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 Subroutines 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 subroutine in
207 all capitals is a loosely-held convention meaning it will be called
208 indirectly by the run-time system itself, usually due to a triggered event.
209 Subroutines that do special, pre-defined things include C<AUTOLOAD>, C<CLONE>,
210 C<DESTROY> plus all functions mentioned in L<perltie> and L<PerlIO::via>.
212 The C<BEGIN>, C<CHECK>, C<INIT> and C<END> subroutines are not so much
213 subroutines as named special code blocks, of which you can have more
214 than one in a package, and which you can B<not> call explicitely. See
215 L<perlmod/"BEGIN, CHECK, INIT and END">
217 =head2 Private Variables via my()
221 my $foo; # declare $foo lexically local
222 my (@wid, %get); # declare list of variables local
223 my $foo = "flurp"; # declare $foo lexical, and init it
224 my @oof = @bar; # declare @oof lexical, and init it
225 my $x : Foo = $y; # similar, with an attribute applied
227 B<WARNING>: The use of attribute lists on C<my> declarations is still
228 evolving. The current semantics and interface are subject to change.
229 See L<attributes> and L<Attribute::Handlers>.
231 The C<my> operator declares the listed variables to be lexically
232 confined to the enclosing block, conditional (C<if/unless/elsif/else>),
233 loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
234 or C<do/require/use>'d file. If more than one value is listed, the
235 list must be placed in parentheses. All listed elements must be
236 legal lvalues. Only alphanumeric identifiers may be lexically
237 scoped--magical built-ins like C<$/> must currently be C<local>ized
238 with C<local> instead.
240 Unlike dynamic variables created by the C<local> operator, lexical
241 variables declared with C<my> are totally hidden from the outside
242 world, including any called subroutines. This is true if it's the
243 same subroutine called from itself or elsewhere--every call gets
246 This doesn't mean that a C<my> variable declared in a statically
247 enclosing lexical scope would be invisible. Only dynamic scopes
248 are cut off. For example, the C<bumpx()> function below has access
249 to the lexical $x variable because both the C<my> and the C<sub>
250 occurred at the same scope, presumably file scope.
255 An C<eval()>, however, can see lexical variables of the scope it is
256 being evaluated in, so long as the names aren't hidden by declarations within
257 the C<eval()> itself. See L<perlref>.
259 The parameter list to my() may be assigned to if desired, which allows you
260 to initialize your variables. (If no initializer is given for a
261 particular variable, it is created with the undefined value.) Commonly
262 this is used to name input parameters to a subroutine. Examples:
264 $arg = "fred"; # "global" variable
266 print "$arg thinks the root is $n\n";
267 fred thinks the root is 3
270 my $arg = shift; # name doesn't matter
275 The C<my> is simply a modifier on something you might assign to. So when
276 you do assign to variables in its argument list, C<my> doesn't
277 change whether those variables are viewed as a scalar or an array. So
279 my ($foo) = <STDIN>; # WRONG?
282 both supply a list context to the right-hand side, while
286 supplies a scalar context. But the following declares only one variable:
288 my $foo, $bar = 1; # WRONG
290 That has the same effect as
295 The declared variable is not introduced (is not visible) until after
296 the current statement. Thus,
300 can be used to initialize a new $x with the value of the old $x, and
303 my $x = 123 and $x == 123
305 is false unless the old $x happened to have the value C<123>.
307 Lexical scopes of control structures are not bounded precisely by the
308 braces that delimit their controlled blocks; control expressions are
309 part of that scope, too. Thus in the loop
311 while (my $line = <>) {
317 the scope of $line extends from its declaration throughout the rest of
318 the loop construct (including the C<continue> clause), but not beyond
319 it. Similarly, in the conditional
321 if ((my $answer = <STDIN>) =~ /^yes$/i) {
323 } elsif ($answer =~ /^no$/i) {
327 die "'$answer' is neither 'yes' nor 'no'";
330 the scope of $answer extends from its declaration through the rest
331 of that conditional, including any C<elsif> and C<else> clauses,
332 but not beyond it. See L<perlsyn/"Simple statements"> for information
333 on the scope of variables in statements with modifiers.
335 The C<foreach> loop defaults to scoping its index variable dynamically
336 in the manner of C<local>. However, if the index variable is
337 prefixed with the keyword C<my>, or if there is already a lexical
338 by that name in scope, then a new lexical is created instead. Thus
341 for my $i (1, 2, 3) {
345 the scope of $i extends to the end of the loop, but not beyond it,
346 rendering the value of $i inaccessible within C<some_function()>.
348 Some users may wish to encourage the use of lexically scoped variables.
349 As an aid to catching implicit uses to package variables,
350 which are always global, if you say
354 then any variable mentioned from there to the end of the enclosing
355 block must either refer to a lexical variable, be predeclared via
356 C<our> or C<use vars>, or else must be fully qualified with the package name.
357 A compilation error results otherwise. An inner block may countermand
358 this with C<no strict 'vars'>.
360 A C<my> has both a compile-time and a run-time effect. At compile
361 time, the compiler takes notice of it. The principal usefulness
362 of this is to quiet C<use strict 'vars'>, but it is also essential
363 for generation of closures as detailed in L<perlref>. Actual
364 initialization is delayed until run time, though, so it gets executed
365 at the appropriate time, such as each time through a loop, for
368 Variables declared with C<my> are not part of any package and are therefore
369 never fully qualified with the package name. In particular, you're not
370 allowed to try to make a package variable (or other global) lexical:
372 my $pack::var; # ERROR! Illegal syntax
373 my $_; # also illegal (currently)
375 In fact, a dynamic variable (also known as package or global variables)
376 are still accessible using the fully qualified C<::> notation even while a
377 lexical of the same name is also visible:
382 print "$x and $::x\n";
384 That will print out C<20> and C<10>.
386 You may declare C<my> variables at the outermost scope of a file
387 to hide any such identifiers from the world outside that file. This
388 is similar in spirit to C's static variables when they are used at
389 the file level. To do this with a subroutine requires the use of
390 a closure (an anonymous function that accesses enclosing lexicals).
391 If you want to create a private subroutine that cannot be called
392 from outside that block, it can declare a lexical variable containing
393 an anonymous sub reference:
395 my $secret_version = '1.001-beta';
396 my $secret_sub = sub { print $secret_version };
399 As long as the reference is never returned by any function within the
400 module, no outside module can see the subroutine, because its name is not in
401 any package's symbol table. Remember that it's not I<REALLY> called
402 C<$some_pack::secret_version> or anything; it's just $secret_version,
403 unqualified and unqualifiable.
405 This does not work with object methods, however; all object methods
406 have to be in the symbol table of some package to be found. See
407 L<perlref/"Function Templates"> for something of a work-around to
410 =head2 Persistent Private Variables
412 Just because a lexical variable is lexically (also called statically)
413 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
414 within a function it works like a C static. It normally works more
415 like a C auto, but with implicit garbage collection.
417 Unlike local variables in C or C++, Perl's lexical variables don't
418 necessarily get recycled just because their scope has exited.
419 If something more permanent is still aware of the lexical, it will
420 stick around. So long as something else references a lexical, that
421 lexical won't be freed--which is as it should be. You wouldn't want
422 memory being free until you were done using it, or kept around once you
423 were done. Automatic garbage collection takes care of this for you.
425 This means that you can pass back or save away references to lexical
426 variables, whereas to return a pointer to a C auto is a grave error.
427 It also gives us a way to simulate C's function statics. Here's a
428 mechanism for giving a function private variables with both lexical
429 scoping and a static lifetime. If you do want to create something like
430 C's static variables, just enclose the whole function in an extra block,
431 and put the static variable outside the function but in the block.
436 return ++$secret_val;
439 # $secret_val now becomes unreachable by the outside
440 # world, but retains its value between calls to gimme_another
442 If this function is being sourced in from a separate file
443 via C<require> or C<use>, then this is probably just fine. If it's
444 all in the main program, you'll need to arrange for the C<my>
445 to be executed early, either by putting the whole block above
446 your main program, or more likely, placing merely a C<BEGIN>
447 code block around it to make sure it gets executed before your program
453 return ++$secret_val;
457 See L<perlmod/"BEGIN, CHECK, INIT and END"> about the
458 special triggered code blocks, C<BEGIN>, C<CHECK>, C<INIT> and C<END>.
460 If declared at the outermost scope (the file scope), then lexicals
461 work somewhat like C's file statics. They are available to all
462 functions in that same file declared below them, but are inaccessible
463 from outside that file. This strategy is sometimes used in modules
464 to create private variables that the whole module can see.
466 =head2 Temporary Values via local()
468 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
469 it's faster and safer. Exceptions to this include the global punctuation
470 variables, global filehandles and formats, and direct manipulation of the
471 Perl symbol table itself. C<local> is mostly used when the current value
472 of a variable must be visible to called subroutines.
476 # localization of values
478 local $foo; # make $foo dynamically local
479 local (@wid, %get); # make list of variables local
480 local $foo = "flurp"; # make $foo dynamic, and init it
481 local @oof = @bar; # make @oof dynamic, and init it
483 local $hash{key} = "val"; # sets a local value for this hash entry
484 local ($cond ? $v1 : $v2); # several types of lvalues support
487 # localization of symbols
489 local *FH; # localize $FH, @FH, %FH, &FH ...
490 local *merlyn = *randal; # now $merlyn is really $randal, plus
491 # @merlyn is really @randal, etc
492 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
493 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
495 A C<local> modifies its listed variables to be "local" to the
496 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
497 called from within that block>. A C<local> just gives temporary
498 values to global (meaning package) variables. It does I<not> create
499 a local variable. This is known as dynamic scoping. Lexical scoping
500 is done with C<my>, which works more like C's auto declarations.
502 Some types of lvalues can be localized as well : hash and array elements
503 and slices, conditionals (provided that their result is always
504 localizable), and symbolic references. As for simple variables, this
505 creates new, dynamically scoped values.
507 If more than one variable or expression is given to C<local>, they must be
508 placed in parentheses. This operator works
509 by saving the current values of those variables in its argument list on a
510 hidden stack and restoring them upon exiting the block, subroutine, or
511 eval. This means that called subroutines can also reference the local
512 variable, but not the global one. The argument list may be assigned to if
513 desired, which allows you to initialize your local variables. (If no
514 initializer is given for a particular variable, it is created with an
517 Because C<local> is a run-time operator, it gets executed each time
518 through a loop. Consequently, it's more efficient to localize your
519 variables outside the loop.
521 =head3 Grammatical note on local()
523 A C<local> is simply a modifier on an lvalue expression. When you assign to
524 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
525 as a scalar or an array. So
527 local($foo) = <STDIN>;
528 local @FOO = <STDIN>;
530 both supply a list context to the right-hand side, while
532 local $foo = <STDIN>;
534 supplies a scalar context.
536 =head3 Localization of special variables
538 If you localize a special variable, you'll be giving a new value to it,
539 but its magic won't go away. That means that all side-effects related
540 to this magic still work with the localized value.
542 This feature allows code like this to work :
544 # Read the whole contents of FILE in $slurp
545 { local $/ = undef; $slurp = <FILE>; }
547 Note, however, that this restricts localization of some values ; for
548 example, the following statement dies, as of perl 5.9.0, with an error
549 I<Modification of a read-only value attempted>, because the $1 variable is
550 magical and read-only :
554 Similarly, but in a way more difficult to spot, the following snippet will
557 sub f { local $_ = "foo"; print }
559 # now $_ is aliased to $1, thus is magic and readonly
563 See next section for an alternative to this situation.
565 B<WARNING>: Localization of tied arrays and hashes does not currently
567 This will be fixed in a future release of Perl; in the meantime, avoid
568 code that relies on any particular behaviour of localising tied arrays
569 or hashes (localising individual elements is still okay).
570 See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
573 =head3 Localization of globs
579 creates a whole new symbol table entry for the glob C<name> in the
580 current package. That means that all variables in its glob slot ($name,
581 @name, %name, &name, and the C<name> filehandle) are dynamically reset.
583 This implies, among other things, that any magic eventually carried by
584 those variables is locally lost. In other words, saying C<local */>
585 will not have any effect on the internal value of the input record
588 Notably, if you want to work with a brand new value of the default scalar
589 $_, and avoid the potential problem listed above about $_ previously
590 carrying a magic value, you should use C<local *_> instead of C<local $_>.
592 =head3 Localization of elements of composite types
594 It's also worth taking a moment to explain what happens when you
595 C<local>ize a member of a composite type (i.e. an array or hash element).
596 In this case, the element is C<local>ized I<by name>. This means that
597 when the scope of the C<local()> ends, the saved value will be
598 restored to the hash element whose key was named in the C<local()>, or
599 the array element whose index was named in the C<local()>. If that
600 element was deleted while the C<local()> was in effect (e.g. by a
601 C<delete()> from a hash or a C<shift()> of an array), it will spring
602 back into existence, possibly extending an array and filling in the
603 skipped elements with C<undef>. For instance, if you say
605 %hash = ( 'This' => 'is', 'a' => 'test' );
609 local($hash{'a'}) = 'drill';
610 while (my $e = pop(@ary)) {
615 $hash{'only a'} = 'test';
619 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
620 print "The array has ",scalar(@ary)," elements: ",
621 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
628 This is a test only a test.
629 The array has 6 elements: 0, 1, 2, undef, undef, 5
631 The behavior of local() on non-existent members of composite
632 types is subject to change in future.
634 =head2 Lvalue subroutines
636 B<WARNING>: Lvalue subroutines are still experimental and the
637 implementation may change in future versions of Perl.
639 It is possible to return a modifiable value from a subroutine.
640 To do this, you have to declare the subroutine to return an lvalue.
643 sub canmod : lvalue {
644 # return $val; this doesn't work, don't say "return"
651 canmod() = 5; # assigns to $val
654 The scalar/list context for the subroutine and for the right-hand
655 side of assignment is determined as if the subroutine call is replaced
656 by a scalar. For example, consider:
658 data(2,3) = get_data(3,4);
660 Both subroutines here are called in a scalar context, while in:
662 (data(2,3)) = get_data(3,4);
666 (data(2),data(3)) = get_data(3,4);
668 all the subroutines are called in a list context.
672 =item Lvalue subroutines are EXPERIMENTAL
674 They appear to be convenient, but there are several reasons to be
677 You can't use the return keyword, you must pass out the value before
678 falling out of subroutine scope. (see comment in example above). This
679 is usually not a problem, but it disallows an explicit return out of a
680 deeply nested loop, which is sometimes a nice way out.
682 They violate encapsulation. A normal mutator can check the supplied
683 argument before setting the attribute it is protecting, an lvalue
684 subroutine never gets that chance. Consider;
686 my $some_array_ref = []; # protected by mutators ??
688 sub set_arr { # normal mutator
690 die("expected array, you supplied ", ref $val)
691 unless ref $val eq 'ARRAY';
692 $some_array_ref = $val;
694 sub set_arr_lv : lvalue { # lvalue mutator
698 # set_arr_lv cannot stop this !
699 set_arr_lv() = { a => 1 };
703 =head2 Passing Symbol Table Entries (typeglobs)
705 B<WARNING>: The mechanism described in this section was originally
706 the only way to simulate pass-by-reference in older versions of
707 Perl. While it still works fine in modern versions, the new reference
708 mechanism is generally easier to work with. See below.
710 Sometimes you don't want to pass the value of an array to a subroutine
711 but rather the name of it, so that the subroutine can modify the global
712 copy of it rather than working with a local copy. In perl you can
713 refer to all objects of a particular name by prefixing the name
714 with a star: C<*foo>. This is often known as a "typeglob", because the
715 star on the front can be thought of as a wildcard match for all the
716 funny prefix characters on variables and subroutines and such.
718 When evaluated, the typeglob produces a scalar value that represents
719 all the objects of that name, including any filehandle, format, or
720 subroutine. When assigned to, it causes the name mentioned to refer to
721 whatever C<*> value was assigned to it. Example:
724 local(*someary) = @_;
725 foreach $elem (@someary) {
732 Scalars are already passed by reference, so you can modify
733 scalar arguments without using this mechanism by referring explicitly
734 to C<$_[0]> etc. You can modify all the elements of an array by passing
735 all the elements as scalars, but you have to use the C<*> mechanism (or
736 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
737 an array. It will certainly be faster to pass the typeglob (or reference).
739 Even if you don't want to modify an array, this mechanism is useful for
740 passing multiple arrays in a single LIST, because normally the LIST
741 mechanism will merge all the array values so that you can't extract out
742 the individual arrays. For more on typeglobs, see
743 L<perldata/"Typeglobs and Filehandles">.
745 =head2 When to Still Use local()
747 Despite the existence of C<my>, there are still three places where the
748 C<local> operator still shines. In fact, in these three places, you
749 I<must> use C<local> instead of C<my>.
755 You need to give a global variable a temporary value, especially $_.
757 The global variables, like C<@ARGV> or the punctuation variables, must be
758 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
759 it up into chunks separated by lines of equal signs, which are placed
763 local @ARGV = ("/etc/motd");
766 @Fields = split /^\s*=+\s*$/;
769 It particular, it's important to C<local>ize $_ in any routine that assigns
770 to it. Look out for implicit assignments in C<while> conditionals.
774 You need to create a local file or directory handle or a local function.
776 A function that needs a filehandle of its own must use
777 C<local()> on a complete typeglob. This can be used to create new symbol
781 local (*READER, *WRITER); # not my!
782 pipe (READER, WRITER) or die "pipe: $!";
783 return (*READER, *WRITER);
785 ($head, $tail) = ioqueue();
787 See the Symbol module for a way to create anonymous symbol table
790 Because assignment of a reference to a typeglob creates an alias, this
791 can be used to create what is effectively a local function, or at least,
795 local *grow = \&shrink; # only until this block exists
796 grow(); # really calls shrink()
797 move(); # if move() grow()s, it shrink()s too
799 grow(); # get the real grow() again
801 See L<perlref/"Function Templates"> for more about manipulating
802 functions by name in this way.
806 You want to temporarily change just one element of an array or hash.
808 You can C<local>ize just one element of an aggregate. Usually this
812 local $SIG{INT} = 'IGNORE';
813 funct(); # uninterruptible
815 # interruptibility automatically restored here
817 But it also works on lexically declared aggregates. Prior to 5.005,
818 this operation could on occasion misbehave.
822 =head2 Pass by Reference
824 If you want to pass more than one array or hash into a function--or
825 return them from it--and have them maintain their integrity, then
826 you're going to have to use an explicit pass-by-reference. Before you
827 do that, you need to understand references as detailed in L<perlref>.
828 This section may not make much sense to you otherwise.
830 Here are a few simple examples. First, let's pass in several arrays
831 to a function and have it C<pop> all of then, returning a new list
832 of all their former last elements:
834 @tailings = popmany ( \@a, \@b, \@c, \@d );
839 foreach $aref ( @_ ) {
840 push @retlist, pop @$aref;
845 Here's how you might write a function that returns a
846 list of keys occurring in all the hashes passed to it:
848 @common = inter( \%foo, \%bar, \%joe );
850 my ($k, $href, %seen); # locals
852 while ( $k = each %$href ) {
856 return grep { $seen{$_} == @_ } keys %seen;
859 So far, we're using just the normal list return mechanism.
860 What happens if you want to pass or return a hash? Well,
861 if you're using only one of them, or you don't mind them
862 concatenating, then the normal calling convention is ok, although
865 Where people get into trouble is here:
867 (@a, @b) = func(@c, @d);
869 (%a, %b) = func(%c, %d);
871 That syntax simply won't work. It sets just C<@a> or C<%a> and
872 clears the C<@b> or C<%b>. Plus the function didn't get passed
873 into two separate arrays or hashes: it got one long list in C<@_>,
876 If you can arrange for everyone to deal with this through references, it's
877 cleaner code, although not so nice to look at. Here's a function that
878 takes two array references as arguments, returning the two array elements
879 in order of how many elements they have in them:
881 ($aref, $bref) = func(\@c, \@d);
882 print "@$aref has more than @$bref\n";
884 my ($cref, $dref) = @_;
885 if (@$cref > @$dref) {
886 return ($cref, $dref);
888 return ($dref, $cref);
892 It turns out that you can actually do this also:
894 (*a, *b) = func(\@c, \@d);
895 print "@a has more than @b\n";
905 Here we're using the typeglobs to do symbol table aliasing. It's
906 a tad subtle, though, and also won't work if you're using C<my>
907 variables, because only globals (even in disguise as C<local>s)
908 are in the symbol table.
910 If you're passing around filehandles, you could usually just use the bare
911 typeglob, like C<*STDOUT>, but typeglobs references work, too.
917 print $fh "her um well a hmmm\n";
920 $rec = get_rec(\*STDIN);
926 If you're planning on generating new filehandles, you could do this.
927 Notice to pass back just the bare *FH, not its reference.
932 return open (FH, $path) ? *FH : undef;
937 Perl supports a very limited kind of compile-time argument checking
938 using function prototyping. If you declare
942 then C<mypush()> takes arguments exactly like C<push()> does. The
943 function declaration must be visible at compile time. The prototype
944 affects only interpretation of new-style calls to the function,
945 where new-style is defined as not using the C<&> character. In
946 other words, if you call it like a built-in function, then it behaves
947 like a built-in function. If you call it like an old-fashioned
948 subroutine, then it behaves like an old-fashioned subroutine. It
949 naturally falls out from this rule that prototypes have no influence
950 on subroutine references like C<\&foo> or on indirect subroutine
951 calls like C<&{$subref}> or C<< $subref->() >>.
953 Method calls are not influenced by prototypes either, because the
954 function to be called is indeterminate at compile time, since
955 the exact code called depends on inheritance.
957 Because the intent of this feature is primarily to let you define
958 subroutines that work like built-in functions, here are prototypes
959 for some other functions that parse almost exactly like the
960 corresponding built-in.
962 Declared as Called as
964 sub mylink ($$) mylink $old, $new
965 sub myvec ($$$) myvec $var, $offset, 1
966 sub myindex ($$;$) myindex &getstring, "substr"
967 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
968 sub myreverse (@) myreverse $a, $b, $c
969 sub myjoin ($@) myjoin ":", $a, $b, $c
970 sub mypop (\@) mypop @array
971 sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme
972 sub mykeys (\%) mykeys %{$hashref}
973 sub myopen (*;$) myopen HANDLE, $name
974 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
975 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
976 sub myrand ($) myrand 42
979 Any backslashed prototype character represents an actual argument
980 that absolutely must start with that character. The value passed
981 as part of C<@_> will be a reference to the actual argument given
982 in the subroutine call, obtained by applying C<\> to that argument.
984 You can also backslash several argument types simultaneously by using
989 will allow calling myref() as
997 and the first argument of myref() will be a reference to
998 a scalar, an array, a hash, a code, or a glob.
1000 Unbackslashed prototype characters have special meanings. Any
1001 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
1002 list context. An argument represented by C<$> forces scalar context. An
1003 C<&> requires an anonymous subroutine, which, if passed as the first
1004 argument, does not require the C<sub> keyword or a subsequent comma.
1006 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
1007 typeglob, or a reference to a typeglob in that slot. The value will be
1008 available to the subroutine either as a simple scalar, or (in the latter
1009 two cases) as a reference to the typeglob. If you wish to always convert
1010 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1013 use Symbol 'qualify_to_ref';
1016 my $fh = qualify_to_ref(shift, caller);
1020 A semicolon separates mandatory arguments from optional arguments.
1021 It is redundant before C<@> or C<%>, which gobble up everything else.
1023 Note how the last three examples in the table above are treated
1024 specially by the parser. C<mygrep()> is parsed as a true list
1025 operator, C<myrand()> is parsed as a true unary operator with unary
1026 precedence the same as C<rand()>, and C<mytime()> is truly without
1027 arguments, just like C<time()>. That is, if you say
1031 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1032 without a prototype.
1034 The interesting thing about C<&> is that you can generate new syntax with it,
1035 provided it's in the initial position:
1038 my($try,$catch) = @_;
1045 sub catch (&) { $_[0] }
1050 /phooey/ and print "unphooey\n";
1053 That prints C<"unphooey">. (Yes, there are still unresolved
1054 issues having to do with visibility of C<@_>. I'm ignoring that
1055 question for the moment. (But note that if we make C<@_> lexically
1056 scoped, those anonymous subroutines can act like closures... (Gee,
1057 is this sounding a little Lispish? (Never mind.))))
1059 And here's a reimplementation of the Perl C<grep> operator:
1065 push(@result, $_) if &$code;
1070 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1071 been intentionally left out of prototypes for the express purpose of
1072 someday in the future adding named, formal parameters. The current
1073 mechanism's main goal is to let module writers provide better diagnostics
1074 for module users. Larry feels the notation quite understandable to Perl
1075 programmers, and that it will not intrude greatly upon the meat of the
1076 module, nor make it harder to read. The line noise is visually
1077 encapsulated into a small pill that's easy to swallow.
1079 If you try to use an alphanumeric sequence in a prototype you will
1080 generate an optional warning - "Illegal character in prototype...".
1081 Unfortunately earlier versions of Perl allowed the prototype to be
1082 used as long as its prefix was a valid prototype. The warning may be
1083 upgraded to a fatal error in a future version of Perl once the
1084 majority of offending code is fixed.
1086 It's probably best to prototype new functions, not retrofit prototyping
1087 into older ones. That's because you must be especially careful about
1088 silent impositions of differing list versus scalar contexts. For example,
1089 if you decide that a function should take just one parameter, like this:
1093 print "you gave me $n\n";
1096 and someone has been calling it with an array or expression
1102 Then you've just supplied an automatic C<scalar> in front of their
1103 argument, which can be more than a bit surprising. The old C<@foo>
1104 which used to hold one thing doesn't get passed in. Instead,
1105 C<func()> now gets passed in a C<1>; that is, the number of elements
1106 in C<@foo>. And the C<split> gets called in scalar context so it
1107 starts scribbling on your C<@_> parameter list. Ouch!
1109 This is all very powerful, of course, and should be used only in moderation
1110 to make the world a better place.
1112 =head2 Constant Functions
1114 Functions with a prototype of C<()> are potential candidates for
1115 inlining. If the result after optimization and constant folding
1116 is either a constant or a lexically-scoped scalar which has no other
1117 references, then it will be used in place of function calls made
1118 without C<&>. Calls made using C<&> are never inlined. (See
1119 F<constant.pm> for an easy way to declare most constants.)
1121 The following functions would all be inlined:
1123 sub pi () { 3.14159 } # Not exact, but close.
1124 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1125 # and it's inlined, too!
1129 sub FLAG_FOO () { 1 << 8 }
1130 sub FLAG_BAR () { 1 << 9 }
1131 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1133 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1143 sub N () { int(BAZ_VAL) / 3 }
1146 for (1..N) { $prod *= $_ }
1147 sub N_FACTORIAL () { $prod }
1150 If you redefine a subroutine that was eligible for inlining, you'll get
1151 a mandatory warning. (You can use this warning to tell whether or not a
1152 particular subroutine is considered constant.) The warning is
1153 considered severe enough not to be optional because previously compiled
1154 invocations of the function will still be using the old value of the
1155 function. If you need to be able to redefine the subroutine, you need to
1156 ensure that it isn't inlined, either by dropping the C<()> prototype
1157 (which changes calling semantics, so beware) or by thwarting the
1158 inlining mechanism in some other way, such as
1160 sub not_inlined () {
1164 =head2 Overriding Built-in Functions
1166 Many built-in functions may be overridden, though this should be tried
1167 only occasionally and for good reason. Typically this might be
1168 done by a package attempting to emulate missing built-in functionality
1169 on a non-Unix system.
1171 Overriding may be done only by importing the name from a module at
1172 compile time--ordinary predeclaration isn't good enough. However, the
1173 C<use subs> pragma lets you, in effect, predeclare subs
1174 via the import syntax, and these names may then override built-in ones:
1176 use subs 'chdir', 'chroot', 'chmod', 'chown';
1180 To unambiguously refer to the built-in form, precede the
1181 built-in name with the special package qualifier C<CORE::>. For example,
1182 saying C<CORE::open()> always refers to the built-in C<open()>, even
1183 if the current package has imported some other subroutine called
1184 C<&open()> from elsewhere. Even though it looks like a regular
1185 function call, it isn't: you can't take a reference to it, such as
1186 the incorrect C<\&CORE::open> might appear to produce.
1188 Library modules should not in general export built-in names like C<open>
1189 or C<chdir> as part of their default C<@EXPORT> list, because these may
1190 sneak into someone else's namespace and change the semantics unexpectedly.
1191 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1192 possible for a user to import the name explicitly, but not implicitly.
1193 That is, they could say
1197 and it would import the C<open> override. But if they said
1201 they would get the default imports without overrides.
1203 The foregoing mechanism for overriding built-in is restricted, quite
1204 deliberately, to the package that requests the import. There is a second
1205 method that is sometimes applicable when you wish to override a built-in
1206 everywhere, without regard to namespace boundaries. This is achieved by
1207 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1208 example that quite brazenly replaces the C<glob> operator with something
1209 that understands regular expressions.
1214 @EXPORT_OK = 'glob';
1220 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1221 $pkg->export($where, $sym, @_);
1228 if (opendir D, '.') {
1229 @got = grep /$pat/, readdir D;
1236 And here's how it could be (ab)used:
1238 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1240 use REGlob 'glob'; # override glob() in Foo:: only
1241 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1243 The initial comment shows a contrived, even dangerous example.
1244 By overriding C<glob> globally, you would be forcing the new (and
1245 subversive) behavior for the C<glob> operator for I<every> namespace,
1246 without the complete cognizance or cooperation of the modules that own
1247 those namespaces. Naturally, this should be done with extreme caution--if
1248 it must be done at all.
1250 The C<REGlob> example above does not implement all the support needed to
1251 cleanly override perl's C<glob> operator. The built-in C<glob> has
1252 different behaviors depending on whether it appears in a scalar or list
1253 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1254 context sensitive behaviors, and these must be adequately supported by
1255 a properly written override. For a fully functional example of overriding
1256 C<glob>, study the implementation of C<File::DosGlob> in the standard
1259 When you override a built-in, your replacement should be consistent (if
1260 possible) with the built-in native syntax. You can achieve this by using
1261 a suitable prototype. To get the prototype of an overridable built-in,
1262 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1263 (see L<perlfunc/prototype>).
1265 Note however that some built-ins can't have their syntax expressed by a
1266 prototype (such as C<system> or C<chomp>). If you override them you won't
1267 be able to fully mimic their original syntax.
1269 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1270 to special magic, their original syntax is preserved, and you don't have
1271 to define a prototype for their replacements. (You can't override the
1272 C<do BLOCK> syntax, though).
1274 C<require> has special additional dark magic: if you invoke your
1275 C<require> replacement as C<require Foo::Bar>, it will actually receive
1276 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1278 And, as you'll have noticed from the previous example, if you override
1279 C<glob>, the C<< <*> >> glob operator is overridden as well.
1281 In a similar fashion, overriding the C<readline> function also overrides
1282 the equivalent I/O operator C<< <FILEHANDLE> >>.
1284 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1288 If you call a subroutine that is undefined, you would ordinarily
1289 get an immediate, fatal error complaining that the subroutine doesn't
1290 exist. (Likewise for subroutines being used as methods, when the
1291 method doesn't exist in any base class of the class's package.)
1292 However, if an C<AUTOLOAD> subroutine is defined in the package or
1293 packages used to locate the original subroutine, then that
1294 C<AUTOLOAD> subroutine is called with the arguments that would have
1295 been passed to the original subroutine. The fully qualified name
1296 of the original subroutine magically appears in the global $AUTOLOAD
1297 variable of the same package as the C<AUTOLOAD> routine. The name
1298 is not passed as an ordinary argument because, er, well, just
1299 because, that's why. (As an exception, a method call to a nonexistent
1300 C<import> or C<unimport> method is just skipped instead.)
1302 Many C<AUTOLOAD> routines load in a definition for the requested
1303 subroutine using eval(), then execute that subroutine using a special
1304 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1305 without a trace. (See the source to the standard module documented
1306 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1307 also just emulate the routine and never define it. For example,
1308 let's pretend that a function that wasn't defined should just invoke
1309 C<system> with those arguments. All you'd do is:
1312 my $program = $AUTOLOAD;
1313 $program =~ s/.*:://;
1314 system($program, @_);
1320 In fact, if you predeclare functions you want to call that way, you don't
1321 even need parentheses:
1323 use subs qw(date who ls);
1328 A more complete example of this is the standard Shell module, which
1329 can treat undefined subroutine calls as calls to external programs.
1331 Mechanisms are available to help modules writers split their modules
1332 into autoloadable files. See the standard AutoLoader module
1333 described in L<AutoLoader> and in L<AutoSplit>, the standard
1334 SelfLoader modules in L<SelfLoader>, and the document on adding C
1335 functions to Perl code in L<perlxs>.
1337 =head2 Subroutine Attributes
1339 A subroutine declaration or definition may have a list of attributes
1340 associated with it. If such an attribute list is present, it is
1341 broken up at space or colon boundaries and treated as though a
1342 C<use attributes> had been seen. See L<attributes> for details
1343 about what attributes are currently supported.
1344 Unlike the limitation with the obsolescent C<use attrs>, the
1345 C<sub : ATTRLIST> syntax works to associate the attributes with
1346 a pre-declaration, and not just with a subroutine definition.
1348 The attributes must be valid as simple identifier names (without any
1349 punctuation other than the '_' character). They may have a parameter
1350 list appended, which is only checked for whether its parentheses ('(',')')
1353 Examples of valid syntax (even though the attributes are unknown):
1355 sub fnord (&\%) : switch(10,foo(7,3)) : expensive ;
1356 sub plugh () : Ugly('\(") :Bad ;
1357 sub xyzzy : _5x5 { ... }
1359 Examples of invalid syntax:
1361 sub fnord : switch(10,foo() ; # ()-string not balanced
1362 sub snoid : Ugly('(') ; # ()-string not balanced
1363 sub xyzzy : 5x5 ; # "5x5" not a valid identifier
1364 sub plugh : Y2::north ; # "Y2::north" not a simple identifier
1365 sub snurt : foo + bar ; # "+" not a colon or space
1367 The attribute list is passed as a list of constant strings to the code
1368 which associates them with the subroutine. In particular, the second example
1369 of valid syntax above currently looks like this in terms of how it's
1372 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1374 For further details on attribute lists and their manipulation,
1375 see L<attributes> and L<Attribute::Handlers>.
1379 See L<perlref/"Function Templates"> for more about references and closures.
1380 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1381 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1382 See L<perlmod> to learn about bundling up your functions in separate files.
1383 See L<perlmodlib> to learn what library modules come standard on your system.
1384 See L<perltoot> to learn how to make object method calls.