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 = $y; # similar, with an attribute applied
223 B<WARNING>: The use of attribute lists on C<my> declarations is still
224 evolving. The current semantics and interface are subject to change.
225 See L<attributes> and L<Attribute::Handlers>.
227 The C<my> operator declares the listed variables to be lexically
228 confined to the enclosing block, conditional (C<if/unless/elsif/else>),
229 loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
230 or C<do/require/use>'d file. If more than one value is listed, the
231 list must be placed in parentheses. All listed elements must be
232 legal lvalues. Only alphanumeric identifiers may be lexically
233 scoped--magical built-ins like C<$/> 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,
328 but not beyond it. See L<perlsyn/"Simple statements"> for information
329 on the scope of variables in statements with modifiers.
331 The C<foreach> loop defaults to scoping its index variable dynamically
332 in the manner of C<local>. However, if the index variable is
333 prefixed with the keyword C<my>, or if there is already a lexical
334 by that name in scope, then a new lexical is created instead. Thus
337 for my $i (1, 2, 3) {
341 the scope of $i extends to the end of the loop, but not beyond it,
342 rendering the value of $i inaccessible within C<some_function()>.
344 Some users may wish to encourage the use of lexically scoped variables.
345 As an aid to catching implicit uses to package variables,
346 which are always global, if you say
350 then any variable mentioned from there to the end of the enclosing
351 block must either refer to a lexical variable, be predeclared via
352 C<our> or C<use vars>, or else must be fully qualified with the package name.
353 A compilation error results otherwise. An inner block may countermand
354 this with C<no strict 'vars'>.
356 A C<my> has both a compile-time and a run-time effect. At compile
357 time, the compiler takes notice of it. The principal usefulness
358 of this is to quiet C<use strict 'vars'>, but it is also essential
359 for generation of closures as detailed in L<perlref>. Actual
360 initialization is delayed until run time, though, so it gets executed
361 at the appropriate time, such as each time through a loop, for
364 Variables declared with C<my> are not part of any package and are therefore
365 never fully qualified with the package name. In particular, you're not
366 allowed to try to make a package variable (or other global) lexical:
368 my $pack::var; # ERROR! Illegal syntax
369 my $_; # also illegal (currently)
371 In fact, a dynamic variable (also known as package or global variables)
372 are still accessible using the fully qualified C<::> notation even while a
373 lexical of the same name is also visible:
378 print "$x and $::x\n";
380 That will print out C<20> and C<10>.
382 You may declare C<my> variables at the outermost scope of a file
383 to hide any such identifiers from the world outside that file. This
384 is similar in spirit to C's static variables when they are used at
385 the file level. To do this with a subroutine requires the use of
386 a closure (an anonymous function that accesses enclosing lexicals).
387 If you want to create a private subroutine that cannot be called
388 from outside that block, it can declare a lexical variable containing
389 an anonymous sub reference:
391 my $secret_version = '1.001-beta';
392 my $secret_sub = sub { print $secret_version };
395 As long as the reference is never returned by any function within the
396 module, no outside module can see the subroutine, because its name is not in
397 any package's symbol table. Remember that it's not I<REALLY> called
398 C<$some_pack::secret_version> or anything; it's just $secret_version,
399 unqualified and unqualifiable.
401 This does not work with object methods, however; all object methods
402 have to be in the symbol table of some package to be found. See
403 L<perlref/"Function Templates"> for something of a work-around to
406 =head2 Persistent Private Variables
408 Just because a lexical variable is lexically (also called statically)
409 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
410 within a function it works like a C static. It normally works more
411 like a C auto, but with implicit garbage collection.
413 Unlike local variables in C or C++, Perl's lexical variables don't
414 necessarily get recycled just because their scope has exited.
415 If something more permanent is still aware of the lexical, it will
416 stick around. So long as something else references a lexical, that
417 lexical won't be freed--which is as it should be. You wouldn't want
418 memory being free until you were done using it, or kept around once you
419 were done. Automatic garbage collection takes care of this for you.
421 This means that you can pass back or save away references to lexical
422 variables, whereas to return a pointer to a C auto is a grave error.
423 It also gives us a way to simulate C's function statics. Here's a
424 mechanism for giving a function private variables with both lexical
425 scoping and a static lifetime. If you do want to create something like
426 C's static variables, just enclose the whole function in an extra block,
427 and put the static variable outside the function but in the block.
432 return ++$secret_val;
435 # $secret_val now becomes unreachable by the outside
436 # world, but retains its value between calls to gimme_another
438 If this function is being sourced in from a separate file
439 via C<require> or C<use>, then this is probably just fine. If it's
440 all in the main program, you'll need to arrange for the C<my>
441 to be executed early, either by putting the whole block above
442 your main program, or more likely, placing merely a C<BEGIN>
443 sub around it to make sure it gets executed before your program
449 return ++$secret_val;
453 See L<perlmod/"Package Constructors and Destructors"> about the
454 special triggered functions, C<BEGIN>, C<CHECK>, C<INIT> and C<END>.
456 If declared at the outermost scope (the file scope), then lexicals
457 work somewhat like C's file statics. They are available to all
458 functions in that same file declared below them, but are inaccessible
459 from outside that file. This strategy is sometimes used in modules
460 to create private variables that the whole module can see.
462 =head2 Temporary Values via local()
464 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
465 it's faster and safer. Exceptions to this include the global punctuation
466 variables, filehandles and formats, and direct manipulation of the Perl
467 symbol table itself. Format variables often use C<local> though, as do
468 other variables whose current value must be visible to called
473 local $foo; # declare $foo dynamically local
474 local (@wid, %get); # declare list of variables local
475 local $foo = "flurp"; # declare $foo dynamic, and init it
476 local @oof = @bar; # declare @oof dynamic, and init it
478 local *FH; # localize $FH, @FH, %FH, &FH ...
479 local *merlyn = *randal; # now $merlyn is really $randal, plus
480 # @merlyn is really @randal, etc
481 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
482 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
484 A C<local> modifies its listed variables to be "local" to the
485 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
486 called from within that block>. A C<local> just gives temporary
487 values to global (meaning package) variables. It does I<not> create
488 a local variable. This is known as dynamic scoping. Lexical scoping
489 is done with C<my>, which works more like C's auto declarations.
491 If more than one variable is given to C<local>, they must be placed in
492 parentheses. All listed elements must be legal lvalues. This operator works
493 by saving the current values of those variables in its argument list on a
494 hidden stack and restoring them upon exiting the block, subroutine, or
495 eval. This means that called subroutines can also reference the local
496 variable, but not the global one. The argument list may be assigned to if
497 desired, which allows you to initialize your local variables. (If no
498 initializer is given for a particular variable, it is created with an
499 undefined value.) Commonly this is used to name the parameters to a
500 subroutine. Examples:
505 # assume this function uses global %digits hash
508 # now temporarily add to %digits hash
510 # (NOTE: not claiming this is efficient!)
511 local %digits = (%digits, 't' => 10, 'e' => 11);
512 parse_num(); # parse_num gets this new %digits!
514 # old %digits restored here
516 Because C<local> is a run-time operator, it gets executed each time
517 through a loop. In releases of Perl previous to 5.0, this used more stack
518 storage each time until the loop was exited. Perl now reclaims the space
519 each time through, but it's still more efficient to declare your variables
522 A C<local> is simply a modifier on an lvalue expression. When you assign to
523 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
524 as a scalar or an array. So
526 local($foo) = <STDIN>;
527 local @FOO = <STDIN>;
529 both supply a list context to the right-hand side, while
531 local $foo = <STDIN>;
533 supplies a scalar context.
535 A note about C<local()> and composite types is in order. Something
536 like C<local(%foo)> works by temporarily placing a brand new hash in
537 the symbol table. The old hash is left alone, but is hidden "behind"
540 This means the old variable is completely invisible via the symbol
541 table (i.e. the hash entry in the C<*foo> typeglob) for the duration
542 of the dynamic scope within which the C<local()> was seen. This
543 has the effect of allowing one to temporarily occlude any magic on
544 composite types. For instance, this will briefly alter a tied
545 hash to some other implementation:
547 tie %ahash, 'APackage';
551 tie %ahash, 'BPackage';
552 [..called code will see %ahash tied to 'BPackage'..]
555 [..%ahash is a normal (untied) hash here..]
558 [..%ahash back to its initial tied self again..]
560 B<WARNING> The code example above does not currently work as described.
561 This will be fixed in a future release of Perl; in the meantime, avoid
562 code that relies on any particular behaviour of localising tied arrays
563 or hashes (localising individual elements is still okay).
564 See L<perldelta/"Localising Tied Arrays and Hashes Is Broken"> for more
567 As another example, a custom implementation of C<%ENV> might look
572 tie %ENV, 'MyOwnEnv';
573 [..do your own fancy %ENV manipulation here..]
575 [..normal %ENV behavior here..]
577 It's also worth taking a moment to explain what happens when you
578 C<local>ize a member of a composite type (i.e. an array or hash element).
579 In this case, the element is C<local>ized I<by name>. This means that
580 when the scope of the C<local()> ends, the saved value will be
581 restored to the hash element whose key was named in the C<local()>, or
582 the array element whose index was named in the C<local()>. If that
583 element was deleted while the C<local()> was in effect (e.g. by a
584 C<delete()> from a hash or a C<shift()> of an array), it will spring
585 back into existence, possibly extending an array and filling in the
586 skipped elements with C<undef>. For instance, if you say
588 %hash = ( 'This' => 'is', 'a' => 'test' );
592 local($hash{'a'}) = 'drill';
593 while (my $e = pop(@ary)) {
598 $hash{'only a'} = 'test';
602 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
603 print "The array has ",scalar(@ary)," elements: ",
604 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
611 This is a test only a test.
612 The array has 6 elements: 0, 1, 2, undef, undef, 5
614 The behavior of local() on non-existent members of composite
615 types is subject to change in future.
617 =head2 Lvalue subroutines
619 B<WARNING>: Lvalue subroutines are still experimental and the
620 implementation may change in future versions of Perl.
622 It is possible to return a modifiable value from a subroutine.
623 To do this, you have to declare the subroutine to return an lvalue.
626 sub canmod : lvalue {
627 # return $val; this doesn't work, don't say "return"
634 canmod() = 5; # assigns to $val
637 The scalar/list context for the subroutine and for the right-hand
638 side of assignment is determined as if the subroutine call is replaced
639 by a scalar. For example, consider:
641 data(2,3) = get_data(3,4);
643 Both subroutines here are called in a scalar context, while in:
645 (data(2,3)) = get_data(3,4);
649 (data(2),data(3)) = get_data(3,4);
651 all the subroutines are called in a list context.
655 =item Lvalue subroutines are EXPERIMENTAL
657 They appear to be convenient, but there are several reasons to be
660 You can't use the return keyword, you must pass out the value before
661 falling out of subroutine scope. (see comment in example above). This
662 is usually not a problem, but it disallows an explicit return out of a
663 deeply nested loop, which is sometimes a nice way out.
665 They violate encapsulation. A normal mutator can check the supplied
666 argument before setting the attribute it is protecting, an lvalue
667 subroutine never gets that chance. Consider;
669 my $some_array_ref = []; # protected by mutators ??
671 sub set_arr { # normal mutator
673 die("expected array, you supplied ", ref $val)
674 unless ref $val eq 'ARRAY';
675 $some_array_ref = $val;
677 sub set_arr_lv : lvalue { # lvalue mutator
681 # set_arr_lv cannot stop this !
682 set_arr_lv() = { a => 1 };
686 =head2 Passing Symbol Table Entries (typeglobs)
688 B<WARNING>: The mechanism described in this section was originally
689 the only way to simulate pass-by-reference in older versions of
690 Perl. While it still works fine in modern versions, the new reference
691 mechanism is generally easier to work with. See below.
693 Sometimes you don't want to pass the value of an array to a subroutine
694 but rather the name of it, so that the subroutine can modify the global
695 copy of it rather than working with a local copy. In perl you can
696 refer to all objects of a particular name by prefixing the name
697 with a star: C<*foo>. This is often known as a "typeglob", because the
698 star on the front can be thought of as a wildcard match for all the
699 funny prefix characters on variables and subroutines and such.
701 When evaluated, the typeglob produces a scalar value that represents
702 all the objects of that name, including any filehandle, format, or
703 subroutine. When assigned to, it causes the name mentioned to refer to
704 whatever C<*> value was assigned to it. Example:
707 local(*someary) = @_;
708 foreach $elem (@someary) {
715 Scalars are already passed by reference, so you can modify
716 scalar arguments without using this mechanism by referring explicitly
717 to C<$_[0]> etc. You can modify all the elements of an array by passing
718 all the elements as scalars, but you have to use the C<*> mechanism (or
719 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
720 an array. It will certainly be faster to pass the typeglob (or reference).
722 Even if you don't want to modify an array, this mechanism is useful for
723 passing multiple arrays in a single LIST, because normally the LIST
724 mechanism will merge all the array values so that you can't extract out
725 the individual arrays. For more on typeglobs, see
726 L<perldata/"Typeglobs and Filehandles">.
728 =head2 When to Still Use local()
730 Despite the existence of C<my>, there are still three places where the
731 C<local> operator still shines. In fact, in these three places, you
732 I<must> use C<local> instead of C<my>.
738 You need to give a global variable a temporary value, especially $_.
740 The global variables, like C<@ARGV> or the punctuation variables, must be
741 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
742 it up into chunks separated by lines of equal signs, which are placed
746 local @ARGV = ("/etc/motd");
749 @Fields = split /^\s*=+\s*$/;
752 It particular, it's important to C<local>ize $_ in any routine that assigns
753 to it. Look out for implicit assignments in C<while> conditionals.
757 You need to create a local file or directory handle or a local function.
759 A function that needs a filehandle of its own must use
760 C<local()> on a complete typeglob. This can be used to create new symbol
764 local (*READER, *WRITER); # not my!
765 pipe (READER, WRITER) or die "pipe: $!";
766 return (*READER, *WRITER);
768 ($head, $tail) = ioqueue();
770 See the Symbol module for a way to create anonymous symbol table
773 Because assignment of a reference to a typeglob creates an alias, this
774 can be used to create what is effectively a local function, or at least,
778 local *grow = \&shrink; # only until this block exists
779 grow(); # really calls shrink()
780 move(); # if move() grow()s, it shrink()s too
782 grow(); # get the real grow() again
784 See L<perlref/"Function Templates"> for more about manipulating
785 functions by name in this way.
789 You want to temporarily change just one element of an array or hash.
791 You can C<local>ize just one element of an aggregate. Usually this
795 local $SIG{INT} = 'IGNORE';
796 funct(); # uninterruptible
798 # interruptibility automatically restored here
800 But it also works on lexically declared aggregates. Prior to 5.005,
801 this operation could on occasion misbehave.
805 =head2 Pass by Reference
807 If you want to pass more than one array or hash into a function--or
808 return them from it--and have them maintain their integrity, then
809 you're going to have to use an explicit pass-by-reference. Before you
810 do that, you need to understand references as detailed in L<perlref>.
811 This section may not make much sense to you otherwise.
813 Here are a few simple examples. First, let's pass in several arrays
814 to a function and have it C<pop> all of then, returning a new list
815 of all their former last elements:
817 @tailings = popmany ( \@a, \@b, \@c, \@d );
822 foreach $aref ( @_ ) {
823 push @retlist, pop @$aref;
828 Here's how you might write a function that returns a
829 list of keys occurring in all the hashes passed to it:
831 @common = inter( \%foo, \%bar, \%joe );
833 my ($k, $href, %seen); # locals
835 while ( $k = each %$href ) {
839 return grep { $seen{$_} == @_ } keys %seen;
842 So far, we're using just the normal list return mechanism.
843 What happens if you want to pass or return a hash? Well,
844 if you're using only one of them, or you don't mind them
845 concatenating, then the normal calling convention is ok, although
848 Where people get into trouble is here:
850 (@a, @b) = func(@c, @d);
852 (%a, %b) = func(%c, %d);
854 That syntax simply won't work. It sets just C<@a> or C<%a> and
855 clears the C<@b> or C<%b>. Plus the function didn't get passed
856 into two separate arrays or hashes: it got one long list in C<@_>,
859 If you can arrange for everyone to deal with this through references, it's
860 cleaner code, although not so nice to look at. Here's a function that
861 takes two array references as arguments, returning the two array elements
862 in order of how many elements they have in them:
864 ($aref, $bref) = func(\@c, \@d);
865 print "@$aref has more than @$bref\n";
867 my ($cref, $dref) = @_;
868 if (@$cref > @$dref) {
869 return ($cref, $dref);
871 return ($dref, $cref);
875 It turns out that you can actually do this also:
877 (*a, *b) = func(\@c, \@d);
878 print "@a has more than @b\n";
888 Here we're using the typeglobs to do symbol table aliasing. It's
889 a tad subtle, though, and also won't work if you're using C<my>
890 variables, because only globals (even in disguise as C<local>s)
891 are in the symbol table.
893 If you're passing around filehandles, you could usually just use the bare
894 typeglob, like C<*STDOUT>, but typeglobs references work, too.
900 print $fh "her um well a hmmm\n";
903 $rec = get_rec(\*STDIN);
909 If you're planning on generating new filehandles, you could do this.
910 Notice to pass back just the bare *FH, not its reference.
915 return open (FH, $path) ? *FH : undef;
920 Perl supports a very limited kind of compile-time argument checking
921 using function prototyping. If you declare
925 then C<mypush()> takes arguments exactly like C<push()> does. The
926 function declaration must be visible at compile time. The prototype
927 affects only interpretation of new-style calls to the function,
928 where new-style is defined as not using the C<&> character. In
929 other words, if you call it like a built-in function, then it behaves
930 like a built-in function. If you call it like an old-fashioned
931 subroutine, then it behaves like an old-fashioned subroutine. It
932 naturally falls out from this rule that prototypes have no influence
933 on subroutine references like C<\&foo> or on indirect subroutine
934 calls like C<&{$subref}> or C<< $subref->() >>.
936 Method calls are not influenced by prototypes either, because the
937 function to be called is indeterminate at compile time, since
938 the exact code called depends on inheritance.
940 Because the intent of this feature is primarily to let you define
941 subroutines that work like built-in functions, here are prototypes
942 for some other functions that parse almost exactly like the
943 corresponding built-in.
945 Declared as Called as
947 sub mylink ($$) mylink $old, $new
948 sub myvec ($$$) myvec $var, $offset, 1
949 sub myindex ($$;$) myindex &getstring, "substr"
950 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
951 sub myreverse (@) myreverse $a, $b, $c
952 sub myjoin ($@) myjoin ":", $a, $b, $c
953 sub mypop (\@) mypop @array
954 sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme
955 sub mykeys (\%) mykeys %{$hashref}
956 sub myopen (*;$) myopen HANDLE, $name
957 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
958 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
959 sub myrand ($) myrand 42
962 Any backslashed prototype character represents an actual argument
963 that absolutely must start with that character. The value passed
964 as part of C<@_> will be a reference to the actual argument given
965 in the subroutine call, obtained by applying C<\> to that argument.
967 You can also backslash several argument types simultaneously by using
972 will allow calling myref() as
980 and the first argument of myref() will be a reference to
981 a scalar, an array, a hash, a code, or a glob.
983 Unbackslashed prototype characters have special meanings. Any
984 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
985 list context. An argument represented by C<$> forces scalar context. An
986 C<&> requires an anonymous subroutine, which, if passed as the first
987 argument, does not require the C<sub> keyword or a subsequent comma.
989 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
990 typeglob, or a reference to a typeglob in that slot. The value will be
991 available to the subroutine either as a simple scalar, or (in the latter
992 two cases) as a reference to the typeglob. If you wish to always convert
993 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
996 use Symbol 'qualify_to_ref';
999 my $fh = qualify_to_ref(shift, caller);
1003 A semicolon separates mandatory arguments from optional arguments.
1004 It is redundant before C<@> or C<%>, which gobble up everything else.
1006 Note how the last three examples in the table above are treated
1007 specially by the parser. C<mygrep()> is parsed as a true list
1008 operator, C<myrand()> is parsed as a true unary operator with unary
1009 precedence the same as C<rand()>, and C<mytime()> is truly without
1010 arguments, just like C<time()>. That is, if you say
1014 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1015 without a prototype.
1017 The interesting thing about C<&> is that you can generate new syntax with it,
1018 provided it's in the initial position:
1021 my($try,$catch) = @_;
1028 sub catch (&) { $_[0] }
1033 /phooey/ and print "unphooey\n";
1036 That prints C<"unphooey">. (Yes, there are still unresolved
1037 issues having to do with visibility of C<@_>. I'm ignoring that
1038 question for the moment. (But note that if we make C<@_> lexically
1039 scoped, those anonymous subroutines can act like closures... (Gee,
1040 is this sounding a little Lispish? (Never mind.))))
1042 And here's a reimplementation of the Perl C<grep> operator:
1048 push(@result, $_) if &$code;
1053 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1054 been intentionally left out of prototypes for the express purpose of
1055 someday in the future adding named, formal parameters. The current
1056 mechanism's main goal is to let module writers provide better diagnostics
1057 for module users. Larry feels the notation quite understandable to Perl
1058 programmers, and that it will not intrude greatly upon the meat of the
1059 module, nor make it harder to read. The line noise is visually
1060 encapsulated into a small pill that's easy to swallow.
1062 If you try to use an alphanumeric sequence in a prototype you will
1063 generate an optional warning - "Illegal character in prototype...".
1064 Unfortunately earlier versions of Perl allowed the prototype to be
1065 used as long as its prefix was a valid prototype. The warning may be
1066 upgraded to a fatal error in a future version of Perl once the
1067 majority of offending code is fixed.
1069 It's probably best to prototype new functions, not retrofit prototyping
1070 into older ones. That's because you must be especially careful about
1071 silent impositions of differing list versus scalar contexts. For example,
1072 if you decide that a function should take just one parameter, like this:
1076 print "you gave me $n\n";
1079 and someone has been calling it with an array or expression
1085 Then you've just supplied an automatic C<scalar> in front of their
1086 argument, which can be more than a bit surprising. The old C<@foo>
1087 which used to hold one thing doesn't get passed in. Instead,
1088 C<func()> now gets passed in a C<1>; that is, the number of elements
1089 in C<@foo>. And the C<split> gets called in scalar context so it
1090 starts scribbling on your C<@_> parameter list. Ouch!
1092 This is all very powerful, of course, and should be used only in moderation
1093 to make the world a better place.
1095 =head2 Constant Functions
1097 Functions with a prototype of C<()> are potential candidates for
1098 inlining. If the result after optimization and constant folding
1099 is either a constant or a lexically-scoped scalar which has no other
1100 references, then it will be used in place of function calls made
1101 without C<&>. Calls made using C<&> are never inlined. (See
1102 F<constant.pm> for an easy way to declare most constants.)
1104 The following functions would all be inlined:
1106 sub pi () { 3.14159 } # Not exact, but close.
1107 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1108 # and it's inlined, too!
1112 sub FLAG_FOO () { 1 << 8 }
1113 sub FLAG_BAR () { 1 << 9 }
1114 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1116 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1126 sub N () { int(BAZ_VAL) / 3 }
1129 for (1..N) { $prod *= $_ }
1130 sub N_FACTORIAL () { $prod }
1133 If you redefine a subroutine that was eligible for inlining, you'll get
1134 a mandatory warning. (You can use this warning to tell whether or not a
1135 particular subroutine is considered constant.) The warning is
1136 considered severe enough not to be optional because previously compiled
1137 invocations of the function will still be using the old value of the
1138 function. If you need to be able to redefine the subroutine, you need to
1139 ensure that it isn't inlined, either by dropping the C<()> prototype
1140 (which changes calling semantics, so beware) or by thwarting the
1141 inlining mechanism in some other way, such as
1143 sub not_inlined () {
1147 =head2 Overriding Built-in Functions
1149 Many built-in functions may be overridden, though this should be tried
1150 only occasionally and for good reason. Typically this might be
1151 done by a package attempting to emulate missing built-in functionality
1152 on a non-Unix system.
1154 Overriding may be done only by importing the name from a module at
1155 compile time--ordinary predeclaration isn't good enough. However, the
1156 C<use subs> pragma lets you, in effect, predeclare subs
1157 via the import syntax, and these names may then override built-in ones:
1159 use subs 'chdir', 'chroot', 'chmod', 'chown';
1163 To unambiguously refer to the built-in form, precede the
1164 built-in name with the special package qualifier C<CORE::>. For example,
1165 saying C<CORE::open()> always refers to the built-in C<open()>, even
1166 if the current package has imported some other subroutine called
1167 C<&open()> from elsewhere. Even though it looks like a regular
1168 function call, it isn't: you can't take a reference to it, such as
1169 the incorrect C<\&CORE::open> might appear to produce.
1171 Library modules should not in general export built-in names like C<open>
1172 or C<chdir> as part of their default C<@EXPORT> list, because these may
1173 sneak into someone else's namespace and change the semantics unexpectedly.
1174 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1175 possible for a user to import the name explicitly, but not implicitly.
1176 That is, they could say
1180 and it would import the C<open> override. But if they said
1184 they would get the default imports without overrides.
1186 The foregoing mechanism for overriding built-in is restricted, quite
1187 deliberately, to the package that requests the import. There is a second
1188 method that is sometimes applicable when you wish to override a built-in
1189 everywhere, without regard to namespace boundaries. This is achieved by
1190 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1191 example that quite brazenly replaces the C<glob> operator with something
1192 that understands regular expressions.
1197 @EXPORT_OK = 'glob';
1203 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1204 $pkg->export($where, $sym, @_);
1211 if (opendir D, '.') {
1212 @got = grep /$pat/, readdir D;
1219 And here's how it could be (ab)used:
1221 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1223 use REGlob 'glob'; # override glob() in Foo:: only
1224 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1226 The initial comment shows a contrived, even dangerous example.
1227 By overriding C<glob> globally, you would be forcing the new (and
1228 subversive) behavior for the C<glob> operator for I<every> namespace,
1229 without the complete cognizance or cooperation of the modules that own
1230 those namespaces. Naturally, this should be done with extreme caution--if
1231 it must be done at all.
1233 The C<REGlob> example above does not implement all the support needed to
1234 cleanly override perl's C<glob> operator. The built-in C<glob> has
1235 different behaviors depending on whether it appears in a scalar or list
1236 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1237 context sensitive behaviors, and these must be adequately supported by
1238 a properly written override. For a fully functional example of overriding
1239 C<glob>, study the implementation of C<File::DosGlob> in the standard
1242 When you override a built-in, your replacement should be consistent (if
1243 possible) with the built-in native syntax. You can achieve this by using
1244 a suitable prototype. To get the prototype of an overridable built-in,
1245 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1246 (see L<perlfunc/prototype>).
1248 Note however that some built-ins can't have their syntax expressed by a
1249 prototype (such as C<system> or C<chomp>). If you override them you won't
1250 be able to fully mimic their original syntax.
1252 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1253 to special magic, their original syntax is preserved, and you don't have
1254 to define a prototype for their replacements. (You can't override the
1255 C<do BLOCK> syntax, though).
1257 C<require> has special additional dark magic: if you invoke your
1258 C<require> replacement as C<require Foo::Bar>, it will actually receive
1259 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1261 And, as you'll have noticed from the previous example, if you override
1262 C<glob>, the C<E<lt>*E<gt>> glob operator is overridden as well.
1264 In a similar fashion, overriding the C<readline> function also overrides
1265 the equivalent I/O operator C<< <FILEHANDLE> >>.
1267 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1271 If you call a subroutine that is undefined, you would ordinarily
1272 get an immediate, fatal error complaining that the subroutine doesn't
1273 exist. (Likewise for subroutines being used as methods, when the
1274 method doesn't exist in any base class of the class's package.)
1275 However, if an C<AUTOLOAD> subroutine is defined in the package or
1276 packages used to locate the original subroutine, then that
1277 C<AUTOLOAD> subroutine is called with the arguments that would have
1278 been passed to the original subroutine. The fully qualified name
1279 of the original subroutine magically appears in the global $AUTOLOAD
1280 variable of the same package as the C<AUTOLOAD> routine. The name
1281 is not passed as an ordinary argument because, er, well, just
1282 because, that's why...
1284 Many C<AUTOLOAD> routines load in a definition for the requested
1285 subroutine using eval(), then execute that subroutine using a special
1286 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1287 without a trace. (See the source to the standard module documented
1288 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1289 also just emulate the routine and never define it. For example,
1290 let's pretend that a function that wasn't defined should just invoke
1291 C<system> with those arguments. All you'd do is:
1294 my $program = $AUTOLOAD;
1295 $program =~ s/.*:://;
1296 system($program, @_);
1302 In fact, if you predeclare functions you want to call that way, you don't
1303 even need parentheses:
1305 use subs qw(date who ls);
1310 A more complete example of this is the standard Shell module, which
1311 can treat undefined subroutine calls as calls to external programs.
1313 Mechanisms are available to help modules writers split their modules
1314 into autoloadable files. See the standard AutoLoader module
1315 described in L<AutoLoader> and in L<AutoSplit>, the standard
1316 SelfLoader modules in L<SelfLoader>, and the document on adding C
1317 functions to Perl code in L<perlxs>.
1319 =head2 Subroutine Attributes
1321 A subroutine declaration or definition may have a list of attributes
1322 associated with it. If such an attribute list is present, it is
1323 broken up at space or colon boundaries and treated as though a
1324 C<use attributes> had been seen. See L<attributes> for details
1325 about what attributes are currently supported.
1326 Unlike the limitation with the obsolescent C<use attrs>, the
1327 C<sub : ATTRLIST> syntax works to associate the attributes with
1328 a pre-declaration, and not just with a subroutine definition.
1330 The attributes must be valid as simple identifier names (without any
1331 punctuation other than the '_' character). They may have a parameter
1332 list appended, which is only checked for whether its parentheses ('(',')')
1335 Examples of valid syntax (even though the attributes are unknown):
1337 sub fnord (&\%) : switch(10,foo(7,3)) : expensive ;
1338 sub plugh () : Ugly('\(") :Bad ;
1339 sub xyzzy : _5x5 { ... }
1341 Examples of invalid syntax:
1343 sub fnord : switch(10,foo() ; # ()-string not balanced
1344 sub snoid : Ugly('(') ; # ()-string not balanced
1345 sub xyzzy : 5x5 ; # "5x5" not a valid identifier
1346 sub plugh : Y2::north ; # "Y2::north" not a simple identifier
1347 sub snurt : foo + bar ; # "+" not a colon or space
1349 The attribute list is passed as a list of constant strings to the code
1350 which associates them with the subroutine. In particular, the second example
1351 of valid syntax above currently looks like this in terms of how it's
1354 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1356 For further details on attribute lists and their manipulation,
1357 see L<attributes> and L<Attribute::Handlers>.
1361 See L<perlref/"Function Templates"> for more about references and closures.
1362 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1363 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1364 See L<perlmod> to learn about bundling up your functions in separate files.
1365 See L<perlmodlib> to learn what library modules come standard on your system.
1366 See L<perltoot> to learn how to make object method calls.