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 /:/, $var );
166 Do not, however, be tempted to do this:
168 (@a, @b) = upcase(@list1, @list2);
170 Like the flattened incoming parameter list, the return list is also
171 flattened on return. So all you have managed to do here is stored
172 everything in C<@a> and made C<@b> an empty list. See
173 L<Pass by Reference> for alternatives.
175 A subroutine may be called using an explicit C<&> prefix. The
176 C<&> is optional in modern Perl, as are parentheses if the
177 subroutine has been predeclared. The C<&> is I<not> optional
178 when just naming the subroutine, such as when it's used as
179 an argument to defined() or undef(). Nor is it optional when you
180 want to do an indirect subroutine call with a subroutine name or
181 reference using the C<&$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>, and
211 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 None of the foregoing text applies to C<if/unless> or C<while/until>
331 modifiers appended to simple statements. Such modifiers are not
332 control structures and have no effect on scoping.
334 The C<foreach> loop defaults to scoping its index variable dynamically
335 in the manner of C<local>. However, if the index variable is
336 prefixed with the keyword C<my>, or if there is already a lexical
337 by that name in scope, then a new lexical is created instead. Thus
340 for my $i (1, 2, 3) {
344 the scope of $i extends to the end of the loop, but not beyond it,
345 rendering the value of $i inaccessible within C<some_function()>.
347 Some users may wish to encourage the use of lexically scoped variables.
348 As an aid to catching implicit uses to package variables,
349 which are always global, if you say
353 then any variable mentioned from there to the end of the enclosing
354 block must either refer to a lexical variable, be predeclared via
355 C<our> or C<use vars>, or else must be fully qualified with the package name.
356 A compilation error results otherwise. An inner block may countermand
357 this with C<no strict 'vars'>.
359 A C<my> has both a compile-time and a run-time effect. At compile
360 time, the compiler takes notice of it. The principal usefulness
361 of this is to quiet C<use strict 'vars'>, but it is also essential
362 for generation of closures as detailed in L<perlref>. Actual
363 initialization is delayed until run time, though, so it gets executed
364 at the appropriate time, such as each time through a loop, for
367 Variables declared with C<my> are not part of any package and are therefore
368 never fully qualified with the package name. In particular, you're not
369 allowed to try to make a package variable (or other global) lexical:
371 my $pack::var; # ERROR! Illegal syntax
372 my $_; # also illegal (currently)
374 In fact, a dynamic variable (also known as package or global variables)
375 are still accessible using the fully qualified C<::> notation even while a
376 lexical of the same name is also visible:
381 print "$x and $::x\n";
383 That will print out C<20> and C<10>.
385 You may declare C<my> variables at the outermost scope of a file
386 to hide any such identifiers from the world outside that file. This
387 is similar in spirit to C's static variables when they are used at
388 the file level. To do this with a subroutine requires the use of
389 a closure (an anonymous function that accesses enclosing lexicals).
390 If you want to create a private subroutine that cannot be called
391 from outside that block, it can declare a lexical variable containing
392 an anonymous sub reference:
394 my $secret_version = '1.001-beta';
395 my $secret_sub = sub { print $secret_version };
398 As long as the reference is never returned by any function within the
399 module, no outside module can see the subroutine, because its name is not in
400 any package's symbol table. Remember that it's not I<REALLY> called
401 C<$some_pack::secret_version> or anything; it's just $secret_version,
402 unqualified and unqualifiable.
404 This does not work with object methods, however; all object methods
405 have to be in the symbol table of some package to be found. See
406 L<perlref/"Function Templates"> for something of a work-around to
409 =head2 Persistent Private Variables
411 Just because a lexical variable is lexically (also called statically)
412 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
413 within a function it works like a C static. It normally works more
414 like a C auto, but with implicit garbage collection.
416 Unlike local variables in C or C++, Perl's lexical variables don't
417 necessarily get recycled just because their scope has exited.
418 If something more permanent is still aware of the lexical, it will
419 stick around. So long as something else references a lexical, that
420 lexical won't be freed--which is as it should be. You wouldn't want
421 memory being free until you were done using it, or kept around once you
422 were done. Automatic garbage collection takes care of this for you.
424 This means that you can pass back or save away references to lexical
425 variables, whereas to return a pointer to a C auto is a grave error.
426 It also gives us a way to simulate C's function statics. Here's a
427 mechanism for giving a function private variables with both lexical
428 scoping and a static lifetime. If you do want to create something like
429 C's static variables, just enclose the whole function in an extra block,
430 and put the static variable outside the function but in the block.
435 return ++$secret_val;
438 # $secret_val now becomes unreachable by the outside
439 # world, but retains its value between calls to gimme_another
441 If this function is being sourced in from a separate file
442 via C<require> or C<use>, then this is probably just fine. If it's
443 all in the main program, you'll need to arrange for the C<my>
444 to be executed early, either by putting the whole block above
445 your main program, or more likely, placing merely a C<BEGIN>
446 sub around it to make sure it gets executed before your program
452 return ++$secret_val;
456 See L<perlmod/"Package Constructors and Destructors"> about the
457 special triggered functions, C<BEGIN>, C<CHECK>, C<INIT> and C<END>.
459 If declared at the outermost scope (the file scope), then lexicals
460 work somewhat like C's file statics. They are available to all
461 functions in that same file declared below them, but are inaccessible
462 from outside that file. This strategy is sometimes used in modules
463 to create private variables that the whole module can see.
465 =head2 Temporary Values via local()
467 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
468 it's faster and safer. Exceptions to this include the global punctuation
469 variables, filehandles and formats, and direct manipulation of the Perl
470 symbol table itself. Format variables often use C<local> though, as do
471 other variables whose current value must be visible to called
476 local $foo; # declare $foo dynamically local
477 local (@wid, %get); # declare list of variables local
478 local $foo = "flurp"; # declare $foo dynamic, and init it
479 local @oof = @bar; # declare @oof dynamic, and init it
481 local *FH; # localize $FH, @FH, %FH, &FH ...
482 local *merlyn = *randal; # now $merlyn is really $randal, plus
483 # @merlyn is really @randal, etc
484 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
485 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
487 A C<local> modifies its listed variables to be "local" to the
488 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
489 called from within that block>. A C<local> just gives temporary
490 values to global (meaning package) variables. It does I<not> create
491 a local variable. This is known as dynamic scoping. Lexical scoping
492 is done with C<my>, which works more like C's auto declarations.
494 If more than one variable is given to C<local>, they must be placed in
495 parentheses. All listed elements must be legal lvalues. This operator works
496 by saving the current values of those variables in its argument list on a
497 hidden stack and restoring them upon exiting the block, subroutine, or
498 eval. This means that called subroutines can also reference the local
499 variable, but not the global one. The argument list may be assigned to if
500 desired, which allows you to initialize your local variables. (If no
501 initializer is given for a particular variable, it is created with an
502 undefined value.) Commonly this is used to name the parameters to a
503 subroutine. Examples:
508 # assume this function uses global %digits hash
511 # now temporarily add to %digits hash
513 # (NOTE: not claiming this is efficient!)
514 local %digits = (%digits, 't' => 10, 'e' => 11);
515 parse_num(); # parse_num gets this new %digits!
517 # old %digits restored here
519 Because C<local> is a run-time operator, it gets executed each time
520 through a loop. In releases of Perl previous to 5.0, this used more stack
521 storage each time until the loop was exited. Perl now reclaims the space
522 each time through, but it's still more efficient to declare your variables
525 A C<local> is simply a modifier on an lvalue expression. When you assign to
526 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
527 as a scalar or an array. So
529 local($foo) = <STDIN>;
530 local @FOO = <STDIN>;
532 both supply a list context to the right-hand side, while
534 local $foo = <STDIN>;
536 supplies a scalar context.
538 A note about C<local()> and composite types is in order. Something
539 like C<local(%foo)> works by temporarily placing a brand new hash in
540 the symbol table. The old hash is left alone, but is hidden "behind"
543 This means the old variable is completely invisible via the symbol
544 table (i.e. the hash entry in the C<*foo> typeglob) for the duration
545 of the dynamic scope within which the C<local()> was seen. This
546 has the effect of allowing one to temporarily occlude any magic on
547 composite types. For instance, this will briefly alter a tied
548 hash to some other implementation:
550 tie %ahash, 'APackage';
554 tie %ahash, 'BPackage';
555 [..called code will see %ahash tied to 'BPackage'..]
558 [..%ahash is a normal (untied) hash here..]
561 [..%ahash back to its initial tied self again..]
563 As another example, a custom implementation of C<%ENV> might look
568 tie %ENV, 'MyOwnEnv';
569 [..do your own fancy %ENV manipulation here..]
571 [..normal %ENV behavior here..]
573 It's also worth taking a moment to explain what happens when you
574 C<local>ize a member of a composite type (i.e. an array or hash element).
575 In this case, the element is C<local>ized I<by name>. This means that
576 when the scope of the C<local()> ends, the saved value will be
577 restored to the hash element whose key was named in the C<local()>, or
578 the array element whose index was named in the C<local()>. If that
579 element was deleted while the C<local()> was in effect (e.g. by a
580 C<delete()> from a hash or a C<shift()> of an array), it will spring
581 back into existence, possibly extending an array and filling in the
582 skipped elements with C<undef>. For instance, if you say
584 %hash = ( 'This' => 'is', 'a' => 'test' );
588 local($hash{'a'}) = 'drill';
589 while (my $e = pop(@ary)) {
594 $hash{'only a'} = 'test';
598 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
599 print "The array has ",scalar(@ary)," elements: ",
600 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
607 This is a test only a test.
608 The array has 6 elements: 0, 1, 2, undef, undef, 5
610 The behavior of local() on non-existent members of composite
611 types is subject to change in future.
613 =head2 Lvalue subroutines
615 B<WARNING>: Lvalue subroutines are still experimental and the implementation
616 may change in future versions of Perl.
618 It is possible to return a modifiable value from a subroutine.
619 To do this, you have to declare the subroutine to return an lvalue.
622 sub canmod : lvalue {
629 canmod() = 5; # assigns to $val
632 The scalar/list context for the subroutine and for the right-hand
633 side of assignment is determined as if the subroutine call is replaced
634 by a scalar. For example, consider:
636 data(2,3) = get_data(3,4);
638 Both subroutines here are called in a scalar context, while in:
640 (data(2,3)) = get_data(3,4);
644 (data(2),data(3)) = get_data(3,4);
646 all the subroutines are called in a list context.
648 The current implementation does not allow arrays and hashes to be
649 returned from lvalue subroutines directly. You may return a
650 reference instead. This restriction may be lifted in future.
652 =head2 Passing Symbol Table Entries (typeglobs)
654 B<WARNING>: The mechanism described in this section was originally
655 the only way to simulate pass-by-reference in older versions of
656 Perl. While it still works fine in modern versions, the new reference
657 mechanism is generally easier to work with. See below.
659 Sometimes you don't want to pass the value of an array to a subroutine
660 but rather the name of it, so that the subroutine can modify the global
661 copy of it rather than working with a local copy. In perl you can
662 refer to all objects of a particular name by prefixing the name
663 with a star: C<*foo>. This is often known as a "typeglob", because the
664 star on the front can be thought of as a wildcard match for all the
665 funny prefix characters on variables and subroutines and such.
667 When evaluated, the typeglob produces a scalar value that represents
668 all the objects of that name, including any filehandle, format, or
669 subroutine. When assigned to, it causes the name mentioned to refer to
670 whatever C<*> value was assigned to it. Example:
673 local(*someary) = @_;
674 foreach $elem (@someary) {
681 Scalars are already passed by reference, so you can modify
682 scalar arguments without using this mechanism by referring explicitly
683 to C<$_[0]> etc. You can modify all the elements of an array by passing
684 all the elements as scalars, but you have to use the C<*> mechanism (or
685 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
686 an array. It will certainly be faster to pass the typeglob (or reference).
688 Even if you don't want to modify an array, this mechanism is useful for
689 passing multiple arrays in a single LIST, because normally the LIST
690 mechanism will merge all the array values so that you can't extract out
691 the individual arrays. For more on typeglobs, see
692 L<perldata/"Typeglobs and Filehandles">.
694 =head2 When to Still Use local()
696 Despite the existence of C<my>, there are still three places where the
697 C<local> operator still shines. In fact, in these three places, you
698 I<must> use C<local> instead of C<my>.
704 You need to give a global variable a temporary value, especially $_.
706 The global variables, like C<@ARGV> or the punctuation variables, must be
707 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
708 it up into chunks separated by lines of equal signs, which are placed
712 local @ARGV = ("/etc/motd");
715 @Fields = split /^\s*=+\s*$/;
718 It particular, it's important to C<local>ize $_ in any routine that assigns
719 to it. Look out for implicit assignments in C<while> conditionals.
723 You need to create a local file or directory handle or a local function.
725 A function that needs a filehandle of its own must use
726 C<local()> on a complete typeglob. This can be used to create new symbol
730 local (*READER, *WRITER); # not my!
731 pipe (READER, WRITER); or die "pipe: $!";
732 return (*READER, *WRITER);
734 ($head, $tail) = ioqueue();
736 See the Symbol module for a way to create anonymous symbol table
739 Because assignment of a reference to a typeglob creates an alias, this
740 can be used to create what is effectively a local function, or at least,
744 local *grow = \&shrink; # only until this block exists
745 grow(); # really calls shrink()
746 move(); # if move() grow()s, it shrink()s too
748 grow(); # get the real grow() again
750 See L<perlref/"Function Templates"> for more about manipulating
751 functions by name in this way.
755 You want to temporarily change just one element of an array or hash.
757 You can C<local>ize just one element of an aggregate. Usually this
761 local $SIG{INT} = 'IGNORE';
762 funct(); # uninterruptible
764 # interruptibility automatically restored here
766 But it also works on lexically declared aggregates. Prior to 5.005,
767 this operation could on occasion misbehave.
771 =head2 Pass by Reference
773 If you want to pass more than one array or hash into a function--or
774 return them from it--and have them maintain their integrity, then
775 you're going to have to use an explicit pass-by-reference. Before you
776 do that, you need to understand references as detailed in L<perlref>.
777 This section may not make much sense to you otherwise.
779 Here are a few simple examples. First, let's pass in several arrays
780 to a function and have it C<pop> all of then, returning a new list
781 of all their former last elements:
783 @tailings = popmany ( \@a, \@b, \@c, \@d );
788 foreach $aref ( @_ ) {
789 push @retlist, pop @$aref;
794 Here's how you might write a function that returns a
795 list of keys occurring in all the hashes passed to it:
797 @common = inter( \%foo, \%bar, \%joe );
799 my ($k, $href, %seen); # locals
801 while ( $k = each %$href ) {
805 return grep { $seen{$_} == @_ } keys %seen;
808 So far, we're using just the normal list return mechanism.
809 What happens if you want to pass or return a hash? Well,
810 if you're using only one of them, or you don't mind them
811 concatenating, then the normal calling convention is ok, although
814 Where people get into trouble is here:
816 (@a, @b) = func(@c, @d);
818 (%a, %b) = func(%c, %d);
820 That syntax simply won't work. It sets just C<@a> or C<%a> and
821 clears the C<@b> or C<%b>. Plus the function didn't get passed
822 into two separate arrays or hashes: it got one long list in C<@_>,
825 If you can arrange for everyone to deal with this through references, it's
826 cleaner code, although not so nice to look at. Here's a function that
827 takes two array references as arguments, returning the two array elements
828 in order of how many elements they have in them:
830 ($aref, $bref) = func(\@c, \@d);
831 print "@$aref has more than @$bref\n";
833 my ($cref, $dref) = @_;
834 if (@$cref > @$dref) {
835 return ($cref, $dref);
837 return ($dref, $cref);
841 It turns out that you can actually do this also:
843 (*a, *b) = func(\@c, \@d);
844 print "@a has more than @b\n";
854 Here we're using the typeglobs to do symbol table aliasing. It's
855 a tad subtle, though, and also won't work if you're using C<my>
856 variables, because only globals (even in disguise as C<local>s)
857 are in the symbol table.
859 If you're passing around filehandles, you could usually just use the bare
860 typeglob, like C<*STDOUT>, but typeglobs references work, too.
866 print $fh "her um well a hmmm\n";
869 $rec = get_rec(\*STDIN);
875 If you're planning on generating new filehandles, you could do this.
876 Notice to pass back just the bare *FH, not its reference.
881 return open (FH, $path) ? *FH : undef;
886 Perl supports a very limited kind of compile-time argument checking
887 using function prototyping. If you declare
891 then C<mypush()> takes arguments exactly like C<push()> does. The
892 function declaration must be visible at compile time. The prototype
893 affects only interpretation of new-style calls to the function,
894 where new-style is defined as not using the C<&> character. In
895 other words, if you call it like a built-in function, then it behaves
896 like a built-in function. If you call it like an old-fashioned
897 subroutine, then it behaves like an old-fashioned subroutine. It
898 naturally falls out from this rule that prototypes have no influence
899 on subroutine references like C<\&foo> or on indirect subroutine
900 calls like C<&{$subref}> or C<< $subref->() >>.
902 Method calls are not influenced by prototypes either, because the
903 function to be called is indeterminate at compile time, since
904 the exact code called depends on inheritance.
906 Because the intent of this feature is primarily to let you define
907 subroutines that work like built-in functions, here are prototypes
908 for some other functions that parse almost exactly like the
909 corresponding built-in.
911 Declared as Called as
913 sub mylink ($$) mylink $old, $new
914 sub myvec ($$$) myvec $var, $offset, 1
915 sub myindex ($$;$) myindex &getstring, "substr"
916 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
917 sub myreverse (@) myreverse $a, $b, $c
918 sub myjoin ($@) myjoin ":", $a, $b, $c
919 sub mypop (\@) mypop @array
920 sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme
921 sub mykeys (\%) mykeys %{$hashref}
922 sub myopen (*;$) myopen HANDLE, $name
923 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
924 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
925 sub myrand ($) myrand 42
928 Any backslashed prototype character represents an actual argument
929 that absolutely must start with that character. The value passed
930 as part of C<@_> will be a reference to the actual argument given
931 in the subroutine call, obtained by applying C<\> to that argument.
933 Unbackslashed prototype characters have special meanings. Any
934 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
935 list context. An argument represented by C<$> forces scalar context. An
936 C<&> requires an anonymous subroutine, which, if passed as the first
937 argument, does not require the C<sub> keyword or a subsequent comma.
939 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
940 typeglob, or a reference to a typeglob in that slot. The value will be
941 available to the subroutine either as a simple scalar, or (in the latter
942 two cases) as a reference to the typeglob. If you wish to always convert
943 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
946 use Symbol 'qualify_to_ref';
949 my $fh = qualify_to_ref(shift, caller);
953 A semicolon separates mandatory arguments from optional arguments.
954 It is redundant before C<@> or C<%>, which gobble up everything else.
956 Note how the last three examples in the table above are treated
957 specially by the parser. C<mygrep()> is parsed as a true list
958 operator, C<myrand()> is parsed as a true unary operator with unary
959 precedence the same as C<rand()>, and C<mytime()> is truly without
960 arguments, just like C<time()>. That is, if you say
964 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
967 The interesting thing about C<&> is that you can generate new syntax with it,
968 provided it's in the initial position:
971 my($try,$catch) = @_;
978 sub catch (&) { $_[0] }
983 /phooey/ and print "unphooey\n";
986 That prints C<"unphooey">. (Yes, there are still unresolved
987 issues having to do with visibility of C<@_>. I'm ignoring that
988 question for the moment. (But note that if we make C<@_> lexically
989 scoped, those anonymous subroutines can act like closures... (Gee,
990 is this sounding a little Lispish? (Never mind.))))
992 And here's a reimplementation of the Perl C<grep> operator:
998 push(@result, $_) if &$code;
1003 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1004 been intentionally left out of prototypes for the express purpose of
1005 someday in the future adding named, formal parameters. The current
1006 mechanism's main goal is to let module writers provide better diagnostics
1007 for module users. Larry feels the notation quite understandable to Perl
1008 programmers, and that it will not intrude greatly upon the meat of the
1009 module, nor make it harder to read. The line noise is visually
1010 encapsulated into a small pill that's easy to swallow.
1012 It's probably best to prototype new functions, not retrofit prototyping
1013 into older ones. That's because you must be especially careful about
1014 silent impositions of differing list versus scalar contexts. For example,
1015 if you decide that a function should take just one parameter, like this:
1019 print "you gave me $n\n";
1022 and someone has been calling it with an array or expression
1028 Then you've just supplied an automatic C<scalar> in front of their
1029 argument, which can be more than a bit surprising. The old C<@foo>
1030 which used to hold one thing doesn't get passed in. Instead,
1031 C<func()> now gets passed in a C<1>; that is, the number of elements
1032 in C<@foo>. And the C<split> gets called in scalar context so it
1033 starts scribbling on your C<@_> parameter list. Ouch!
1035 This is all very powerful, of course, and should be used only in moderation
1036 to make the world a better place.
1038 =head2 Constant Functions
1040 Functions with a prototype of C<()> are potential candidates for
1041 inlining. If the result after optimization and constant folding
1042 is either a constant or a lexically-scoped scalar which has no other
1043 references, then it will be used in place of function calls made
1044 without C<&>. Calls made using C<&> are never inlined. (See
1045 F<constant.pm> for an easy way to declare most constants.)
1047 The following functions would all be inlined:
1049 sub pi () { 3.14159 } # Not exact, but close.
1050 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1051 # and it's inlined, too!
1055 sub FLAG_FOO () { 1 << 8 }
1056 sub FLAG_BAR () { 1 << 9 }
1057 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1059 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1069 sub N () { int(BAZ_VAL) / 3 }
1072 for (1..N) { $prod *= $_ }
1073 sub N_FACTORIAL () { $prod }
1076 If you redefine a subroutine that was eligible for inlining, you'll get
1077 a mandatory warning. (You can use this warning to tell whether or not a
1078 particular subroutine is considered constant.) The warning is
1079 considered severe enough not to be optional because previously compiled
1080 invocations of the function will still be using the old value of the
1081 function. If you need to be able to redefine the subroutine, you need to
1082 ensure that it isn't inlined, either by dropping the C<()> prototype
1083 (which changes calling semantics, so beware) or by thwarting the
1084 inlining mechanism in some other way, such as
1086 sub not_inlined () {
1090 =head2 Overriding Built-in Functions
1092 Many built-in functions may be overridden, though this should be tried
1093 only occasionally and for good reason. Typically this might be
1094 done by a package attempting to emulate missing built-in functionality
1095 on a non-Unix system.
1097 Overriding may be done only by importing the name from a
1098 module--ordinary predeclaration isn't good enough. However, the
1099 C<use subs> pragma lets you, in effect, predeclare subs
1100 via the import syntax, and these names may then override built-in ones:
1102 use subs 'chdir', 'chroot', 'chmod', 'chown';
1106 To unambiguously refer to the built-in form, precede the
1107 built-in name with the special package qualifier C<CORE::>. For example,
1108 saying C<CORE::open()> always refers to the built-in C<open()>, even
1109 if the current package has imported some other subroutine called
1110 C<&open()> from elsewhere. Even though it looks like a regular
1111 function call, it isn't: you can't take a reference to it, such as
1112 the incorrect C<\&CORE::open> might appear to produce.
1114 Library modules should not in general export built-in names like C<open>
1115 or C<chdir> as part of their default C<@EXPORT> list, because these may
1116 sneak into someone else's namespace and change the semantics unexpectedly.
1117 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1118 possible for a user to import the name explicitly, but not implicitly.
1119 That is, they could say
1123 and it would import the C<open> override. But if they said
1127 they would get the default imports without overrides.
1129 The foregoing mechanism for overriding built-in is restricted, quite
1130 deliberately, to the package that requests the import. There is a second
1131 method that is sometimes applicable when you wish to override a built-in
1132 everywhere, without regard to namespace boundaries. This is achieved by
1133 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1134 example that quite brazenly replaces the C<glob> operator with something
1135 that understands regular expressions.
1140 @EXPORT_OK = 'glob';
1146 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1147 $pkg->export($where, $sym, @_);
1154 if (opendir D, '.') {
1155 @got = grep /$pat/, readdir D;
1162 And here's how it could be (ab)used:
1164 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1166 use REGlob 'glob'; # override glob() in Foo:: only
1167 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1169 The initial comment shows a contrived, even dangerous example.
1170 By overriding C<glob> globally, you would be forcing the new (and
1171 subversive) behavior for the C<glob> operator for I<every> namespace,
1172 without the complete cognizance or cooperation of the modules that own
1173 those namespaces. Naturally, this should be done with extreme caution--if
1174 it must be done at all.
1176 The C<REGlob> example above does not implement all the support needed to
1177 cleanly override perl's C<glob> operator. The built-in C<glob> has
1178 different behaviors depending on whether it appears in a scalar or list
1179 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1180 context sensitive behaviors, and these must be adequately supported by
1181 a properly written override. For a fully functional example of overriding
1182 C<glob>, study the implementation of C<File::DosGlob> in the standard
1187 If you call a subroutine that is undefined, you would ordinarily
1188 get an immediate, fatal error complaining that the subroutine doesn't
1189 exist. (Likewise for subroutines being used as methods, when the
1190 method doesn't exist in any base class of the class's package.)
1191 However, if an C<AUTOLOAD> subroutine is defined in the package or
1192 packages used to locate the original subroutine, then that
1193 C<AUTOLOAD> subroutine is called with the arguments that would have
1194 been passed to the original subroutine. The fully qualified name
1195 of the original subroutine magically appears in the global $AUTOLOAD
1196 variable of the same package as the C<AUTOLOAD> routine. The name
1197 is not passed as an ordinary argument because, er, well, just
1198 because, that's why...
1200 Many C<AUTOLOAD> routines load in a definition for the requested
1201 subroutine using eval(), then execute that subroutine using a special
1202 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1203 without a trace. (See the source to the standard module documented
1204 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1205 also just emulate the routine and never define it. For example,
1206 let's pretend that a function that wasn't defined should just invoke
1207 C<system> with those arguments. All you'd do is:
1210 my $program = $AUTOLOAD;
1211 $program =~ s/.*:://;
1212 system($program, @_);
1218 In fact, if you predeclare functions you want to call that way, you don't
1219 even need parentheses:
1221 use subs qw(date who ls);
1226 A more complete example of this is the standard Shell module, which
1227 can treat undefined subroutine calls as calls to external programs.
1229 Mechanisms are available to help modules writers split their modules
1230 into autoloadable files. See the standard AutoLoader module
1231 described in L<AutoLoader> and in L<AutoSplit>, the standard
1232 SelfLoader modules in L<SelfLoader>, and the document on adding C
1233 functions to Perl code in L<perlxs>.
1235 =head2 Subroutine Attributes
1237 A subroutine declaration or definition may have a list of attributes
1238 associated with it. If such an attribute list is present, it is
1239 broken up at space or colon boundaries and treated as though a
1240 C<use attributes> had been seen. See L<attributes> for details
1241 about what attributes are currently supported.
1242 Unlike the limitation with the obsolescent C<use attrs>, the
1243 C<sub : ATTRLIST> syntax works to associate the attributes with
1244 a pre-declaration, and not just with a subroutine definition.
1246 The attributes must be valid as simple identifier names (without any
1247 punctuation other than the '_' character). They may have a parameter
1248 list appended, which is only checked for whether its parentheses ('(',')')
1251 Examples of valid syntax (even though the attributes are unknown):
1253 sub fnord (&\%) : switch(10,foo(7,3)) : expensive ;
1254 sub plugh () : Ugly('\(") :Bad ;
1255 sub xyzzy : _5x5 { ... }
1257 Examples of invalid syntax:
1259 sub fnord : switch(10,foo() ; # ()-string not balanced
1260 sub snoid : Ugly('(') ; # ()-string not balanced
1261 sub xyzzy : 5x5 ; # "5x5" not a valid identifier
1262 sub plugh : Y2::north ; # "Y2::north" not a simple identifier
1263 sub snurt : foo + bar ; # "+" not a colon or space
1265 The attribute list is passed as a list of constant strings to the code
1266 which associates them with the subroutine. In particular, the second example
1267 of valid syntax above currently looks like this in terms of how it's
1270 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1272 For further details on attribute lists and their manipulation,
1277 See L<perlref/"Function Templates"> for more about references and closures.
1278 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1279 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1280 See L<perlmod> to learn about bundling up your functions in separate files.
1281 See L<perlmodlib> to learn what library modules come standard on your system.
1282 See L<perltoot> to learn how to make object method calls.