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(SIG) BLOCK # with a signature instead
19 sub NAME : ATTRS BLOCK # with attributes
20 sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
21 sub NAME(SIG) : ATTRS BLOCK # with a signature and attributes
23 To define an anonymous subroutine at runtime:
24 X<subroutine, anonymous>
26 $subref = sub BLOCK; # no proto
27 $subref = sub (PROTO) BLOCK; # with proto
28 $subref = sub (SIG) BLOCK; # with signature
29 $subref = sub : ATTRS BLOCK; # with attributes
30 $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
31 $subref = sub (SIG) : ATTRS BLOCK; # with signature and attributes
33 To import subroutines:
36 use MODULE qw(NAME1 NAME2 NAME3);
39 X<subroutine, call> X<call>
41 NAME(LIST); # & is optional with parentheses.
42 NAME LIST; # Parentheses optional if predeclared/imported.
43 &NAME(LIST); # Circumvent prototypes.
44 &NAME; # Makes current @_ visible to called subroutine.
48 Like many languages, Perl provides for user-defined subroutines.
49 These may be located anywhere in the main program, loaded in from
50 other files via the C<do>, C<require>, or C<use> keywords, or
51 generated on the fly using C<eval> or anonymous subroutines.
52 You can even call a function indirectly using a variable containing
53 its name or a CODE reference.
55 The Perl model for function call and return values is simple: all
56 functions are passed as parameters one single flat list of scalars, and
57 all functions likewise return to their caller one single flat list of
58 scalars. Any arrays or hashes in these call and return lists will
59 collapse, losing their identities--but you may always use
60 pass-by-reference instead to avoid this. Both call and return lists may
61 contain as many or as few scalar elements as you'd like. (Often a
62 function without an explicit return statement is called a subroutine, but
63 there's really no difference from Perl's perspective.)
64 X<subroutine, parameter> X<parameter>
66 Any arguments passed in show up in the array C<@_>.
67 (They may also show up in lexical variables introduced by a signature;
68 see L</Signatures> below.) Therefore, if
69 you called a function with two arguments, those would be stored in
70 C<$_[0]> and C<$_[1]>. The array C<@_> is a local array, but its
71 elements are aliases for the actual scalar parameters. In particular,
72 if an element C<$_[0]> is updated, the corresponding argument is
73 updated (or an error occurs if it is not updatable). If an argument
74 is an array or hash element which did not exist when the function
75 was called, that element is created only when (and if) it is modified
76 or a reference to it is taken. (Some earlier versions of Perl
77 created the element whether or not the element was assigned to.)
78 Assigning to the whole array C<@_> removes that aliasing, and does
79 not update any arguments.
80 X<subroutine, argument> X<argument> X<@_>
82 A C<return> statement may be used to exit a subroutine, optionally
83 specifying the returned value, which will be evaluated in the
84 appropriate context (list, scalar, or void) depending on the context of
85 the subroutine call. If you specify no return value, the subroutine
86 returns an empty list in list context, the undefined value in scalar
87 context, or nothing in void context. If you return one or more
88 aggregates (arrays and hashes), these will be flattened together into
89 one large indistinguishable list.
91 If no C<return> is found and if the last statement is an expression, its
92 value is returned. If the last statement is a loop control structure
93 like a C<foreach> or a C<while>, the returned value is unspecified. The
94 empty sub returns the empty list.
95 X<subroutine, return value> X<return value> X<return>
97 Aside from an experimental facility (see L</Signatures> below),
98 Perl does not have named formal parameters. In practice all you
99 do is assign to a C<my()> list of these. Variables that aren't
100 declared to be private are global variables. For gory details
101 on creating private variables, see L</"Private Variables via my()">
102 and L</"Temporary Values via local()">. To create protected
103 environments for a set of functions in a separate package (and
104 probably a separate file), see L<perlmod/"Packages">.
105 X<formal parameter> X<parameter, formal>
112 $max = $foo if $max < $foo;
116 $bestday = max($mon,$tue,$wed,$thu,$fri);
120 # get a line, combining continuation lines
121 # that start with whitespace
124 $thisline = $lookahead; # global variables!
125 LINE: while (defined($lookahead = <STDIN>)) {
126 if ($lookahead =~ /^[ \t]/) {
127 $thisline .= $lookahead;
136 $lookahead = <STDIN>; # get first line
137 while (defined($line = get_line())) {
141 Assigning to a list of private variables to name your arguments:
144 my($key, $value) = @_;
145 $Foo{$key} = $value unless $Foo{$key};
148 Because the assignment copies the values, this also has the effect
149 of turning call-by-reference into call-by-value. Otherwise a
150 function is free to do in-place modifications of C<@_> and change
152 X<call-by-reference> X<call-by-value>
154 upcase_in($v1, $v2); # this changes $v1 and $v2
156 for (@_) { tr/a-z/A-Z/ }
159 You aren't allowed to modify constants in this way, of course. If an
160 argument were actually literal and you tried to change it, you'd take a
161 (presumably fatal) exception. For example, this won't work:
162 X<call-by-reference> X<call-by-value>
164 upcase_in("frederick");
166 It would be much safer if the C<upcase_in()> function
167 were written to return a copy of its parameters instead
168 of changing them in place:
170 ($v3, $v4) = upcase($v1, $v2); # this doesn't change $v1 and $v2
172 return unless defined wantarray; # void context, do nothing
174 for (@parms) { tr/a-z/A-Z/ }
175 return wantarray ? @parms : $parms[0];
178 Notice how this (unprototyped) function doesn't care whether it was
179 passed real scalars or arrays. Perl sees all arguments as one big,
180 long, flat parameter list in C<@_>. This is one area where
181 Perl's simple argument-passing style shines. The C<upcase()>
182 function would work perfectly well without changing the C<upcase()>
183 definition even if we fed it things like this:
185 @newlist = upcase(@list1, @list2);
186 @newlist = upcase( split /:/, $var );
188 Do not, however, be tempted to do this:
190 (@a, @b) = upcase(@list1, @list2);
192 Like the flattened incoming parameter list, the return list is also
193 flattened on return. So all you have managed to do here is stored
194 everything in C<@a> and made C<@b> empty. See
195 L</Pass by Reference> for alternatives.
197 A subroutine may be called using an explicit C<&> prefix. The
198 C<&> is optional in modern Perl, as are parentheses if the
199 subroutine has been predeclared. The C<&> is I<not> optional
200 when just naming the subroutine, such as when it's used as
201 an argument to defined() or undef(). Nor is it optional when you
202 want to do an indirect subroutine call with a subroutine name or
203 reference using the C<&$subref()> or C<&{$subref}()> constructs,
204 although the C<< $subref->() >> notation solves that problem.
205 See L<perlref> for more about all that.
208 Subroutines may be called recursively. If a subroutine is called
209 using the C<&> form, the argument list is optional, and if omitted,
210 no C<@_> array is set up for the subroutine: the C<@_> array at the
211 time of the call is visible to subroutine instead. This is an
212 efficiency mechanism that new users may wish to avoid.
215 &foo(1,2,3); # pass three arguments
216 foo(1,2,3); # the same
218 foo(); # pass a null list
221 &foo; # foo() get current args, like foo(@_) !!
222 foo; # like foo() IFF sub foo predeclared, else "foo"
224 Not only does the C<&> form make the argument list optional, it also
225 disables any prototype checking on arguments you do provide. This
226 is partly for historical reasons, and partly for having a convenient way
227 to cheat if you know what you're doing. See L</Prototypes> below.
230 Since Perl 5.16.0, the C<__SUB__> token is available under C<use feature
231 'current_sub'> and C<use 5.16.0>. It will evaluate to a reference to the
232 currently-running sub, which allows for recursive calls without knowing
233 your subroutine's name.
236 my $factorial = sub {
239 return($x * __SUB__->( $x - 1 ) );
242 The behavior of C<__SUB__> within a regex code block (such as C</(?{...})/>)
243 is subject to change.
245 Subroutines whose names are in all upper case are reserved to the Perl
246 core, as are modules whose names are in all lower case. A subroutine in
247 all capitals is a loosely-held convention meaning it will be called
248 indirectly by the run-time system itself, usually due to a triggered event.
249 Subroutines whose name start with a left parenthesis are also reserved the
250 same way. The following is a list of some subroutines that currently do
251 special, pre-defined things.
255 =item documented later in this document
259 =item documented in L<perlmod>
261 C<CLONE>, C<CLONE_SKIP>
263 =item documented in L<perlobj>
267 =item documented in L<perltie>
269 C<BINMODE>, C<CLEAR>, C<CLOSE>, C<DELETE>, C<DESTROY>, C<EOF>, C<EXISTS>,
270 C<EXTEND>, C<FETCH>, C<FETCHSIZE>, C<FILENO>, C<FIRSTKEY>, C<GETC>,
271 C<NEXTKEY>, C<OPEN>, C<POP>, C<PRINT>, C<PRINTF>, C<PUSH>, C<READ>,
272 C<READLINE>, C<SCALAR>, C<SEEK>, C<SHIFT>, C<SPLICE>, C<STORE>,
273 C<STORESIZE>, C<TELL>, C<TIEARRAY>, C<TIEHANDLE>, C<TIEHASH>,
274 C<TIESCALAR>, C<UNSHIFT>, C<UNTIE>, C<WRITE>
276 =item documented in L<PerlIO::via>
278 C<BINMODE>, C<CLEARERR>, C<CLOSE>, C<EOF>, C<ERROR>, C<FDOPEN>, C<FILENO>,
279 C<FILL>, C<FLUSH>, C<OPEN>, C<POPPED>, C<PUSHED>, C<READ>, C<SEEK>,
280 C<SETLINEBUF>, C<SYSOPEN>, C<TELL>, C<UNREAD>, C<UTF8>, C<WRITE>
282 =item documented in L<perlfunc>
284 L<< C<import> | perlfunc/use >>, L<< C<unimport> | perlfunc/use >>,
285 L<< C<INC> | perlfunc/require >>
287 =item documented in L<UNIVERSAL>
291 =item documented in L<perldebguts>
293 C<DB::DB>, C<DB::sub>, C<DB::lsub>, C<DB::goto>, C<DB::postponed>
295 =item undocumented, used internally by the L<overload> feature
297 any starting with C<(>
301 The C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> and C<END> subroutines
302 are not so much subroutines as named special code blocks, of which you
303 can have more than one in a package, and which you can B<not> call
304 explicitly. See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END">
308 B<WARNING>: Subroutine signatures are experimental. The feature may be
309 modified or removed in future versions of Perl.
311 Perl has an experimental facility to allow a subroutine's formal
312 parameters to be introduced by special syntax, separate from the
313 procedural code of the subroutine body. The formal parameter list
314 is known as a I<signature>. The facility must be enabled first by a
315 pragmatic declaration, C<use feature 'signatures'>, and it will produce
316 a warning unless the "experimental::signatures" warnings category is
319 The signature is part of a subroutine's body. Normally the body of a
320 subroutine is simply a braced block of code. When using a signature,
321 the signature is a parenthesised list that goes immediately after
322 the subroutine name (or, for anonymous subroutines, immediately after
323 the C<sub> keyword). The signature declares lexical variables that are
324 in scope for the block. When the subroutine is called, the signature
325 takes control first. It populates the signature variables from the
326 list of arguments that were passed. If the argument list doesn't meet
327 the requirements of the signature, then it will throw an exception.
328 When the signature processing is complete, control passes to the block.
330 Positional parameters are handled by simply naming scalar variables in
331 the signature. For example,
333 sub foo ($left, $right) {
334 return $left + $right;
337 takes two positional parameters, which must be filled at runtime by
338 two arguments. By default the parameters are mandatory, and it is
339 not permitted to pass more arguments than expected. So the above is
343 die "Too many arguments for subroutine" unless @_ <= 2;
344 die "Too few arguments for subroutine" unless @_ >= 2;
347 return $left + $right;
350 An argument can be ignored by omitting the main part of the name from
351 a parameter declaration, leaving just a bare C<$> sigil. For example,
353 sub foo ($first, $, $third) {
354 return "first=$first, third=$third";
357 Although the ignored argument doesn't go into a variable, it is still
358 mandatory for the caller to pass it.
360 A positional parameter is made optional by giving a default value,
361 separated from the parameter name by C<=>:
363 sub foo ($left, $right = 0) {
364 return $left + $right;
367 The above subroutine may be called with either one or two arguments.
368 The default value expression is evaluated when the subroutine is called,
369 so it may provide different default values for different calls. It is
370 only evaluated if the argument was actually omitted from the call.
374 sub foo ($thing, $id = $auto_id++) {
375 print "$thing has ID $id";
378 automatically assigns distinct sequential IDs to things for which no
379 ID was supplied by the caller. A default value expression may also
380 refer to parameters earlier in the signature, making the default for
381 one parameter vary according to the earlier parameters. For example,
383 sub foo ($first_name, $surname, $nickname = $first_name) {
384 print "$first_name $surname is known as \"$nickname\"";
387 An optional parameter can be nameless just like a mandatory parameter.
390 sub foo ($thing, $ = 1) {
394 The parameter's default value will still be evaluated if the corresponding
395 argument isn't supplied, even though the value won't be stored anywhere.
396 This is in case evaluating it has important side effects. However, it
397 will be evaluated in void context, so if it doesn't have side effects
398 and is not trivial it will generate a warning if the "void" warning
399 category is enabled. If a nameless optional parameter's default value
400 is not important, it may be omitted just as the parameter's name was:
402 sub foo ($thing, $=) {
406 Optional positional parameters must come after all mandatory positional
407 parameters. (If there are no mandatory positional parameters then an
408 optional positional parameters can be the first thing in the signature.)
409 If there are multiple optional positional parameters and not enough
410 arguments are supplied to fill them all, they will be filled from left
413 After positional parameters, additional arguments may be captured in a
414 slurpy parameter. The simplest form of this is just an array variable:
416 sub foo ($filter, @inputs) {
417 print $filter->($_) foreach @inputs;
420 With a slurpy parameter in the signature, there is no upper limit on how
421 many arguments may be passed. A slurpy array parameter may be nameless
422 just like a positional parameter, in which case its only effect is to
423 turn off the argument limit that would otherwise apply:
425 sub foo ($thing, @) {
429 A slurpy parameter may instead be a hash, in which case the arguments
430 available to it are interpreted as alternating keys and values.
431 There must be as many keys as values: if there is an odd argument then
432 an exception will be thrown. Keys will be stringified, and if there are
433 duplicates then the later instance takes precedence over the earlier,
434 as with standard hash construction.
436 sub foo ($filter, %inputs) {
437 print $filter->($_, $inputs{$_}) foreach sort keys %inputs;
440 A slurpy hash parameter may be nameless just like other kinds of
441 parameter. It still insists that the number of arguments available to
442 it be even, even though they're not being put into a variable.
444 sub foo ($thing, %) {
448 A slurpy parameter, either array or hash, must be the last thing in the
449 signature. It may follow mandatory and optional positional parameters;
450 it may also be the only thing in the signature. Slurpy parameters cannot
451 have default values: if no arguments are supplied for them then you get
452 an empty array or empty hash.
454 A signature may be entirely empty, in which case all it does is check
455 that the caller passed no arguments:
461 When using a signature, the arguments are still available in the special
462 array variable C<@_>, in addition to the lexical variables of the
463 signature. There is a difference between the two ways of accessing the
464 arguments: C<@_> I<aliases> the arguments, but the signature variables
465 get I<copies> of the arguments. So writing to a signature variable
466 only changes that variable, and has no effect on the caller's variables,
467 but writing to an element of C<@_> modifies whatever the caller used to
468 supply that argument.
470 There is a potential syntactic ambiguity between signatures and prototypes
471 (see L</Prototypes>), because both start with an opening parenthesis and
472 both can appear in some of the same places, such as just after the name
473 in a subroutine declaration. For historical reasons, when signatures
474 are not enabled, any opening parenthesis in such a context will trigger
475 very forgiving prototype parsing. Most signatures will be interpreted
476 as prototypes in those circumstances, but won't be valid prototypes.
477 (A valid prototype cannot contain any alphabetic character.) This will
478 lead to somewhat confusing error messages.
480 To avoid ambiguity, when signatures are enabled the special syntax
481 for prototypes is disabled. There is no attempt to guess whether a
482 parenthesised group was intended to be a prototype or a signature.
483 To give a subroutine a prototype under these circumstances, use a
484 L<prototype attribute|attributes/Built-in Attributes>. For example,
486 sub foo :prototype($) { $_[0] }
488 It is entirely possible for a subroutine to have both a prototype and
489 a signature. They do different jobs: the prototype affects compilation
490 of calls to the subroutine, and the signature puts argument values into
491 lexical variables at runtime. You can therefore write
493 sub foo ($left, $right) : prototype($$) {
494 return $left + $right;
497 The prototype attribute, and any other attributes, come after
500 =head2 Private Variables via my()
501 X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical>
502 X<lexical scope> X<attributes, my>
506 my $foo; # declare $foo lexically local
507 my (@wid, %get); # declare list of variables local
508 my $foo = "flurp"; # declare $foo lexical, and init it
509 my @oof = @bar; # declare @oof lexical, and init it
510 my $x : Foo = $y; # similar, with an attribute applied
512 B<WARNING>: The use of attribute lists on C<my> declarations is still
513 evolving. The current semantics and interface are subject to change.
514 See L<attributes> and L<Attribute::Handlers>.
516 The C<my> operator declares the listed variables to be lexically
517 confined to the enclosing block, conditional
518 (C<if>/C<unless>/C<elsif>/C<else>), loop
519 (C<for>/C<foreach>/C<while>/C<until>/C<continue>), subroutine, C<eval>,
520 or C<do>/C<require>/C<use>'d file. If more than one value is listed, the
521 list must be placed in parentheses. All listed elements must be
522 legal lvalues. Only alphanumeric identifiers may be lexically
523 scoped--magical built-ins like C<$/> must currently be C<local>ized
524 with C<local> instead.
526 Unlike dynamic variables created by the C<local> operator, lexical
527 variables declared with C<my> are totally hidden from the outside
528 world, including any called subroutines. This is true if it's the
529 same subroutine called from itself or elsewhere--every call gets
533 This doesn't mean that a C<my> variable declared in a statically
534 enclosing lexical scope would be invisible. Only dynamic scopes
535 are cut off. For example, the C<bumpx()> function below has access
536 to the lexical $x variable because both the C<my> and the C<sub>
537 occurred at the same scope, presumably file scope.
542 An C<eval()>, however, can see lexical variables of the scope it is
543 being evaluated in, so long as the names aren't hidden by declarations within
544 the C<eval()> itself. See L<perlref>.
547 The parameter list to my() may be assigned to if desired, which allows you
548 to initialize your variables. (If no initializer is given for a
549 particular variable, it is created with the undefined value.) Commonly
550 this is used to name input parameters to a subroutine. Examples:
552 $arg = "fred"; # "global" variable
554 print "$arg thinks the root is $n\n";
555 fred thinks the root is 3
558 my $arg = shift; # name doesn't matter
563 The C<my> is simply a modifier on something you might assign to. So when
564 you do assign to variables in its argument list, C<my> doesn't
565 change whether those variables are viewed as a scalar or an array. So
567 my ($foo) = <STDIN>; # WRONG?
570 both supply a list context to the right-hand side, while
574 supplies a scalar context. But the following declares only one variable:
576 my $foo, $bar = 1; # WRONG
578 That has the same effect as
583 The declared variable is not introduced (is not visible) until after
584 the current statement. Thus,
588 can be used to initialize a new $x with the value of the old $x, and
591 my $x = 123 and $x == 123
593 is false unless the old $x happened to have the value C<123>.
595 Lexical scopes of control structures are not bounded precisely by the
596 braces that delimit their controlled blocks; control expressions are
597 part of that scope, too. Thus in the loop
599 while (my $line = <>) {
605 the scope of $line extends from its declaration throughout the rest of
606 the loop construct (including the C<continue> clause), but not beyond
607 it. Similarly, in the conditional
609 if ((my $answer = <STDIN>) =~ /^yes$/i) {
611 } elsif ($answer =~ /^no$/i) {
615 die "'$answer' is neither 'yes' nor 'no'";
618 the scope of $answer extends from its declaration through the rest
619 of that conditional, including any C<elsif> and C<else> clauses,
620 but not beyond it. See L<perlsyn/"Simple Statements"> for information
621 on the scope of variables in statements with modifiers.
623 The C<foreach> loop defaults to scoping its index variable dynamically
624 in the manner of C<local>. However, if the index variable is
625 prefixed with the keyword C<my>, or if there is already a lexical
626 by that name in scope, then a new lexical is created instead. Thus
630 for my $i (1, 2, 3) {
634 the scope of $i extends to the end of the loop, but not beyond it,
635 rendering the value of $i inaccessible within C<some_function()>.
638 Some users may wish to encourage the use of lexically scoped variables.
639 As an aid to catching implicit uses to package variables,
640 which are always global, if you say
644 then any variable mentioned from there to the end of the enclosing
645 block must either refer to a lexical variable, be predeclared via
646 C<our> or C<use vars>, or else must be fully qualified with the package name.
647 A compilation error results otherwise. An inner block may countermand
648 this with C<no strict 'vars'>.
650 A C<my> has both a compile-time and a run-time effect. At compile
651 time, the compiler takes notice of it. The principal usefulness
652 of this is to quiet C<use strict 'vars'>, but it is also essential
653 for generation of closures as detailed in L<perlref>. Actual
654 initialization is delayed until run time, though, so it gets executed
655 at the appropriate time, such as each time through a loop, for
658 Variables declared with C<my> are not part of any package and are therefore
659 never fully qualified with the package name. In particular, you're not
660 allowed to try to make a package variable (or other global) lexical:
662 my $pack::var; # ERROR! Illegal syntax
664 In fact, a dynamic variable (also known as package or global variables)
665 are still accessible using the fully qualified C<::> notation even while a
666 lexical of the same name is also visible:
671 print "$x and $::x\n";
673 That will print out C<20> and C<10>.
675 You may declare C<my> variables at the outermost scope of a file
676 to hide any such identifiers from the world outside that file. This
677 is similar in spirit to C's static variables when they are used at
678 the file level. To do this with a subroutine requires the use of
679 a closure (an anonymous function that accesses enclosing lexicals).
680 If you want to create a private subroutine that cannot be called
681 from outside that block, it can declare a lexical variable containing
682 an anonymous sub reference:
684 my $secret_version = '1.001-beta';
685 my $secret_sub = sub { print $secret_version };
688 As long as the reference is never returned by any function within the
689 module, no outside module can see the subroutine, because its name is not in
690 any package's symbol table. Remember that it's not I<REALLY> called
691 C<$some_pack::secret_version> or anything; it's just $secret_version,
692 unqualified and unqualifiable.
694 This does not work with object methods, however; all object methods
695 have to be in the symbol table of some package to be found. See
696 L<perlref/"Function Templates"> for something of a work-around to
699 =head2 Persistent Private Variables
700 X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure>
702 There are two ways to build persistent private variables in Perl 5.10.
703 First, you can simply use the C<state> feature. Or, you can use closures,
704 if you want to stay compatible with releases older than 5.10.
706 =head3 Persistent variables via state()
708 Beginning with Perl 5.10.0, you can declare variables with the C<state>
709 keyword in place of C<my>. For that to work, though, you must have
710 enabled that feature beforehand, either by using the C<feature> pragma, or
711 by using C<-E> on one-liners (see L<feature>). Beginning with Perl 5.16,
712 the C<CORE::state> form does not require the
715 The C<state> keyword creates a lexical variable (following the same scoping
716 rules as C<my>) that persists from one subroutine call to the next. If a
717 state variable resides inside an anonymous subroutine, then each copy of
718 the subroutine has its own copy of the state variable. However, the value
719 of the state variable will still persist between calls to the same copy of
720 the anonymous subroutine. (Don't forget that C<sub { ... }> creates a new
721 subroutine each time it is executed.)
723 For example, the following code maintains a private counter, incremented
724 each time the gimme_another() function is called:
727 sub gimme_another { state $x; return ++$x }
729 And this example uses anonymous subroutines to create separate counters:
733 return sub { state $x; return ++$x }
736 Also, since C<$x> is lexical, it can't be reached or modified by any Perl
739 When combined with variable declaration, simple scalar assignment to C<state>
740 variables (as in C<state $x = 42>) is executed only the first time. When such
741 statements are evaluated subsequent times, the assignment is ignored. The
742 behavior of this sort of assignment to non-scalar variables is undefined.
744 =head3 Persistent variables with closures
746 Just because a lexical variable is lexically (also called statically)
747 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
748 within a function it works like a C static. It normally works more
749 like a C auto, but with implicit garbage collection.
751 Unlike local variables in C or C++, Perl's lexical variables don't
752 necessarily get recycled just because their scope has exited.
753 If something more permanent is still aware of the lexical, it will
754 stick around. So long as something else references a lexical, that
755 lexical won't be freed--which is as it should be. You wouldn't want
756 memory being free until you were done using it, or kept around once you
757 were done. Automatic garbage collection takes care of this for you.
759 This means that you can pass back or save away references to lexical
760 variables, whereas to return a pointer to a C auto is a grave error.
761 It also gives us a way to simulate C's function statics. Here's a
762 mechanism for giving a function private variables with both lexical
763 scoping and a static lifetime. If you do want to create something like
764 C's static variables, just enclose the whole function in an extra block,
765 and put the static variable outside the function but in the block.
770 return ++$secret_val;
773 # $secret_val now becomes unreachable by the outside
774 # world, but retains its value between calls to gimme_another
776 If this function is being sourced in from a separate file
777 via C<require> or C<use>, then this is probably just fine. If it's
778 all in the main program, you'll need to arrange for the C<my>
779 to be executed early, either by putting the whole block above
780 your main program, or more likely, placing merely a C<BEGIN>
781 code block around it to make sure it gets executed before your program
787 return ++$secret_val;
791 See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the
792 special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>,
795 If declared at the outermost scope (the file scope), then lexicals
796 work somewhat like C's file statics. They are available to all
797 functions in that same file declared below them, but are inaccessible
798 from outside that file. This strategy is sometimes used in modules
799 to create private variables that the whole module can see.
801 =head2 Temporary Values via local()
802 X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
803 X<variable, temporary>
805 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
806 it's faster and safer. Exceptions to this include the global punctuation
807 variables, global filehandles and formats, and direct manipulation of the
808 Perl symbol table itself. C<local> is mostly used when the current value
809 of a variable must be visible to called subroutines.
813 # localization of values
815 local $foo; # make $foo dynamically local
816 local (@wid, %get); # make list of variables local
817 local $foo = "flurp"; # make $foo dynamic, and init it
818 local @oof = @bar; # make @oof dynamic, and init it
820 local $hash{key} = "val"; # sets a local value for this hash entry
821 delete local $hash{key}; # delete this entry for the current block
822 local ($cond ? $v1 : $v2); # several types of lvalues support
825 # localization of symbols
827 local *FH; # localize $FH, @FH, %FH, &FH ...
828 local *merlyn = *randal; # now $merlyn is really $randal, plus
829 # @merlyn is really @randal, etc
830 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
831 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
833 A C<local> modifies its listed variables to be "local" to the
834 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
835 called from within that block>. A C<local> just gives temporary
836 values to global (meaning package) variables. It does I<not> create
837 a local variable. This is known as dynamic scoping. Lexical scoping
838 is done with C<my>, which works more like C's auto declarations.
840 Some types of lvalues can be localized as well: hash and array elements
841 and slices, conditionals (provided that their result is always
842 localizable), and symbolic references. As for simple variables, this
843 creates new, dynamically scoped values.
845 If more than one variable or expression is given to C<local>, they must be
846 placed in parentheses. This operator works
847 by saving the current values of those variables in its argument list on a
848 hidden stack and restoring them upon exiting the block, subroutine, or
849 eval. This means that called subroutines can also reference the local
850 variable, but not the global one. The argument list may be assigned to if
851 desired, which allows you to initialize your local variables. (If no
852 initializer is given for a particular variable, it is created with an
855 Because C<local> is a run-time operator, it gets executed each time
856 through a loop. Consequently, it's more efficient to localize your
857 variables outside the loop.
859 =head3 Grammatical note on local()
862 A C<local> is simply a modifier on an lvalue expression. When you assign to
863 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
864 as a scalar or an array. So
866 local($foo) = <STDIN>;
867 local @FOO = <STDIN>;
869 both supply a list context to the right-hand side, while
871 local $foo = <STDIN>;
873 supplies a scalar context.
875 =head3 Localization of special variables
876 X<local, special variable>
878 If you localize a special variable, you'll be giving a new value to it,
879 but its magic won't go away. That means that all side-effects related
880 to this magic still work with the localized value.
882 This feature allows code like this to work :
884 # Read the whole contents of FILE in $slurp
885 { local $/ = undef; $slurp = <FILE>; }
887 Note, however, that this restricts localization of some values ; for
888 example, the following statement dies, as of perl 5.10.0, with an error
889 I<Modification of a read-only value attempted>, because the $1 variable is
890 magical and read-only :
894 One exception is the default scalar variable: starting with perl 5.14
895 C<local($_)> will always strip all magic from $_, to make it possible
896 to safely reuse $_ in a subroutine.
898 B<WARNING>: Localization of tied arrays and hashes does not currently
900 This will be fixed in a future release of Perl; in the meantime, avoid
901 code that relies on any particular behavior of localising tied arrays
902 or hashes (localising individual elements is still okay).
903 See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
907 =head3 Localization of globs
908 X<local, glob> X<glob>
914 creates a whole new symbol table entry for the glob C<name> in the
915 current package. That means that all variables in its glob slot ($name,
916 @name, %name, &name, and the C<name> filehandle) are dynamically reset.
918 This implies, among other things, that any magic eventually carried by
919 those variables is locally lost. In other words, saying C<local */>
920 will not have any effect on the internal value of the input record
923 =head3 Localization of elements of composite types
924 X<local, composite type element> X<local, array element> X<local, hash element>
926 It's also worth taking a moment to explain what happens when you
927 C<local>ize a member of a composite type (i.e. an array or hash element).
928 In this case, the element is C<local>ized I<by name>. This means that
929 when the scope of the C<local()> ends, the saved value will be
930 restored to the hash element whose key was named in the C<local()>, or
931 the array element whose index was named in the C<local()>. If that
932 element was deleted while the C<local()> was in effect (e.g. by a
933 C<delete()> from a hash or a C<shift()> of an array), it will spring
934 back into existence, possibly extending an array and filling in the
935 skipped elements with C<undef>. For instance, if you say
937 %hash = ( 'This' => 'is', 'a' => 'test' );
941 local($hash{'a'}) = 'drill';
942 while (my $e = pop(@ary)) {
947 $hash{'only a'} = 'test';
951 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
952 print "The array has ",scalar(@ary)," elements: ",
953 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
960 This is a test only a test.
961 The array has 6 elements: 0, 1, 2, undef, undef, 5
963 The behavior of local() on non-existent members of composite
964 types is subject to change in future.
966 =head3 Localized deletion of elements of composite types
967 X<delete> X<local, composite type element> X<local, array element> X<local, hash element>
969 You can use the C<delete local $array[$idx]> and C<delete local $hash{key}>
970 constructs to delete a composite type entry for the current block and restore
971 it when it ends. They return the array/hash value before the localization,
972 which means that they are respectively equivalent to
975 my $val = $array[$idx];
984 my $val = $hash{key};
990 except that for those the C<local> is
991 scoped to the C<do> block. Slices are
1000 my $a = delete local $hash{a};
1005 my @nums = delete local @$a[0, 2]
1007 # $a is [ undef, 8 ]
1009 $a[0] = 999; # will be erased when the scope ends
1011 # $a is back to [ 7, 8, 9 ]
1014 # %hash is back to its original state
1016 =head2 Lvalue subroutines
1017 X<lvalue> X<subroutine, lvalue>
1019 It is possible to return a modifiable value from a subroutine.
1020 To do this, you have to declare the subroutine to return an lvalue.
1023 sub canmod : lvalue {
1024 $val; # or: return $val;
1030 canmod() = 5; # assigns to $val
1031 nomod() = 5; # ERROR
1033 The scalar/list context for the subroutine and for the right-hand
1034 side of assignment is determined as if the subroutine call is replaced
1035 by a scalar. For example, consider:
1037 data(2,3) = get_data(3,4);
1039 Both subroutines here are called in a scalar context, while in:
1041 (data(2,3)) = get_data(3,4);
1045 (data(2),data(3)) = get_data(3,4);
1047 all the subroutines are called in a list context.
1049 Lvalue subroutines are convenient, but you have to keep in mind that,
1050 when used with objects, they may violate encapsulation. A normal
1051 mutator can check the supplied argument before setting the attribute
1052 it is protecting, an lvalue subroutine cannot. If you require any
1053 special processing when storing and retrieving the values, consider
1054 using the CPAN module Sentinel or something similar.
1056 =head2 Lexical Subroutines
1057 X<my sub> X<state sub> X<our sub> X<subroutine, lexical>
1059 Beginning with Perl 5.18, you can declare a private subroutine with C<my>
1060 or C<state>. As with state variables, the C<state> keyword is only
1061 available under C<use feature 'state'> or C<use 5.010> or higher.
1063 Prior to Perl 5.26, lexical subroutines were deemed experimental and were
1064 available only under the C<use feature 'lexical_subs'> pragma. They also
1065 produced a warning unless the "experimental::lexical_subs" warnings
1066 category was disabled.
1068 These subroutines are only visible within the block in which they are
1069 declared, and only after that declaration:
1071 # Include these two lines if your code is intended to run under Perl
1072 # versions earlier than 5.26.
1073 no warnings "experimental::lexical_subs";
1074 use feature 'lexical_subs';
1076 foo(); # calls the package/global subroutine
1078 foo(); # also calls the package subroutine
1080 foo(); # calls "state" sub
1081 my $ref = \&foo; # take a reference to "state" sub
1084 bar(); # calls "my" sub
1086 To use a lexical subroutine from inside the subroutine itself, you must
1087 predeclare it. The C<sub foo {...}> subroutine definition syntax respects
1088 any previous C<my sub;> or C<state sub;> declaration.
1090 my sub baz; # predeclaration
1091 sub baz { # define the "my" sub
1092 baz(); # recursive call
1095 =head3 C<state sub> vs C<my sub>
1097 What is the difference between "state" subs and "my" subs? Each time that
1098 execution enters a block when "my" subs are declared, a new copy of each
1099 sub is created. "State" subroutines persist from one execution of the
1100 containing block to the next.
1102 So, in general, "state" subroutines are faster. But "my" subs are
1103 necessary if you want to create closures:
1108 ... do something with $x ...
1113 In this example, a new C<$x> is created when C<whatever> is called, and
1114 also a new C<inner>, which can see the new C<$x>. A "state" sub will only
1115 see the C<$x> from the first call to C<whatever>.
1117 =head3 C<our> subroutines
1119 Like C<our $variable>, C<our sub> creates a lexical alias to the package
1120 subroutine of the same name.
1122 The two main uses for this are to switch back to using the package sub
1123 inside an inner scope:
1130 # need to use the outer foo here
1136 and to make a subroutine visible to other packages in the same scope:
1138 package MySneakyModule;
1140 our sub do_something { ... }
1142 sub do_something_with_caller {
1144 () = caller 1; # sets @DB::args
1145 do_something(@args); # uses MySneakyModule::do_something
1148 =head2 Passing Symbol Table Entries (typeglobs)
1151 B<WARNING>: The mechanism described in this section was originally
1152 the only way to simulate pass-by-reference in older versions of
1153 Perl. While it still works fine in modern versions, the new reference
1154 mechanism is generally easier to work with. See below.
1156 Sometimes you don't want to pass the value of an array to a subroutine
1157 but rather the name of it, so that the subroutine can modify the global
1158 copy of it rather than working with a local copy. In perl you can
1159 refer to all objects of a particular name by prefixing the name
1160 with a star: C<*foo>. This is often known as a "typeglob", because the
1161 star on the front can be thought of as a wildcard match for all the
1162 funny prefix characters on variables and subroutines and such.
1164 When evaluated, the typeglob produces a scalar value that represents
1165 all the objects of that name, including any filehandle, format, or
1166 subroutine. When assigned to, it causes the name mentioned to refer to
1167 whatever C<*> value was assigned to it. Example:
1170 local(*someary) = @_;
1171 foreach $elem (@someary) {
1178 Scalars are already passed by reference, so you can modify
1179 scalar arguments without using this mechanism by referring explicitly
1180 to C<$_[0]> etc. You can modify all the elements of an array by passing
1181 all the elements as scalars, but you have to use the C<*> mechanism (or
1182 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
1183 an array. It will certainly be faster to pass the typeglob (or reference).
1185 Even if you don't want to modify an array, this mechanism is useful for
1186 passing multiple arrays in a single LIST, because normally the LIST
1187 mechanism will merge all the array values so that you can't extract out
1188 the individual arrays. For more on typeglobs, see
1189 L<perldata/"Typeglobs and Filehandles">.
1191 =head2 When to Still Use local()
1192 X<local> X<variable, local>
1194 Despite the existence of C<my>, there are still three places where the
1195 C<local> operator still shines. In fact, in these three places, you
1196 I<must> use C<local> instead of C<my>.
1202 You need to give a global variable a temporary value, especially $_.
1204 The global variables, like C<@ARGV> or the punctuation variables, must be
1205 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
1206 it up into chunks separated by lines of equal signs, which are placed
1210 local @ARGV = ("/etc/motd");
1213 @Fields = split /^\s*=+\s*$/;
1216 It particular, it's important to C<local>ize $_ in any routine that assigns
1217 to it. Look out for implicit assignments in C<while> conditionals.
1221 You need to create a local file or directory handle or a local function.
1223 A function that needs a filehandle of its own must use
1224 C<local()> on a complete typeglob. This can be used to create new symbol
1228 local (*READER, *WRITER); # not my!
1229 pipe (READER, WRITER) or die "pipe: $!";
1230 return (*READER, *WRITER);
1232 ($head, $tail) = ioqueue();
1234 See the Symbol module for a way to create anonymous symbol table
1237 Because assignment of a reference to a typeglob creates an alias, this
1238 can be used to create what is effectively a local function, or at least,
1242 local *grow = \&shrink; # only until this block exits
1243 grow(); # really calls shrink()
1244 move(); # if move() grow()s, it shrink()s too
1246 grow(); # get the real grow() again
1248 See L<perlref/"Function Templates"> for more about manipulating
1249 functions by name in this way.
1253 You want to temporarily change just one element of an array or hash.
1255 You can C<local>ize just one element of an aggregate. Usually this
1256 is done on dynamics:
1259 local $SIG{INT} = 'IGNORE';
1260 funct(); # uninterruptible
1262 # interruptibility automatically restored here
1264 But it also works on lexically declared aggregates.
1268 =head2 Pass by Reference
1269 X<pass by reference> X<pass-by-reference> X<reference>
1271 If you want to pass more than one array or hash into a function--or
1272 return them from it--and have them maintain their integrity, then
1273 you're going to have to use an explicit pass-by-reference. Before you
1274 do that, you need to understand references as detailed in L<perlref>.
1275 This section may not make much sense to you otherwise.
1277 Here are a few simple examples. First, let's pass in several arrays
1278 to a function and have it C<pop> all of then, returning a new list
1279 of all their former last elements:
1281 @tailings = popmany ( \@a, \@b, \@c, \@d );
1286 foreach $aref ( @_ ) {
1287 push @retlist, pop @$aref;
1292 Here's how you might write a function that returns a
1293 list of keys occurring in all the hashes passed to it:
1295 @common = inter( \%foo, \%bar, \%joe );
1297 my ($k, $href, %seen); # locals
1298 foreach $href (@_) {
1299 while ( $k = each %$href ) {
1303 return grep { $seen{$_} == @_ } keys %seen;
1306 So far, we're using just the normal list return mechanism.
1307 What happens if you want to pass or return a hash? Well,
1308 if you're using only one of them, or you don't mind them
1309 concatenating, then the normal calling convention is ok, although
1312 Where people get into trouble is here:
1314 (@a, @b) = func(@c, @d);
1316 (%a, %b) = func(%c, %d);
1318 That syntax simply won't work. It sets just C<@a> or C<%a> and
1319 clears the C<@b> or C<%b>. Plus the function didn't get passed
1320 into two separate arrays or hashes: it got one long list in C<@_>,
1323 If you can arrange for everyone to deal with this through references, it's
1324 cleaner code, although not so nice to look at. Here's a function that
1325 takes two array references as arguments, returning the two array elements
1326 in order of how many elements they have in them:
1328 ($aref, $bref) = func(\@c, \@d);
1329 print "@$aref has more than @$bref\n";
1331 my ($cref, $dref) = @_;
1332 if (@$cref > @$dref) {
1333 return ($cref, $dref);
1335 return ($dref, $cref);
1339 It turns out that you can actually do this also:
1341 (*a, *b) = func(\@c, \@d);
1342 print "@a has more than @b\n";
1344 local (*c, *d) = @_;
1352 Here we're using the typeglobs to do symbol table aliasing. It's
1353 a tad subtle, though, and also won't work if you're using C<my>
1354 variables, because only globals (even in disguise as C<local>s)
1355 are in the symbol table.
1357 If you're passing around filehandles, you could usually just use the bare
1358 typeglob, like C<*STDOUT>, but typeglobs references work, too.
1364 print $fh "her um well a hmmm\n";
1367 $rec = get_rec(\*STDIN);
1370 return scalar <$fh>;
1373 If you're planning on generating new filehandles, you could do this.
1374 Notice to pass back just the bare *FH, not its reference.
1379 return open (FH, $path) ? *FH : undef;
1383 X<prototype> X<subroutine, prototype>
1385 Perl supports a very limited kind of compile-time argument checking
1386 using function prototyping. This can be declared in either the PROTO
1387 section or with a L<prototype attribute|attributes/Built-in Attributes>.
1388 If you declare either of
1391 sub mypush :prototype(\@@)
1393 then C<mypush()> takes arguments exactly like C<push()> does.
1395 If subroutine signatures are enabled (see L</Signatures>), then
1396 the shorter PROTO syntax is unavailable, because it would clash with
1397 signatures. In that case, a prototype can only be declared in the form
1401 function declaration must be visible at compile time. The prototype
1402 affects only interpretation of new-style calls to the function,
1403 where new-style is defined as not using the C<&> character. In
1404 other words, if you call it like a built-in function, then it behaves
1405 like a built-in function. If you call it like an old-fashioned
1406 subroutine, then it behaves like an old-fashioned subroutine. It
1407 naturally falls out from this rule that prototypes have no influence
1408 on subroutine references like C<\&foo> or on indirect subroutine
1409 calls like C<&{$subref}> or C<< $subref->() >>.
1411 Method calls are not influenced by prototypes either, because the
1412 function to be called is indeterminate at compile time, since
1413 the exact code called depends on inheritance.
1415 Because the intent of this feature is primarily to let you define
1416 subroutines that work like built-in functions, here are prototypes
1417 for some other functions that parse almost exactly like the
1418 corresponding built-in.
1420 Declared as Called as
1422 sub mylink ($$) mylink $old, $new
1423 sub myvec ($$$) myvec $var, $offset, 1
1424 sub myindex ($$;$) myindex &getstring, "substr"
1425 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
1426 sub myreverse (@) myreverse $a, $b, $c
1427 sub myjoin ($@) myjoin ":", $a, $b, $c
1428 sub mypop (\@) mypop @array
1429 sub mysplice (\@$$@) mysplice @array, 0, 2, @pushme
1430 sub mykeys (\[%@]) mykeys %{$hashref}
1431 sub myopen (*;$) myopen HANDLE, $name
1432 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
1433 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
1434 sub myrand (;$) myrand 42
1435 sub mytime () mytime
1437 Any backslashed prototype character represents an actual argument
1438 that must start with that character (optionally preceded by C<my>,
1439 C<our> or C<local>), with the exception of C<$>, which will
1440 accept any scalar lvalue expression, such as C<$foo = 7> or
1441 C<< my_function()->[0] >>. The value passed as part of C<@_> will be a
1442 reference to the actual argument given in the subroutine call,
1443 obtained by applying C<\> to that argument.
1445 You can use the C<\[]> backslash group notation to specify more than one
1446 allowed argument type. For example:
1448 sub myref (\[$@%&*])
1450 will allow calling myref() as
1458 and the first argument of myref() will be a reference to
1459 a scalar, an array, a hash, a code, or a glob.
1461 Unbackslashed prototype characters have special meanings. Any
1462 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
1463 list context. An argument represented by C<$> forces scalar context. An
1464 C<&> requires an anonymous subroutine, which, if passed as the first
1465 argument, does not require the C<sub> keyword or a subsequent comma.
1467 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
1468 typeglob, or a reference to a typeglob in that slot. The value will be
1469 available to the subroutine either as a simple scalar, or (in the latter
1470 two cases) as a reference to the typeglob. If you wish to always convert
1471 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1474 use Symbol 'qualify_to_ref';
1477 my $fh = qualify_to_ref(shift, caller);
1481 The C<+> prototype is a special alternative to C<$> that will act like
1482 C<\[@%]> when given a literal array or hash variable, but will otherwise
1483 force scalar context on the argument. This is useful for functions which
1484 should accept either a literal array or an array reference as the argument:
1488 die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
1492 When using the C<+> prototype, your function must check that the argument
1493 is of an acceptable type.
1495 A semicolon (C<;>) separates mandatory arguments from optional arguments.
1496 It is redundant before C<@> or C<%>, which gobble up everything else.
1498 As the last character of a prototype, or just before a semicolon, a C<@>
1499 or a C<%>, you can use C<_> in place of C<$>: if this argument is not
1500 provided, C<$_> will be used instead.
1502 Note how the last three examples in the table above are treated
1503 specially by the parser. C<mygrep()> is parsed as a true list
1504 operator, C<myrand()> is parsed as a true unary operator with unary
1505 precedence the same as C<rand()>, and C<mytime()> is truly without
1506 arguments, just like C<time()>. That is, if you say
1510 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1511 without a prototype. If you want to force a unary function to have the
1512 same precedence as a list operator, add C<;> to the end of the prototype:
1514 sub mygetprotobynumber($;);
1515 mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)
1517 The interesting thing about C<&> is that you can generate new syntax with it,
1518 provided it's in the initial position:
1522 my($try,$catch) = @_;
1529 sub catch (&) { $_[0] }
1534 /phooey/ and print "unphooey\n";
1537 That prints C<"unphooey">. (Yes, there are still unresolved
1538 issues having to do with visibility of C<@_>. I'm ignoring that
1539 question for the moment. (But note that if we make C<@_> lexically
1540 scoped, those anonymous subroutines can act like closures... (Gee,
1541 is this sounding a little Lispish? (Never mind.))))
1543 And here's a reimplementation of the Perl C<grep> operator:
1550 push(@result, $_) if &$code;
1555 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1556 been intentionally left out of prototypes for the express purpose of
1557 someday in the future adding named, formal parameters. The current
1558 mechanism's main goal is to let module writers provide better diagnostics
1559 for module users. Larry feels the notation quite understandable to Perl
1560 programmers, and that it will not intrude greatly upon the meat of the
1561 module, nor make it harder to read. The line noise is visually
1562 encapsulated into a small pill that's easy to swallow.
1564 If you try to use an alphanumeric sequence in a prototype you will
1565 generate an optional warning - "Illegal character in prototype...".
1566 Unfortunately earlier versions of Perl allowed the prototype to be
1567 used as long as its prefix was a valid prototype. The warning may be
1568 upgraded to a fatal error in a future version of Perl once the
1569 majority of offending code is fixed.
1571 It's probably best to prototype new functions, not retrofit prototyping
1572 into older ones. That's because you must be especially careful about
1573 silent impositions of differing list versus scalar contexts. For example,
1574 if you decide that a function should take just one parameter, like this:
1578 print "you gave me $n\n";
1581 and someone has been calling it with an array or expression
1585 func( $text =~ /\w+/g );
1587 Then you've just supplied an automatic C<scalar> in front of their
1588 argument, which can be more than a bit surprising. The old C<@foo>
1589 which used to hold one thing doesn't get passed in. Instead,
1590 C<func()> now gets passed in a C<1>; that is, the number of elements
1591 in C<@foo>. And the C<m//g> gets called in scalar context so instead of a
1592 list of words it returns a boolean result and advances C<pos($text)>. Ouch!
1594 If a sub has both a PROTO and a BLOCK, the prototype is not applied
1595 until after the BLOCK is completely defined. This means that a recursive
1596 function with a prototype has to be predeclared for the prototype to take
1604 This is all very powerful, of course, and should be used only in moderation
1605 to make the world a better place.
1607 =head2 Constant Functions
1610 Functions with a prototype of C<()> are potential candidates for
1611 inlining. If the result after optimization and constant folding
1612 is either a constant or a lexically-scoped scalar which has no other
1613 references, then it will be used in place of function calls made
1614 without C<&>. Calls made using C<&> are never inlined. (See
1615 F<constant.pm> for an easy way to declare most constants.)
1617 The following functions would all be inlined:
1619 sub pi () { 3.14159 } # Not exact, but close.
1620 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1621 # and it's inlined, too!
1625 sub FLAG_FOO () { 1 << 8 }
1626 sub FLAG_BAR () { 1 << 9 }
1627 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1629 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1631 sub N () { int(OPT_BAZ) / 3 }
1633 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1634 sub FOO_SET2 () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1636 (Be aware that the last example was not always inlined in Perl 5.20 and
1637 earlier, which did not behave consistently with subroutines containing
1638 inner scopes.) You can countermand inlining by using an explicit
1649 sub bonk_val () { return 12345 }
1651 As alluded to earlier you can also declare inlined subs dynamically at
1652 BEGIN time if their body consists of a lexically-scoped scalar which
1653 has no other references. Only the first example here will be inlined:
1658 *INLINED = sub () { $var };
1665 *NOT_INLINED = sub () { $var };
1668 A not so obvious caveat with this (see [RT #79908]) is that the
1669 variable will be immediately inlined, and will stop behaving like a
1670 normal lexical variable, e.g. this will print C<79907>, not C<79908>:
1674 *RT_79908 = sub () { $x };
1677 print RT_79908(); # prints 79907
1679 As of Perl 5.22, this buggy behavior, while preserved for backward
1680 compatibility, is detected and emits a deprecation warning. If you want
1681 the subroutine to be inlined (with no warning), make sure the variable is
1682 not used in a context where it could be modified aside from where it is
1688 *INLINED = sub () { $x };
1690 # Warns. Future Perl versions will stop inlining it.
1694 *ALSO_INLINED = sub () { $x };
1697 Perl 5.22 also introduces the experimental "const" attribute as an
1698 alternative. (Disable the "experimental::const_attr" warnings if you want
1699 to use it.) When applied to an anonymous subroutine, it forces the sub to
1700 be called when the C<sub> expression is evaluated. The return value is
1701 captured and turned into a constant subroutine:
1704 *INLINED = sub : const { $x };
1707 The return value of C<INLINED> in this example will always be 54321,
1708 regardless of later modifications to $x. You can also put any arbitrary
1709 code inside the sub, at it will be executed immediately and its return
1710 value captured the same way.
1712 If you really want a subroutine with a C<()> prototype that returns a
1713 lexical variable you can easily force it to not be inlined by adding
1714 an explicit C<return>:
1718 *RT_79908 = sub () { return $x };
1721 print RT_79908(); # prints 79908
1723 The easiest way to tell if a subroutine was inlined is by using
1724 L<B::Deparse>. Consider this example of two subroutines returning
1725 C<1>, one with a C<()> prototype causing it to be inlined, and one
1726 without (with deparse output truncated for clarity):
1728 $ perl -MO=Deparse -le 'sub ONE { 1 } if (ONE) { print ONE if ONE }'
1733 print ONE() if ONE ;
1735 $ perl -MO=Deparse -le 'sub ONE () { 1 } if (ONE) { print ONE if ONE }'
1741 If you redefine a subroutine that was eligible for inlining, you'll
1742 get a warning by default. You can use this warning to tell whether or
1743 not a particular subroutine is considered inlinable, since it's
1744 different than the warning for overriding non-inlined subroutines:
1746 $ perl -e 'sub one () {1} sub one () {2}'
1747 Constant subroutine one redefined at -e line 1.
1748 $ perl -we 'sub one {1} sub one {2}'
1749 Subroutine one redefined at -e line 1.
1751 The warning is considered severe enough not to be affected by the
1752 B<-w> switch (or its absence) because previously compiled invocations
1753 of the function will still be using the old value of the function. If
1754 you need to be able to redefine the subroutine, you need to ensure
1755 that it isn't inlined, either by dropping the C<()> prototype (which
1756 changes calling semantics, so beware) or by thwarting the inlining
1757 mechanism in some other way, e.g. by adding an explicit C<return>, as
1760 sub not_inlined () { return 23 }
1762 =head2 Overriding Built-in Functions
1763 X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
1765 Many built-in functions may be overridden, though this should be tried
1766 only occasionally and for good reason. Typically this might be
1767 done by a package attempting to emulate missing built-in functionality
1768 on a non-Unix system.
1770 Overriding may be done only by importing the name from a module at
1771 compile time--ordinary predeclaration isn't good enough. However, the
1772 C<use subs> pragma lets you, in effect, predeclare subs
1773 via the import syntax, and these names may then override built-in ones:
1775 use subs 'chdir', 'chroot', 'chmod', 'chown';
1779 To unambiguously refer to the built-in form, precede the
1780 built-in name with the special package qualifier C<CORE::>. For example,
1781 saying C<CORE::open()> always refers to the built-in C<open()>, even
1782 if the current package has imported some other subroutine called
1783 C<&open()> from elsewhere. Even though it looks like a regular
1784 function call, it isn't: the CORE:: prefix in that case is part of Perl's
1785 syntax, and works for any keyword, regardless of what is in the CORE
1786 package. Taking a reference to it, that is, C<\&CORE::open>, only works
1787 for some keywords. See L<CORE>.
1789 Library modules should not in general export built-in names like C<open>
1790 or C<chdir> as part of their default C<@EXPORT> list, because these may
1791 sneak into someone else's namespace and change the semantics unexpectedly.
1792 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1793 possible for a user to import the name explicitly, but not implicitly.
1794 That is, they could say
1798 and it would import the C<open> override. But if they said
1802 they would get the default imports without overrides.
1804 The foregoing mechanism for overriding built-in is restricted, quite
1805 deliberately, to the package that requests the import. There is a second
1806 method that is sometimes applicable when you wish to override a built-in
1807 everywhere, without regard to namespace boundaries. This is achieved by
1808 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1809 example that quite brazenly replaces the C<glob> operator with something
1810 that understands regular expressions.
1815 @EXPORT_OK = 'glob';
1821 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1822 $pkg->export($where, $sym, @_);
1828 if (opendir my $d, '.') {
1829 @got = grep /$pat/, readdir $d;
1836 And here's how it could be (ab)used:
1838 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1840 use REGlob 'glob'; # override glob() in Foo:: only
1841 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1843 The initial comment shows a contrived, even dangerous example.
1844 By overriding C<glob> globally, you would be forcing the new (and
1845 subversive) behavior for the C<glob> operator for I<every> namespace,
1846 without the complete cognizance or cooperation of the modules that own
1847 those namespaces. Naturally, this should be done with extreme caution--if
1848 it must be done at all.
1850 The C<REGlob> example above does not implement all the support needed to
1851 cleanly override perl's C<glob> operator. The built-in C<glob> has
1852 different behaviors depending on whether it appears in a scalar or list
1853 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1854 context sensitive behaviors, and these must be adequately supported by
1855 a properly written override. For a fully functional example of overriding
1856 C<glob>, study the implementation of C<File::DosGlob> in the standard
1859 When you override a built-in, your replacement should be consistent (if
1860 possible) with the built-in native syntax. You can achieve this by using
1861 a suitable prototype. To get the prototype of an overridable built-in,
1862 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1863 (see L<perlfunc/prototype>).
1865 Note however that some built-ins can't have their syntax expressed by a
1866 prototype (such as C<system> or C<chomp>). If you override them you won't
1867 be able to fully mimic their original syntax.
1869 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1870 to special magic, their original syntax is preserved, and you don't have
1871 to define a prototype for their replacements. (You can't override the
1872 C<do BLOCK> syntax, though).
1874 C<require> has special additional dark magic: if you invoke your
1875 C<require> replacement as C<require Foo::Bar>, it will actually receive
1876 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1878 And, as you'll have noticed from the previous example, if you override
1879 C<glob>, the C<< <*> >> glob operator is overridden as well.
1881 In a similar fashion, overriding the C<readline> function also overrides
1882 the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding
1883 C<readpipe> also overrides the operators C<``> and C<qx//>.
1885 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1888 X<autoloading> X<AUTOLOAD>
1890 If you call a subroutine that is undefined, you would ordinarily
1891 get an immediate, fatal error complaining that the subroutine doesn't
1892 exist. (Likewise for subroutines being used as methods, when the
1893 method doesn't exist in any base class of the class's package.)
1894 However, if an C<AUTOLOAD> subroutine is defined in the package or
1895 packages used to locate the original subroutine, then that
1896 C<AUTOLOAD> subroutine is called with the arguments that would have
1897 been passed to the original subroutine. The fully qualified name
1898 of the original subroutine magically appears in the global $AUTOLOAD
1899 variable of the same package as the C<AUTOLOAD> routine. The name
1900 is not passed as an ordinary argument because, er, well, just
1901 because, that's why. (As an exception, a method call to a nonexistent
1902 C<import> or C<unimport> method is just skipped instead. Also, if
1903 the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the
1904 subroutine name. See L<perlguts/Autoloading with XSUBs> for details.)
1907 Many C<AUTOLOAD> routines load in a definition for the requested
1908 subroutine using eval(), then execute that subroutine using a special
1909 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1910 without a trace. (See the source to the standard module documented
1911 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1912 also just emulate the routine and never define it. For example,
1913 let's pretend that a function that wasn't defined should just invoke
1914 C<system> with those arguments. All you'd do is:
1917 my $program = $AUTOLOAD;
1918 $program =~ s/.*:://;
1919 system($program, @_);
1925 In fact, if you predeclare functions you want to call that way, you don't
1926 even need parentheses:
1928 use subs qw(date who ls);
1933 A more complete example of this is the Shell module on CPAN, which
1934 can treat undefined subroutine calls as calls to external programs.
1936 Mechanisms are available to help modules writers split their modules
1937 into autoloadable files. See the standard AutoLoader module
1938 described in L<AutoLoader> and in L<AutoSplit>, the standard
1939 SelfLoader modules in L<SelfLoader>, and the document on adding C
1940 functions to Perl code in L<perlxs>.
1942 =head2 Subroutine Attributes
1943 X<attribute> X<subroutine, attribute> X<attrs>
1945 A subroutine declaration or definition may have a list of attributes
1946 associated with it. If such an attribute list is present, it is
1947 broken up at space or colon boundaries and treated as though a
1948 C<use attributes> had been seen. See L<attributes> for details
1949 about what attributes are currently supported.
1950 Unlike the limitation with the obsolescent C<use attrs>, the
1951 C<sub : ATTRLIST> syntax works to associate the attributes with
1952 a pre-declaration, and not just with a subroutine definition.
1954 The attributes must be valid as simple identifier names (without any
1955 punctuation other than the '_' character). They may have a parameter
1956 list appended, which is only checked for whether its parentheses ('(',')')
1959 Examples of valid syntax (even though the attributes are unknown):
1961 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1962 sub plugh () : Ugly('\(") :Bad;
1963 sub xyzzy : _5x5 { ... }
1965 Examples of invalid syntax:
1967 sub fnord : switch(10,foo(); # ()-string not balanced
1968 sub snoid : Ugly('('); # ()-string not balanced
1969 sub xyzzy : 5x5; # "5x5" not a valid identifier
1970 sub plugh : Y2::north; # "Y2::north" not a simple identifier
1971 sub snurt : foo + bar; # "+" not a colon or space
1973 The attribute list is passed as a list of constant strings to the code
1974 which associates them with the subroutine. In particular, the second example
1975 of valid syntax above currently looks like this in terms of how it's
1978 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1980 For further details on attribute lists and their manipulation,
1981 see L<attributes> and L<Attribute::Handlers>.
1985 See L<perlref/"Function Templates"> for more about references and closures.
1986 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1987 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1988 See L<perlmod> to learn about bundling up your functions in separate files.
1989 See L<perlmodlib> to learn what library modules come standard on your system.
1990 See L<perlootut> to learn how to make object method calls.