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 B<WARNING> The code example above does not currently work as described.
567 This will be fixed in a future release of Perl; in the meantime, avoid
568 code that relies on any particular behaviour of localising tied arrays
569 or hashes (localising individual elements is still okay).
570 See L<perldelta/"Localising Tied Arrays and Hashes Is Broken"> for more
573 As another example, a custom implementation of C<%ENV> might look
578 tie %ENV, 'MyOwnEnv';
579 [..do your own fancy %ENV manipulation here..]
581 [..normal %ENV behavior here..]
583 It's also worth taking a moment to explain what happens when you
584 C<local>ize a member of a composite type (i.e. an array or hash element).
585 In this case, the element is C<local>ized I<by name>. This means that
586 when the scope of the C<local()> ends, the saved value will be
587 restored to the hash element whose key was named in the C<local()>, or
588 the array element whose index was named in the C<local()>. If that
589 element was deleted while the C<local()> was in effect (e.g. by a
590 C<delete()> from a hash or a C<shift()> of an array), it will spring
591 back into existence, possibly extending an array and filling in the
592 skipped elements with C<undef>. For instance, if you say
594 %hash = ( 'This' => 'is', 'a' => 'test' );
598 local($hash{'a'}) = 'drill';
599 while (my $e = pop(@ary)) {
604 $hash{'only a'} = 'test';
608 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
609 print "The array has ",scalar(@ary)," elements: ",
610 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
617 This is a test only a test.
618 The array has 6 elements: 0, 1, 2, undef, undef, 5
620 The behavior of local() on non-existent members of composite
621 types is subject to change in future.
623 =head2 Lvalue subroutines
625 B<WARNING>: Lvalue subroutines are still experimental and the
626 implementation may change in future versions of Perl.
628 It is possible to return a modifiable value from a subroutine.
629 To do this, you have to declare the subroutine to return an lvalue.
632 sub canmod : lvalue {
633 # return $val; this doesn't work, don't say "return"
640 canmod() = 5; # assigns to $val
643 The scalar/list context for the subroutine and for the right-hand
644 side of assignment is determined as if the subroutine call is replaced
645 by a scalar. For example, consider:
647 data(2,3) = get_data(3,4);
649 Both subroutines here are called in a scalar context, while in:
651 (data(2,3)) = get_data(3,4);
655 (data(2),data(3)) = get_data(3,4);
657 all the subroutines are called in a list context.
661 =item Lvalue subroutines are EXPERIMENTAL
663 They appear to be convenient, but there are several reasons to be
666 You can't use the return keyword, you must pass out the value before
667 falling out of subroutine scope. (see comment in example above). This
668 is usually not a problem, but it disallows an explicit return out of a
669 deeply nested loop, which is sometimes a nice way out.
671 They violate encapsulation. A normal mutator can check the supplied
672 argument before setting the attribute it is protecting, an lvalue
673 subroutine never gets that chance. Consider;
675 my $some_array_ref = []; # protected by mutators ??
677 sub set_arr { # normal mutator
679 die("expected array, you supplied ", ref $val)
680 unless ref $val eq 'ARRAY';
681 $some_array_ref = $val;
683 sub set_arr_lv : lvalue { # lvalue mutator
687 # set_arr_lv cannot stop this !
688 set_arr_lv() = { a => 1 };
692 =head2 Passing Symbol Table Entries (typeglobs)
694 B<WARNING>: The mechanism described in this section was originally
695 the only way to simulate pass-by-reference in older versions of
696 Perl. While it still works fine in modern versions, the new reference
697 mechanism is generally easier to work with. See below.
699 Sometimes you don't want to pass the value of an array to a subroutine
700 but rather the name of it, so that the subroutine can modify the global
701 copy of it rather than working with a local copy. In perl you can
702 refer to all objects of a particular name by prefixing the name
703 with a star: C<*foo>. This is often known as a "typeglob", because the
704 star on the front can be thought of as a wildcard match for all the
705 funny prefix characters on variables and subroutines and such.
707 When evaluated, the typeglob produces a scalar value that represents
708 all the objects of that name, including any filehandle, format, or
709 subroutine. When assigned to, it causes the name mentioned to refer to
710 whatever C<*> value was assigned to it. Example:
713 local(*someary) = @_;
714 foreach $elem (@someary) {
721 Scalars are already passed by reference, so you can modify
722 scalar arguments without using this mechanism by referring explicitly
723 to C<$_[0]> etc. You can modify all the elements of an array by passing
724 all the elements as scalars, but you have to use the C<*> mechanism (or
725 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
726 an array. It will certainly be faster to pass the typeglob (or reference).
728 Even if you don't want to modify an array, this mechanism is useful for
729 passing multiple arrays in a single LIST, because normally the LIST
730 mechanism will merge all the array values so that you can't extract out
731 the individual arrays. For more on typeglobs, see
732 L<perldata/"Typeglobs and Filehandles">.
734 =head2 When to Still Use local()
736 Despite the existence of C<my>, there are still three places where the
737 C<local> operator still shines. In fact, in these three places, you
738 I<must> use C<local> instead of C<my>.
744 You need to give a global variable a temporary value, especially $_.
746 The global variables, like C<@ARGV> or the punctuation variables, must be
747 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
748 it up into chunks separated by lines of equal signs, which are placed
752 local @ARGV = ("/etc/motd");
755 @Fields = split /^\s*=+\s*$/;
758 It particular, it's important to C<local>ize $_ in any routine that assigns
759 to it. Look out for implicit assignments in C<while> conditionals.
763 You need to create a local file or directory handle or a local function.
765 A function that needs a filehandle of its own must use
766 C<local()> on a complete typeglob. This can be used to create new symbol
770 local (*READER, *WRITER); # not my!
771 pipe (READER, WRITER) or die "pipe: $!";
772 return (*READER, *WRITER);
774 ($head, $tail) = ioqueue();
776 See the Symbol module for a way to create anonymous symbol table
779 Because assignment of a reference to a typeglob creates an alias, this
780 can be used to create what is effectively a local function, or at least,
784 local *grow = \&shrink; # only until this block exists
785 grow(); # really calls shrink()
786 move(); # if move() grow()s, it shrink()s too
788 grow(); # get the real grow() again
790 See L<perlref/"Function Templates"> for more about manipulating
791 functions by name in this way.
795 You want to temporarily change just one element of an array or hash.
797 You can C<local>ize just one element of an aggregate. Usually this
801 local $SIG{INT} = 'IGNORE';
802 funct(); # uninterruptible
804 # interruptibility automatically restored here
806 But it also works on lexically declared aggregates. Prior to 5.005,
807 this operation could on occasion misbehave.
811 =head2 Pass by Reference
813 If you want to pass more than one array or hash into a function--or
814 return them from it--and have them maintain their integrity, then
815 you're going to have to use an explicit pass-by-reference. Before you
816 do that, you need to understand references as detailed in L<perlref>.
817 This section may not make much sense to you otherwise.
819 Here are a few simple examples. First, let's pass in several arrays
820 to a function and have it C<pop> all of then, returning a new list
821 of all their former last elements:
823 @tailings = popmany ( \@a, \@b, \@c, \@d );
828 foreach $aref ( @_ ) {
829 push @retlist, pop @$aref;
834 Here's how you might write a function that returns a
835 list of keys occurring in all the hashes passed to it:
837 @common = inter( \%foo, \%bar, \%joe );
839 my ($k, $href, %seen); # locals
841 while ( $k = each %$href ) {
845 return grep { $seen{$_} == @_ } keys %seen;
848 So far, we're using just the normal list return mechanism.
849 What happens if you want to pass or return a hash? Well,
850 if you're using only one of them, or you don't mind them
851 concatenating, then the normal calling convention is ok, although
854 Where people get into trouble is here:
856 (@a, @b) = func(@c, @d);
858 (%a, %b) = func(%c, %d);
860 That syntax simply won't work. It sets just C<@a> or C<%a> and
861 clears the C<@b> or C<%b>. Plus the function didn't get passed
862 into two separate arrays or hashes: it got one long list in C<@_>,
865 If you can arrange for everyone to deal with this through references, it's
866 cleaner code, although not so nice to look at. Here's a function that
867 takes two array references as arguments, returning the two array elements
868 in order of how many elements they have in them:
870 ($aref, $bref) = func(\@c, \@d);
871 print "@$aref has more than @$bref\n";
873 my ($cref, $dref) = @_;
874 if (@$cref > @$dref) {
875 return ($cref, $dref);
877 return ($dref, $cref);
881 It turns out that you can actually do this also:
883 (*a, *b) = func(\@c, \@d);
884 print "@a has more than @b\n";
894 Here we're using the typeglobs to do symbol table aliasing. It's
895 a tad subtle, though, and also won't work if you're using C<my>
896 variables, because only globals (even in disguise as C<local>s)
897 are in the symbol table.
899 If you're passing around filehandles, you could usually just use the bare
900 typeglob, like C<*STDOUT>, but typeglobs references work, too.
906 print $fh "her um well a hmmm\n";
909 $rec = get_rec(\*STDIN);
915 If you're planning on generating new filehandles, you could do this.
916 Notice to pass back just the bare *FH, not its reference.
921 return open (FH, $path) ? *FH : undef;
926 Perl supports a very limited kind of compile-time argument checking
927 using function prototyping. If you declare
931 then C<mypush()> takes arguments exactly like C<push()> does. The
932 function declaration must be visible at compile time. The prototype
933 affects only interpretation of new-style calls to the function,
934 where new-style is defined as not using the C<&> character. In
935 other words, if you call it like a built-in function, then it behaves
936 like a built-in function. If you call it like an old-fashioned
937 subroutine, then it behaves like an old-fashioned subroutine. It
938 naturally falls out from this rule that prototypes have no influence
939 on subroutine references like C<\&foo> or on indirect subroutine
940 calls like C<&{$subref}> or C<< $subref->() >>.
942 Method calls are not influenced by prototypes either, because the
943 function to be called is indeterminate at compile time, since
944 the exact code called depends on inheritance.
946 Because the intent of this feature is primarily to let you define
947 subroutines that work like built-in functions, here are prototypes
948 for some other functions that parse almost exactly like the
949 corresponding built-in.
951 Declared as Called as
953 sub mylink ($$) mylink $old, $new
954 sub myvec ($$$) myvec $var, $offset, 1
955 sub myindex ($$;$) myindex &getstring, "substr"
956 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
957 sub myreverse (@) myreverse $a, $b, $c
958 sub myjoin ($@) myjoin ":", $a, $b, $c
959 sub mypop (\@) mypop @array
960 sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme
961 sub mykeys (\%) mykeys %{$hashref}
962 sub myopen (*;$) myopen HANDLE, $name
963 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
964 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
965 sub myrand ($) myrand 42
968 Any backslashed prototype character represents an actual argument
969 that absolutely must start with that character. The value passed
970 as part of C<@_> will be a reference to the actual argument given
971 in the subroutine call, obtained by applying C<\> to that argument.
973 You can also backslash several argument types simultaneously by using
978 will allow calling myref() as
986 and the first argument of myref() will be a reference to
987 a scalar, an array, a hash, a code, or a glob.
989 Unbackslashed prototype characters have special meanings. Any
990 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
991 list context. An argument represented by C<$> forces scalar context. An
992 C<&> requires an anonymous subroutine, which, if passed as the first
993 argument, does not require the C<sub> keyword or a subsequent comma.
995 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
996 typeglob, or a reference to a typeglob in that slot. The value will be
997 available to the subroutine either as a simple scalar, or (in the latter
998 two cases) as a reference to the typeglob. If you wish to always convert
999 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1002 use Symbol 'qualify_to_ref';
1005 my $fh = qualify_to_ref(shift, caller);
1009 A semicolon separates mandatory arguments from optional arguments.
1010 It is redundant before C<@> or C<%>, which gobble up everything else.
1012 Note how the last three examples in the table above are treated
1013 specially by the parser. C<mygrep()> is parsed as a true list
1014 operator, C<myrand()> is parsed as a true unary operator with unary
1015 precedence the same as C<rand()>, and C<mytime()> is truly without
1016 arguments, just like C<time()>. That is, if you say
1020 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1021 without a prototype.
1023 The interesting thing about C<&> is that you can generate new syntax with it,
1024 provided it's in the initial position:
1027 my($try,$catch) = @_;
1034 sub catch (&) { $_[0] }
1039 /phooey/ and print "unphooey\n";
1042 That prints C<"unphooey">. (Yes, there are still unresolved
1043 issues having to do with visibility of C<@_>. I'm ignoring that
1044 question for the moment. (But note that if we make C<@_> lexically
1045 scoped, those anonymous subroutines can act like closures... (Gee,
1046 is this sounding a little Lispish? (Never mind.))))
1048 And here's a reimplementation of the Perl C<grep> operator:
1054 push(@result, $_) if &$code;
1059 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1060 been intentionally left out of prototypes for the express purpose of
1061 someday in the future adding named, formal parameters. The current
1062 mechanism's main goal is to let module writers provide better diagnostics
1063 for module users. Larry feels the notation quite understandable to Perl
1064 programmers, and that it will not intrude greatly upon the meat of the
1065 module, nor make it harder to read. The line noise is visually
1066 encapsulated into a small pill that's easy to swallow.
1068 If you try to use an alphanumeric sequence in a prototype you will
1069 generate an optional warning - "Illegal character in prototype...".
1070 Unfortunately earlier versions of Perl allowed the prototype to be
1071 used as long as its prefix was a valid prototype. The warning may be
1072 upgraded to a fatal error in a future version of Perl once the
1073 majority of offending code is fixed.
1075 It's probably best to prototype new functions, not retrofit prototyping
1076 into older ones. That's because you must be especially careful about
1077 silent impositions of differing list versus scalar contexts. For example,
1078 if you decide that a function should take just one parameter, like this:
1082 print "you gave me $n\n";
1085 and someone has been calling it with an array or expression
1091 Then you've just supplied an automatic C<scalar> in front of their
1092 argument, which can be more than a bit surprising. The old C<@foo>
1093 which used to hold one thing doesn't get passed in. Instead,
1094 C<func()> now gets passed in a C<1>; that is, the number of elements
1095 in C<@foo>. And the C<split> gets called in scalar context so it
1096 starts scribbling on your C<@_> parameter list. Ouch!
1098 This is all very powerful, of course, and should be used only in moderation
1099 to make the world a better place.
1101 =head2 Constant Functions
1103 Functions with a prototype of C<()> are potential candidates for
1104 inlining. If the result after optimization and constant folding
1105 is either a constant or a lexically-scoped scalar which has no other
1106 references, then it will be used in place of function calls made
1107 without C<&>. Calls made using C<&> are never inlined. (See
1108 F<constant.pm> for an easy way to declare most constants.)
1110 The following functions would all be inlined:
1112 sub pi () { 3.14159 } # Not exact, but close.
1113 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1114 # and it's inlined, too!
1118 sub FLAG_FOO () { 1 << 8 }
1119 sub FLAG_BAR () { 1 << 9 }
1120 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1122 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1132 sub N () { int(BAZ_VAL) / 3 }
1135 for (1..N) { $prod *= $_ }
1136 sub N_FACTORIAL () { $prod }
1139 If you redefine a subroutine that was eligible for inlining, you'll get
1140 a mandatory warning. (You can use this warning to tell whether or not a
1141 particular subroutine is considered constant.) The warning is
1142 considered severe enough not to be optional because previously compiled
1143 invocations of the function will still be using the old value of the
1144 function. If you need to be able to redefine the subroutine, you need to
1145 ensure that it isn't inlined, either by dropping the C<()> prototype
1146 (which changes calling semantics, so beware) or by thwarting the
1147 inlining mechanism in some other way, such as
1149 sub not_inlined () {
1153 =head2 Overriding Built-in Functions
1155 Many built-in functions may be overridden, though this should be tried
1156 only occasionally and for good reason. Typically this might be
1157 done by a package attempting to emulate missing built-in functionality
1158 on a non-Unix system.
1160 Overriding may be done only by importing the name from a
1161 module--ordinary predeclaration isn't good enough. However, the
1162 C<use subs> pragma lets you, in effect, predeclare subs
1163 via the import syntax, and these names may then override built-in ones:
1165 use subs 'chdir', 'chroot', 'chmod', 'chown';
1169 To unambiguously refer to the built-in form, precede the
1170 built-in name with the special package qualifier C<CORE::>. For example,
1171 saying C<CORE::open()> always refers to the built-in C<open()>, even
1172 if the current package has imported some other subroutine called
1173 C<&open()> from elsewhere. Even though it looks like a regular
1174 function call, it isn't: you can't take a reference to it, such as
1175 the incorrect C<\&CORE::open> might appear to produce.
1177 Library modules should not in general export built-in names like C<open>
1178 or C<chdir> as part of their default C<@EXPORT> list, because these may
1179 sneak into someone else's namespace and change the semantics unexpectedly.
1180 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1181 possible for a user to import the name explicitly, but not implicitly.
1182 That is, they could say
1186 and it would import the C<open> override. But if they said
1190 they would get the default imports without overrides.
1192 The foregoing mechanism for overriding built-in is restricted, quite
1193 deliberately, to the package that requests the import. There is a second
1194 method that is sometimes applicable when you wish to override a built-in
1195 everywhere, without regard to namespace boundaries. This is achieved by
1196 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1197 example that quite brazenly replaces the C<glob> operator with something
1198 that understands regular expressions.
1203 @EXPORT_OK = 'glob';
1209 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1210 $pkg->export($where, $sym, @_);
1217 if (opendir D, '.') {
1218 @got = grep /$pat/, readdir D;
1225 And here's how it could be (ab)used:
1227 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1229 use REGlob 'glob'; # override glob() in Foo:: only
1230 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1232 The initial comment shows a contrived, even dangerous example.
1233 By overriding C<glob> globally, you would be forcing the new (and
1234 subversive) behavior for the C<glob> operator for I<every> namespace,
1235 without the complete cognizance or cooperation of the modules that own
1236 those namespaces. Naturally, this should be done with extreme caution--if
1237 it must be done at all.
1239 The C<REGlob> example above does not implement all the support needed to
1240 cleanly override perl's C<glob> operator. The built-in C<glob> has
1241 different behaviors depending on whether it appears in a scalar or list
1242 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1243 context sensitive behaviors, and these must be adequately supported by
1244 a properly written override. For a fully functional example of overriding
1245 C<glob>, study the implementation of C<File::DosGlob> in the standard
1248 When you override a built-in, your replacement should be consistent (if
1249 possible) with the built-in native syntax. You can achieve this by using
1250 a suitable prototype. To get the prototype of an overridable built-in,
1251 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1252 (see L<perlfunc/prototype>).
1254 Note however that some built-ins can't have their syntax expressed by a
1255 prototype (such as C<system> or C<chomp>). If you override them you won't
1256 be able to fully mimic their original syntax.
1258 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1259 to special magic, their original syntax is preserved, and you don't have
1260 to define a prototype for their replacements. (You can't override the
1261 C<do BLOCK> syntax, though).
1263 C<require> has special additional dark magic: if you invoke your
1264 C<require> replacement as C<require Foo::Bar>, it will actually receive
1265 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1267 And, as you'll have noticed from the previous example, if you override
1268 C<glob>, the C<E<lt>*E<gt>> glob operator is overridden as well.
1270 In a similar fashion, overriding the C<readline> function also overrides
1271 the equivalent I/O operator C<< <FILEHANDLE> >>.
1273 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1277 If you call a subroutine that is undefined, you would ordinarily
1278 get an immediate, fatal error complaining that the subroutine doesn't
1279 exist. (Likewise for subroutines being used as methods, when the
1280 method doesn't exist in any base class of the class's package.)
1281 However, if an C<AUTOLOAD> subroutine is defined in the package or
1282 packages used to locate the original subroutine, then that
1283 C<AUTOLOAD> subroutine is called with the arguments that would have
1284 been passed to the original subroutine. The fully qualified name
1285 of the original subroutine magically appears in the global $AUTOLOAD
1286 variable of the same package as the C<AUTOLOAD> routine. The name
1287 is not passed as an ordinary argument because, er, well, just
1288 because, that's why...
1290 Many C<AUTOLOAD> routines load in a definition for the requested
1291 subroutine using eval(), then execute that subroutine using a special
1292 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1293 without a trace. (See the source to the standard module documented
1294 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1295 also just emulate the routine and never define it. For example,
1296 let's pretend that a function that wasn't defined should just invoke
1297 C<system> with those arguments. All you'd do is:
1300 my $program = $AUTOLOAD;
1301 $program =~ s/.*:://;
1302 system($program, @_);
1308 In fact, if you predeclare functions you want to call that way, you don't
1309 even need parentheses:
1311 use subs qw(date who ls);
1316 A more complete example of this is the standard Shell module, which
1317 can treat undefined subroutine calls as calls to external programs.
1319 Mechanisms are available to help modules writers split their modules
1320 into autoloadable files. See the standard AutoLoader module
1321 described in L<AutoLoader> and in L<AutoSplit>, the standard
1322 SelfLoader modules in L<SelfLoader>, and the document on adding C
1323 functions to Perl code in L<perlxs>.
1325 =head2 Subroutine Attributes
1327 A subroutine declaration or definition may have a list of attributes
1328 associated with it. If such an attribute list is present, it is
1329 broken up at space or colon boundaries and treated as though a
1330 C<use attributes> had been seen. See L<attributes> for details
1331 about what attributes are currently supported.
1332 Unlike the limitation with the obsolescent C<use attrs>, the
1333 C<sub : ATTRLIST> syntax works to associate the attributes with
1334 a pre-declaration, and not just with a subroutine definition.
1336 The attributes must be valid as simple identifier names (without any
1337 punctuation other than the '_' character). They may have a parameter
1338 list appended, which is only checked for whether its parentheses ('(',')')
1341 Examples of valid syntax (even though the attributes are unknown):
1343 sub fnord (&\%) : switch(10,foo(7,3)) : expensive ;
1344 sub plugh () : Ugly('\(") :Bad ;
1345 sub xyzzy : _5x5 { ... }
1347 Examples of invalid syntax:
1349 sub fnord : switch(10,foo() ; # ()-string not balanced
1350 sub snoid : Ugly('(') ; # ()-string not balanced
1351 sub xyzzy : 5x5 ; # "5x5" not a valid identifier
1352 sub plugh : Y2::north ; # "Y2::north" not a simple identifier
1353 sub snurt : foo + bar ; # "+" not a colon or space
1355 The attribute list is passed as a list of constant strings to the code
1356 which associates them with the subroutine. In particular, the second example
1357 of valid syntax above currently looks like this in terms of how it's
1360 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1362 For further details on attribute lists and their manipulation,
1367 See L<perlref/"Function Templates"> for more about references and closures.
1368 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1369 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1370 See L<perlmod> to learn about bundling up your functions in separate files.
1371 See L<perlmodlib> to learn what library modules come standard on your system.
1372 See L<perltoot> to learn how to make object method calls.