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 signature
19 sub NAME : ATTRS BLOCK # with attributes
20 sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
21 sub NAME : ATTRS SIG BLOCK # with attributes and signature
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 $subref = sub : ATTRS SIG BLOCK; # with attribs and signature
33 To import subroutines:
36 use MODULE qw(NAME1 NAME2 NAME3);
39 X<subroutine, call> X<call>
41 NAME(LIST); # & is optional with parentheses.
42 NAME LIST; # Parentheses optional if predeclared/imported.
43 &NAME(LIST); # Circumvent prototypes.
44 &NAME; # Makes current @_ visible to called subroutine.
48 Like many languages, Perl provides for user-defined subroutines.
49 These may be located anywhere in the main program, loaded in from
50 other files via the C<do>, C<require>, or C<use> keywords, or
51 generated on the fly using C<eval> or anonymous subroutines.
52 You can even call a function indirectly using a variable containing
53 its name or a CODE reference.
55 The Perl model for function call and return values is simple: all
56 functions are passed as parameters one single flat list of scalars, and
57 all functions likewise return to their caller one single flat list of
58 scalars. Any arrays or hashes in these call and return lists will
59 collapse, losing their identities--but you may always use
60 pass-by-reference instead to avoid this. Both call and return lists may
61 contain as many or as few scalar elements as you'd like. (Often a
62 function without an explicit return statement is called a subroutine, but
63 there's really no difference from Perl's perspective.)
64 X<subroutine, parameter> X<parameter>
66 Any arguments passed in show up in the array C<@_>.
67 (They may also show up in lexical variables introduced by a signature;
68 see L</Signatures> below.) Therefore, if
69 you called a function with two arguments, those would be stored in
70 C<$_[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 behaviour 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 before
322 the braced block. The signature declares lexical variables that are
323 in scope for the block. When the subroutine is called, the signature
324 takes control first. It populates the signature variables from the
325 list of arguments that were passed. If the argument list doesn't meet
326 the requirements of the signature, then it will throw an exception.
327 When the signature processing is complete, control passes to the block.
329 Positional parameters are handled by simply naming scalar variables in
330 the signature. For example,
332 sub foo ($left, $right) {
333 return $left + $right;
336 takes two positional parameters, which must be filled at runtime by
337 two arguments. By default the parameters are mandatory, and it is
338 not permitted to pass more arguments than expected. So the above is
342 die "Too many arguments for subroutine" unless @_ <= 2;
343 die "Too few arguments for subroutine" unless @_ >= 2;
346 return $left + $right;
349 An argument can be ignored by omitting the main part of the name from
350 a parameter declaration, leaving just a bare C<$> sigil. For example,
352 sub foo ($first, $, $third) {
353 return "first=$first, third=$third";
356 Although the ignored argument doesn't go into a variable, it is still
357 mandatory for the caller to pass it.
359 A positional parameter is made optional by giving a default value,
360 separated from the parameter name by C<=>:
362 sub foo ($left, $right = 0) {
363 return $left + $right;
366 The above subroutine may be called with either one or two arguments.
367 The default value expression is evaluated when the subroutine is called,
368 so it may provide different default values for different calls. It is
369 only evaluated if the argument was actually omitted from the call.
373 sub foo ($thing, $id = $auto_id++) {
374 print "$thing has ID $id";
377 automatically assigns distinct sequential IDs to things for which no
378 ID was supplied by the caller. A default value expression may also
379 refer to parameters earlier in the signature, making the default for
380 one parameter vary according to the earlier parameters. For example,
382 sub foo ($first_name, $surname, $nickname = $first_name) {
383 print "$first_name $surname is known as \"$nickname\"";
386 An optional parameter can be nameless just like a mandatory parameter.
389 sub foo ($thing, $ = 1) {
393 The parameter's default value will still be evaluated if the corresponding
394 argument isn't supplied, even though the value won't be stored anywhere.
395 This is in case evaluating it has important side effects. However, it
396 will be evaluated in void context, so if it doesn't have side effects
397 and is not trivial it will generate a warning if the "void" warning
398 category is enabled. If a nameless optional parameter's default value
399 is not important, it may be omitted just as the parameter's name was:
401 sub foo ($thing, $=) {
405 Optional positional parameters must come after all mandatory positional
406 parameters. (If there are no mandatory positional parameters then an
407 optional positional parameters can be the first thing in the signature.)
408 If there are multiple optional positional parameters and not enough
409 arguments are supplied to fill them all, they will be filled from left
412 After positional parameters, additional arguments may be captured in a
413 slurpy parameter. The simplest form of this is just an array variable:
415 sub foo ($filter, @inputs) {
416 print $filter->($_) foreach @inputs;
419 With a slurpy parameter in the signature, there is no upper limit on how
420 many arguments may be passed. A slurpy array parameter may be nameless
421 just like a positional parameter, in which case its only effect is to
422 turn off the argument limit that would otherwise apply:
424 sub foo ($thing, @) {
428 A slurpy parameter may instead be a hash, in which case the arguments
429 available to it are interpreted as alternating keys and values.
430 There must be as many keys as values: if there is an odd argument then
431 an exception will be thrown. Keys will be stringified, and if there are
432 duplicates then the later instance takes precedence over the earlier,
433 as with standard hash construction.
435 sub foo ($filter, %inputs) {
436 print $filter->($_, $inputs{$_}) foreach sort keys %inputs;
439 A slurpy hash parameter may be nameless just like other kinds of
440 parameter. It still insists that the number of arguments available to
441 it be even, even though they're not being put into a variable.
443 sub foo ($thing, %) {
447 A slurpy parameter, either array or hash, must be the last thing in the
448 signature. It may follow mandatory and optional positional parameters;
449 it may also be the only thing in the signature. Slurpy parameters cannot
450 have default values: if no arguments are supplied for them then you get
451 an empty array or empty hash.
453 A signature may be entirely empty, in which case all it does is check
454 that the caller passed no arguments:
460 When using a signature, the arguments are still available in the special
461 array variable C<@_>, in addition to the lexical variables of the
462 signature. There is a difference between the two ways of accessing the
463 arguments: C<@_> I<aliases> the arguments, but the signature variables
464 get I<copies> of the arguments. So writing to a signature variable
465 only changes that variable, and has no effect on the caller's variables,
466 but writing to an element of C<@_> modifies whatever the caller used to
467 supply that argument.
469 There is a potential syntactic ambiguity between signatures and prototypes
470 (see L</Prototypes>), because both start with an opening parenthesis and
471 both can appear in some of the same places, such as just after the name
472 in a subroutine declaration. For historical reasons, when signatures
473 are not enabled, any opening parenthesis in such a context will trigger
474 very forgiving prototype parsing. Most signatures will be interpreted
475 as prototypes in those circumstances, but won't be valid prototypes.
476 (A valid prototype cannot contain any alphabetic character.) This will
477 lead to somewhat confusing error messages.
479 To avoid ambiguity, when signatures are enabled the special syntax
480 for prototypes is disabled. There is no attempt to guess whether a
481 parenthesised group was intended to be a prototype or a signature.
482 To give a subroutine a prototype under these circumstances, use a
483 L<prototype attribute|attributes/Built-in Attributes>. For example,
485 sub foo :prototype($) { $_[0] }
487 It is entirely possible for a subroutine to have both a prototype and
488 a signature. They do different jobs: the prototype affects compilation
489 of calls to the subroutine, and the signature puts argument values into
490 lexical variables at runtime. You can therefore write
492 sub foo :prototype($$) ($left, $right) {
493 return $left + $right;
496 The prototype attribute, and any other attributes, must come before
497 the signature. The signature always immediately precedes the block of
498 the subroutine's body.
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 (C<if/unless/elsif/else>),
518 loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
519 or C<do/require/use>'d file. If more than one value is listed, the
520 list must be placed in parentheses. All listed elements must be
521 legal lvalues. Only alphanumeric identifiers may be lexically
522 scoped--magical built-ins like C<$/> must currently be C<local>ized
523 with C<local> instead.
525 Unlike dynamic variables created by the C<local> operator, lexical
526 variables declared with C<my> are totally hidden from the outside
527 world, including any called subroutines. This is true if it's the
528 same subroutine called from itself or elsewhere--every call gets
532 This doesn't mean that a C<my> variable declared in a statically
533 enclosing lexical scope would be invisible. Only dynamic scopes
534 are cut off. For example, the C<bumpx()> function below has access
535 to the lexical $x variable because both the C<my> and the C<sub>
536 occurred at the same scope, presumably file scope.
541 An C<eval()>, however, can see lexical variables of the scope it is
542 being evaluated in, so long as the names aren't hidden by declarations within
543 the C<eval()> itself. See L<perlref>.
546 The parameter list to my() may be assigned to if desired, which allows you
547 to initialize your variables. (If no initializer is given for a
548 particular variable, it is created with the undefined value.) Commonly
549 this is used to name input parameters to a subroutine. Examples:
551 $arg = "fred"; # "global" variable
553 print "$arg thinks the root is $n\n";
554 fred thinks the root is 3
557 my $arg = shift; # name doesn't matter
562 The C<my> is simply a modifier on something you might assign to. So when
563 you do assign to variables in its argument list, C<my> doesn't
564 change whether those variables are viewed as a scalar or an array. So
566 my ($foo) = <STDIN>; # WRONG?
569 both supply a list context to the right-hand side, while
573 supplies a scalar context. But the following declares only one variable:
575 my $foo, $bar = 1; # WRONG
577 That has the same effect as
582 The declared variable is not introduced (is not visible) until after
583 the current statement. Thus,
587 can be used to initialize a new $x with the value of the old $x, and
590 my $x = 123 and $x == 123
592 is false unless the old $x happened to have the value C<123>.
594 Lexical scopes of control structures are not bounded precisely by the
595 braces that delimit their controlled blocks; control expressions are
596 part of that scope, too. Thus in the loop
598 while (my $line = <>) {
604 the scope of $line extends from its declaration throughout the rest of
605 the loop construct (including the C<continue> clause), but not beyond
606 it. Similarly, in the conditional
608 if ((my $answer = <STDIN>) =~ /^yes$/i) {
610 } elsif ($answer =~ /^no$/i) {
614 die "'$answer' is neither 'yes' nor 'no'";
617 the scope of $answer extends from its declaration through the rest
618 of that conditional, including any C<elsif> and C<else> clauses,
619 but not beyond it. See L<perlsyn/"Simple Statements"> for information
620 on the scope of variables in statements with modifiers.
622 The C<foreach> loop defaults to scoping its index variable dynamically
623 in the manner of C<local>. However, if the index variable is
624 prefixed with the keyword C<my>, or if there is already a lexical
625 by that name in scope, then a new lexical is created instead. Thus
629 for my $i (1, 2, 3) {
633 the scope of $i extends to the end of the loop, but not beyond it,
634 rendering the value of $i inaccessible within C<some_function()>.
637 Some users may wish to encourage the use of lexically scoped variables.
638 As an aid to catching implicit uses to package variables,
639 which are always global, if you say
643 then any variable mentioned from there to the end of the enclosing
644 block must either refer to a lexical variable, be predeclared via
645 C<our> or C<use vars>, or else must be fully qualified with the package name.
646 A compilation error results otherwise. An inner block may countermand
647 this with C<no strict 'vars'>.
649 A C<my> has both a compile-time and a run-time effect. At compile
650 time, the compiler takes notice of it. The principal usefulness
651 of this is to quiet C<use strict 'vars'>, but it is also essential
652 for generation of closures as detailed in L<perlref>. Actual
653 initialization is delayed until run time, though, so it gets executed
654 at the appropriate time, such as each time through a loop, for
657 Variables declared with C<my> are not part of any package and are therefore
658 never fully qualified with the package name. In particular, you're not
659 allowed to try to make a package variable (or other global) lexical:
661 my $pack::var; # ERROR! Illegal syntax
663 In fact, a dynamic variable (also known as package or global variables)
664 are still accessible using the fully qualified C<::> notation even while a
665 lexical of the same name is also visible:
670 print "$x and $::x\n";
672 That will print out C<20> and C<10>.
674 You may declare C<my> variables at the outermost scope of a file
675 to hide any such identifiers from the world outside that file. This
676 is similar in spirit to C's static variables when they are used at
677 the file level. To do this with a subroutine requires the use of
678 a closure (an anonymous function that accesses enclosing lexicals).
679 If you want to create a private subroutine that cannot be called
680 from outside that block, it can declare a lexical variable containing
681 an anonymous sub reference:
683 my $secret_version = '1.001-beta';
684 my $secret_sub = sub { print $secret_version };
687 As long as the reference is never returned by any function within the
688 module, no outside module can see the subroutine, because its name is not in
689 any package's symbol table. Remember that it's not I<REALLY> called
690 C<$some_pack::secret_version> or anything; it's just $secret_version,
691 unqualified and unqualifiable.
693 This does not work with object methods, however; all object methods
694 have to be in the symbol table of some package to be found. See
695 L<perlref/"Function Templates"> for something of a work-around to
698 =head2 Persistent Private Variables
699 X<state> X<state variable> X<static> X<variable, persistent> X<variable, static> X<closure>
701 There are two ways to build persistent private variables in Perl 5.10.
702 First, you can simply use the C<state> feature. Or, you can use closures,
703 if you want to stay compatible with releases older than 5.10.
705 =head3 Persistent variables via state()
707 Beginning with Perl 5.10.0, you can declare variables with the C<state>
708 keyword in place of C<my>. For that to work, though, you must have
709 enabled that feature beforehand, either by using the C<feature> pragma, or
710 by using C<-E> on one-liners (see L<feature>). Beginning with Perl 5.16,
711 the C<CORE::state> form does not require the
714 The C<state> keyword creates a lexical variable (following the same scoping
715 rules as C<my>) that persists from one subroutine call to the next. If a
716 state variable resides inside an anonymous subroutine, then each copy of
717 the subroutine has its own copy of the state variable. However, the value
718 of the state variable will still persist between calls to the same copy of
719 the anonymous subroutine. (Don't forget that C<sub { ... }> creates a new
720 subroutine each time it is executed.)
722 For example, the following code maintains a private counter, incremented
723 each time the gimme_another() function is called:
726 sub gimme_another { state $x; return ++$x }
728 And this example uses anonymous subroutines to create separate counters:
732 return sub { state $x; return ++$x }
735 Also, since C<$x> is lexical, it can't be reached or modified by any Perl
738 When combined with variable declaration, simple scalar assignment to C<state>
739 variables (as in C<state $x = 42>) is executed only the first time. When such
740 statements are evaluated subsequent times, the assignment is ignored. The
741 behavior of this sort of assignment to non-scalar variables is undefined.
743 =head3 Persistent variables with closures
745 Just because a lexical variable is lexically (also called statically)
746 scoped to its enclosing block, C<eval>, or C<do> FILE, this doesn't mean that
747 within a function it works like a C static. It normally works more
748 like a C auto, but with implicit garbage collection.
750 Unlike local variables in C or C++, Perl's lexical variables don't
751 necessarily get recycled just because their scope has exited.
752 If something more permanent is still aware of the lexical, it will
753 stick around. So long as something else references a lexical, that
754 lexical won't be freed--which is as it should be. You wouldn't want
755 memory being free until you were done using it, or kept around once you
756 were done. Automatic garbage collection takes care of this for you.
758 This means that you can pass back or save away references to lexical
759 variables, whereas to return a pointer to a C auto is a grave error.
760 It also gives us a way to simulate C's function statics. Here's a
761 mechanism for giving a function private variables with both lexical
762 scoping and a static lifetime. If you do want to create something like
763 C's static variables, just enclose the whole function in an extra block,
764 and put the static variable outside the function but in the block.
769 return ++$secret_val;
772 # $secret_val now becomes unreachable by the outside
773 # world, but retains its value between calls to gimme_another
775 If this function is being sourced in from a separate file
776 via C<require> or C<use>, then this is probably just fine. If it's
777 all in the main program, you'll need to arrange for the C<my>
778 to be executed early, either by putting the whole block above
779 your main program, or more likely, placing merely a C<BEGIN>
780 code block around it to make sure it gets executed before your program
786 return ++$secret_val;
790 See L<perlmod/"BEGIN, UNITCHECK, CHECK, INIT and END"> about the
791 special triggered code blocks, C<BEGIN>, C<UNITCHECK>, C<CHECK>,
794 If declared at the outermost scope (the file scope), then lexicals
795 work somewhat like C's file statics. They are available to all
796 functions in that same file declared below them, but are inaccessible
797 from outside that file. This strategy is sometimes used in modules
798 to create private variables that the whole module can see.
800 =head2 Temporary Values via local()
801 X<local> X<scope, dynamic> X<dynamic scope> X<variable, local>
802 X<variable, temporary>
804 B<WARNING>: In general, you should be using C<my> instead of C<local>, because
805 it's faster and safer. Exceptions to this include the global punctuation
806 variables, global filehandles and formats, and direct manipulation of the
807 Perl symbol table itself. C<local> is mostly used when the current value
808 of a variable must be visible to called subroutines.
812 # localization of values
814 local $foo; # make $foo dynamically local
815 local (@wid, %get); # make list of variables local
816 local $foo = "flurp"; # make $foo dynamic, and init it
817 local @oof = @bar; # make @oof dynamic, and init it
819 local $hash{key} = "val"; # sets a local value for this hash entry
820 delete local $hash{key}; # delete this entry for the current block
821 local ($cond ? $v1 : $v2); # several types of lvalues support
824 # localization of symbols
826 local *FH; # localize $FH, @FH, %FH, &FH ...
827 local *merlyn = *randal; # now $merlyn is really $randal, plus
828 # @merlyn is really @randal, etc
829 local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal
830 local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
832 A C<local> modifies its listed variables to be "local" to the
833 enclosing block, C<eval>, or C<do FILE>--and to I<any subroutine
834 called from within that block>. A C<local> just gives temporary
835 values to global (meaning package) variables. It does I<not> create
836 a local variable. This is known as dynamic scoping. Lexical scoping
837 is done with C<my>, which works more like C's auto declarations.
839 Some types of lvalues can be localized as well: hash and array elements
840 and slices, conditionals (provided that their result is always
841 localizable), and symbolic references. As for simple variables, this
842 creates new, dynamically scoped values.
844 If more than one variable or expression is given to C<local>, they must be
845 placed in parentheses. This operator works
846 by saving the current values of those variables in its argument list on a
847 hidden stack and restoring them upon exiting the block, subroutine, or
848 eval. This means that called subroutines can also reference the local
849 variable, but not the global one. The argument list may be assigned to if
850 desired, which allows you to initialize your local variables. (If no
851 initializer is given for a particular variable, it is created with an
854 Because C<local> is a run-time operator, it gets executed each time
855 through a loop. Consequently, it's more efficient to localize your
856 variables outside the loop.
858 =head3 Grammatical note on local()
861 A C<local> is simply a modifier on an lvalue expression. When you assign to
862 a C<local>ized variable, the C<local> doesn't change whether its list is viewed
863 as a scalar or an array. So
865 local($foo) = <STDIN>;
866 local @FOO = <STDIN>;
868 both supply a list context to the right-hand side, while
870 local $foo = <STDIN>;
872 supplies a scalar context.
874 =head3 Localization of special variables
875 X<local, special variable>
877 If you localize a special variable, you'll be giving a new value to it,
878 but its magic won't go away. That means that all side-effects related
879 to this magic still work with the localized value.
881 This feature allows code like this to work :
883 # Read the whole contents of FILE in $slurp
884 { local $/ = undef; $slurp = <FILE>; }
886 Note, however, that this restricts localization of some values ; for
887 example, the following statement dies, as of perl 5.10.0, with an error
888 I<Modification of a read-only value attempted>, because the $1 variable is
889 magical and read-only :
893 One exception is the default scalar variable: starting with perl 5.14
894 C<local($_)> will always strip all magic from $_, to make it possible
895 to safely reuse $_ in a subroutine.
897 B<WARNING>: Localization of tied arrays and hashes does not currently
899 This will be fixed in a future release of Perl; in the meantime, avoid
900 code that relies on any particular behaviour of localising tied arrays
901 or hashes (localising individual elements is still okay).
902 See L<perl58delta/"Localising Tied Arrays and Hashes Is Broken"> for more
906 =head3 Localization of globs
907 X<local, glob> X<glob>
913 creates a whole new symbol table entry for the glob C<name> in the
914 current package. That means that all variables in its glob slot ($name,
915 @name, %name, &name, and the C<name> filehandle) are dynamically reset.
917 This implies, among other things, that any magic eventually carried by
918 those variables is locally lost. In other words, saying C<local */>
919 will not have any effect on the internal value of the input record
922 =head3 Localization of elements of composite types
923 X<local, composite type element> X<local, array element> X<local, hash element>
925 It's also worth taking a moment to explain what happens when you
926 C<local>ize a member of a composite type (i.e. an array or hash element).
927 In this case, the element is C<local>ized I<by name>. This means that
928 when the scope of the C<local()> ends, the saved value will be
929 restored to the hash element whose key was named in the C<local()>, or
930 the array element whose index was named in the C<local()>. If that
931 element was deleted while the C<local()> was in effect (e.g. by a
932 C<delete()> from a hash or a C<shift()> of an array), it will spring
933 back into existence, possibly extending an array and filling in the
934 skipped elements with C<undef>. For instance, if you say
936 %hash = ( 'This' => 'is', 'a' => 'test' );
940 local($hash{'a'}) = 'drill';
941 while (my $e = pop(@ary)) {
946 $hash{'only a'} = 'test';
950 print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
951 print "The array has ",scalar(@ary)," elements: ",
952 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";
959 This is a test only a test.
960 The array has 6 elements: 0, 1, 2, undef, undef, 5
962 The behavior of local() on non-existent members of composite
963 types is subject to change in future.
965 =head3 Localized deletion of elements of composite types
966 X<delete> X<local, composite type element> X<local, array element> X<local, hash element>
968 You can use the C<delete local $array[$idx]> and C<delete local $hash{key}>
969 constructs to delete a composite type entry for the current block and restore
970 it when it ends. They return the array/hash value before the localization,
971 which means that they are respectively equivalent to
974 my $val = $array[$idx];
983 my $val = $hash{key};
989 except that for those the C<local> is scoped to the C<do> block. Slices are
998 my $a = delete local $hash{a};
1003 my @nums = delete local @$a[0, 2]
1005 # $a is [ undef, 8 ]
1007 $a[0] = 999; # will be erased when the scope ends
1009 # $a is back to [ 7, 8, 9 ]
1012 # %hash is back to its original state
1014 =head2 Lvalue subroutines
1015 X<lvalue> X<subroutine, lvalue>
1017 It is possible to return a modifiable value from a subroutine.
1018 To do this, you have to declare the subroutine to return an lvalue.
1021 sub canmod : lvalue {
1022 $val; # or: return $val;
1028 canmod() = 5; # assigns to $val
1029 nomod() = 5; # ERROR
1031 The scalar/list context for the subroutine and for the right-hand
1032 side of assignment is determined as if the subroutine call is replaced
1033 by a scalar. For example, consider:
1035 data(2,3) = get_data(3,4);
1037 Both subroutines here are called in a scalar context, while in:
1039 (data(2,3)) = get_data(3,4);
1043 (data(2),data(3)) = get_data(3,4);
1045 all the subroutines are called in a list context.
1047 Lvalue subroutines are convenient, but you have to keep in mind that,
1048 when used with objects, they may violate encapsulation. A normal
1049 mutator can check the supplied argument before setting the attribute
1050 it is protecting, an lvalue subroutine cannot. If you require any
1051 special processing when storing and retrieving the values, consider
1052 using the CPAN module Sentinel or something similar.
1054 =head2 Lexical Subroutines
1055 X<my sub> X<state sub> X<our sub> X<subroutine, lexical>
1057 B<WARNING>: Lexical subroutines are still experimental. The feature may be
1058 modified or removed in future versions of Perl.
1060 Lexical subroutines are only available under the C<use feature
1061 'lexical_subs'> pragma, which produces a warning unless the
1062 "experimental::lexical_subs" warnings category is disabled.
1064 Beginning with Perl 5.18, you can declare a private subroutine with C<my>
1065 or C<state>. As with state variables, the C<state> keyword is only
1066 available under C<use feature 'state'> or C<use 5.010> or higher.
1068 These subroutines are only visible within the block in which they are
1069 declared, and only after that declaration:
1071 no warnings "experimental::lexical_subs";
1072 use feature 'lexical_subs';
1074 foo(); # calls the package/global subroutine
1076 foo(); # also calls the package subroutine
1078 foo(); # calls "state" sub
1079 my $ref = \&foo; # take a reference to "state" sub
1082 bar(); # calls "my" sub
1084 To use a lexical subroutine from inside the subroutine itself, you must
1085 predeclare it. The C<sub foo {...}> subroutine definition syntax respects
1086 any previous C<my sub;> or C<state sub;> declaration.
1088 my sub baz; # predeclaration
1089 sub baz { # define the "my" sub
1090 baz(); # recursive call
1093 =head3 C<state sub> vs C<my sub>
1095 What is the difference between "state" subs and "my" subs? Each time that
1096 execution enters a block when "my" subs are declared, a new copy of each
1097 sub is created. "State" subroutines persist from one execution of the
1098 containing block to the next.
1100 So, in general, "state" subroutines are faster. But "my" subs are
1101 necessary if you want to create closures:
1103 no warnings "experimental::lexical_subs";
1104 use feature 'lexical_subs';
1109 ... do something with $x ...
1114 In this example, a new C<$x> is created when C<whatever> is called, and
1115 also a new C<inner>, which can see the new C<$x>. A "state" sub will only
1116 see the C<$x> from the first call to C<whatever>.
1118 =head3 C<our> subroutines
1120 Like C<our $variable>, C<our sub> creates a lexical alias to the package
1121 subroutine of the same name.
1123 The two main uses for this are to switch back to using the package sub
1124 inside an inner scope:
1126 no warnings "experimental::lexical_subs";
1127 use feature 'lexical_subs';
1134 # need to use the outer foo here
1140 and to make a subroutine visible to other packages in the same scope:
1142 package MySneakyModule;
1144 no warnings "experimental::lexical_subs";
1145 use feature 'lexical_subs';
1147 our sub do_something { ... }
1149 sub do_something_with_caller {
1151 () = caller 1; # sets @DB::args
1152 do_something(@args); # uses MySneakyModule::do_something
1155 =head2 Passing Symbol Table Entries (typeglobs)
1158 B<WARNING>: The mechanism described in this section was originally
1159 the only way to simulate pass-by-reference in older versions of
1160 Perl. While it still works fine in modern versions, the new reference
1161 mechanism is generally easier to work with. See below.
1163 Sometimes you don't want to pass the value of an array to a subroutine
1164 but rather the name of it, so that the subroutine can modify the global
1165 copy of it rather than working with a local copy. In perl you can
1166 refer to all objects of a particular name by prefixing the name
1167 with a star: C<*foo>. This is often known as a "typeglob", because the
1168 star on the front can be thought of as a wildcard match for all the
1169 funny prefix characters on variables and subroutines and such.
1171 When evaluated, the typeglob produces a scalar value that represents
1172 all the objects of that name, including any filehandle, format, or
1173 subroutine. When assigned to, it causes the name mentioned to refer to
1174 whatever C<*> value was assigned to it. Example:
1177 local(*someary) = @_;
1178 foreach $elem (@someary) {
1185 Scalars are already passed by reference, so you can modify
1186 scalar arguments without using this mechanism by referring explicitly
1187 to C<$_[0]> etc. You can modify all the elements of an array by passing
1188 all the elements as scalars, but you have to use the C<*> mechanism (or
1189 the equivalent reference mechanism) to C<push>, C<pop>, or change the size of
1190 an array. It will certainly be faster to pass the typeglob (or reference).
1192 Even if you don't want to modify an array, this mechanism is useful for
1193 passing multiple arrays in a single LIST, because normally the LIST
1194 mechanism will merge all the array values so that you can't extract out
1195 the individual arrays. For more on typeglobs, see
1196 L<perldata/"Typeglobs and Filehandles">.
1198 =head2 When to Still Use local()
1199 X<local> X<variable, local>
1201 Despite the existence of C<my>, there are still three places where the
1202 C<local> operator still shines. In fact, in these three places, you
1203 I<must> use C<local> instead of C<my>.
1209 You need to give a global variable a temporary value, especially $_.
1211 The global variables, like C<@ARGV> or the punctuation variables, must be
1212 C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
1213 it up into chunks separated by lines of equal signs, which are placed
1217 local @ARGV = ("/etc/motd");
1220 @Fields = split /^\s*=+\s*$/;
1223 It particular, it's important to C<local>ize $_ in any routine that assigns
1224 to it. Look out for implicit assignments in C<while> conditionals.
1228 You need to create a local file or directory handle or a local function.
1230 A function that needs a filehandle of its own must use
1231 C<local()> on a complete typeglob. This can be used to create new symbol
1235 local (*READER, *WRITER); # not my!
1236 pipe (READER, WRITER) or die "pipe: $!";
1237 return (*READER, *WRITER);
1239 ($head, $tail) = ioqueue();
1241 See the Symbol module for a way to create anonymous symbol table
1244 Because assignment of a reference to a typeglob creates an alias, this
1245 can be used to create what is effectively a local function, or at least,
1249 local *grow = \&shrink; # only until this block exits
1250 grow(); # really calls shrink()
1251 move(); # if move() grow()s, it shrink()s too
1253 grow(); # get the real grow() again
1255 See L<perlref/"Function Templates"> for more about manipulating
1256 functions by name in this way.
1260 You want to temporarily change just one element of an array or hash.
1262 You can C<local>ize just one element of an aggregate. Usually this
1263 is done on dynamics:
1266 local $SIG{INT} = 'IGNORE';
1267 funct(); # uninterruptible
1269 # interruptibility automatically restored here
1271 But it also works on lexically declared aggregates.
1275 =head2 Pass by Reference
1276 X<pass by reference> X<pass-by-reference> X<reference>
1278 If you want to pass more than one array or hash into a function--or
1279 return them from it--and have them maintain their integrity, then
1280 you're going to have to use an explicit pass-by-reference. Before you
1281 do that, you need to understand references as detailed in L<perlref>.
1282 This section may not make much sense to you otherwise.
1284 Here are a few simple examples. First, let's pass in several arrays
1285 to a function and have it C<pop> all of then, returning a new list
1286 of all their former last elements:
1288 @tailings = popmany ( \@a, \@b, \@c, \@d );
1293 foreach $aref ( @_ ) {
1294 push @retlist, pop @$aref;
1299 Here's how you might write a function that returns a
1300 list of keys occurring in all the hashes passed to it:
1302 @common = inter( \%foo, \%bar, \%joe );
1304 my ($k, $href, %seen); # locals
1305 foreach $href (@_) {
1306 while ( $k = each %$href ) {
1310 return grep { $seen{$_} == @_ } keys %seen;
1313 So far, we're using just the normal list return mechanism.
1314 What happens if you want to pass or return a hash? Well,
1315 if you're using only one of them, or you don't mind them
1316 concatenating, then the normal calling convention is ok, although
1319 Where people get into trouble is here:
1321 (@a, @b) = func(@c, @d);
1323 (%a, %b) = func(%c, %d);
1325 That syntax simply won't work. It sets just C<@a> or C<%a> and
1326 clears the C<@b> or C<%b>. Plus the function didn't get passed
1327 into two separate arrays or hashes: it got one long list in C<@_>,
1330 If you can arrange for everyone to deal with this through references, it's
1331 cleaner code, although not so nice to look at. Here's a function that
1332 takes two array references as arguments, returning the two array elements
1333 in order of how many elements they have in them:
1335 ($aref, $bref) = func(\@c, \@d);
1336 print "@$aref has more than @$bref\n";
1338 my ($cref, $dref) = @_;
1339 if (@$cref > @$dref) {
1340 return ($cref, $dref);
1342 return ($dref, $cref);
1346 It turns out that you can actually do this also:
1348 (*a, *b) = func(\@c, \@d);
1349 print "@a has more than @b\n";
1351 local (*c, *d) = @_;
1359 Here we're using the typeglobs to do symbol table aliasing. It's
1360 a tad subtle, though, and also won't work if you're using C<my>
1361 variables, because only globals (even in disguise as C<local>s)
1362 are in the symbol table.
1364 If you're passing around filehandles, you could usually just use the bare
1365 typeglob, like C<*STDOUT>, but typeglobs references work, too.
1371 print $fh "her um well a hmmm\n";
1374 $rec = get_rec(\*STDIN);
1377 return scalar <$fh>;
1380 If you're planning on generating new filehandles, you could do this.
1381 Notice to pass back just the bare *FH, not its reference.
1386 return open (FH, $path) ? *FH : undef;
1390 X<prototype> X<subroutine, prototype>
1392 Perl supports a very limited kind of compile-time argument checking
1393 using function prototyping. This can be declared in either the PROTO
1394 section or with a L<prototype attribute|attributes/Built-in Attributes>.
1395 If you declare either of
1398 sub mypush :prototype(+@)
1400 then C<mypush()> takes arguments exactly like C<push()> does.
1402 If subroutine signatures are enabled (see L</Signatures>), then
1403 the shorter PROTO syntax is unavailable, because it would clash with
1404 signatures. In that case, a prototype can only be declared in the form
1408 function declaration must be visible at compile time. The prototype
1409 affects only interpretation of new-style calls to the function,
1410 where new-style is defined as not using the C<&> character. In
1411 other words, if you call it like a built-in function, then it behaves
1412 like a built-in function. If you call it like an old-fashioned
1413 subroutine, then it behaves like an old-fashioned subroutine. It
1414 naturally falls out from this rule that prototypes have no influence
1415 on subroutine references like C<\&foo> or on indirect subroutine
1416 calls like C<&{$subref}> or C<< $subref->() >>.
1418 Method calls are not influenced by prototypes either, because the
1419 function to be called is indeterminate at compile time, since
1420 the exact code called depends on inheritance.
1422 Because the intent of this feature is primarily to let you define
1423 subroutines that work like built-in functions, here are prototypes
1424 for some other functions that parse almost exactly like the
1425 corresponding built-in.
1427 Declared as Called as
1429 sub mylink ($$) mylink $old, $new
1430 sub myvec ($$$) myvec $var, $offset, 1
1431 sub myindex ($$;$) myindex &getstring, "substr"
1432 sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off
1433 sub myreverse (@) myreverse $a, $b, $c
1434 sub myjoin ($@) myjoin ":", $a, $b, $c
1435 sub mypop (+) mypop @array
1436 sub mysplice (+$$@) mysplice @array, 0, 2, @pushme
1437 sub mykeys (+) mykeys %{$hashref}
1438 sub myopen (*;$) myopen HANDLE, $name
1439 sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
1440 sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
1441 sub myrand (;$) myrand 42
1442 sub mytime () mytime
1444 Any backslashed prototype character represents an actual argument
1445 that must start with that character (optionally preceded by C<my>,
1446 C<our> or C<local>), with the exception of C<$>, which will
1447 accept any scalar lvalue expression, such as C<$foo = 7> or
1448 C<< my_function()->[0] >>. The value passed as part of C<@_> will be a
1449 reference to the actual argument given in the subroutine call,
1450 obtained by applying C<\> to that argument.
1452 You can use the C<\[]> backslash group notation to specify more than one
1453 allowed argument type. For example:
1455 sub myref (\[$@%&*])
1457 will allow calling myref() as
1465 and the first argument of myref() will be a reference to
1466 a scalar, an array, a hash, a code, or a glob.
1468 Unbackslashed prototype characters have special meanings. Any
1469 unbackslashed C<@> or C<%> eats all remaining arguments, and forces
1470 list context. An argument represented by C<$> forces scalar context. An
1471 C<&> requires an anonymous subroutine, which, if passed as the first
1472 argument, does not require the C<sub> keyword or a subsequent comma.
1474 A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
1475 typeglob, or a reference to a typeglob in that slot. The value will be
1476 available to the subroutine either as a simple scalar, or (in the latter
1477 two cases) as a reference to the typeglob. If you wish to always convert
1478 such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
1481 use Symbol 'qualify_to_ref';
1484 my $fh = qualify_to_ref(shift, caller);
1488 The C<+> prototype is a special alternative to C<$> that will act like
1489 C<\[@%]> when given a literal array or hash variable, but will otherwise
1490 force scalar context on the argument. This is useful for functions which
1491 should accept either a literal array or an array reference as the argument:
1495 die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
1499 When using the C<+> prototype, your function must check that the argument
1500 is of an acceptable type.
1502 A semicolon (C<;>) separates mandatory arguments from optional arguments.
1503 It is redundant before C<@> or C<%>, which gobble up everything else.
1505 As the last character of a prototype, or just before a semicolon, a C<@>
1506 or a C<%>, you can use C<_> in place of C<$>: if this argument is not
1507 provided, C<$_> will be used instead.
1509 Note how the last three examples in the table above are treated
1510 specially by the parser. C<mygrep()> is parsed as a true list
1511 operator, C<myrand()> is parsed as a true unary operator with unary
1512 precedence the same as C<rand()>, and C<mytime()> is truly without
1513 arguments, just like C<time()>. That is, if you say
1517 you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
1518 without a prototype. If you want to force a unary function to have the
1519 same precedence as a list operator, add C<;> to the end of the prototype:
1521 sub mygetprotobynumber($;);
1522 mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)
1524 The interesting thing about C<&> is that you can generate new syntax with it,
1525 provided it's in the initial position:
1529 my($try,$catch) = @_;
1536 sub catch (&) { $_[0] }
1541 /phooey/ and print "unphooey\n";
1544 That prints C<"unphooey">. (Yes, there are still unresolved
1545 issues having to do with visibility of C<@_>. I'm ignoring that
1546 question for the moment. (But note that if we make C<@_> lexically
1547 scoped, those anonymous subroutines can act like closures... (Gee,
1548 is this sounding a little Lispish? (Never mind.))))
1550 And here's a reimplementation of the Perl C<grep> operator:
1557 push(@result, $_) if &$code;
1562 Some folks would prefer full alphanumeric prototypes. Alphanumerics have
1563 been intentionally left out of prototypes for the express purpose of
1564 someday in the future adding named, formal parameters. The current
1565 mechanism's main goal is to let module writers provide better diagnostics
1566 for module users. Larry feels the notation quite understandable to Perl
1567 programmers, and that it will not intrude greatly upon the meat of the
1568 module, nor make it harder to read. The line noise is visually
1569 encapsulated into a small pill that's easy to swallow.
1571 If you try to use an alphanumeric sequence in a prototype you will
1572 generate an optional warning - "Illegal character in prototype...".
1573 Unfortunately earlier versions of Perl allowed the prototype to be
1574 used as long as its prefix was a valid prototype. The warning may be
1575 upgraded to a fatal error in a future version of Perl once the
1576 majority of offending code is fixed.
1578 It's probably best to prototype new functions, not retrofit prototyping
1579 into older ones. That's because you must be especially careful about
1580 silent impositions of differing list versus scalar contexts. For example,
1581 if you decide that a function should take just one parameter, like this:
1585 print "you gave me $n\n";
1588 and someone has been calling it with an array or expression
1594 Then you've just supplied an automatic C<scalar> in front of their
1595 argument, which can be more than a bit surprising. The old C<@foo>
1596 which used to hold one thing doesn't get passed in. Instead,
1597 C<func()> now gets passed in a C<1>; that is, the number of elements
1598 in C<@foo>. And the C<split> gets called in scalar context so it
1599 starts scribbling on your C<@_> parameter list. Ouch!
1601 If a sub has both a PROTO and a BLOCK, the prototype is not applied
1602 until after the BLOCK is completely defined. This means that a recursive
1603 function with a prototype has to be predeclared for the prototype to take
1611 This is all very powerful, of course, and should be used only in moderation
1612 to make the world a better place.
1614 =head2 Constant Functions
1617 Functions with a prototype of C<()> are potential candidates for
1618 inlining. If the result after optimization and constant folding
1619 is either a constant or a lexically-scoped scalar which has no other
1620 references, then it will be used in place of function calls made
1621 without C<&>. Calls made using C<&> are never inlined. (See
1622 F<constant.pm> for an easy way to declare most constants.)
1624 The following functions would all be inlined:
1626 sub pi () { 3.14159 } # Not exact, but close.
1627 sub PI () { 4 * atan2 1, 1 } # As good as it gets,
1628 # and it's inlined, too!
1632 sub FLAG_FOO () { 1 << 8 }
1633 sub FLAG_BAR () { 1 << 9 }
1634 sub FLAG_MASK () { FLAG_FOO | FLAG_BAR }
1636 sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
1638 sub N () { int(OPT_BAZ) / 3 }
1640 sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }
1642 Be aware that these will not be inlined; as they contain inner scopes,
1643 the constant folding doesn't reduce them to a single constant:
1645 sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }
1656 As alluded to earlier you can also declare inlined subs dynamically at
1657 BEGIN time if their body consists of a lexically-scoped scalar which
1658 has no other references. Only the first example here will be inlined:
1663 *INLINED = sub () { $var };
1670 *NOT_INLINED = sub () { $var };
1673 A not so obvious caveat with this (see [RT #79908]) is that the
1674 variable will be immediately inlined, and will stop behaving like a
1675 normal lexical variable, e.g. this will print C<79907>, not C<79908>:
1679 *RT_79908 = sub () { $x };
1682 print RT_79908(); # prints 79907
1684 If you really want a subroutine with a C<()> prototype that returns a
1685 lexical variable you can easily force it to not be inlined by adding
1686 an explicit C<return>:
1690 *RT_79908 = sub () { return $x };
1693 print RT_79908(); # prints 79908
1695 The easiest way to tell if a subroutine was inlined is by using
1696 L<B::Deparse>, consider this example of two subroutines returning
1697 C<1>, one with a C<()> prototype causing it to be inlined, and one
1698 without (with deparse output truncated for clarity):
1700 $ perl -MO=Deparse -le 'sub ONE { 1 } if (ONE) { print ONE if ONE }'
1705 print ONE() if ONE ;
1707 $ perl -MO=Deparse -le 'sub ONE () { 1 } if (ONE) { print ONE if ONE }'
1713 If you redefine a subroutine that was eligible for inlining, you'll
1714 get a warning by default. You can use this warning to tell whether or
1715 not a particular subroutine is considered inlinable, since it's
1716 different than the warning for overriding non-inlined subroutines:
1718 $ perl -e 'sub one () {1} sub one () {2}'
1719 Constant subroutine one redefined at -e line 1.
1720 $ perl -we 'sub one {1} sub one {2}'
1721 Subroutine one redefined at -e line 1.
1723 The warning is considered severe enough not to be affected by the
1724 B<-w> switch (or its absence) because previously compiled invocations
1725 of the function will still be using the old value of the function. If
1726 you need to be able to redefine the subroutine, you need to ensure
1727 that it isn't inlined, either by dropping the C<()> prototype (which
1728 changes calling semantics, so beware) or by thwarting the inlining
1729 mechanism in some other way, e.g. by adding an explicit C<return>:
1731 sub not_inlined () { return 23 }
1733 =head2 Overriding Built-in Functions
1734 X<built-in> X<override> X<CORE> X<CORE::GLOBAL>
1736 Many built-in functions may be overridden, though this should be tried
1737 only occasionally and for good reason. Typically this might be
1738 done by a package attempting to emulate missing built-in functionality
1739 on a non-Unix system.
1741 Overriding may be done only by importing the name from a module at
1742 compile time--ordinary predeclaration isn't good enough. However, the
1743 C<use subs> pragma lets you, in effect, predeclare subs
1744 via the import syntax, and these names may then override built-in ones:
1746 use subs 'chdir', 'chroot', 'chmod', 'chown';
1750 To unambiguously refer to the built-in form, precede the
1751 built-in name with the special package qualifier C<CORE::>. For example,
1752 saying C<CORE::open()> always refers to the built-in C<open()>, even
1753 if the current package has imported some other subroutine called
1754 C<&open()> from elsewhere. Even though it looks like a regular
1755 function call, it isn't: the CORE:: prefix in that case is part of Perl's
1756 syntax, and works for any keyword, regardless of what is in the CORE
1757 package. Taking a reference to it, that is, C<\&CORE::open>, only works
1758 for some keywords. See L<CORE>.
1760 Library modules should not in general export built-in names like C<open>
1761 or C<chdir> as part of their default C<@EXPORT> list, because these may
1762 sneak into someone else's namespace and change the semantics unexpectedly.
1763 Instead, if the module adds that name to C<@EXPORT_OK>, then it's
1764 possible for a user to import the name explicitly, but not implicitly.
1765 That is, they could say
1769 and it would import the C<open> override. But if they said
1773 they would get the default imports without overrides.
1775 The foregoing mechanism for overriding built-in is restricted, quite
1776 deliberately, to the package that requests the import. There is a second
1777 method that is sometimes applicable when you wish to override a built-in
1778 everywhere, without regard to namespace boundaries. This is achieved by
1779 importing a sub into the special namespace C<CORE::GLOBAL::>. Here is an
1780 example that quite brazenly replaces the C<glob> operator with something
1781 that understands regular expressions.
1786 @EXPORT_OK = 'glob';
1792 my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
1793 $pkg->export($where, $sym, @_);
1799 if (opendir my $d, '.') {
1800 @got = grep /$pat/, readdir $d;
1807 And here's how it could be (ab)used:
1809 #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces
1811 use REGlob 'glob'; # override glob() in Foo:: only
1812 print for <^[a-z_]+\.pm\$>; # show all pragmatic modules
1814 The initial comment shows a contrived, even dangerous example.
1815 By overriding C<glob> globally, you would be forcing the new (and
1816 subversive) behavior for the C<glob> operator for I<every> namespace,
1817 without the complete cognizance or cooperation of the modules that own
1818 those namespaces. Naturally, this should be done with extreme caution--if
1819 it must be done at all.
1821 The C<REGlob> example above does not implement all the support needed to
1822 cleanly override perl's C<glob> operator. The built-in C<glob> has
1823 different behaviors depending on whether it appears in a scalar or list
1824 context, but our C<REGlob> doesn't. Indeed, many perl built-in have such
1825 context sensitive behaviors, and these must be adequately supported by
1826 a properly written override. For a fully functional example of overriding
1827 C<glob>, study the implementation of C<File::DosGlob> in the standard
1830 When you override a built-in, your replacement should be consistent (if
1831 possible) with the built-in native syntax. You can achieve this by using
1832 a suitable prototype. To get the prototype of an overridable built-in,
1833 use the C<prototype> function with an argument of C<"CORE::builtin_name">
1834 (see L<perlfunc/prototype>).
1836 Note however that some built-ins can't have their syntax expressed by a
1837 prototype (such as C<system> or C<chomp>). If you override them you won't
1838 be able to fully mimic their original syntax.
1840 The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
1841 to special magic, their original syntax is preserved, and you don't have
1842 to define a prototype for their replacements. (You can't override the
1843 C<do BLOCK> syntax, though).
1845 C<require> has special additional dark magic: if you invoke your
1846 C<require> replacement as C<require Foo::Bar>, it will actually receive
1847 the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
1849 And, as you'll have noticed from the previous example, if you override
1850 C<glob>, the C<< <*> >> glob operator is overridden as well.
1852 In a similar fashion, overriding the C<readline> function also overrides
1853 the equivalent I/O operator C<< <FILEHANDLE> >>. Also, overriding
1854 C<readpipe> also overrides the operators C<``> and C<qx//>.
1856 Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
1859 X<autoloading> X<AUTOLOAD>
1861 If you call a subroutine that is undefined, you would ordinarily
1862 get an immediate, fatal error complaining that the subroutine doesn't
1863 exist. (Likewise for subroutines being used as methods, when the
1864 method doesn't exist in any base class of the class's package.)
1865 However, if an C<AUTOLOAD> subroutine is defined in the package or
1866 packages used to locate the original subroutine, then that
1867 C<AUTOLOAD> subroutine is called with the arguments that would have
1868 been passed to the original subroutine. The fully qualified name
1869 of the original subroutine magically appears in the global $AUTOLOAD
1870 variable of the same package as the C<AUTOLOAD> routine. The name
1871 is not passed as an ordinary argument because, er, well, just
1872 because, that's why. (As an exception, a method call to a nonexistent
1873 C<import> or C<unimport> method is just skipped instead. Also, if
1874 the AUTOLOAD subroutine is an XSUB, there are other ways to retrieve the
1875 subroutine name. See L<perlguts/Autoloading with XSUBs> for details.)
1878 Many C<AUTOLOAD> routines load in a definition for the requested
1879 subroutine using eval(), then execute that subroutine using a special
1880 form of goto() that erases the stack frame of the C<AUTOLOAD> routine
1881 without a trace. (See the source to the standard module documented
1882 in L<AutoLoader>, for example.) But an C<AUTOLOAD> routine can
1883 also just emulate the routine and never define it. For example,
1884 let's pretend that a function that wasn't defined should just invoke
1885 C<system> with those arguments. All you'd do is:
1888 my $program = $AUTOLOAD;
1889 $program =~ s/.*:://;
1890 system($program, @_);
1896 In fact, if you predeclare functions you want to call that way, you don't
1897 even need parentheses:
1899 use subs qw(date who ls);
1904 A more complete example of this is the Shell module on CPAN, which
1905 can treat undefined subroutine calls as calls to external programs.
1907 Mechanisms are available to help modules writers split their modules
1908 into autoloadable files. See the standard AutoLoader module
1909 described in L<AutoLoader> and in L<AutoSplit>, the standard
1910 SelfLoader modules in L<SelfLoader>, and the document on adding C
1911 functions to Perl code in L<perlxs>.
1913 =head2 Subroutine Attributes
1914 X<attribute> X<subroutine, attribute> X<attrs>
1916 A subroutine declaration or definition may have a list of attributes
1917 associated with it. If such an attribute list is present, it is
1918 broken up at space or colon boundaries and treated as though a
1919 C<use attributes> had been seen. See L<attributes> for details
1920 about what attributes are currently supported.
1921 Unlike the limitation with the obsolescent C<use attrs>, the
1922 C<sub : ATTRLIST> syntax works to associate the attributes with
1923 a pre-declaration, and not just with a subroutine definition.
1925 The attributes must be valid as simple identifier names (without any
1926 punctuation other than the '_' character). They may have a parameter
1927 list appended, which is only checked for whether its parentheses ('(',')')
1930 Examples of valid syntax (even though the attributes are unknown):
1932 sub fnord (&\%) : switch(10,foo(7,3)) : expensive;
1933 sub plugh () : Ugly('\(") :Bad;
1934 sub xyzzy : _5x5 { ... }
1936 Examples of invalid syntax:
1938 sub fnord : switch(10,foo(); # ()-string not balanced
1939 sub snoid : Ugly('('); # ()-string not balanced
1940 sub xyzzy : 5x5; # "5x5" not a valid identifier
1941 sub plugh : Y2::north; # "Y2::north" not a simple identifier
1942 sub snurt : foo + bar; # "+" not a colon or space
1944 The attribute list is passed as a list of constant strings to the code
1945 which associates them with the subroutine. In particular, the second example
1946 of valid syntax above currently looks like this in terms of how it's
1949 use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
1951 For further details on attribute lists and their manipulation,
1952 see L<attributes> and L<Attribute::Handlers>.
1956 See L<perlref/"Function Templates"> for more about references and closures.
1957 See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
1958 See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
1959 See L<perlmod> to learn about bundling up your functions in separate files.
1960 See L<perlmodlib> to learn what library modules come standard on your system.
1961 See L<perlootut> to learn how to make object method calls.