2 X<subroutine> X<function>
4 perlsub - Perl subroutines
8 To declare subroutines:
9 X<subroutine, declaration> X<sub>
11 sub NAME; # A "forward" declaration.
12 sub NAME(PROTO); # ditto, but with prototypes
13 sub NAME : ATTRS; # with attributes
14 sub NAME(PROTO) : ATTRS; # with attributes and prototypes
16 sub NAME BLOCK # A declaration and a definition.
17 sub NAME(PROTO) BLOCK # ditto, but with prototypes
18 sub NAME : ATTRS BLOCK # with attributes
19 sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
21 use feature 'signatures';
22 sub NAME(SIG) BLOCK # with signature
23 sub NAME :ATTRS (SIG) BLOCK # with signature, attributes
24 sub NAME :prototype(PROTO) (SIG) BLOCK # with signature, prototype
26 To define an anonymous subroutine at runtime:
27 X<subroutine, anonymous>
29 $subref = sub BLOCK; # no proto
30 $subref = sub (PROTO) BLOCK; # with proto
31 $subref = sub : ATTRS BLOCK; # with attributes
32 $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
34 use feature 'signatures';
35 $subref = sub (SIG) BLOCK; # with signature
36 $subref = sub : ATTRS(SIG) BLOCK; # with signature, attributes
38 To import subroutines:
41 use MODULE qw(NAME1 NAME2 NAME3);
44 X<subroutine, call> X<call>
46 NAME(LIST); # & is optional with parentheses.
47 NAME LIST; # Parentheses optional if predeclared/imported.
48 &NAME(LIST); # Circumvent prototypes.
49 &NAME; # Makes current @_ visible to called subroutine.
53 Like many languages, Perl provides for user-defined subroutines.
54 These may be located anywhere in the main program, loaded in from
55 other files via the C<do>, C<require>, or C<use> keywords, or
56 generated on the fly using C<eval> or anonymous subroutines.
57 You can even call a function indirectly using a variable containing
58 its name or a CODE reference.
60 The Perl model for function call and return values is simple: all
61 functions are passed as parameters one single flat list of scalars, and
62 all functions likewise return to their caller one single flat list of
63 scalars. Any arrays or hashes in these call and return lists will
64 collapse, losing their identities--but you may always use
65 pass-by-reference instead to avoid this. Both call and return lists may
66 contain as many or as few scalar elements as you'd like. (Often a
67 function without an explicit return statement is called a subroutine, but
68 there's really no difference from Perl's perspective.)
69 X<subroutine, parameter> X<parameter>
71 Any arguments passed in show up in the array C<@_>.
72 (They may also show up in lexical variables introduced by a signature;
73 see L</Signatures> below.) Therefore, if
74 you called a function with two arguments, those would be stored in
75 C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its
76 elements are aliases for the actual scalar parameters. In particular,
77 if an element C<$_[0]> is updated, the corresponding argument is
78 updated (or an error occurs if it is not updatable). If an argument
79 is an array or hash element which did not exist when the function
80 was called, that element is created only when (and if) it is modified
81 or a reference to it is taken. (Some earlier versions of Perl
82 created the element whether or not the element was assigned to.)
83 Assigning to the whole array C<@_> removes that aliasing, and does
84 not update any arguments.
85 X<subroutine, argument> X<argument> X<@_>
87 A C<return> statement may be used to exit a subroutine, optionally
88 specifying the returned value, which will be evaluated in the
89 appropriate context (list, scalar, or void) depending on the context of
90 the subroutine call. If you specify no return value, the subroutine
91 returns an empty list in list context, the undefined value in scalar
92 context, or nothing in void context. If you return one or more
93 aggregates (arrays and hashes), these will be flattened together into
94 one large indistinguishable list.
96 If no C<return> is found and if the last statement is an expression, its
97 value is returned. If the last statement is a loop control structure
98 like a C<foreach> or a C<while>, the returned value is unspecified. The
99 empty sub returns the empty list.
100 X<subroutine, return value> X<return value> X<return>
102 Aside from an experimental facility (see L</Signatures> below),
103 Perl does not have named formal parameters. In practice all you
104 do is assign to a C<my()> list of these. Variables that aren't
105 declared to be private are global variables. For gory details
106 on creating private variables, see L</"Private Variables via my()">
107 and L</"Temporary Values via local()">. To create protected
108 environments for a set of functions in a separate package (and
109 probably a separate file), see L<perlmod/"Packages">.
110 X<formal parameter> X<parameter, formal>
117 $max = $foo if $max < $foo;
121 $bestday = max($mon,$tue,$wed,$thu,$fri);
125 # get a line, combining continuation lines
126 # that start with whitespace
129 $thisline = $lookahead; # global variables!
130 LINE: while (defined($lookahead = <STDIN>)) {
131 if ($lookahead =~ /^[ \t]/) {
132 $thisline .= $lookahead;
141 $lookahead = <STDIN>; # get first line
142 while (defined($line = get_line())) {
146 Assigning to a list of private variables to name your arguments:
149 my($key, $value) = @_;
150 $Foo{$key} = $value unless $Foo{$key};
153 Because the assignment copies the values, this also has the effect
154 of turning call-by-reference into call-by-value. Otherwise a
155 function is free to do in-place modifications of C<@_> and change
157 X<call-by-reference> X<call-by-value>
159 upcase_in($v1, $v2); # this changes $v1 and $v2
161 for (@_) { tr/a-z/A-Z/ }
164 You aren't allowed to modify constants in this way, of course. If an
165 argument were actually literal and you tried to change it, you'd take a
166 (presumably fatal) exception. For example, this won't work:
167 X<call-by-reference> X<call-by-value>
169 upcase_in("frederick");
171 It would be much safer if the C<upcase_in()> function
172 were written to return a copy of its parameters instead
173 of changing them in place:
175 ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
177 return unless defined wantarray; # void context, do nothing
179 for (@parms) { tr/a-z/A-Z/ }
180 return wantarray ? @parms : $parms[0];
183 Notice how this (unprototyped) function doesn't care whether it was
184 passed real scalars or arrays. Perl sees all arguments as one big,
185 long, flat parameter list in C<@_>. This is one area where
186 Perl's simple argument-passing style shines. The C<upcase()>
187 function would work perfectly well without changing the C<upcase()>
188 definition even if we fed it things like this:
190 @newlist = upcase(@list1, @list2);
191 @newlist = upcase( split /:/, $var );
193 Do not, however, be tempted to do this:
195 (@a, @b) = upcase(@list1, @list2);
197 Like the flattened incoming parameter list, the return list is also
198 flattened on return. So all you have managed to do here is stored
199 everything in C<@a> and made C<@b> empty. See
200 L</Pass by Reference> for alternatives.
202 A subroutine may be called using an explicit C<&> prefix. The
203 C<&> is optional in modern Perl, as are parentheses if the
204 subroutine has been predeclared. The C<&> is I<not> optional
205 when just naming the subroutine, such as when it's used as
206 an argument to defined() or undef(). Nor is it optional when you
207 want to do an indirect subroutine call with a subroutine name or
208 reference using the C<&$subref()> or C<&{$subref}()> constructs,
209 although the C<< $subref->() >> notation solves that problem.
210 See L<perlref> for more about all that.
213 Subroutines may be called recursively. If a subroutine is called
214 using the C<&> form, the argument list is optional, and if omitted,
215 no C<@_> array is set up for the subroutine: the C<@_> array at the
216 time of the call is visible to subroutine instead. This is an
217 efficiency mechanism that new users may wish to avoid.
220 &foo(1,2,3); # pass three arguments
221 foo(1,2,3); # the same
223 foo(); # pass a null list
226 &foo; # foo() get current args, like foo(@_) !!
227 foo; # like foo() IFF sub foo predeclared, else "foo"
229 Not only does the C<&> form make the argument list optional, it also
230 disables any prototype checking on arguments you do provide. This
231 is partly for historical reasons, and partly for having a convenient way
232 to cheat if you know what you're doing. See L</Prototypes> below.
235 Since Perl 5.16.0, the C<__SUB__> token is available under C<use feature
236 'current_sub'> and C<use 5.16.0>. It will evaluate to a reference to the
237 currently-running sub, which allows for recursive calls without knowing
238 your subroutine's name.
241 my $factorial = sub {
244 return($x * __SUB__->( $x - 1 ) );
247 The behavior of C<__SUB__> within a regex code block (such as C</(?{...})/>)
248 is subject to change.
250 Subroutines whose names are in all upper case are reserved to the Perl
251 core, as are modules whose names are in all lower case. A subroutine in
252 all capitals is a loosely-held convention meaning it will be called
253 indirectly by the run-time system itself, usually due to a triggered event.
254 Subroutines whose name start with a left parenthesis are also reserved the
255 same way. The following is a list of some subroutines that currently do
256 special, pre-defined things.
260 =item documented later in this document
264 =item documented in L<perlmod>
266 C<CLONE>, C<CLONE_SKIP>
268 =item documented in L<perlobj>
272 =item documented in L<perltie>
274 C<BINMODE>, C<CLEAR>, C<CLOSE>, C<DELETE>, C<DESTROY>, C<EOF>, C<EXISTS>,
275 C<EXTEND>, C<FETCH>, C<FETCHSIZE>, C<FILENO>, C<FIRSTKEY>, C<GETC>,
276 C<NEXTKEY>, C<OPEN>, C<POP>, C<PRINT>, C<PRINTF>, C<PUSH>, C<READ>,
277 C<READLINE>, C<SCALAR>, C<SEEK>, C<SHIFT>, C<SPLICE>, C<STORE>,
278 C<STORESIZE>, C<TELL>, C<TIEARRAY>, C<TIEHANDLE>, C<TIEHASH>,
279 C<TIESCALAR>, C<UNSHIFT>, C<UNTIE>, C<WRITE>
281 =item documented in L<PerlIO::via>
283 C<BINMODE>, C<CLEARERR>, C<CLOSE>, C<EOF>, C<ERROR>, C<FDOPEN>, C<FILENO>,
284 C<FILL>, C<FLUSH>, C<OPEN>, C<POPPED>, C<PUSHED>, C<READ>, C<SEEK>,
285 C<SETLINEBUF>, C<SYSOPEN>, C<TELL>, C<UNREAD>, C<UTF8>, C<WRITE>
287 =item documented in L<perlfunc>
289 L<< C<import> | perlfunc/use >>, L<< C<unimport> | perlfunc/use >>,
290 L<< C<INC> | perlfunc/require >>
292 =item documented in L<UNIVERSAL>
296 =item documented in L<perldebguts>
298 C<DB::DB>, C<DB::sub>, C<DB::lsub>, C<DB::goto>, C<DB::postponed>
300 =item undocumented, used internally by the L<overload> feature
302 any starting with C<(>
306 The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines
307 are not so much subroutines as named special code blocks, of which you
308 can have more than one in a package, and which you can B<not> call
309 explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">
313 B<WARNING>: Subroutine signatures are experimental. The feature may be
314 modified or removed in future versions of Perl.
316 Perl has an experimental facility to allow a subroutine's formal
317 parameters to be introduced by special syntax, separate from the
318 procedural code of the subroutine body. The formal parameter list
319 is known as a I<signature>. The facility must be enabled first by a
320 pragmatic declaration, C<use feature 'signatures'>, and it will produce
321 a warning unless the "experimental::signatures" warnings category is
324 The signature is part of a subroutine's body. Normally the body of a
325 subroutine is simply a braced block of code, but when using a signature,
326 the signature is a parenthesised list that goes immediately before the
327 block, after any name or attributes.
331 sub foo :lvalue ($a, $b = 1, @c) { .... }
333 The signature declares lexical variables that are
334 in scope for the block. When the subroutine is called, the signature
335 takes control first. It populates the signature variables from the
336 list of arguments that were passed. If the argument list doesn't meet
337 the requirements of the signature, then it will throw an exception.
338 When the signature processing is complete, control passes to the block.
340 Positional parameters are handled by simply naming scalar variables in
341 the signature. For example,
343 sub foo ($left, $right) {
344 return $left + $right;
347 takes two positional parameters, which must be filled at runtime by
348 two arguments. By default the parameters are mandatory, and it is
349 not permitted to pass more arguments than expected. So the above is
353 die "Too many arguments for subroutine" unless @_ <= 2;
354 die "Too few arguments for subroutine" unless @_ >= 2;
357 return $left + $right;
360 An argument can be ignored by omitting the main part of the name from
361 a parameter declaration, leaving just a bare C<$> sigil. For example,
363 sub foo ($first, $, $third) {
364 return "first=$first, third=$third";
367 Although the ignored argument doesn't go into a variable, it is still
368 mandatory for the caller to pass it.
370 A positional parameter is made optional by giving a default value,
371 separated from the parameter name by C<=>:
373 sub foo ($left, $right = 0) {
374 return $left + $right;
377 The above subroutine may be called with either one or two arguments.
378 The default value expression is evaluated when the subroutine is called,
379 so it may provide different default values for different calls. It is
380 only evaluated if the argument was actually omitted from the call.
384 sub foo ($thing, $id = $auto_id++) {
385 print "$thing has ID $id";
388 automatically assigns distinct sequential IDs to things for which no
389 ID was supplied by the caller. A default value expression may also
390 refer to parameters earlier in the signature, making the default for
391 one parameter vary according to the earlier parameters. For example,
393 sub foo ($first_name, $surname, $nickname = $first_name) {
394 print "$first_name $surname is known as \"$nickname\"";
397 An optional parameter can be nameless just like a mandatory parameter.
400 sub foo ($thing, $ = 1) {
404 The parameter's default value will still be evaluated if the corresponding
405 argument isn't supplied, even though the value won't be stored anywhere.
406 This is in case evaluating it has important side effects. However, it
407 will be evaluated in void context, so if it doesn't have side effects
408 and is not trivial it will generate a warning if the "void" warning
409 category is enabled. If a nameless optional parameter's default value
410 is not important, it may be omitted just as the parameter's name was:
412 sub foo ($thing, $=) {
416 Optional positional parameters must come after all mandatory positional
417 parameters. (If there are no mandatory positional parameters then an
418 optional positional parameters can be the first thing in the signature.)
419 If there are multiple optional positional parameters and not enough
420 arguments are supplied to fill them all, they will be filled from left
423 After positional parameters, additional arguments may be captured in a
424 slurpy parameter. The simplest form of this is just an array variable:
426 sub foo ($filter, @inputs) {
427 print $filter->($_) foreach @inputs;
430 With a slurpy parameter in the signature, there is no upper limit on how
431 many arguments may be passed. A slurpy array parameter may be nameless
432 just like a positional parameter, in which case its only effect is to
433 turn off the argument limit that would otherwise apply:
435 sub foo ($thing, @) {
439 A slurpy parameter may instead be a hash, in which case the arguments
440 available to it are interpreted as alternating keys and values.
441 There must be as many keys as values: if there is an odd argument then
442 an exception will be thrown. Keys will be stringified, and if there are
443 duplicates then the later instance takes precedence over the earlier,
444 as with standard hash construction.
446 sub foo ($filter, %inputs) {
447 print $filter->($_, $inputs{$_}) foreach sort keys %inputs;
450 A slurpy hash parameter may be nameless just like other kinds of
451 parameter. It still insists that the number of arguments available to
452 it be even, even though they're not being put into a variable.
454 sub foo ($thing, %) {
458 A slurpy parameter, either array or hash, must be the last thing in the
459 signature. It may follow mandatory and optional positional parameters;
460 it may also be the only thing in the signature. Slurpy parameters cannot
461 have default values: if no arguments are supplied for them then you get
462 an empty array or empty hash.
464 A signature may be entirely empty, in which case all it does is check
465 that the caller passed no arguments:
471 When using a signature, the arguments are still available in the special
472 array variable C<@_>, in addition to the lexical variables of the
473 signature. There is a difference between the two ways of accessing the
474 arguments: C<@_> I<aliases> the arguments, but the signature variables
475 get I<copies> of the arguments. So writing to a signature variable
476 only changes that variable, and has no effect on the caller's variables,
477 but writing to an element of C<@_> modifies whatever the caller used to
478 supply that argument.
480 There is a potential syntactic ambiguity between signatures and prototypes
481 (see L</Prototypes>), because both start with an opening parenthesis and
482 both can appear in some of the same places, such as just after the name
483 in a subroutine declaration. For historical reasons, when signatures
484 are not enabled, any opening parenthesis in such a context will trigger
485 very forgiving prototype parsing. Most signatures will be interpreted
486 as prototypes in those circumstances, but won't be valid prototypes.
487 (A valid prototype cannot contain any alphabetic character.) This will
488 lead to somewhat confusing error messages.
490 To avoid ambiguity, when signatures are enabled the special syntax
491 for prototypes is disabled. There is no attempt to guess whether a
492 parenthesised group was intended to be a prototype or a signature.
493 To give a subroutine a prototype under these circumstances, use a
494 L<prototype attribute|attributes/Built-in Attributes>. For example,
496 sub foo :prototype($) { $_[0] }
498 It is entirely possible for a subroutine to have both a prototype and
499 a signature. They do different jobs: the prototype affects compilation
500 of calls to the subroutine, and the signature puts argument values into
501 lexical variables at runtime. You can therefore write
503 sub foo :prototype($$) ($left, $right) {
504 return $left + $right;
507 The prototype attribute, and any other attributes, must come before
508 the signature. The signature always immediately precedes the block of
509 the subroutine's body.
511 =head2 Private Variables via my()
512 X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical>
513 X<lexical scope> X<attributes, my>
517 my $foo; # declare $foo lexically local
518 my (@wid, %get); # declare list of variables local
519 my $foo = "flurp"; # declare $foo lexical, and init it
520 my @oof = @bar; # declare @oof lexical, and init it
521 my $x : Foo = $y; # similar, with an attribute applied
523 B<WARNING>: The use of attribute lists on C<my> declarations is still
524 evolving. The current semantics and interface are subject to change.
525 See L<attributes> and L<Attribute::Handlers>.
527 The C<my> operator declares the listed variables to be lexically
528 confined to the enclosing block, conditional
529 (C<if>/C<unless>/C<elsif>/C<else>), loop
530 (C<for>/C<foreach>/C<while>/C<until>/C<continue>), subroutine, C<eval>,
531 or C<do>/C<require>/C<use>'d file. If more than one value is listed, the
532 list must be placed in parentheses. All listed elements must be
533 legal lvalues. Only alphanumeric identifiers may be lexically
534 scoped--magical built-ins like C<$/> must currently be C<local>ized
535 with C<local> instead.
537 Unlike dynamic variables created by the C<local> operator, lexical
538 variables declared with C<my> are totally hidden from the outside
539 world, including any called subroutines. This is true if it's the
540 same subroutine called from itself or elsewhere--every call gets
544 This doesn't mean that a C<my> variable declared in a statically
545 enclosing lexical scope would be invisible. Only dynamic scopes
546 are cut off. For example, the C<bumpx()> function below has access
547 to the lexical $x variable because both the C<my> and the C<sub>
548 occurred at the same scope, presumably file scope.
553 An C<eval()>, however, can see lexical variables of the scope it is
554 being evaluated in, so long as the names aren't hidden by declarations within
555 the C<eval()> itself. See L<perlref>.
558 The parameter list to my() may be assigned to if desired, which allows you
559 to initialize your variables. (If no initializer is given for a
560 particular variable, it is created with the undefined value.) Commonly
561 this is used to name input parameters to a subroutine. Examples:
563 $arg = "fred"; # "global" variable
565 print "$arg thinks the root is $n\n";
566 fred thinks the root is 3
569 my $arg = shift; # name doesn't matter
574 The C<my> is simply a modifier on something you might assign to. So when
575 you do assign to variables in its argument list, C<my> doesn't
576 change whether those variables are viewed as a scalar or an array. So
578 my ($foo) = <STDIN>; # WRONG?
581 both supply a list context to the right-hand side, while
585 supplies a scalar context. But the following declares only one variable:
587 my $foo, $bar = 1; # WRONG
589 That has the same effect as
594 The declared variable is not introduced (is not visible) until after
595 the current statement. Thus,
599 can be used to initialize a new $x with the value of the old $x, and
602 my $x = 123 and $x == 123
604 is false unless the old $x happened to have the value C<123>.
606 Lexical scopes of control structures are not bounded precisely by the
607 braces that delimit their controlled blocks; control expressions are
608 part of that scope, too. Thus in the loop
610 while (my $line = <>) {
616 the scope of $line extends from its declaration throughout the rest of
617 the loop construct (including the C<continue> clause), but not beyond
618 it. Similarly, in the conditional
620 if ((my $answer = <STDIN>) =~ /^yes$/i) {
622 } elsif ($answer =~ /^no$/i) {
626 die "'$answer' is neither 'yes' nor 'no'";
629 the scope of $answer extends from its declaration through the rest
630 of that conditional, including any C<elsif> and C<else> clauses,
631 but not beyond it. See L<perlsyn/"Simple Statements"> for information
632 on the scope of variables in statements with modifiers.
634 The C<foreach> loop defaults to scoping its index variable dynamically
635 in the manner of C<local>. However, if the index variable is
636 prefixed with the keyword C<my>, or if there is already a lexical
637 by that name in scope, then a new lexical is created instead. Thus
641 for my $i (1, 2, 3) {
645 the scope of $i extends to the end of the loop, but not beyond it,
646 rendering the value of $i inaccessible within C<some_function()>.
649 Some users may wish to encourage the use of lexically scoped variables.
650 As an aid to catching implicit uses to package variables,
651 which are always global, if you say
655 then any variable mentioned from there to the end of the enclosing
656 block must either refer to a lexical variable, be predeclared via
657 C<our> or C<use vars>, or else must be fully qualified with the package name.
658 A compilation error results otherwise. An inner block may countermand
659 this with C<no strict 'vars'>.
661 A C<my> has both a compile-time and a run-time effect. At compile
662 time, the compiler takes notice of it. The principal usefulness
663 of this is to quiet C<use strict 'vars'>, but it is also essential
664 for generation of closures as detailed in L<perlref>. Actual
665 initialization is delayed until run time, though, so it gets executed
666 at the appropriate time, such as each time through a loop, for
669 Variables declared with C<my> are not part of any package and are therefore
670 never fully qualified with the package name. In particular, you're not
671 allowed to try to make a package variable (or other global) lexical:
673 my $pack::var; # ERROR! Illegal syntax
675 In fact, a dynamic variable (also known as package or global variables)
676 are still accessible using the fully qualified C<::> notation even while a
677 lexical of the same name is also visible:
682 print "$x and $::x\n";
684 That will print out C<20> and C<10>.
686 You may declare C<my> variables at the outermost scope of a file
687 to hide any such identifiers from the world outside that file. This
688 is similar in spirit to C's static variables when they are used at
689 the file level. To do this with a subroutine requires the use of
690 a closure (an anonymous function that accesses enclosing lexicals).
691 If you want to create a private subroutine that cannot be called
692 from outside that block, it can declare a lexical variable containing
693 an anonymous sub reference:
695 my $secret_version = '1.001-beta';
696 my $secret_sub = sub { print $secret_version };
699 As long as the reference is never returned by any function within the
700 module, no outside module can see the subroutine, because its name is not in
701 any package's symbol table. Remember that it's not I<REALLY> called
702 C<$some_pack::secret_version> or anything; it's just $secret_version,
703 unqualified and unqualifiable.
705 This does not work with object methods, however; all object methods
706 have to be in the symbol table of some package to be found. See
707 L<perlref/"Function Templates"> for something of a work-around to
710 =head2 Persistent Private Variables
711 X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure>
713 There are two ways to build persistent private variables in Perl 5.10.
714 First, you can simply use the C<state> feature. Or, you can use closures,
715 if you want to stay compatible with releases older than 5.10.
717 =head3 Persistent variables via state()
719 Beginning with Perl 5.10.0, you can declare variables with the C<state>
720 keyword in place of C<my>. For that to work, though, you must have
721 enabled that feature beforehand, either by using the C<feature> pragma, or
722 by using C<-E> on one-liners (see L<feature>). Beginning with Perl 5.16,
723 the C<CORE::state> form does not require the
726 The C<state> keyword creates a lexical variable (following the same scoping
727 rules as C<my>) that persists from one subroutine call to the next. If a
728 state variable resides inside an anonymous subroutine, then each copy of
729 the subroutine has its own copy of the state variable. However, the value
730 of the state variable will still persist between calls to the same copy of
731 the anonymous subroutine. (Don't forget that C<sub { ... }> creates a new
732 subroutine each time it is executed.)
734 For example, the following code maintains a private counter, incremented
735 each time the gimme_another() function is called:
738 sub gimme_another { state $x; return ++$x }
740 And this example uses anonymous subroutines to create separate counters:
744 return sub { state $x; return ++$x }
747 Also, since C<$x> is lexical, it can't be reached or modified by any Perl
750 When combined with variable declaration, simple assignment to C<state>
751 variables (as in C<state $x = 42>) is executed only the first time. When such
752 statements are evaluated subsequent times, the assignment is ignored. The
753 behavior of assignment to C<state> declarations where the left hand side
754 of the assignment involves any parentheses is currently undefined.
756 =head3 Persistent variables with closures
758 Just because a lexical variable is lexically (also called statically)
759 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
760 within a function it works like a C static. It normally works more
761 like a C auto, but with implicit garbage collection.
763 Unlike local variables in C or C++, Perl's lexical variables don't
764 necessarily get recycled just because their scope has exited.
765 If something more permanent is still aware of the lexical, it will
766 stick around. So long as something else references a lexical, that
767 lexical won't be freed--which is as it should be. You wouldn't want
768 memory being free until you were done using it, or kept around once you
769 were done. Automatic garbage collection takes care of this for you.
771 This means that you can pass back or save away references to lexical
772 variables, whereas to return a pointer to a C auto is a grave error.
773 It also gives us a way to simulate C's function statics. Here's a
774 mechanism for giving a function private variables with both lexical
775 scoping and a static lifetime. If you do want to create something like
776 C's static variables, just enclose the whole function in an extra block,
777 and put the static variable outside the function but in the block.
782 return ++$secret_val;
785 # $secret_val now becomes unreachable by the outside
786 # world, but retains its value between calls to gimme_another
788 If this function is being sourced in from a separate file
789 via C<require> or C<use>, then this is probably just fine. If it's
790 all in the main program, you'll need to arrange for the C<my>
791 to be executed early, either by putting the whole block above
792 your main program, or more likely, placing merely a C<BEGIN>
793 code block around it to make sure it gets executed before your program
799 return ++$secret_val;
803 See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the
804 special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>,
807 If declared at the outermost scope (the file scope), then lexicals
808 work somewhat like C's file statics. They are available to all
809 functions in that same file declared below them, but are inaccessible
810 from outside that file. This strategy is sometimes used in modules
811 to create private variables that the whole module can see.
813 =head2 Temporary Values via local()
814 X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
815 X<variable, temporary>
817 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
818 it's faster and safer. Exceptions to this include the global punctuation
819 variables, global filehandles and formats, and direct manipulation of the
820 Perl symbol table itself. C<local> is mostly used when the current value
821 of a variable must be visible to called subroutines.
825 # localization of values
827 local $foo; # make $foo dynamically local
828 local (@wid, %get); # make list of variables local
829 local $foo = "flurp"; # make $foo dynamic, and init it
830 local @oof = @bar; # make @oof dynamic, and init it
832 local $hash{key} = "val"; # sets a local value for this hash entry
833 delete local $hash{key}; # delete this entry for the current block
834 local ($cond ? $v1 : $v2); # several types of lvalues support
837 # localization of symbols
839 local *FH; # localize $FH, @FH, %FH, &FH ...
840 local *merlyn = *randal; # now $merlyn is really $randal, plus
841 # @merlyn is really @randal, etc
842 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
843 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
845 A C<local> modifies its listed variables to be "local" to the
846 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
847 called from within that block>. A C<local> just gives temporary
848 values to global (meaning package) variables. It does I<not> create
849 a local variable. This is known as dynamic scoping. Lexical scoping
850 is done with C<my>, which works more like C's auto declarations.
852 Some types of lvalues can be localized as well: hash and array elements
853 and slices, conditionals (provided that their result is always
854 localizable), and symbolic references. As for simple variables, this
855 creates new, dynamically scoped values.
857 If more than one variable or expression is given to C<local>, they must be
858 placed in parentheses. This operator works
859 by saving the current values of those variables in its argument list on a
860 hidden stack and restoring them upon exiting the block, subroutine, or
861 eval. This means that called subroutines can also reference the local
862 variable, but not the global one. The argument list may be assigned to if
863 desired, which allows you to initialize your local variables. (If no
864 initializer is given for a particular variable, it is created with an
867 Because C<local> is a run-time operator, it gets executed each time
868 through a loop. Consequently, it's more efficient to localize your
869 variables outside the loop.
871 =head3 Grammatical note on local()
874 A C<local> is simply a modifier on an lvalue expression. When you assign to
875 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
876 as a scalar or an array. So
878 local($foo) = <STDIN>;
879 local @FOO = <STDIN>;
881 both supply a list context to the right-hand side, while
883 local $foo = <STDIN>;
885 supplies a scalar context.
887 =head3 Localization of special variables
888 X<local, special variable>
890 If you localize a special variable, you'll be giving a new value to it,
891 but its magic won't go away. That means that all side-effects related
892 to this magic still work with the localized value.
894 This feature allows code like this to work :
896 # Read the whole contents of FILE in $slurp
897 { local $/ = undef; $slurp = <FILE>; }
899 Note, however, that this restricts localization of some values ; for
900 example, the following statement dies, as of perl 5.10.0, with an error
901 I<Modification of a read-only value attempted>, because the $1 variable is
902 magical and read-only :
906 One exception is the default scalar variable: starting with perl 5.14
907 C<local($_)> will always strip all magic from $_, to make it possible
908 to safely reuse $_ in a subroutine.
910 B<WARNING>: Localization of tied arrays and hashes does not currently
912 This will be fixed in a future release of Perl; in the meantime, avoid
913 code that relies on any particular behavior of localising tied arrays
914 or hashes (localising individual elements is still okay).
915 See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
919 =head3 Localization of globs
920 X<local, glob> X<glob>
926 creates a whole new symbol table entry for the glob C<name> in the
927 current package. That means that all variables in its glob slot ($name,
928 @name, %name, &name, and the C<name> filehandle) are dynamically reset.
930 This implies, among other things, that any magic eventually carried by
931 those variables is locally lost. In other words, saying C<local */>
932 will not have any effect on the internal value of the input record
935 =head3 Localization of elements of composite types
936 X<local, composite type element> X<local, array element> X<local, hash element>
938 It's also worth taking a moment to explain what happens when you
939 C<local>ize a member of a composite type (i.e. an array or hash element).
940 In this case, the element is C<local>ized I<by name>. This means that
941 when the scope of the C<local()> ends, the saved value will be
942 restored to the hash element whose key was named in the C<local()>, or
943 the array element whose index was named in the C<local()>. If that
944 element was deleted while the C<local()> was in effect (e.g. by a
945 C<delete()> from a hash or a C<shift()> of an array), it will spring
946 back into existence, possibly extending an array and filling in the
947 skipped elements with C<undef>. For instance, if you say
949 %hash = ( 'This' => 'is', 'a' => 'test' );
953 local($hash{'a'}) = 'drill';
954 while (my $e = pop(@ary)) {
959 $hash{'only a'} = 'test';
963 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
964 print "The array has ",scalar(@ary)," elements: ",
965 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
972 This is a test only a test.
973 The array has 6 elements: 0, 1, 2, undef, undef, 5
975 The behavior of local() on non-existent members of composite
976 types is subject to change in future. The behavior of local()
977 on array elements specified using negative indexes is particularly
978 surprising, and is very likely to change.
980 =head3 Localized deletion of elements of composite types
981 X<delete> X<local, composite type element> X<local, array element> X<local, hash element>
983 You can use the C<delete local $array[$idx]> and C<delete local $hash{key}>
984 constructs to delete a composite type entry for the current block and restore
985 it when it ends. They return the array/hash value before the localization,
986 which means that they are respectively equivalent to
989 my $val = $array[$idx];
998 my $val = $hash{key};
1004 except that for those the C<local> is
1005 scoped to the C<do> block. Slices are
1014 my $a = delete local $hash{a};
1019 my @nums = delete local @$a[0, 2]
1021 # $a is [ undef, 8 ]
1023 $a[0] = 999; # will be erased when the scope ends
1025 # $a is back to [ 7, 8, 9 ]
1028 # %hash is back to its original state
1030 =head2 Lvalue subroutines
1031 X<lvalue> X<subroutine, lvalue>
1033 It is possible to return a modifiable value from a subroutine.
1034 To do this, you have to declare the subroutine to return an lvalue.
1037 sub canmod : lvalue {
1038 $val; # or: return $val;
1044 canmod() = 5; # assigns to $val
1045 nomod() = 5; # ERROR
1047 The scalar/list context for the subroutine and for the right-hand
1048 side of assignment is determined as if the subroutine call is replaced
1049 by a scalar. For example, consider:
1051 data(2,3) = get_data(3,4);
1053 Both subroutines here are called in a scalar context, while in:
1055 (data(2,3)) = get_data(3,4);
1059 (data(2),data(3)) = get_data(3,4);
1061 all the subroutines are called in a list context.
1063 Lvalue subroutines are convenient, but you have to keep in mind that,
1064 when used with objects, they may violate encapsulation. A normal
1065 mutator can check the supplied argument before setting the attribute
1066 it is protecting, an lvalue subroutine cannot. If you require any
1067 special processing when storing and retrieving the values, consider
1068 using the CPAN module Sentinel or something similar.
1070 =head2 Lexical Subroutines
1071 X<my sub> X<state sub> X<our sub> X<subroutine, lexical>
1073 Beginning with Perl 5.18, you can declare a private subroutine with C<my>
1074 or C<state>. As with state variables, the C<state> keyword is only
1075 available under C<use feature 'state'> or C<use 5.010> or higher.
1077 Prior to Perl 5.26, lexical subroutines were deemed experimental and were
1078 available only under the C<use feature 'lexical_subs'> pragma. They also
1079 produced a warning unless the "experimental::lexical_subs" warnings
1080 category was disabled.
1082 These subroutines are only visible within the block in which they are
1083 declared, and only after that declaration:
1085 # Include these two lines if your code is intended to run under Perl
1086 # versions earlier than 5.26.
1087 no warnings "experimental::lexical_subs";
1088 use feature 'lexical_subs';
1090 foo(); # calls the package/global subroutine
1092 foo(); # also calls the package subroutine
1094 foo(); # calls "state" sub
1095 my $ref = \&foo; # take a reference to "state" sub
1098 bar(); # calls "my" sub
1100 You can't (directly) write a recursive lexical subroutine:
1107 This example fails because C<baz()> refers to the package/global subroutine
1108 C<baz>, not the lexical subroutine currently being defined.
1110 The solution is to use L<C<__SUB__>|perlfunc/__SUB__>:
1113 __SUB__->(); # calls itself
1116 It is possible to predeclare a lexical subroutine. The C<sub foo {...}>
1117 subroutine definition syntax respects any previous C<my sub;> or C<state sub;>
1118 declaration. Using this to define recursive subroutines is a bad idea,
1121 my sub baz; # predeclaration
1122 sub baz { # define the "my" sub
1123 baz(); # WRONG: calls itself, but leaks memory
1126 Just like C<< my $f; $f = sub { $f->() } >>, this example leaks memory. The
1127 name C<baz> is a reference to the subroutine, and the subroutine uses the name
1128 C<baz>; they keep each other alive (see L<perlref/Circular References>).
1130 =head3 C<state sub> vs C<my sub>
1132 What is the difference between "state" subs and "my" subs? Each time that
1133 execution enters a block when "my" subs are declared, a new copy of each
1134 sub is created. "State" subroutines persist from one execution of the
1135 containing block to the next.
1137 So, in general, "state" subroutines are faster. But "my" subs are
1138 necessary if you want to create closures:
1143 ... do something with $x ...
1148 In this example, a new C<$x> is created when C<whatever> is called, and
1149 also a new C<inner>, which can see the new C<$x>. A "state" sub will only
1150 see the C<$x> from the first call to C<whatever>.
1152 =head3 C<our> subroutines
1154 Like C<our $variable>, C<our sub> creates a lexical alias to the package
1155 subroutine of the same name.
1157 The two main uses for this are to switch back to using the package sub
1158 inside an inner scope:
1165 # need to use the outer foo here
1171 and to make a subroutine visible to other packages in the same scope:
1173 package MySneakyModule;
1175 our sub do_something { ... }
1177 sub do_something_with_caller {
1179 () = caller 1; # sets @DB::args
1180 do_something(@args); # uses MySneakyModule::do_something
1183 =head2 Passing Symbol Table Entries (typeglobs)
1186 B<WARNING>: The mechanism described in this section was originally
1187 the only way to simulate pass-by-reference in older versions of
1188 Perl. While it still works fine in modern versions, the new reference
1189 mechanism is generally easier to work with. See below.
1191 Sometimes you don't want to pass the value of an array to a subroutine
1192 but rather the name of it, so that the subroutine can modify the global
1193 copy of it rather than working with a local copy. In perl you can
1194 refer to all objects of a particular name by prefixing the name
1195 with a star: C<*foo>. This is often known as a "typeglob", because the
1196 star on the front can be thought of as a wildcard match for all the
1197 funny prefix characters on variables and subroutines and such.
1199 When evaluated, the typeglob produces a scalar value that represents
1200 all the objects of that name, including any filehandle, format, or
1201 subroutine. When assigned to, it causes the name mentioned to refer to
1202 whatever C<*> value was assigned to it. Example:
1205 local(*someary) = @_;
1206 foreach $elem (@someary) {
1213 Scalars are already passed by reference, so you can modify
1214 scalar arguments without using this mechanism by referring explicitly
1215 to C<$_[0]> etc. You can modify all the elements of an array by passing
1216 all the elements as scalars, but you have to use the C<*> mechanism (or
1217 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
1218 an array. It will certainly be faster to pass the typeglob (or reference).
1220 Even if you don't want to modify an array, this mechanism is useful for
1221 passing multiple arrays in a single LIST, because normally the LIST
1222 mechanism will merge all the array values so that you can't extract out
1223 the individual arrays. For more on typeglobs, see
1224 L<perldata/"Typeglobs and Filehandles">.
1226 =head2 When to Still Use local()
1227 X<local> X<variable, local>
1229 Despite the existence of C<my>, there are still three places where the
1230 C<local> operator still shines. In fact, in these three places, you
1231 I<must> use C<local> instead of C<my>.
1237 You need to give a global variable a temporary value, especially $_.
1239 The global variables, like C<@ARGV> or the punctuation variables, must be
1240 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
1241 it up into chunks separated by lines of equal signs, which are placed
1245 local @ARGV = ("/etc/motd");
1248 @Fields = split /^\s*=+\s*$/;
1251 It particular, it's important to C<local>ize $_ in any routine that assigns
1252 to it. Look out for implicit assignments in C<while> conditionals.
1256 You need to create a local file or directory handle or a local function.
1258 A function that needs a filehandle of its own must use
1259 C<local()> on a complete typeglob. This can be used to create new symbol
1263 local (*READER, *WRITER); # not my!
1264 pipe (READER, WRITER) or die "pipe: $!";
1265 return (*READER, *WRITER);
1267 ($head, $tail) = ioqueue();
1269 See the Symbol module for a way to create anonymous symbol table
1272 Because assignment of a reference to a typeglob creates an alias, this
1273 can be used to create what is effectively a local function, or at least,
1277 local *grow = \&shrink; # only until this block exits
1278 grow(); # really calls shrink()
1279 move(); # if move() grow()s, it shrink()s too
1281 grow(); # get the real grow() again
1283 See L<perlref/"Function Templates"> for more about manipulating
1284 functions by name in this way.
1288 You want to temporarily change just one element of an array or hash.
1290 You can C<local>ize just one element of an aggregate. Usually this
1291 is done on dynamics:
1294 local $SIG{INT} = 'IGNORE';
1295 funct(); # uninterruptible
1297 # interruptibility automatically restored here
1299 But it also works on lexically declared aggregates.
1303 =head2 Pass by Reference
1304 X<pass by reference> X<pass-by-reference> X<reference>
1306 If you want to pass more than one array or hash into a function--or
1307 return them from it--and have them maintain their integrity, then
1308 you're going to have to use an explicit pass-by-reference. Before you
1309 do that, you need to understand references as detailed in L<perlref>.
1310 This section may not make much sense to you otherwise.
1312 Here are a few simple examples. First, let's pass in several arrays
1313 to a function and have it C<pop> all of then, returning a new list
1314 of all their former last elements:
1316 @tailings = popmany ( \@a, \@b, \@c, \@d );
1321 foreach $aref ( @_ ) {
1322 push @retlist, pop @$aref;
1327 Here's how you might write a function that returns a
1328 list of keys occurring in all the hashes passed to it:
1330 @common = inter( \%foo, \%bar, \%joe );
1332 my ($k, $href, %seen); # locals
1333 foreach $href (@_) {
1334 while ( $k = each %$href ) {
1338 return grep { $seen{$_} == @_ } keys %seen;
1341 So far, we're using just the normal list return mechanism.
1342 What happens if you want to pass or return a hash? Well,
1343 if you're using only one of them, or you don't mind them
1344 concatenating, then the normal calling convention is ok, although
1347 Where people get into trouble is here:
1349 (@a, @b) = func(@c, @d);
1351 (%a, %b) = func(%c, %d);
1353 That syntax simply won't work. It sets just C<@a> or C<%a> and
1354 clears the C<@b> or C<%b>. Plus the function didn't get passed
1355 into two separate arrays or hashes: it got one long list in C<@_>,
1358 If you can arrange for everyone to deal with this through references, it's
1359 cleaner code, although not so nice to look at. Here's a function that
1360 takes two array references as arguments, returning the two array elements
1361 in order of how many elements they have in them:
1363 ($aref, $bref) = func(\@c, \@d);
1364 print "@$aref has more than @$bref\n";
1366 my ($cref, $dref) = @_;
1367 if (@$cref > @$dref) {
1368 return ($cref, $dref);
1370 return ($dref, $cref);
1374 It turns out that you can actually do this also:
1376 (*a, *b) = func(\@c, \@d);
1377 print "@a has more than @b\n";
1379 local (*c, *d) = @_;
1387 Here we're using the typeglobs to do symbol table aliasing. It's
1388 a tad subtle, though, and also won't work if you're using C<my>
1389 variables, because only globals (even in disguise as C<local>s)
1390 are in the symbol table.
1392 If you're passing around filehandles, you could usually just use the bare
1393 typeglob, like C<*STDOUT>, but typeglobs references work, too.
1399 print $fh "her um well a hmmm\n";
1402 $rec = get_rec(\*STDIN);
1405 return scalar <$fh>;
1408 If you're planning on generating new filehandles, you could do this.
1409 Notice to pass back just the bare *FH, not its reference.
1414 return open (FH, $path) ? *FH : undef;
1418 X<prototype> X<subroutine, prototype>
1420 Perl supports a very limited kind of compile-time argument checking
1421 using function prototyping. This can be declared in either the PROTO
1422 section or with a L<prototype attribute|attributes/Built-in Attributes>.
1423 If you declare either of
1426 sub mypush :prototype(\@@)
1428 then C<mypush()> takes arguments exactly like C<push()> does.
1430 If subroutine signatures are enabled (see L</Signatures>), then
1431 the shorter PROTO syntax is unavailable, because it would clash with
1432 signatures. In that case, a prototype can only be declared in the form
1436 function declaration must be visible at compile time. The prototype
1437 affects only interpretation of new-style calls to the function,
1438 where new-style is defined as not using the C<&> character. In
1439 other words, if you call it like a built-in function, then it behaves
1440 like a built-in function. If you call it like an old-fashioned
1441 subroutine, then it behaves like an old-fashioned subroutine. It
1442 naturally falls out from this rule that prototypes have no influence
1443 on subroutine references like C<\&foo> or on indirect subroutine
1444 calls like C<&{$subref}> or C<< $subref->() >>.
1446 Method calls are not influenced by prototypes either, because the
1447 function to be called is indeterminate at compile time, since
1448 the exact code called depends on inheritance.
1450 Because the intent of this feature is primarily to let you define
1451 subroutines that work like built-in functions, here are prototypes
1452 for some other functions that parse almost exactly like the
1453 corresponding built-in.
1455 Declared as Called as
1457 sub mylink ($$) mylink $old, $new
1458 sub myvec ($$$) myvec $var, $offset, 1
1459 sub myindex ($$;$) myindex &getstring, "substr"
1460 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
1461 sub myreverse (@) myreverse $a, $b, $c
1462 sub myjoin ($@) myjoin ":", $a, $b, $c
1463 sub mypop (\@) mypop @array
1464 sub mysplice (\@$$@) mysplice @array, 0, 2, @pushme
1465 sub mykeys (\[%@]) mykeys %{$hashref}
1466 sub myopen (*;$) myopen HANDLE, $name
1467 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
1468 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
1469 sub myrand (;$) myrand 42
1470 sub mytime () mytime
1472 Any backslashed prototype character represents an actual argument
1473 that must start with that character (optionally preceded by C<my>,
1474 C<our> or C<local>), with the exception of C<$>, which will
1475 accept any scalar lvalue expression, such as C<$foo = 7> or
1476 C<< my_function()->[0] >>. The value passed as part of C<@_> will be a
1477 reference to the actual argument given in the subroutine call,
1478 obtained by applying C<\> to that argument.
1480 You can use the C<\[]> backslash group notation to specify more than one
1481 allowed argument type. For example:
1483 sub myref (\[$@%&*])
1485 will allow calling myref() as
1493 and the first argument of myref() will be a reference to
1494 a scalar, an array, a hash, a code, or a glob.
1496 Unbackslashed prototype characters have special meanings. Any
1497 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
1498 list context. An argument represented by C<$> forces scalar context. An
1499 C<&> requires an anonymous subroutine, which, if passed as the first
1500 argument, does not require the C<sub> keyword or a subsequent comma.
1502 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
1503 typeglob, or a reference to a typeglob in that slot. The value will be
1504 available to the subroutine either as a simple scalar, or (in the latter
1505 two cases) as a reference to the typeglob. If you wish to always convert
1506 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1509 use Symbol 'qualify_to_ref';
1512 my $fh = qualify_to_ref(shift, caller);
1516 The C<+> prototype is a special alternative to C<$> that will act like
1517 C<\[@%]> when given a literal array or hash variable, but will otherwise
1518 force scalar context on the argument. This is useful for functions which
1519 should accept either a literal array or an array reference as the argument:
1523 die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
1527 When using the C<+> prototype, your function must check that the argument
1528 is of an acceptable type.
1530 A semicolon (C<;>) separates mandatory arguments from optional arguments.
1531 It is redundant before C<@> or C<%>, which gobble up everything else.
1533 As the last character of a prototype, or just before a semicolon, a C<@>
1534 or a C<%>, you can use C<_> in place of C<$>: if this argument is not
1535 provided, C<$_> will be used instead.
1537 Note how the last three examples in the table above are treated
1538 specially by the parser. C<mygrep()> is parsed as a true list
1539 operator, C<myrand()> is parsed as a true unary operator with unary
1540 precedence the same as C<rand()>, and C<mytime()> is truly without
1541 arguments, just like C<time()>. That is, if you say
1545 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1546 without a prototype. If you want to force a unary function to have the
1547 same precedence as a list operator, add C<;> to the end of the prototype:
1549 sub mygetprotobynumber($;);
1550 mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)
1552 The interesting thing about C<&> is that you can generate new syntax with it,
1553 provided it's in the initial position:
1557 my($try,$catch) = @_;
1564 sub catch (&) { $_[0] }
1569 /phooey/ and print "unphooey\n";
1572 That prints C<"unphooey">. (Yes, there are still unresolved
1573 issues having to do with visibility of C<@_>. I'm ignoring that
1574 question for the moment. (But note that if we make C<@_> lexically
1575 scoped, those anonymous subroutines can act like closures... (Gee,
1576 is this sounding a little Lispish? (Never mind.))))
1578 And here's a reimplementation of the Perl C<grep> operator:
1585 push(@result, $_) if &$code;
1590 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1591 been intentionally left out of prototypes for the express purpose of
1592 someday in the future adding named, formal parameters. The current
1593 mechanism's main goal is to let module writers provide better diagnostics
1594 for module users. Larry feels the notation quite understandable to Perl
1595 programmers, and that it will not intrude greatly upon the meat of the
1596 module, nor make it harder to read. The line noise is visually
1597 encapsulated into a small pill that's easy to swallow.
1599 If you try to use an alphanumeric sequence in a prototype you will
1600 generate an optional warning - "Illegal character in prototype...".
1601 Unfortunately earlier versions of Perl allowed the prototype to be
1602 used as long as its prefix was a valid prototype. The warning may be
1603 upgraded to a fatal error in a future version of Perl once the
1604 majority of offending code is fixed.
1606 It's probably best to prototype new functions, not retrofit prototyping
1607 into older ones. That's because you must be especially careful about
1608 silent impositions of differing list versus scalar contexts. For example,
1609 if you decide that a function should take just one parameter, like this:
1613 print "you gave me $n\n";
1616 and someone has been calling it with an array or expression
1620 func( $text =~ /\w+/g );
1622 Then you've just supplied an automatic C<scalar> in front of their
1623 argument, which can be more than a bit surprising. The old C<@foo>
1624 which used to hold one thing doesn't get passed in. Instead,
1625 C<func()> now gets passed in a C<1>; that is, the number of elements
1626 in C<@foo>. And the C<m//g> gets called in scalar context so instead of a
1627 list of words it returns a boolean result and advances C<pos($text)>. Ouch!
1629 If a sub has both a PROTO and a BLOCK, the prototype is not applied
1630 until after the BLOCK is completely defined. This means that a recursive
1631 function with a prototype has to be predeclared for the prototype to take
1639 This is all very powerful, of course, and should be used only in moderation
1640 to make the world a better place.
1642 =head2 Constant Functions
1645 Functions with a prototype of C<()> are potential candidates for
1646 inlining. If the result after optimization and constant folding
1647 is either a constant or a lexically-scoped scalar which has no other
1648 references, then it will be used in place of function calls made
1649 without C<&>. Calls made using C<&> are never inlined. (See
1650 F<constant.pm> for an easy way to declare most constants.)
1652 The following functions would all be inlined:
1654 sub pi () { 3.14159 } # Not exact, but close.
1655 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1656 # and it's inlined, too!
1660 sub FLAG_FOO () { 1 << 8 }
1661 sub FLAG_BAR () { 1 << 9 }
1662 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1664 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1666 sub N () { int(OPT_BAZ) / 3 }
1668 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1669 sub FOO_SET2 () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1671 (Be aware that the last example was not always inlined in Perl 5.20 and
1672 earlier, which did not behave consistently with subroutines containing
1673 inner scopes.) You can countermand inlining by using an explicit
1684 sub bonk_val () { return 12345 }
1686 As alluded to earlier you can also declare inlined subs dynamically at
1687 BEGIN time if their body consists of a lexically-scoped scalar which
1688 has no other references. Only the first example here will be inlined:
1693 *INLINED = sub () { $var };
1700 *NOT_INLINED = sub () { $var };
1703 A not so obvious caveat with this (see [RT #79908]) is that the
1704 variable will be immediately inlined, and will stop behaving like a
1705 normal lexical variable, e.g. this will print C<79907>, not C<79908>:
1709 *RT_79908 = sub () { $x };
1712 print RT_79908(); # prints 79907
1714 As of Perl 5.22, this buggy behavior, while preserved for backward
1715 compatibility, is detected and emits a deprecation warning. If you want
1716 the subroutine to be inlined (with no warning), make sure the variable is
1717 not used in a context where it could be modified aside from where it is
1723 *INLINED = sub () { $x };
1725 # Warns. Future Perl versions will stop inlining it.
1729 *ALSO_INLINED = sub () { $x };
1732 Perl 5.22 also introduces the experimental "const" attribute as an
1733 alternative. (Disable the "experimental::const_attr" warnings if you want
1734 to use it.) When applied to an anonymous subroutine, it forces the sub to
1735 be called when the C<sub> expression is evaluated. The return value is
1736 captured and turned into a constant subroutine:
1739 *INLINED = sub : const { $x };
1742 The return value of C<INLINED> in this example will always be 54321,
1743 regardless of later modifications to $x. You can also put any arbitrary
1744 code inside the sub, at it will be executed immediately and its return
1745 value captured the same way.
1747 If you really want a subroutine with a C<()> prototype that returns a
1748 lexical variable you can easily force it to not be inlined by adding
1749 an explicit C<return>:
1753 *RT_79908 = sub () { return $x };
1756 print RT_79908(); # prints 79908
1758 The easiest way to tell if a subroutine was inlined is by using
1759 L<B::Deparse>. Consider this example of two subroutines returning
1760 C<1>, one with a C<()> prototype causing it to be inlined, and one
1761 without (with deparse output truncated for clarity):
1763 $ perl -MO=Deparse -le 'sub ONE { 1 } if (ONE) { print ONE if ONE }'
1768 print ONE() if ONE ;
1770 $ perl -MO=Deparse -le 'sub ONE () { 1 } if (ONE) { print ONE if ONE }'
1776 If you redefine a subroutine that was eligible for inlining, you'll
1777 get a warning by default. You can use this warning to tell whether or
1778 not a particular subroutine is considered inlinable, since it's
1779 different than the warning for overriding non-inlined subroutines:
1781 $ perl -e 'sub one () {1} sub one () {2}'
1782 Constant subroutine one redefined at -e line 1.
1783 $ perl -we 'sub one {1} sub one {2}'
1784 Subroutine one redefined at -e line 1.
1786 The warning is considered severe enough not to be affected by the
1787 B<-w> switch (or its absence) because previously compiled invocations
1788 of the function will still be using the old value of the function. If
1789 you need to be able to redefine the subroutine, you need to ensure
1790 that it isn't inlined, either by dropping the C<()> prototype (which
1791 changes calling semantics, so beware) or by thwarting the inlining
1792 mechanism in some other way, e.g. by adding an explicit C<return>, as
1795 sub not_inlined () { return 23 }
1797 =head2 Overriding Built-in Functions
1798 X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
1800 Many built-in functions may be overridden, though this should be tried
1801 only occasionally and for good reason. Typically this might be
1802 done by a package attempting to emulate missing built-in functionality
1803 on a non-Unix system.
1805 Overriding may be done only by importing the name from a module at
1806 compile time--ordinary predeclaration isn't good enough. However, the
1807 C<use subs> pragma lets you, in effect, predeclare subs
1808 via the import syntax, and these names may then override built-in ones:
1810 use subs 'chdir', 'chroot', 'chmod', 'chown';
1814 To unambiguously refer to the built-in form, precede the
1815 built-in name with the special package qualifier C<CORE::>. For example,
1816 saying C<CORE::open()> always refers to the built-in C<open()>, even
1817 if the current package has imported some other subroutine called
1818 C<&open()> from elsewhere. Even though it looks like a regular
1819 function call, it isn't: the CORE:: prefix in that case is part of Perl's
1820 syntax, and works for any keyword, regardless of what is in the CORE
1821 package. Taking a reference to it, that is, C<\&CORE::open>, only works
1822 for some keywords. See L<CORE>.
1824 Library modules should not in general export built-in names like C<open>
1825 or C<chdir> as part of their default C<@EXPORT> list, because these may
1826 sneak into someone else's namespace and change the semantics unexpectedly.
1827 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1828 possible for a user to import the name explicitly, but not implicitly.
1829 That is, they could say
1833 and it would import the C<open> override. But if they said
1837 they would get the default imports without overrides.
1839 The foregoing mechanism for overriding built-in is restricted, quite
1840 deliberately, to the package that requests the import. There is a second
1841 method that is sometimes applicable when you wish to override a built-in
1842 everywhere, without regard to namespace boundaries. This is achieved by
1843 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1844 example that quite brazenly replaces the C<glob> operator with something
1845 that understands regular expressions.
1850 @EXPORT_OK = 'glob';
1856 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1857 $pkg->export($where, $sym, @_);
1863 if (opendir my $d, '.') {
1864 @got = grep /$pat/, readdir $d;
1871 And here's how it could be (ab)used:
1873 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1875 use REGlob 'glob'; # override glob() in Foo:: only
1876 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1878 The initial comment shows a contrived, even dangerous example.
1879 By overriding C<glob> globally, you would be forcing the new (and
1880 subversive) behavior for the C<glob> operator for I<every> namespace,
1881 without the complete cognizance or cooperation of the modules that own
1882 those namespaces. Naturally, this should be done with extreme caution--if
1883 it must be done at all.
1885 The C<REGlob> example above does not implement all the support needed to
1886 cleanly override perl's C<glob> operator. The built-in C<glob> has
1887 different behaviors depending on whether it appears in a scalar or list
1888 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1889 context sensitive behaviors, and these must be adequately supported by
1890 a properly written override. For a fully functional example of overriding
1891 C<glob>, study the implementation of C<File::DosGlob> in the standard
1894 When you override a built-in, your replacement should be consistent (if
1895 possible) with the built-in native syntax. You can achieve this by using
1896 a suitable prototype. To get the prototype of an overridable built-in,
1897 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1898 (see L<perlfunc/prototype>).
1900 Note however that some built-ins can't have their syntax expressed by a
1901 prototype (such as C<system> or C<chomp>). If you override them you won't
1902 be able to fully mimic their original syntax.
1904 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1905 to special magic, their original syntax is preserved, and you don't have
1906 to define a prototype for their replacements. (You can't override the
1907 C<do BLOCK> syntax, though).
1909 C<require> has special additional dark magic: if you invoke your
1910 C<require> replacement as C<require Foo::Bar>, it will actually receive
1911 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1913 And, as you'll have noticed from the previous example, if you override
1914 C<glob>, the C<< <*> >> glob operator is overridden as well.
1916 In a similar fashion, overriding the C<readline> function also overrides
1917 the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding
1918 C<readpipe> also overrides the operators C<``> and C<qx//>.
1920 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1923 X<autoloading> X<AUTOLOAD>
1925 If you call a subroutine that is undefined, you would ordinarily
1926 get an immediate, fatal error complaining that the subroutine doesn't
1927 exist. (Likewise for subroutines being used as methods, when the
1928 method doesn't exist in any base class of the class's package.)
1929 However, if an C<AUTOLOAD> subroutine is defined in the package or
1930 packages used to locate the original subroutine, then that
1931 C<AUTOLOAD> subroutine is called with the arguments that would have
1932 been passed to the original subroutine. The fully qualified name
1933 of the original subroutine magically appears in the global $AUTOLOAD
1934 variable of the same package as the C<AUTOLOAD> routine. The name
1935 is not passed as an ordinary argument because, er, well, just
1936 because, that's why. (As an exception, a method call to a nonexistent
1937 C<import> or C<unimport> method is just skipped instead. Also, if
1938 the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the
1939 subroutine name. See L<perlguts/Autoloading with XSUBs> for details.)
1942 Many C<AUTOLOAD> routines load in a definition for the requested
1943 subroutine using eval(), then execute that subroutine using a special
1944 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1945 without a trace. (See the source to the standard module documented
1946 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1947 also just emulate the routine and never define it. For example,
1948 let's pretend that a function that wasn't defined should just invoke
1949 C<system> with those arguments. All you'd do is:
1952 our $AUTOLOAD; # keep 'use strict' happy
1953 my $program = $AUTOLOAD;
1954 $program =~ s/.*:://;
1955 system($program, @_);
1961 In fact, if you predeclare functions you want to call that way, you don't
1962 even need parentheses:
1964 use subs qw(date who ls);
1969 A more complete example of this is the Shell module on CPAN, which
1970 can treat undefined subroutine calls as calls to external programs.
1972 Mechanisms are available to help modules writers split their modules
1973 into autoloadable files. See the standard AutoLoader module
1974 described in L<AutoLoader> and in L<AutoSplit>, the standard
1975 SelfLoader modules in L<SelfLoader>, and the document on adding C
1976 functions to Perl code in L<perlxs>.
1978 =head2 Subroutine Attributes
1979 X<attribute> X<subroutine, attribute> X<attrs>
1981 A subroutine declaration or definition may have a list of attributes
1982 associated with it. If such an attribute list is present, it is
1983 broken up at space or colon boundaries and treated as though a
1984 C<use attributes> had been seen. See L<attributes> for details
1985 about what attributes are currently supported.
1986 Unlike the limitation with the obsolescent C<use attrs>, the
1987 C<sub : ATTRLIST> syntax works to associate the attributes with
1988 a pre-declaration, and not just with a subroutine definition.
1990 The attributes must be valid as simple identifier names (without any
1991 punctuation other than the '_' character). They may have a parameter
1992 list appended, which is only checked for whether its parentheses ('(',')')
1995 Examples of valid syntax (even though the attributes are unknown):
1997 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1998 sub plugh () : Ugly('\(") :Bad;
1999 sub xyzzy : _5x5 { ... }
2001 Examples of invalid syntax:
2003 sub fnord : switch(10,foo(); # ()-string not balanced
2004 sub snoid : Ugly('('); # ()-string not balanced
2005 sub xyzzy : 5x5; # "5x5" not a valid identifier
2006 sub plugh : Y2::north; # "Y2::north" not a simple identifier
2007 sub snurt : foo + bar; # "+" not a colon or space
2009 The attribute list is passed as a list of constant strings to the code
2010 which associates them with the subroutine. In particular, the second example
2011 of valid syntax above currently looks like this in terms of how it's
2014 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
2016 For further details on attribute lists and their manipulation,
2017 see L<attributes> and L<Attribute::Handlers>.
2021 See L<perlref/"Function Templates"> for more about references and closures.
2022 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
2023 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
2024 See L<perlmod> to learn about bundling up your functions in separate files.
2025 See L<perlmodlib> to learn what library modules come standard on your system.
2026 See L<perlootut> to learn how to make object method calls.