scope of $line extends from its declaration throughout the rest of
355 the loop construct (including the C<continue> clause), but not beyond
356 it. Similarly, in the conditional
358 if ((my $answer = <STDIN>) =~ /^yes$/i) {
360 } elsif ($answer =~ /^no$/i) {
364 die "'$answer' is neither 'yes' nor 'no'";
367 the scope of $answer extends from its declaration through the rest
368 of that conditional, including any C<elsif> and C<else> clauses,
369 but not beyond it. See L<perlsyn/"Simple Statements"> for information
370 on the scope of variables in statements with modifiers.
372 The C<foreach> loop defaults to scoping its index variable dynamically
373 in the manner of C<local>. However, if the index variable is
374 prefixed with the keyword C<my>, or if there is already a lexical
375 by that name in scope, then a new lexical is created instead. Thus
379 for my $i (1, 2, 3) {
383 the scope of $i extends to the end of the loop, but not beyond it,
384 rendering the value of $i inaccessible within C<some_function()>.
387 Some users may wish to encourage the use of lexically scoped variables.
388 As an aid to catching implicit uses to package variables,
389 which are always global, if you say
393 then any variable mentioned from there to the end of the enclosing
394 block must either refer to a lexical variable, be predeclared via
395 C<our> or C<use vars>, or else must be fully qualified with the package name.
396 A compilation error results otherwise. An inner block may countermand
397 this with C<no strict 'vars'>.
399 A C<my> has both a compile-time and a run-time effect. At compile
400 time, the compiler takes notice of it. The principal usefulness
401 of this is to quiet C<use strict 'vars'>, but it is also essential
402 for generation of closures as detailed in L<perlref>. Actual
403 initialization is delayed until run time, though, so it gets executed
404 at the appropriate time, such as each time through a loop, for
407 Variables declared with C<my> are not part of any package and are therefore
408 never fully qualified with the package name. In particular, you're not
409 allowed to try to make a package variable (or other global) lexical:
411 my $pack::var; # ERROR! Illegal syntax
413 In fact, a dynamic variable (also known as package or global variables)
414 are still accessible using the fully qualified C<::> notation even while a
415 lexical of the same name is also visible:
420 print "$x and $::x\n";
422 That will print out C<20> and C<10>.
424 You may declare C<my> variables at the outermost scope of a file
425 to hide any such identifiers from the world outside that file. This
426 is similar in spirit to C's static variables when they are used at
427 the file level. To do this with a subroutine requires the use of
428 a closure (an anonymous function that accesses enclosing lexicals).
429 If you want to create a private subroutine that cannot be called
430 from outside that block, it can declare a lexical variable containing
431 an anonymous sub reference:
433 my $secret_version = '1.001-beta';
434 my $secret_sub = sub { print $secret_version };
437 As long as the reference is never returned by any function within the
438 module, no outside module can see the subroutine, because its name is not in
439 any package's symbol table. Remember that it's not I<REALLY> called
440 C<$some_pack::secret_version> or anything; it's just $secret_version,
441 unqualified and unqualifiable.
443 This does not work with object methods, however; all object methods
444 have to be in the symbol table of some package to be found. See
445 L<perlref/"Function Templates"> for something of a work-around to
448 =head2 Persistent Private Variables
449 X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure>
451 There are two ways to build persistent private variables in Perl 5.10.
452 First, you can simply use the C<state> feature. Or, you can use closures,
453 if you want to stay compatible with releases older than 5.10.
455 =head3 Persistent variables via state()
457 Beginning with Perl 5.10.0, you can declare variables with the C<state>
458 keyword in place of C<my>. For that to work, though, you must have
459 enabled that feature beforehand, either by using the C<feature> pragma, or
460 by using C<-E> on one-liners (see L<feature>). Beginning with Perl 5.16,
461 the C<CORE::state> form does not require the
464 The C<state> keyword creates a lexical variable (following the same scoping
465 rules as C<my>) that persists from one subroutine call to the next. If a
466 state variable resides inside an anonymous subroutine, then each copy of
467 the subroutine has its own copy of the state variable. However, the value
468 of the state variable will still persist between calls to the same copy of
469 the anonymous subroutine. (Don't forget that C<sub { ... }> creates a new
470 subroutine each time it is executed.)
472 For example, the following code maintains a private counter, incremented
473 each time the gimme_another() function is called:
476 sub gimme_another { state $x; return ++$x }
478 And this example uses anonymous subroutines to create separate counters:
482 return sub { state $x; return ++$x }
485 Also, since C<$x> is lexical, it can't be reached or modified by any Perl
488 When combined with variable declaration, simple scalar assignment to C<state>
489 variables (as in C<state $x = 42>) is executed only the first time. When such
490 statements are evaluated subsequent times, the assignment is ignored. The
491 behavior of this sort of assignment to non-scalar variables is undefined.
493 =head3 Persistent variables with closures
495 Just because a lexical variable is lexically (also called statically)
496 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
497 within a function it works like a C static. It normally works more
498 like a C auto, but with implicit garbage collection.
500 Unlike local variables in C or C++, Perl's lexical variables don't
501 necessarily get recycled just because their scope has exited.
502 If something more permanent is still aware of the lexical, it will
503 stick around. So long as something else references a lexical, that
504 lexical won't be freed--which is as it should be. You wouldn't want
505 memory being free until you were done using it, or kept around once you
506 were done. Automatic garbage collection takes care of this for you.
508 This means that you can pass back or save away references to lexical
509 variables, whereas to return a pointer to a C auto is a grave error.
510 It also gives us a way to simulate C's function statics. Here's a
511 mechanism for giving a function private variables with both lexical
512 scoping and a static lifetime. If you do want to create something like
513 C's static variables, just enclose the whole function in an extra block,
514 and put the static variable outside the function but in the block.
519 return ++$secret_val;
522 # $secret_val now becomes unreachable by the outside
523 # world, but retains its value between calls to gimme_another
525 If this function is being sourced in from a separate file
526 via C<require> or C<use>, then this is probably just fine. If it's
527 all in the main program, you'll need to arrange for the C<my>
528 to be executed early, either by putting the whole block above
529 your main program, or more likely, placing merely a C<BEGIN>
530 code block around it to make sure it gets executed before your program
536 return ++$secret_val;
540 See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the
541 special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>,
544 If declared at the outermost scope (the file scope), then lexicals
545 work somewhat like C's file statics. They are available to all
546 functions in that same file declared below them, but are inaccessible
547 from outside that file. This strategy is sometimes used in modules
548 to create private variables that the whole module can see.
550 =head2 Temporary Values via local()
551 X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
552 X<variable, temporary>
554 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
555 it's faster and safer. Exceptions to this include the global punctuation
556 variables, global filehandles and formats, and direct manipulation of the
557 Perl symbol table itself. C<local> is mostly used when the current value
558 of a variable must be visible to called subroutines.
562 # localization of values
564 local $foo; # make $foo dynamically local
565 local (@wid, %get); # make list of variables local
566 local $foo = "flurp"; # make $foo dynamic, and init it
567 local @oof = @bar; # make @oof dynamic, and init it
569 local $hash{key} = "val"; # sets a local value for this hash entry
570 delete local $hash{key}; # delete this entry for the current block
571 local ($cond ? $v1 : $v2); # several types of lvalues support
574 # localization of symbols
576 local *FH; # localize $FH, @FH, %FH, &FH ...
577 local *merlyn = *randal; # now $merlyn is really $randal, plus
578 # @merlyn is really @randal, etc
579 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
580 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
582 A C<local> modifies its listed variables to be "local" to the
583 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
584 called from within that block>. A C<local> just gives temporary
585 values to global (meaning package) variables. It does I<not> create
586 a local variable. This is known as dynamic scoping. Lexical scoping
587 is done with C<my>, which works more like C's auto declarations.
589 Some types of lvalues can be localized as well: hash and array elements
590 and slices, conditionals (provided that their result is always
591 localizable), and symbolic references. As for simple variables, this
592 creates new, dynamically scoped values.
594 If more than one variable or expression is given to C<local>, they must be
595 placed in parentheses. This operator works
596 by saving the current values of those variables in its argument list on a
597 hidden stack and restoring them upon exiting the block, subroutine, or
598 eval. This means that called subroutines can also reference the local
599 variable, but not the global one. The argument list may be assigned to if
600 desired, which allows you to initialize your local variables. (If no
601 initializer is given for a particular variable, it is created with an
604 Because C<local> is a run-time operator, it gets executed each time
605 through a loop. Consequently, it's more efficient to localize your
606 variables outside the loop.
608 =head3 Grammatical note on local()
611 A C<local> is simply a modifier on an lvalue expression. When you assign to
612 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
613 as a scalar or an array. So
615 local($foo) = <STDIN>;
616 local @FOO = <STDIN>;
618 both supply a list context to the right-hand side, while
620 local $foo = <STDIN>;
622 supplies a scalar context.
624 =head3 Localization of special variables
625 X<local, special variable>
627 If you localize a special variable, you'll be giving a new value to it,
628 but its magic won't go away. That means that all side-effects related
629 to this magic still work with the localized value.
631 This feature allows code like this to work :
633 # Read the whole contents of FILE in $slurp
634 { local $/ = undef; $slurp = <FILE>; }
636 Note, however, that this restricts localization of some values ; for
637 example, the following statement dies, as of perl 5.10.0, with an error
638 I<Modification of a read-only value attempted>, because the $1 variable is
639 magical and read-only :
643 One exception is the default scalar variable: starting with perl 5.14
644 C<local($_)> will always strip all magic from $_, to make it possible
645 to safely reuse $_ in a subroutine.
647 B<WARNING>: Localization of tied arrays and hashes does not currently
649 This will be fixed in a future release of Perl; in the meantime, avoid
650 code that relies on any particular behaviour of localising tied arrays
651 or hashes (localising individual elements is still okay).
652 See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
656 =head3 Localization of globs
657 X<local, glob> X<glob>
663 creates a whole new symbol table entry for the glob C<name> in the
664 current package. That means that all variables in its glob slot ($name,
665 @name, %name, &name, and the C<name> filehandle) are dynamically reset.
667 This implies, among other things, that any magic eventually carried by
668 those variables is locally lost. In other words, saying C<local */>
669 will not have any effect on the internal value of the input record
672 =head3 Localization of elements of composite types
673 X<local, composite type element> X<local, array element> X<local, hash element>
675 It's also worth taking a moment to explain what happens when you
676 C<local>ize a member of a composite type (i.e. an array or hash element).
677 In this case, the element is C<local>ized I<by name>. This means that
678 when the scope of the C<local()> ends, the saved value will be
679 restored to the hash element whose key was named in the C<local()>, or
680 the array element whose index was named in the C<local()>. If that
681 element was deleted while the C<local()> was in effect (e.g. by a
682 C<delete()> from a hash or a C<shift()> of an array), it will spring
683 back into existence, possibly extending an array and filling in the
684 skipped elements with C<undef>. For instance, if you say
686 %hash = ( 'This' => 'is', 'a' => 'test' );
690 local($hash{'a'}) = 'drill';
691 while (my $e = pop(@ary)) {
696 $hash{'only a'} = 'test';
700 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
701 print "The array has ",scalar(@ary)," elements: ",
702 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
709 This is a test only a test.
710 The array has 6 elements: 0, 1, 2, undef, undef, 5
712 The behavior of local() on non-existent members of composite
713 types is subject to change in future.
715 =head3 Localized deletion of elements of composite types
716 X<delete> X<local, composite type element> X<local, array element> X<local, hash element>
718 You can use the C<delete local $array[$idx]> and C<delete local $hash{key}>
719 constructs to delete a composite type entry for the current block and restore
720 it when it ends. They return the array/hash value before the localization,
721 which means that they are respectively equivalent to
724 my $val = $array[$idx];
733 my $val = $hash{key};
739 except that for those the C<local> is scoped to the C<do> block. Slices are
748 my $a = delete local $hash{a};
753 my @nums = delete local @$a[0, 2]
757 $a[0] = 999; # will be erased when the scope ends
759 # $a is back to [ 7, 8, 9 ]
762 # %hash is back to its original state
764 =head2 Lvalue subroutines
765 X<lvalue> X<subroutine, lvalue>
767 It is possible to return a modifiable value from a subroutine.
768 To do this, you have to declare the subroutine to return an lvalue.
771 sub canmod : lvalue {
772 $val; # or: return $val;
778 canmod() = 5; # assigns to $val
781 The scalar/list context for the subroutine and for the right-hand
782 side of assignment is determined as if the subroutine call is replaced
783 by a scalar. For example, consider:
785 data(2,3) = get_data(3,4);
787 Both subroutines here are called in a scalar context, while in:
789 (data(2,3)) = get_data(3,4);
793 (data(2),data(3)) = get_data(3,4);
795 all the subroutines are called in a list context.
797 Lvalue subroutines are convenient, but you have to keep in mind that,
798 when used with objects, they may violate encapsulation. A normal
799 mutator can check the supplied argument before setting the attribute
800 it is protecting, an lvalue subroutine cannot. If you require any
801 special processing when storing and retrieving the values, consider
802 using the CPAN module Sentinel or something similar.
804 =head2 Lexical Subroutines
805 X<my sub> X<state sub> X<our sub> X<subroutine, lexical>
807 B<WARNING>: Lexical subroutines are still experimental. The feature may be
808 modified or removed in future versions of Perl.
810 Lexical subroutines are only available under the C<use feature
811 'lexical_subs'> pragma, which produces a warning unless the
812 "experimental::lexical_subs" warnings category is disabled.
814 Beginning with Perl 5.18, you can declare a private subroutine with C<my>
815 or C<state>. As with state variables, the C<state> keyword is only
816 available under C<use feature 'state'> or C<use 5.010> or higher.
818 These subroutines are only visible within the block in which they are
819 declared, and only after that declaration:
821 no warnings "experimental::lexical_subs";
822 use feature 'lexical_subs';
824 foo(); # calls the package/global subroutine
826 foo(); # also calls the package subroutine
828 foo(); # calls "state" sub
829 my $ref = \&foo; # take a reference to "state" sub
832 bar(); # calls "my" sub
834 To use a lexical subroutine from inside the subroutine itself, you must
835 predeclare it. The C<sub foo {...}> subroutine definition syntax respects
836 any previous C<my sub;> or C<state sub;> declaration.
838 my sub baz; # predeclaration
839 sub baz { # define the "my" sub
840 baz(); # recursive call
843 =head3 C<state sub> vs C<my sub>
845 What is the difference between "state" subs and "my" subs? Each time that
846 execution enters a block when "my" subs are declared, a new copy of each
847 sub is created. "State" subroutines persist from one execution of the
848 containing block to the next.
850 So, in general, "state" subroutines are faster. But "my" subs are
851 necessary if you want to create closures:
853 no warnings "experimental::lexical_subs";
854 use feature 'lexical_subs';
859 ... do something with $x ...
864 In this example, a new C<$x> is created when C<whatever> is called, and
865 also a new C<inner>, which can see the new C<$x>. A "state" sub will only
866 see the C<$x> from the first call to C<whatever>.
868 =head3 C<our> subroutines
870 Like C<our $variable>, C<our sub> creates a lexical alias to the package
871 subroutine of the same name.
873 The two main uses for this are to switch back to using the package sub
874 inside an inner scope:
876 no warnings "experimental::lexical_subs";
877 use feature 'lexical_subs';
884 # need to use the outer foo here
890 and to make a subroutine visible to other packages in the same scope:
892 package MySneakyModule;
894 no warnings "experimental::lexical_subs";
895 use feature 'lexical_subs';
897 our sub do_something { ... }
899 sub do_something_with_caller {
901 () = caller 1; # sets @DB::args
902 do_something(@args); # uses MySneakyModule::do_something
905 =head2 Passing Symbol Table Entries (typeglobs)
908 B<WARNING>: The mechanism described in this section was originally
909 the only way to simulate pass-by-reference in older versions of
910 Perl. While it still works fine in modern versions, the new reference
911 mechanism is generally easier to work with. See below.
913 Sometimes you don't want to pass the value of an array to a subroutine
914 but rather the name of it, so that the subroutine can modify the global
915 copy of it rather than working with a local copy. In perl you can
916 refer to all objects of a particular name by prefixing the name
917 with a star: C<*foo>. This is often known as a "typeglob", because the
918 star on the front can be thought of as a wildcard match for all the
919 funny prefix characters on variables and subroutines and such.
921 When evaluated, the typeglob produces a scalar value that represents
922 all the objects of that name, including any filehandle, format, or
923 subroutine. When assigned to, it causes the name mentioned to refer to
924 whatever C<*> value was assigned to it. Example:
927 local(*someary) = @_;
928 foreach $elem (@someary) {
935 Scalars are already passed by reference, so you can modify
936 scalar arguments without using this mechanism by referring explicitly
937 to C<$_[0]> etc. You can modify all the elements of an array by passing
938 all the elements as scalars, but you have to use the C<*> mechanism (or
939 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
940 an array. It will certainly be faster to pass the typeglob (or reference).
942 Even if you don't want to modify an array, this mechanism is useful for
943 passing multiple arrays in a single LIST, because normally the LIST
944 mechanism will merge all the array values so that you can't extract out
945 the individual arrays. For more on typeglobs, see
946 L<perldata/"Typeglobs and Filehandles">.
948 =head2 When to Still Use local()
949 X<local> X<variable, local>
951 Despite the existence of C<my>, there are still three places where the
952 C<local> operator still shines. In fact, in these three places, you
953 I<must> use C<local> instead of C<my>.
959 You need to give a global variable a temporary value, especially $_.
961 The global variables, like C<@ARGV> or the punctuation variables, must be
962 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
963 it up into chunks separated by lines of equal signs, which are placed
967 local @ARGV = ("/etc/motd");
970 @Fields = split /^\s*=+\s*$/;
973 It particular, it's important to C<local>ize $_ in any routine that assigns
974 to it. Look out for implicit assignments in C<while> conditionals.
978 You need to create a local file or directory handle or a local function.
980 A function that needs a filehandle of its own must use
981 C<local()> on a complete typeglob. This can be used to create new symbol
985 local (*READER, *WRITER); # not my!
986 pipe (READER, WRITER) or die "pipe: $!";
987 return (*READER, *WRITER);
989 ($head, $tail) = ioqueue();
991 See the Symbol module for a way to create anonymous symbol table
994 Because assignment of a reference to a typeglob creates an alias, this
995 can be used to create what is effectively a local function, or at least,
999 local *grow = \&shrink; # only until this block exits
1000 grow(); # really calls shrink()
1001 move(); # if move() grow()s, it shrink()s too
1003 grow(); # get the real grow() again
1005 See L<perlref/"Function Templates"> for more about manipulating
1006 functions by name in this way.
1010 You want to temporarily change just one element of an array or hash.
1012 You can C<local>ize just one element of an aggregate. Usually this
1013 is done on dynamics:
1016 local $SIG{INT} = 'IGNORE';
1017 funct(); # uninterruptible
1019 # interruptibility automatically restored here
1021 But it also works on lexically declared aggregates.
1025 =head2 Pass by Reference
1026 X<pass by reference> X<pass-by-reference> X<reference>
1028 If you want to pass more than one array or hash into a function--or
1029 return them from it--and have them maintain their integrity, then
1030 you're going to have to use an explicit pass-by-reference. Before you
1031 do that, you need to understand references as detailed in L<perlref>.
1032 This section may not make much sense to you otherwise.
1034 Here are a few simple examples. First, let's pass in several arrays
1035 to a function and have it C<pop> all of then, returning a new list
1036 of all their former last elements:
1038 @tailings = popmany ( \@a, \@b, \@c, \@d );
1043 foreach $aref ( @_ ) {
1044 push @retlist, pop @$aref;
1049 Here's how you might write a function that returns a
1050 list of keys occurring in all the hashes passed to it:
1052 @common = inter( \%foo, \%bar, \%joe );
1054 my ($k, $href, %seen); # locals
1055 foreach $href (@_) {
1056 while ( $k = each %$href ) {
1060 return grep { $seen{$_} == @_ } keys %seen;
1063 So far, we're using just the normal list return mechanism.
1064 What happens if you want to pass or return a hash? Well,
1065 if you're using only one of them, or you don't mind them
1066 concatenating, then the normal calling convention is ok, although
1069 Where people get into trouble is here:
1071 (@a, @b) = func(@c, @d);
1073 (%a, %b) = func(%c, %d);
1075 That syntax simply won't work. It sets just C<@a> or C<%a> and
1076 clears the C<@b> or C<%b>. Plus the function didn't get passed
1077 into two separate arrays or hashes: it got one long list in C<@_>,
1080 If you can arrange for everyone to deal with this through references, it's
1081 cleaner code, although not so nice to look at. Here's a function that
1082 takes two array references as arguments, returning the two array elements
1083 in order of how many elements they have in them:
1085 ($aref, $bref) = func(\@c, \@d);
1086 print "@$aref has more than @$bref\n";
1088 my ($cref, $dref) = @_;
1089 if (@$cref > @$dref) {
1090 return ($cref, $dref);
1092 return ($dref, $cref);
1096 It turns out that you can actually do this also:
1098 (*a, *b) = func(\@c, \@d);
1099 print "@a has more than @b\n";
1101 local (*c, *d) = @_;
1109 Here we're using the typeglobs to do symbol table aliasing. It's
1110 a tad subtle, though, and also won't work if you're using C<my>
1111 variables, because only globals (even in disguise as C<local>s)
1112 are in the symbol table.
1114 If you're passing around filehandles, you could usually just use the bare
1115 typeglob, like C<*STDOUT>, but typeglobs references work, too.
1121 print $fh "her um well a hmmm\n";
1124 $rec = get_rec(\*STDIN);
1127 return scalar <$fh>;
1130 If you're planning on generating new filehandles, you could do this.
1131 Notice to pass back just the bare *FH, not its reference.
1136 return open (FH, $path) ? *FH : undef;
1140 X<prototype> X<subroutine, prototype>
1142 Perl supports a very limited kind of compile-time argument checking
1143 using function prototyping. If you declare
1147 then C<mypush()> takes arguments exactly like C<push()> does. The
1148 function declaration must be visible at compile time. The prototype
1149 affects only interpretation of new-style calls to the function,
1150 where new-style is defined as not using the C<&> character. In
1151 other words, if you call it like a built-in function, then it behaves
1152 like a built-in function. If you call it like an old-fashioned
1153 subroutine, then it behaves like an old-fashioned subroutine. It
1154 naturally falls out from this rule that prototypes have no influence
1155 on subroutine references like C<\&foo> or on indirect subroutine
1156 calls like C<&{$subref}> or C<< $subref->() >>.
1158 Method calls are not influenced by prototypes either, because the
1159 function to be called is indeterminate at compile time, since
1160 the exact code called depends on inheritance.
1162 Because the intent of this feature is primarily to let you define
1163 subroutines that work like built-in functions, here are prototypes
1164 for some other functions that parse almost exactly like the
1165 corresponding built-in.
1167 Declared as Called as
1169 sub mylink ($$) mylink $old, $new
1170 sub myvec ($$$) myvec $var, $offset, 1
1171 sub myindex ($$;$) myindex &getstring, "substr"
1172 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
1173 sub myreverse (@) myreverse $a, $b, $c
1174 sub myjoin ($@) myjoin ":", $a, $b, $c
1175 sub mypop (+) mypop @array
1176 sub mysplice (+$$@) mysplice @array, 0, 2, @pushme
1177 sub mykeys (+) mykeys %{$hashref}
1178 sub myopen (*;$) myopen HANDLE, $name
1179 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
1180 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
1181 sub myrand (;$) myrand 42
1182 sub mytime () mytime
1184 Any backslashed prototype character represents an actual argument
1185 that must start with that character (optionally preceded by C<my>,
1186 C<our> or C<local>), with the exception of C<$>, which will
1187 accept any scalar lvalue expression, such as C<$foo = 7> or
1188 C<< my_function()->[0] >>. The value passed as part of C<@_> will be a
1189 reference to the actual argument given in the subroutine call,
1190 obtained by applying C<\> to that argument.
1192 You can use the C<\[]> backslash group notation to specify more than one
1193 allowed argument type. For example:
1195 sub myref (\[$@%&*])
1197 will allow calling myref() as
1205 and the first argument of myref() will be a reference to
1206 a scalar, an array, a hash, a code, or a glob.
1208 Unbackslashed prototype characters have special meanings. Any
1209 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
1210 list context. An argument represented by C<$> forces scalar context. An
1211 C<&> requires an anonymous subroutine, which, if passed as the first
1212 argument, does not require the C<sub> keyword or a subsequent comma.
1214 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
1215 typeglob, or a reference to a typeglob in that slot. The value will be
1216 available to the subroutine either as a simple scalar, or (in the latter
1217 two cases) as a reference to the typeglob. If you wish to always convert
1218 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1221 use Symbol 'qualify_to_ref';
1224 my $fh = qualify_to_ref(shift, caller);
1228 The C<+> prototype is a special alternative to C<$> that will act like
1229 C<\[@%]> when given a literal array or hash variable, but will otherwise
1230 force scalar context on the argument. This is useful for functions which
1231 should accept either a literal array or an array reference as the argument:
1235 die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
1239 When using the C<+> prototype, your function must check that the argument
1240 is of an acceptable type.
1242 A semicolon (C<;>) separates mandatory arguments from optional arguments.
1243 It is redundant before C<@> or C<%>, which gobble up everything else.
1245 As the last character of a prototype, or just before a semicolon, a C<@>
1246 or a C<%>, you can use C<_> in place of C<$>: if this argument is not
1247 provided, C<$_> will be used instead.
1249 Note how the last three examples in the table above are treated
1250 specially by the parser. C<mygrep()> is parsed as a true list
1251 operator, C<myrand()> is parsed as a true unary operator with unary
1252 precedence the same as C<rand()>, and C<mytime()> is truly without
1253 arguments, just like C<time()>. That is, if you say
1257 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1258 without a prototype. If you want to force a unary function to have the
1259 same precedence as a list operator, add C<;> to the end of the prototype:
1261 sub mygetprotobynumber($;);
1262 mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)
1264 The interesting thing about C<&> is that you can generate new syntax with it,
1265 provided it's in the initial position:
1269 my($try,$catch) = @_;
1276 sub catch (&) { $_[0] }
1281 /phooey/ and print "unphooey\n";
1284 That prints C<"unphooey">. (Yes, there are still unresolved
1285 issues having to do with visibility of C<@_>. I'm ignoring that
1286 question for the moment. (But note that if we make C<@_> lexically
1287 scoped, those anonymous subroutines can act like closures... (Gee,
1288 is this sounding a little Lispish? (Never mind.))))
1290 And here's a reimplementation of the Perl C<grep> operator:
1297 push(@result, $_) if &$code;
1302 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1303 been intentionally left out of prototypes for the express purpose of
1304 someday in the future adding named, formal parameters. The current
1305 mechanism's main goal is to let module writers provide better diagnostics
1306 for module users. Larry feels the notation quite understandable to Perl
1307 programmers, and that it will not intrude greatly upon the meat of the
1308 module, nor make it harder to read. The line noise is visually
1309 encapsulated into a small pill that's easy to swallow.
1311 If you try to use an alphanumeric sequence in a prototype you will
1312 generate an optional warning - "Illegal character in prototype...".
1313 Unfortunately earlier versions of Perl allowed the prototype to be
1314 used as long as its prefix was a valid prototype. The warning may be
1315 upgraded to a fatal error in a future version of Perl once the
1316 majority of offending code is fixed.
1318 It's probably best to prototype new functions, not retrofit prototyping
1319 into older ones. That's because you must be especially careful about
1320 silent impositions of differing list versus scalar contexts. For example,
1321 if you decide that a function should take just one parameter, like this:
1325 print "you gave me $n\n";
1328 and someone has been calling it with an array or expression
1334 Then you've just supplied an automatic C<scalar> in front of their
1335 argument, which can be more than a bit surprising. The old C<@foo>
1336 which used to hold one thing doesn't get passed in. Instead,
1337 C<func()> now gets passed in a C<1>; that is, the number of elements
1338 in C<@foo>. And the C<split> gets called in scalar context so it
1339 starts scribbling on your C<@_> parameter list. Ouch!
1341 If a sub has both a PROTO and a BLOCK, the prototype is not applied
1342 until after the BLOCK is completely defined. This means that a recursive
1343 function with a prototype has to be predeclared for the prototype to take
1351 This is all very powerful, of course, and should be used only in moderation
1352 to make the world a better place.
1354 =head2 Constant Functions
1357 Functions with a prototype of C<()> are potential candidates for
1358 inlining. If the result after optimization and constant folding
1359 is either a constant or a lexically-scoped scalar which has no other
1360 references, then it will be used in place of function calls made
1361 without C<&>. Calls made using C<&> are never inlined. (See
1362 F<constant.pm> for an easy way to declare most constants.)
1364 The following functions would all be inlined:
1366 sub pi () { 3.14159 } # Not exact, but close.
1367 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1368 # and it's inlined, too!
1372 sub FLAG_FOO () { 1 << 8 }
1373 sub FLAG_BAR () { 1 << 9 }
1374 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1376 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1378 sub N () { int(OPT_BAZ) / 3 }
1380 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1382 Be aware that these will not be inlined; as they contain inner scopes,
1383 the constant folding doesn't reduce them to a single constant:
1385 sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1396 If you redefine a subroutine that was eligible for inlining, you'll get
1397 a warning by default. (You can use this warning to tell whether or not a
1398 particular subroutine is considered inlinable.) The warning is
1399 considered severe enough not to be affected by the B<-w>
1400 switch (or its absence) because previously compiled
1401 invocations of the function will still be using the old value of the
1402 function. If you need to be able to redefine the subroutine, you need to
1403 ensure that it isn't inlined, either by dropping the C<()> prototype
1404 (which changes calling semantics, so beware) or by thwarting the
1405 inlining mechanism in some other way, such as
1407 sub not_inlined () {
1411 =head2 Overriding Built-in Functions
1412 X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
1414 Many built-in functions may be overridden, though this should be tried
1415 only occasionally and for good reason. Typically this might be
1416 done by a package attempting to emulate missing built-in functionality
1417 on a non-Unix system.
1419 Overriding may be done only by importing the name from a module at
1420 compile time--ordinary predeclaration isn't good enough. However, the
1421 C<use subs> pragma lets you, in effect, predeclare subs
1422 via the import syntax, and these names may then override built-in ones:
1424 use subs 'chdir', 'chroot', 'chmod', 'chown';
1428 To unambiguously refer to the built-in form, precede the
1429 built-in name with the special package qualifier C<CORE::>. For example,
1430 saying C<CORE::open()> always refers to the built-in C<open()>, even
1431 if the current package has imported some other subroutine called
1432 C<&open()> from elsewhere. Even though it looks like a regular
1433 function call, it isn't: the CORE:: prefix in that case is part of Perl's
1434 syntax, and works for any keyword, regardless of what is in the CORE
1435 package. Taking a reference to it, that is, C<\&CORE::open>, only works
1436 for some keywords. See L<CORE>.
1438 Library modules should not in general export built-in names like C<open>
1439 or C<chdir> as part of their default C<@EXPORT> list, because these may
1440 sneak into someone else's namespace and change the semantics unexpectedly.
1441 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1442 possible for a user to import the name explicitly, but not implicitly.
1443 That is, they could say
1447 and it would import the C<open> override. But if they said
1451 they would get the default imports without overrides.
1453 The foregoing mechanism for overriding built-in is restricted, quite
1454 deliberately, to the package that requests the import. There is a second
1455 method that is sometimes applicable when you wish to override a built-in
1456 everywhere, without regard to namespace boundaries. This is achieved by
1457 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1458 example that quite brazenly replaces the C<glob> operator with something
1459 that understands regular expressions.
1464 @EXPORT_OK = 'glob';
1470 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1471 $pkg->export($where, $sym, @_);
1477 if (opendir my $d, '.') {
1478 @got = grep /$pat/, readdir $d;
1485 And here's how it could be (ab)used:
1487 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1489 use REGlob 'glob'; # override glob() in Foo:: only
1490 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1492 The initial comment shows a contrived, even dangerous example.
1493 By overriding C<glob> globally, you would be forcing the new (and
1494 subversive) behavior for the C<glob> operator for I<every> namespace,
1495 without the complete cognizance or cooperation of the modules that own
1496 those namespaces. Naturally, this should be done with extreme caution--if
1497 it must be done at all.
1499 The C<REGlob> example above does not implement all the support needed to
1500 cleanly override perl's C<glob> operator. The built-in C<glob> has
1501 different behaviors depending on whether it appears in a scalar or list
1502 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1503 context sensitive behaviors, and these must be adequately supported by
1504 a properly written override. For a fully functional example of overriding
1505 C<glob>, study the implementation of C<File::DosGlob> in the standard
1508 When you override a built-in, your replacement should be consistent (if
1509 possible) with the built-in native syntax. You can achieve this by using
1510 a suitable prototype. To get the prototype of an overridable built-in,
1511 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1512 (see L<perlfunc/prototype>).
1514 Note however that some built-ins can't have their syntax expressed by a
1515 prototype (such as C<system> or C<chomp>). If you override them you won't
1516 be able to fully mimic their original syntax.
1518 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1519 to special magic, their original syntax is preserved, and you don't have
1520 to define a prototype for their replacements. (You can't override the
1521 C<do BLOCK> syntax, though).
1523 C<require> has special additional dark magic: if you invoke your
1524 C<require> replacement as C<require Foo::Bar>, it will actually receive
1525 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1527 And, as you'll have noticed from the previous example, if you override
1528 C<glob>, the C<< <*> >> glob operator is overridden as well.
1530 In a similar fashion, overriding the C<readline> function also overrides
1531 the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding
1532 C<readpipe> also overrides the operators C<``> and C<qx//>.
1534 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1537 X<autoloading> X<AUTOLOAD>
1539 If you call a subroutine that is undefined, you would ordinarily
1540 get an immediate, fatal error complaining that the subroutine doesn't
1541 exist. (Likewise for subroutines being used as methods, when the
1542 method doesn't exist in any base class of the class's package.)
1543 However, if an C<AUTOLOAD> subroutine is defined in the package or
1544 packages used to locate the original subroutine, then that
1545 C<AUTOLOAD> subroutine is called with the arguments that would have
1546 been passed to the original subroutine. The fully qualified name
1547 of the original subroutine magically appears in the global $AUTOLOAD
1548 variable of the same package as the C<AUTOLOAD> routine. The name
1549 is not passed as an ordinary argument because, er, well, just
1550 because, that's why. (As an exception, a method call to a nonexistent
1551 C<import> or C<unimport> method is just skipped instead. Also, if
1552 the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the
1553 subroutine name. See L<perlguts/Autoloading with XSUBs> for details.)
1556 Many C<AUTOLOAD> routines load in a definition for the requested
1557 subroutine using eval(), then execute that subroutine using a special
1558 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1559 without a trace. (See the source to the standard module documented
1560 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1561 also just emulate the routine and never define it. For example,
1562 let's pretend that a function that wasn't defined should just invoke
1563 C<system> with those arguments. All you'd do is:
1566 my $program = $AUTOLOAD;
1567 $program =~ s/.*:://;
1568 system($program, @_);
1574 In fact, if you predeclare functions you want to call that way, you don't
1575 even need parentheses:
1577 use subs qw(date who ls);
1582 A more complete example of this is the Shell module on CPAN, which
1583 can treat undefined subroutine calls as calls to external programs.
1585 Mechanisms are available to help modules writers split their modules
1586 into autoloadable files. See the standard AutoLoader module
1587 described in L<AutoLoader> and in L<AutoSplit>, the standard
1588 SelfLoader modules in L<SelfLoader>, and the document on adding C
1589 functions to Perl code in L<perlxs>.
1591 =head2 Subroutine Attributes
1592 X<attribute> X<subroutine, attribute> X<attrs>
1594 A subroutine declaration or definition may have a list of attributes
1595 associated with it. If such an attribute list is present, it is
1596 broken up at space or colon boundaries and treated as though a
1597 C<use attributes> had been seen. See L<attributes> for details
1598 about what attributes are currently supported.
1599 Unlike the limitation with the obsolescent C<use attrs>, the
1600 C<sub : ATTRLIST> syntax works to associate the attributes with
1601 a pre-declaration, and not just with a subroutine definition.
1603 The attributes must be valid as simple identifier names (without any
1604 punctuation other than the '_' character). They may have a parameter
1605 list appended, which is only checked for whether its parentheses ('(',')')
1608 Examples of valid syntax (even though the attributes are unknown):
1610 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1611 sub plugh () : Ugly('\(") :Bad;
1612 sub xyzzy : _5x5 { ... }
1614 Examples of invalid syntax:
1616 sub fnord : switch(10,foo(); # ()-string not balanced
1617 sub snoid : Ugly('('); # ()-string not balanced
1618 sub xyzzy : 5x5; # "5x5" not a valid identifier
1619 sub plugh : Y2::north; # "Y2::north" not a simple identifier
1620 sub snurt : foo + bar; # "+" not a colon or space
1622 The attribute list is passed as a list of constant strings to the code
1623 which associates them with the subroutine. In particular, the second example
1624 of valid syntax above currently looks like this in terms of how it's
1627 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1629 For further details on attribute lists and their manipulation,
1630 see L<attributes> and L<Attribute::Handlers>.
1634 See L<perlref/"Function Templates"> for more about references and closures.
1635 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1636 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1637 See L<perlmod> to learn about bundling up your functions in separate files.
1638 See L<perlmodlib> to learn what library modules come standard on your system.
1639 See L<perlootut> to learn how to make object method calls.