3 perlsub - Perl subroutines
7 To declare subroutines:
9 sub NAME; # A "forward" declaration.
10 sub NAME(PROTO); # ditto, but with prototypes
11 sub NAME : ATTRS; # with attributes
12 sub NAME(PROTO) : ATTRS; # with attributes and prototypes
14 sub NAME BLOCK # A declaration and a definition.
15 sub NAME(PROTO) BLOCK # ditto, but with prototypes
16 sub NAME : ATTRS BLOCK # with attributes
17 sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
19 To define an anonymous subroutine at runtime:
21 $subref = sub BLOCK; # no proto
22 $subref = sub (PROTO) BLOCK; # with proto
23 $subref = sub : ATTRS BLOCK; # with attributes
24 $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
26 To import subroutines:
28 use MODULE qw(NAME1 NAME2 NAME3);
32 NAME(LIST); # & is optional with parentheses.
33 NAME LIST; # Parentheses optional if predeclared/imported.
34 &NAME(LIST); # Circumvent prototypes.
35 &NAME; # Makes current @_ visible to called subroutine.
39 Like many languages, Perl provides for user-defined subroutines.
40 These may be located anywhere in the main program, loaded in from
41 other files via the C<do>, C<require>, or C<use> keywords, or
42 generated on the fly using C<eval> or anonymous subroutines.
43 You can even call a function indirectly using a variable containing
44 its name or a CODE reference.
46 The Perl model for function call and return values is simple: all
47 functions are passed as parameters one single flat list of scalars, and
48 all functions likewise return to their caller one single flat list of
49 scalars. Any arrays or hashes in these call and return lists will
50 collapse, losing their identities--but you may always use
51 pass-by-reference instead to avoid this. Both call and return lists may
52 contain as many or as few scalar elements as you'd like. (Often a
53 function without an explicit return statement is called a subroutine, but
54 there's really no difference from Perl's perspective.)
56 Any arguments passed in show up in the array C<@_>. Therefore, if
57 you called a function with two arguments, those would be stored in
58 C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its
59 elements are aliases for the actual scalar parameters. In particular,
60 if an element C<$_[0]> is updated, the corresponding argument is
61 updated (or an error occurs if it is not updatable). If an argument
62 is an array or hash element which did not exist when the function
63 was called, that element is created only when (and if) it is modified
64 or a reference to it is taken. (Some earlier versions of Perl
65 created the element whether or not the element was assigned to.)
66 Assigning to the whole array C<@_> removes that aliasing, and does
67 not update any arguments.
69 The return value of a subroutine is the value of the last expression
70 evaluated. More explicitly, a C<return> statement may be used to exit the
71 subroutine, optionally specifying the returned value, which will be
72 evaluated in the appropriate context (list, scalar, or void) depending
73 on the context of the subroutine call. If you specify no return value,
74 the subroutine returns an empty list in list context, the undefined
75 value in scalar context, or nothing in void context. If you return
76 one or more aggregates (arrays and hashes), these will be flattened
77 together into one large indistinguishable list.
79 Perl does not have named formal parameters. In practice all you
80 do is assign to a C<my()> list of these. Variables that aren't
81 declared to be private are global variables. For gory details
82 on creating private variables, see L<"Private Variables via my()">
83 and L<"Temporary Values via local()">. To create protected
84 environments for a set of functions in a separate package (and
85 probably a separate file), see L<perlmod/"Packages">.
92 $max = $foo if $max < $foo;
96 $bestday = max($mon,$tue,$wed,$thu,$fri);
100 # get a line, combining continuation lines
101 # that start with whitespace
104 $thisline = $lookahead; # global variables!
105 LINE: while (defined($lookahead = <STDIN>)) {
106 if ($lookahead =~ /^[ \t]/) {
107 $thisline .= $lookahead;
116 $lookahead = <STDIN>; # get first line
117 while (defined($line = get_line())) {
121 Assigning to a list of private variables to name your arguments:
124 my($key, $value) = @_;
125 $Foo{$key} = $value unless $Foo{$key};
128 Because the assignment copies the values, this also has the effect
129 of turning call-by-reference into call-by-value. Otherwise a
130 function is free to do in-place modifications of C<@_> and change
133 upcase_in($v1, $v2); # this changes $v1 and $v2
135 for (@_) { tr/a-z/A-Z/ }
138 You aren't allowed to modify constants in this way, of course. If an
139 argument were actually literal and you tried to change it, you'd take a
140 (presumably fatal) exception. For example, this won't work:
142 upcase_in("frederick");
144 It would be much safer if the C<upcase_in()> function
145 were written to return a copy of its parameters instead
146 of changing them in place:
148 ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
150 return unless defined wantarray; # void context, do nothing
152 for (@parms) { tr/a-z/A-Z/ }
153 return wantarray ? @parms : $parms[0];
156 Notice how this (unprototyped) function doesn't care whether it was
157 passed real scalars or arrays. Perl sees all arguments as one big,
158 long, flat parameter list in C<@_>. This is one area where
159 Perl's simple argument-passing style shines. The C<upcase()>
160 function would work perfectly well without changing the C<upcase()>
161 definition even if we fed it things like this:
163 @newlist = upcase(@list1, @list2);
164 @newlist = upcase( split /:/, $var );
166 Do not, however, be tempted to do this:
168 (@a, @b) = upcase(@list1, @list2);
170 Like the flattened incoming parameter list, the return list is also
171 flattened on return. So all you have managed to do here is stored
172 everything in C<@a> and made C<@b> empty. See
173 L<Pass by Reference> for alternatives.
175 A subroutine may be called using an explicit C<&> prefix. The
176 C<&> is optional in modern Perl, as are parentheses if the
177 subroutine has been predeclared. The C<&> is I<not> optional
178 when just naming the subroutine, such as when it's used as
179 an argument to defined() or undef(). Nor is it optional when you
180 want to do an indirect subroutine call with a subroutine name or
181 reference using the C<&$subref()> or C<&{$subref}()> constructs,
182 although the C<< $subref->() >> notation solves that problem.
183 See L<perlref> for more about all that.
185 Subroutines may be called recursively. If a subroutine is called
186 using the C<&> form, the argument list is optional, and if omitted,
187 no C<@_> array is set up for the subroutine: the C<@_> array at the
188 time of the call is visible to subroutine instead. This is an
189 efficiency mechanism that new users may wish to avoid.
191 &foo(1,2,3); # pass three arguments
192 foo(1,2,3); # the same
194 foo(); # pass a null list
197 &foo; # foo() get current args, like foo(@_) !!
198 foo; # like foo() IFF sub foo predeclared, else "foo"
200 Not only does the C<&> form make the argument list optional, it also
201 disables any prototype checking on arguments you do provide. This
202 is partly for historical reasons, and partly for having a convenient way
203 to cheat if you know what you're doing. See L<Prototypes> below.
205 Functions whose names are in all upper case are reserved to the Perl
206 core, as are modules whose names are in all lower case. A
207 function in all capitals is a loosely-held convention meaning it
208 will be called indirectly by the run-time system itself, usually
209 due to a triggered event. Functions that do special, pre-defined
210 things include C<BEGIN>, C<CHECK>, C<INIT>, C<END>, C<AUTOLOAD>,
211 C<CLONE> and C<DESTROY>--plus all functions mentioned in L<perltie>.
213 =head2 Private Variables via my()
217 my $foo; # declare $foo lexically local
218 my (@wid, %get); # declare list of variables local
219 my $foo = "flurp"; # declare $foo lexical, and init it
220 my @oof = @bar; # declare @oof lexical, and init it
221 my $x : Foo = $y; # similar, with an attribute applied
223 B<WARNING>: The use of attribute lists on C<my> declarations is
224 experimental. This feature should not be relied upon. It may
225 change or disappear in future releases of Perl. See L<attributes>.
227 The C<my> operator declares the listed variables to be lexically
228 confined to the enclosing block, conditional (C<if/unless/elsif/else>),
229 loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
230 or C<do/require/use>'d file. If more than one value is listed, the
231 list must be placed in parentheses. All listed elements must be
232 legal lvalues. Only alphanumeric identifiers may be lexically
233 scoped--magical built-ins like C<$/> must currently be C<local>ize
234 with C<local> instead.
236 Unlike dynamic variables created by the C<local> operator, lexical
237 variables declared with C<my> are totally hidden from the outside
238 world, including any called subroutines. This is true if it's the
239 same subroutine called from itself or elsewhere--every call gets
242 This doesn't mean that a C<my> variable declared in a statically
243 enclosing lexical scope would be invisible. Only dynamic scopes
244 are cut off. For example, the C<bumpx()> function below has access
245 to the lexical $x variable because both the C<my> and the C<sub>
246 occurred at the same scope, presumably file scope.
251 An C<eval()>, however, can see lexical variables of the scope it is
252 being evaluated in, so long as the names aren't hidden by declarations within
253 the C<eval()> itself. See L<perlref>.
255 The parameter list to my() may be assigned to if desired, which allows you
256 to initialize your variables. (If no initializer is given for a
257 particular variable, it is created with the undefined value.) Commonly
258 this is used to name input parameters to a subroutine. Examples:
260 $arg = "fred"; # "global" variable
262 print "$arg thinks the root is $n\n";
263 fred thinks the root is 3
266 my $arg = shift; # name doesn't matter
271 The C<my> is simply a modifier on something you might assign to. So when
272 you do assign to variables in its argument list, C<my> doesn't
273 change whether those variables are viewed as a scalar or an array. So
275 my ($foo) = <STDIN>; # WRONG?
278 both supply a list context to the right-hand side, while
282 supplies a scalar context. But the following declares only one variable:
284 my $foo, $bar = 1; # WRONG
286 That has the same effect as
291 The declared variable is not introduced (is not visible) until after
292 the current statement. Thus,
296 can be used to initialize a new $x with the value of the old $x, and
299 my $x = 123 and $x == 123
301 is false unless the old $x happened to have the value C<123>.
303 Lexical scopes of control structures are not bounded precisely by the
304 braces that delimit their controlled blocks; control expressions are
305 part of that scope, too. Thus in the loop
307 while (my $line = <>) {
313 the scope of $line extends from its declaration throughout the rest of
314 the loop construct (including the C<continue> clause), but not beyond
315 it. Similarly, in the conditional
317 if ((my $answer = <STDIN>) =~ /^yes$/i) {
319 } elsif ($answer =~ /^no$/i) {
323 die "'$answer' is neither 'yes' nor 'no'";
326 the scope of $answer extends from its declaration through the rest
327 of that conditional, including any C<elsif> and C<else> clauses,
330 B<NOTE:> The behaviour of a C<my> statement modified with a statement
331 modifier conditional or loop construct (e.g. C<my $x if ...>) is
332 B<undefined>. The value of the C<my> variable may be C<undef>, any
333 previously assigned value, or possibly anything else. Don't rely on
334 it. Future versions of perl might do something different from the
335 version of perl you try it out on. Here be dragons.
337 The C<foreach> loop defaults to scoping its index variable dynamically
338 in the manner of C<local>. However, if the index variable is
339 prefixed with the keyword C<my>, or if there is already a lexical
340 by that name in scope, then a new lexical is created instead. Thus
343 for my $i (1, 2, 3) {
347 the scope of $i extends to the end of the loop, but not beyond it,
348 rendering the value of $i inaccessible within C<some_function()>.
350 Some users may wish to encourage the use of lexically scoped variables.
351 As an aid to catching implicit uses to package variables,
352 which are always global, if you say
356 then any variable mentioned from there to the end of the enclosing
357 block must either refer to a lexical variable, be predeclared via
358 C<our> or C<use vars>, or else must be fully qualified with the package name.
359 A compilation error results otherwise. An inner block may countermand
360 this with C<no strict 'vars'>.
362 A C<my> has both a compile-time and a run-time effect. At compile
363 time, the compiler takes notice of it. The principal usefulness
364 of this is to quiet C<use strict 'vars'>, but it is also essential
365 for generation of closures as detailed in L<perlref>. Actual
366 initialization is delayed until run time, though, so it gets executed
367 at the appropriate time, such as each time through a loop, for
370 Variables declared with C<my> are not part of any package and are therefore
371 never fully qualified with the package name. In particular, you're not
372 allowed to try to make a package variable (or other global) lexical:
374 my $pack::var; # ERROR! Illegal syntax
375 my $_; # also illegal (currently)
377 In fact, a dynamic variable (also known as package or global variables)
378 are still accessible using the fully qualified C<::> notation even while a
379 lexical of the same name is also visible:
384 print "$x and $::x\n";
386 That will print out C<20> and C<10>.
388 You may declare C<my> variables at the outermost scope of a file
389 to hide any such identifiers from the world outside that file. This
390 is similar in spirit to C's static variables when they are used at
391 the file level. To do this with a subroutine requires the use of
392 a closure (an anonymous function that accesses enclosing lexicals).
393 If you want to create a private subroutine that cannot be called
394 from outside that block, it can declare a lexical variable containing
395 an anonymous sub reference:
397 my $secret_version = '1.001-beta';
398 my $secret_sub = sub { print $secret_version };
401 As long as the reference is never returned by any function within the
402 module, no outside module can see the subroutine, because its name is not in
403 any package's symbol table. Remember that it's not I<REALLY> called
404 C<$some_pack::secret_version> or anything; it's just $secret_version,
405 unqualified and unqualifiable.
407 This does not work with object methods, however; all object methods
408 have to be in the symbol table of some package to be found. See
409 L<perlref/"Function Templates"> for something of a work-around to
412 =head2 Persistent Private Variables
414 Just because a lexical variable is lexically (also called statically)
415 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
416 within a function it works like a C static. It normally works more
417 like a C auto, but with implicit garbage collection.
419 Unlike local variables in C or C++, Perl's lexical variables don't
420 necessarily get recycled just because their scope has exited.
421 If something more permanent is still aware of the lexical, it will
422 stick around. So long as something else references a lexical, that
423 lexical won't be freed--which is as it should be. You wouldn't want
424 memory being free until you were done using it, or kept around once you
425 were done. Automatic garbage collection takes care of this for you.
427 This means that you can pass back or save away references to lexical
428 variables, whereas to return a pointer to a C auto is a grave error.
429 It also gives us a way to simulate C's function statics. Here's a
430 mechanism for giving a function private variables with both lexical
431 scoping and a static lifetime. If you do want to create something like
432 C's static variables, just enclose the whole function in an extra block,
433 and put the static variable outside the function but in the block.
438 return ++$secret_val;
441 # $secret_val now becomes unreachable by the outside
442 # world, but retains its value between calls to gimme_another
444 If this function is being sourced in from a separate file
445 via C<require> or C<use>, then this is probably just fine. If it's
446 all in the main program, you'll need to arrange for the C<my>
447 to be executed early, either by putting the whole block above
448 your main program, or more likely, placing merely a C<BEGIN>
449 sub around it to make sure it gets executed before your program
455 return ++$secret_val;
459 See L<perlmod/"Package Constructors and Destructors"> about the
460 special triggered functions, C<BEGIN>, C<CHECK>, C<INIT> and C<END>.
462 If declared at the outermost scope (the file scope), then lexicals
463 work somewhat like C's file statics. They are available to all
464 functions in that same file declared below them, but are inaccessible
465 from outside that file. This strategy is sometimes used in modules
466 to create private variables that the whole module can see.
468 =head2 Temporary Values via local()
470 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
471 it's faster and safer. Exceptions to this include the global punctuation
472 variables, filehandles and formats, and direct manipulation of the Perl
473 symbol table itself. Format variables often use C<local> though, as do
474 other variables whose current value must be visible to called
479 local $foo; # declare $foo dynamically local
480 local (@wid, %get); # declare list of variables local
481 local $foo = "flurp"; # declare $foo dynamic, and init it
482 local @oof = @bar; # declare @oof dynamic, and init it
484 local *FH; # localize $FH, @FH, %FH, &FH ...
485 local *merlyn = *randal; # now $merlyn is really $randal, plus
486 # @merlyn is really @randal, etc
487 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
488 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
490 A C<local> modifies its listed variables to be "local" to the
491 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
492 called from within that block>. A C<local> just gives temporary
493 values to global (meaning package) variables. It does I<not> create
494 a local variable. This is known as dynamic scoping. Lexical scoping
495 is done with C<my>, which works more like C's auto declarations.
497 If more than one variable is given to C<local>, they must be placed in
498 parentheses. All listed elements must be legal lvalues. This operator works
499 by saving the current values of those variables in its argument list on a
500 hidden stack and restoring them upon exiting the block, subroutine, or
501 eval. This means that called subroutines can also reference the local
502 variable, but not the global one. The argument list may be assigned to if
503 desired, which allows you to initialize your local variables. (If no
504 initializer is given for a particular variable, it is created with an
505 undefined value.) Commonly this is used to name the parameters to a
506 subroutine. Examples:
511 # assume this function uses global %digits hash
514 # now temporarily add to %digits hash
516 # (NOTE: not claiming this is efficient!)
517 local %digits = (%digits, 't' => 10, 'e' => 11);
518 parse_num(); # parse_num gets this new %digits!
520 # old %digits restored here
522 Because C<local> is a run-time operator, it gets executed each time
523 through a loop. In releases of Perl previous to 5.0, this used more stack
524 storage each time until the loop was exited. Perl now reclaims the space
525 each time through, but it's still more efficient to declare your variables
528 A C<local> is simply a modifier on an lvalue expression. When you assign to
529 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
530 as a scalar or an array. So
532 local($foo) = <STDIN>;
533 local @FOO = <STDIN>;
535 both supply a list context to the right-hand side, while
537 local $foo = <STDIN>;
539 supplies a scalar context.
541 A note about C<local()> and composite types is in order. Something
542 like C<local(%foo)> works by temporarily placing a brand new hash in
543 the symbol table. The old hash is left alone, but is hidden "behind"
546 This means the old variable is completely invisible via the symbol
547 table (i.e. the hash entry in the C<*foo> typeglob) for the duration
548 of the dynamic scope within which the C<local()> was seen. This
549 has the effect of allowing one to temporarily occlude any magic on
550 composite types. For instance, this will briefly alter a tied
551 hash to some other implementation:
553 tie %ahash, 'APackage';
557 tie %ahash, 'BPackage';
558 [..called code will see %ahash tied to 'BPackage'..]
561 [..%ahash is a normal (untied) hash here..]
564 [..%ahash back to its initial tied self again..]
566 As another example, a custom implementation of C<%ENV> might look
571 tie %ENV, 'MyOwnEnv';
572 [..do your own fancy %ENV manipulation here..]
574 [..normal %ENV behavior here..]
576 It's also worth taking a moment to explain what happens when you
577 C<local>ize a member of a composite type (i.e. an array or hash element).
578 In this case, the element is C<local>ized I<by name>. This means that
579 when the scope of the C<local()> ends, the saved value will be
580 restored to the hash element whose key was named in the C<local()>, or
581 the array element whose index was named in the C<local()>. If that
582 element was deleted while the C<local()> was in effect (e.g. by a
583 C<delete()> from a hash or a C<shift()> of an array), it will spring
584 back into existence, possibly extending an array and filling in the
585 skipped elements with C<undef>. For instance, if you say
587 %hash = ( 'This' => 'is', 'a' => 'test' );
591 local($hash{'a'}) = 'drill';
592 while (my $e = pop(@ary)) {
597 $hash{'only a'} = 'test';
601 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
602 print "The array has ",scalar(@ary)," elements: ",
603 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
610 This is a test only a test.
611 The array has 6 elements: 0, 1, 2, undef, undef, 5
613 The behavior of local() on non-existent members of composite
614 types is subject to change in future.
616 =head2 Lvalue subroutines
618 B<WARNING>: Lvalue subroutines are still experimental and the
619 implementation may change in future versions of Perl.
621 It is possible to return a modifiable value from a subroutine.
622 To do this, you have to declare the subroutine to return an lvalue.
625 sub canmod : lvalue {
626 # return $val; this doesn't work, don't say "return"
633 canmod() = 5; # assigns to $val
636 The scalar/list context for the subroutine and for the right-hand
637 side of assignment is determined as if the subroutine call is replaced
638 by a scalar. For example, consider:
640 data(2,3) = get_data(3,4);
642 Both subroutines here are called in a scalar context, while in:
644 (data(2,3)) = get_data(3,4);
648 (data(2),data(3)) = get_data(3,4);
650 all the subroutines are called in a list context.
654 =item Lvalue subroutines are EXPERIMENTAL
656 They appear to be convenient, but there are several reasons to be
659 You can't use the return keyword, you must pass out the value before
660 falling out of subroutine scope. (see comment in example above). This
661 is usually not a problem, but it disallows an explicit return out of a
662 deeply nested loop, which is sometimes a nice way out.
664 They violate encapsulation. A normal mutator can check the supplied
665 argument before setting the attribute it is protecting, an lvalue
666 subroutine never gets that chance. Consider;
668 my $some_array_ref = []; # protected by mutators ??
670 sub set_arr { # normal mutator
672 die("expected array, you supplied ", ref $val)
673 unless ref $val eq 'ARRAY';
674 $some_array_ref = $val;
676 sub set_arr_lv : lvalue { # lvalue mutator
680 # set_arr_lv cannot stop this !
681 set_arr_lv() = { a => 1 };
685 =head2 Passing Symbol Table Entries (typeglobs)
687 B<WARNING>: The mechanism described in this section was originally
688 the only way to simulate pass-by-reference in older versions of
689 Perl. While it still works fine in modern versions, the new reference
690 mechanism is generally easier to work with. See below.
692 Sometimes you don't want to pass the value of an array to a subroutine
693 but rather the name of it, so that the subroutine can modify the global
694 copy of it rather than working with a local copy. In perl you can
695 refer to all objects of a particular name by prefixing the name
696 with a star: C<*foo>. This is often known as a "typeglob", because the
697 star on the front can be thought of as a wildcard match for all the
698 funny prefix characters on variables and subroutines and such.
700 When evaluated, the typeglob produces a scalar value that represents
701 all the objects of that name, including any filehandle, format, or
702 subroutine. When assigned to, it causes the name mentioned to refer to
703 whatever C<*> value was assigned to it. Example:
706 local(*someary) = @_;
707 foreach $elem (@someary) {
714 Scalars are already passed by reference, so you can modify
715 scalar arguments without using this mechanism by referring explicitly
716 to C<$_[0]> etc. You can modify all the elements of an array by passing
717 all the elements as scalars, but you have to use the C<*> mechanism (or
718 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
719 an array. It will certainly be faster to pass the typeglob (or reference).
721 Even if you don't want to modify an array, this mechanism is useful for
722 passing multiple arrays in a single LIST, because normally the LIST
723 mechanism will merge all the array values so that you can't extract out
724 the individual arrays. For more on typeglobs, see
725 L<perldata/"Typeglobs and Filehandles">.
727 =head2 When to Still Use local()
729 Despite the existence of C<my>, there are still three places where the
730 C<local> operator still shines. In fact, in these three places, you
731 I<must> use C<local> instead of C<my>.
737 You need to give a global variable a temporary value, especially $_.
739 The global variables, like C<@ARGV> or the punctuation variables, must be
740 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
741 it up into chunks separated by lines of equal signs, which are placed
745 local @ARGV = ("/etc/motd");
748 @Fields = split /^\s*=+\s*$/;
751 It particular, it's important to C<local>ize $_ in any routine that assigns
752 to it. Look out for implicit assignments in C<while> conditionals.
756 You need to create a local file or directory handle or a local function.
758 A function that needs a filehandle of its own must use
759 C<local()> on a complete typeglob. This can be used to create new symbol
763 local (*READER, *WRITER); # not my!
764 pipe (READER, WRITER) or die "pipe: $!";
765 return (*READER, *WRITER);
767 ($head, $tail) = ioqueue();
769 See the Symbol module for a way to create anonymous symbol table
772 Because assignment of a reference to a typeglob creates an alias, this
773 can be used to create what is effectively a local function, or at least,
777 local *grow = \&shrink; # only until this block exists
778 grow(); # really calls shrink()
779 move(); # if move() grow()s, it shrink()s too
781 grow(); # get the real grow() again
783 See L<perlref/"Function Templates"> for more about manipulating
784 functions by name in this way.
788 You want to temporarily change just one element of an array or hash.
790 You can C<local>ize just one element of an aggregate. Usually this
794 local $SIG{INT} = 'IGNORE';
795 funct(); # uninterruptible
797 # interruptibility automatically restored here
799 But it also works on lexically declared aggregates. Prior to 5.005,
800 this operation could on occasion misbehave.
804 =head2 Pass by Reference
806 If you want to pass more than one array or hash into a function--or
807 return them from it--and have them maintain their integrity, then
808 you're going to have to use an explicit pass-by-reference. Before you
809 do that, you need to understand references as detailed in L<perlref>.
810 This section may not make much sense to you otherwise.
812 Here are a few simple examples. First, let's pass in several arrays
813 to a function and have it C<pop> all of then, returning a new list
814 of all their former last elements:
816 @tailings = popmany ( \@a, \@b, \@c, \@d );
821 foreach $aref ( @_ ) {
822 push @retlist, pop @$aref;
827 Here's how you might write a function that returns a
828 list of keys occurring in all the hashes passed to it:
830 @common = inter( \%foo, \%bar, \%joe );
832 my ($k, $href, %seen); # locals
834 while ( $k = each %$href ) {
838 return grep { $seen{$_} == @_ } keys %seen;
841 So far, we're using just the normal list return mechanism.
842 What happens if you want to pass or return a hash? Well,
843 if you're using only one of them, or you don't mind them
844 concatenating, then the normal calling convention is ok, although
847 Where people get into trouble is here:
849 (@a, @b) = func(@c, @d);
851 (%a, %b) = func(%c, %d);
853 That syntax simply won't work. It sets just C<@a> or C<%a> and
854 clears the C<@b> or C<%b>. Plus the function didn't get passed
855 into two separate arrays or hashes: it got one long list in C<@_>,
858 If you can arrange for everyone to deal with this through references, it's
859 cleaner code, although not so nice to look at. Here's a function that
860 takes two array references as arguments, returning the two array elements
861 in order of how many elements they have in them:
863 ($aref, $bref) = func(\@c, \@d);
864 print "@$aref has more than @$bref\n";
866 my ($cref, $dref) = @_;
867 if (@$cref > @$dref) {
868 return ($cref, $dref);
870 return ($dref, $cref);
874 It turns out that you can actually do this also:
876 (*a, *b) = func(\@c, \@d);
877 print "@a has more than @b\n";
887 Here we're using the typeglobs to do symbol table aliasing. It's
888 a tad subtle, though, and also won't work if you're using C<my>
889 variables, because only globals (even in disguise as C<local>s)
890 are in the symbol table.
892 If you're passing around filehandles, you could usually just use the bare
893 typeglob, like C<*STDOUT>, but typeglobs references work, too.
899 print $fh "her um well a hmmm\n";
902 $rec = get_rec(\*STDIN);
908 If you're planning on generating new filehandles, you could do this.
909 Notice to pass back just the bare *FH, not its reference.
914 return open (FH, $path) ? *FH : undef;
919 Perl supports a very limited kind of compile-time argument checking
920 using function prototyping. If you declare
924 then C<mypush()> takes arguments exactly like C<push()> does. The
925 function declaration must be visible at compile time. The prototype
926 affects only interpretation of new-style calls to the function,
927 where new-style is defined as not using the C<&> character. In
928 other words, if you call it like a built-in function, then it behaves
929 like a built-in function. If you call it like an old-fashioned
930 subroutine, then it behaves like an old-fashioned subroutine. It
931 naturally falls out from this rule that prototypes have no influence
932 on subroutine references like C<\&foo> or on indirect subroutine
933 calls like C<&{$subref}> or C<< $subref->() >>.
935 Method calls are not influenced by prototypes either, because the
936 function to be called is indeterminate at compile time, since
937 the exact code called depends on inheritance.
939 Because the intent of this feature is primarily to let you define
940 subroutines that work like built-in functions, here are prototypes
941 for some other functions that parse almost exactly like the
942 corresponding built-in.
944 Declared as Called as
946 sub mylink ($$) mylink $old, $new
947 sub myvec ($$$) myvec $var, $offset, 1
948 sub myindex ($$;$) myindex &getstring, "substr"
949 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
950 sub myreverse (@) myreverse $a, $b, $c
951 sub myjoin ($@) myjoin ":", $a, $b, $c
952 sub mypop (\@) mypop @array
953 sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme
954 sub mykeys (\%) mykeys %{$hashref}
955 sub myopen (*;$) myopen HANDLE, $name
956 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
957 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
958 sub myrand ($) myrand 42
961 Any backslashed prototype character represents an actual argument
962 that absolutely must start with that character. The value passed
963 as part of C<@_> will be a reference to the actual argument given
964 in the subroutine call, obtained by applying C<\> to that argument.
966 You can also backslash several argument types simultaneously by using
971 will allow calling myref() as
979 and the first argument of myref() will be a reference to
980 a scalar, an array, a hash, a code, or a glob.
982 Unbackslashed prototype characters have special meanings. Any
983 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
984 list context. An argument represented by C<$> forces scalar context. An
985 C<&> requires an anonymous subroutine, which, if passed as the first
986 argument, does not require the C<sub> keyword or a subsequent comma.
988 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
989 typeglob, or a reference to a typeglob in that slot. The value will be
990 available to the subroutine either as a simple scalar, or (in the latter
991 two cases) as a reference to the typeglob. If you wish to always convert
992 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
995 use Symbol 'qualify_to_ref';
998 my $fh = qualify_to_ref(shift, caller);
1002 A semicolon separates mandatory arguments from optional arguments.
1003 It is redundant before C<@> or C<%>, which gobble up everything else.
1005 Note how the last three examples in the table above are treated
1006 specially by the parser. C<mygrep()> is parsed as a true list
1007 operator, C<myrand()> is parsed as a true unary operator with unary
1008 precedence the same as C<rand()>, and C<mytime()> is truly without
1009 arguments, just like C<time()>. That is, if you say
1013 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1014 without a prototype.
1016 The interesting thing about C<&> is that you can generate new syntax with it,
1017 provided it's in the initial position:
1020 my($try,$catch) = @_;
1027 sub catch (&) { $_[0] }
1032 /phooey/ and print "unphooey\n";
1035 That prints C<"unphooey">. (Yes, there are still unresolved
1036 issues having to do with visibility of C<@_>. I'm ignoring that
1037 question for the moment. (But note that if we make C<@_> lexically
1038 scoped, those anonymous subroutines can act like closures... (Gee,
1039 is this sounding a little Lispish? (Never mind.))))
1041 And here's a reimplementation of the Perl C<grep> operator:
1047 push(@result, $_) if &$code;
1052 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1053 been intentionally left out of prototypes for the express purpose of
1054 someday in the future adding named, formal parameters. The current
1055 mechanism's main goal is to let module writers provide better diagnostics
1056 for module users. Larry feels the notation quite understandable to Perl
1057 programmers, and that it will not intrude greatly upon the meat of the
1058 module, nor make it harder to read. The line noise is visually
1059 encapsulated into a small pill that's easy to swallow.
1061 If you try to use an alphanumeric sequence in a prototype you will
1062 generate an optional warning - "Illegal character in prototype...".
1063 Unfortunately earlier versions of Perl allowed the prototype to be
1064 used as long as its prefix was a valid prototype. The warning may be
1065 upgraded to a fatal error in a future version of Perl once the
1066 majority of offending code is fixed.
1068 It's probably best to prototype new functions, not retrofit prototyping
1069 into older ones. That's because you must be especially careful about
1070 silent impositions of differing list versus scalar contexts. For example,
1071 if you decide that a function should take just one parameter, like this:
1075 print "you gave me $n\n";
1078 and someone has been calling it with an array or expression
1084 Then you've just supplied an automatic C<scalar> in front of their
1085 argument, which can be more than a bit surprising. The old C<@foo>
1086 which used to hold one thing doesn't get passed in. Instead,
1087 C<func()> now gets passed in a C<1>; that is, the number of elements
1088 in C<@foo>. And the C<split> gets called in scalar context so it
1089 starts scribbling on your C<@_> parameter list. Ouch!
1091 This is all very powerful, of course, and should be used only in moderation
1092 to make the world a better place.
1094 =head2 Constant Functions
1096 Functions with a prototype of C<()> are potential candidates for
1097 inlining. If the result after optimization and constant folding
1098 is either a constant or a lexically-scoped scalar which has no other
1099 references, then it will be used in place of function calls made
1100 without C<&>. Calls made using C<&> are never inlined. (See
1101 F<constant.pm> for an easy way to declare most constants.)
1103 The following functions would all be inlined:
1105 sub pi () { 3.14159 } # Not exact, but close.
1106 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1107 # and it's inlined, too!
1111 sub FLAG_FOO () { 1 << 8 }
1112 sub FLAG_BAR () { 1 << 9 }
1113 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1115 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1125 sub N () { int(BAZ_VAL) / 3 }
1128 for (1..N) { $prod *= $_ }
1129 sub N_FACTORIAL () { $prod }
1132 If you redefine a subroutine that was eligible for inlining, you'll get
1133 a mandatory warning. (You can use this warning to tell whether or not a
1134 particular subroutine is considered constant.) The warning is
1135 considered severe enough not to be optional because previously compiled
1136 invocations of the function will still be using the old value of the
1137 function. If you need to be able to redefine the subroutine, you need to
1138 ensure that it isn't inlined, either by dropping the C<()> prototype
1139 (which changes calling semantics, so beware) or by thwarting the
1140 inlining mechanism in some other way, such as
1142 sub not_inlined () {
1146 =head2 Overriding Built-in Functions
1148 Many built-in functions may be overridden, though this should be tried
1149 only occasionally and for good reason. Typically this might be
1150 done by a package attempting to emulate missing built-in functionality
1151 on a non-Unix system.
1153 Overriding may be done only by importing the name from a
1154 module--ordinary predeclaration isn't good enough. However, the
1155 C<use subs> pragma lets you, in effect, predeclare subs
1156 via the import syntax, and these names may then override built-in ones:
1158 use subs 'chdir', 'chroot', 'chmod', 'chown';
1162 To unambiguously refer to the built-in form, precede the
1163 built-in name with the special package qualifier C<CORE::>. For example,
1164 saying C<CORE::open()> always refers to the built-in C<open()>, even
1165 if the current package has imported some other subroutine called
1166 C<&open()> from elsewhere. Even though it looks like a regular
1167 function call, it isn't: you can't take a reference to it, such as
1168 the incorrect C<\&CORE::open> might appear to produce.
1170 Library modules should not in general export built-in names like C<open>
1171 or C<chdir> as part of their default C<@EXPORT> list, because these may
1172 sneak into someone else's namespace and change the semantics unexpectedly.
1173 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1174 possible for a user to import the name explicitly, but not implicitly.
1175 That is, they could say
1179 and it would import the C<open> override. But if they said
1183 they would get the default imports without overrides.
1185 The foregoing mechanism for overriding built-in is restricted, quite
1186 deliberately, to the package that requests the import. There is a second
1187 method that is sometimes applicable when you wish to override a built-in
1188 everywhere, without regard to namespace boundaries. This is achieved by
1189 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1190 example that quite brazenly replaces the C<glob> operator with something
1191 that understands regular expressions.
1196 @EXPORT_OK = 'glob';
1202 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1203 $pkg->export($where, $sym, @_);
1210 if (opendir D, '.') {
1211 @got = grep /$pat/, readdir D;
1218 And here's how it could be (ab)used:
1220 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1222 use REGlob 'glob'; # override glob() in Foo:: only
1223 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1225 The initial comment shows a contrived, even dangerous example.
1226 By overriding C<glob> globally, you would be forcing the new (and
1227 subversive) behavior for the C<glob> operator for I<every> namespace,
1228 without the complete cognizance or cooperation of the modules that own
1229 those namespaces. Naturally, this should be done with extreme caution--if
1230 it must be done at all.
1232 The C<REGlob> example above does not implement all the support needed to
1233 cleanly override perl's C<glob> operator. The built-in C<glob> has
1234 different behaviors depending on whether it appears in a scalar or list
1235 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1236 context sensitive behaviors, and these must be adequately supported by
1237 a properly written override. For a fully functional example of overriding
1238 C<glob>, study the implementation of C<File::DosGlob> in the standard
1241 When you override a built-in, your replacement should be consistent (if
1242 possible) with the built-in native syntax. You can achieve this by using
1243 a suitable prototype. To get the prototype of an overridable built-in,
1244 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1245 (see L<perlfunc/prototype>).
1247 Note however that some built-ins can't have their syntax expressed by a
1248 prototype (such as C<system> or C<chomp>). If you override them you won't
1249 be able to fully mimic their original syntax.
1251 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1252 to special magic, their original syntax is preserved, and you don't have
1253 to define a prototype for their replacements. (You can't override the
1254 C<do BLOCK> syntax, though).
1256 C<require> has special additional dark magic: if you invoke your
1257 C<require> replacement as C<require Foo::Bar>, it will actually receive
1258 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1260 And, as you'll have noticed from the previous example, if you override
1261 C<glob>, the C<E<lt>*E<gt>> glob operator is overridden as well.
1263 In a similar fashion, overriding the C<readline> function also overrides
1264 the equivalent I/O operator C<< <FILEHANDLE> >>.
1266 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1270 If you call a subroutine that is undefined, you would ordinarily
1271 get an immediate, fatal error complaining that the subroutine doesn't
1272 exist. (Likewise for subroutines being used as methods, when the
1273 method doesn't exist in any base class of the class's package.)
1274 However, if an C<AUTOLOAD> subroutine is defined in the package or
1275 packages used to locate the original subroutine, then that
1276 C<AUTOLOAD> subroutine is called with the arguments that would have
1277 been passed to the original subroutine. The fully qualified name
1278 of the original subroutine magically appears in the global $AUTOLOAD
1279 variable of the same package as the C<AUTOLOAD> routine. The name
1280 is not passed as an ordinary argument because, er, well, just
1281 because, that's why...
1283 Many C<AUTOLOAD> routines load in a definition for the requested
1284 subroutine using eval(), then execute that subroutine using a special
1285 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1286 without a trace. (See the source to the standard module documented
1287 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1288 also just emulate the routine and never define it. For example,
1289 let's pretend that a function that wasn't defined should just invoke
1290 C<system> with those arguments. All you'd do is:
1293 my $program = $AUTOLOAD;
1294 $program =~ s/.*:://;
1295 system($program, @_);
1301 In fact, if you predeclare functions you want to call that way, you don't
1302 even need parentheses:
1304 use subs qw(date who ls);
1309 A more complete example of this is the standard Shell module, which
1310 can treat undefined subroutine calls as calls to external programs.
1312 Mechanisms are available to help modules writers split their modules
1313 into autoloadable files. See the standard AutoLoader module
1314 described in L<AutoLoader> and in L<AutoSplit>, the standard
1315 SelfLoader modules in L<SelfLoader>, and the document on adding C
1316 functions to Perl code in L<perlxs>.
1318 =head2 Subroutine Attributes
1320 A subroutine declaration or definition may have a list of attributes
1321 associated with it. If such an attribute list is present, it is
1322 broken up at space or colon boundaries and treated as though a
1323 C<use attributes> had been seen. See L<attributes> for details
1324 about what attributes are currently supported.
1325 Unlike the limitation with the obsolescent C<use attrs>, the
1326 C<sub : ATTRLIST> syntax works to associate the attributes with
1327 a pre-declaration, and not just with a subroutine definition.
1329 The attributes must be valid as simple identifier names (without any
1330 punctuation other than the '_' character). They may have a parameter
1331 list appended, which is only checked for whether its parentheses ('(',')')
1334 Examples of valid syntax (even though the attributes are unknown):
1336 sub fnord (&\%) : switch(10,foo(7,3)) : expensive ;
1337 sub plugh () : Ugly('\(") :Bad ;
1338 sub xyzzy : _5x5 { ... }
1340 Examples of invalid syntax:
1342 sub fnord : switch(10,foo() ; # ()-string not balanced
1343 sub snoid : Ugly('(') ; # ()-string not balanced
1344 sub xyzzy : 5x5 ; # "5x5" not a valid identifier
1345 sub plugh : Y2::north ; # "Y2::north" not a simple identifier
1346 sub snurt : foo + bar ; # "+" not a colon or space
1348 The attribute list is passed as a list of constant strings to the code
1349 which associates them with the subroutine. In particular, the second example
1350 of valid syntax above currently looks like this in terms of how it's
1353 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1355 For further details on attribute lists and their manipulation,
1360 See L<perlref/"Function Templates"> for more about references and closures.
1361 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1362 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1363 See L<perlmod> to learn about bundling up your functions in separate files.
1364 See L<perlmodlib> to learn what library modules come standard on your system.
1365 See L<perltoot> to learn how to make object method calls.