sub NAME BLOCK # A declaration and a definition.
sub NAME(PROTO) BLOCK # ditto, but with prototypes
- sub NAME(SIG) BLOCK # with a signature instead
sub NAME : ATTRS BLOCK # with attributes
sub NAME(PROTO) : ATTRS BLOCK # with prototypes and attributes
- sub NAME(SIG) : ATTRS BLOCK # with a signature and attributes
+
+ use feature 'signatures';
+ sub NAME(SIG) BLOCK # with signature
+ sub NAME :ATTRS (SIG) BLOCK # with signature, attributes
+ sub NAME :prototype(PROTO) (SIG) BLOCK # with signature, prototype
To define an anonymous subroutine at runtime:
X<subroutine, anonymous>
$subref = sub BLOCK; # no proto
$subref = sub (PROTO) BLOCK; # with proto
- $subref = sub (SIG) BLOCK; # with signature
$subref = sub : ATTRS BLOCK; # with attributes
$subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
- $subref = sub (SIG) : ATTRS BLOCK; # with signature and attributes
+
+ use feature 'signatures';
+ $subref = sub (SIG) BLOCK; # with signature
+ $subref = sub : ATTRS(SIG) BLOCK; # with signature, attributes
To import subroutines:
X<import>
Perl does not have named formal parameters. In practice all you
do is assign to a C<my()> list of these. Variables that aren't
declared to be private are global variables. For gory details
-on creating private variables, see L<"Private Variables via my()">
-and L<"Temporary Values via local()">. To create protected
+on creating private variables, see L</"Private Variables via my()">
+and L</"Temporary Values via local()">. To create protected
environments for a set of functions in a separate package (and
probably a separate file), see L<perlmod/"Packages">.
X<formal parameter> X<parameter, formal>
Like the flattened incoming parameter list, the return list is also
flattened on return. So all you have managed to do here is stored
everything in C<@a> and made C<@b> empty. See
-L<Pass by Reference> for alternatives.
+L</Pass by Reference> for alternatives.
A subroutine may be called using an explicit C<&> prefix. The
C<&> is optional in modern Perl, as are parentheses if the
&foo(); # the same
&foo; # foo() get current args, like foo(@_) !!
- foo; # like foo() IFF sub foo predeclared, else "foo"
+ use strict 'subs';
+ foo; # like foo() iff sub foo predeclared, else
+ # a compile-time error
+ no strict 'subs';
+ foo; # like foo() iff sub foo predeclared, else
+ # a literal string "foo"
Not only does the C<&> form make the argument list optional, it also
disables any prototype checking on arguments you do provide. This
disabled.
The signature is part of a subroutine's body. Normally the body of a
-subroutine is simply a braced block of code. When using a signature,
-the signature is a parenthesised list that goes immediately after
-the subroutine name. The signature declares lexical variables that are
+subroutine is simply a braced block of code, but when using a signature,
+the signature is a parenthesised list that goes immediately before the
+block, after any name or attributes.
+
+For example,
+
+ sub foo :lvalue ($a, $b = 1, @c) { .... }
+
+The signature declares lexical variables that are
in scope for the block. When the subroutine is called, the signature
takes control first. It populates the signature variables from the
list of arguments that were passed. If the argument list doesn't meet
of calls to the subroutine, and the signature puts argument values into
lexical variables at runtime. You can therefore write
- sub foo ($left, $right) : prototype($$) {
+ sub foo :prototype($$) ($left, $right) {
return $left + $right;
}
-The prototype attribute, and any other attributes, come after
-the signature.
+The prototype attribute, and any other attributes, must come before
+the signature. The signature always immediately precedes the block of
+the subroutine's body.
=head2 Private Variables via my()
X<my> X<variable, lexical> X<lexical> X<lexical variable> X<scope, lexical>
See L<attributes> and L<Attribute::Handlers>.
The C<my> operator declares the listed variables to be lexically
-confined to the enclosing block, conditional (C<if/unless/elsif/else>),
-loop (C<for/foreach/while/until/continue>), subroutine, C<eval>,
-or C<do/require/use>'d file. If more than one value is listed, the
+confined to the enclosing block, conditional
+(C<if>/C<unless>/C<elsif>/C<else>), loop
+(C<for>/C<foreach>/C<while>/C<until>/C<continue>), subroutine, C<eval>,
+or C<do>/C<require>/C<use>'d file. If more than one value is listed, the
list must be placed in parentheses. All listed elements must be
legal lvalues. Only alphanumeric identifiers may be lexically
scoped--magical built-ins like C<$/> must currently be C<local>ized
Also, since C<$x> is lexical, it can't be reached or modified by any Perl
code outside.
-When combined with variable declaration, simple scalar assignment to C<state>
+When combined with variable declaration, simple assignment to C<state>
variables (as in C<state $x = 42>) is executed only the first time. When such
statements are evaluated subsequent times, the assignment is ignored. The
-behavior of this sort of assignment to non-scalar variables is undefined.
+behavior of assignment to C<state> declarations where the left hand side
+of the assignment involves any parentheses is currently undefined.
=head3 Persistent variables with closures
The array has 6 elements: 0, 1, 2, undef, undef, 5
The behavior of local() on non-existent members of composite
-types is subject to change in future.
+types is subject to change in future. The behavior of local()
+on array elements specified using negative indexes is particularly
+surprising, and is very likely to change.
=head3 Localized deletion of elements of composite types
X<delete> X<local, composite type element> X<local, array element> X<local, hash element>
=head2 Lexical Subroutines
X<my sub> X<state sub> X<our sub> X<subroutine, lexical>
-B<WARNING>: Lexical subroutines are still experimental. The feature may be
-modified or removed in future versions of Perl.
-
-Lexical subroutines are only available under the C<use feature
-'lexical_subs'> pragma, which produces a warning unless the
-"experimental::lexical_subs" warnings category is disabled.
-
Beginning with Perl 5.18, you can declare a private subroutine with C<my>
or C<state>. As with state variables, the C<state> keyword is only
available under C<use feature 'state'> or C<use 5.010> or higher.
+Prior to Perl 5.26, lexical subroutines were deemed experimental and were
+available only under the C<use feature 'lexical_subs'> pragma. They also
+produced a warning unless the "experimental::lexical_subs" warnings
+category was disabled.
+
These subroutines are only visible within the block in which they are
declared, and only after that declaration:
+ # Include these two lines if your code is intended to run under Perl
+ # versions earlier than 5.26.
no warnings "experimental::lexical_subs";
use feature 'lexical_subs';
- foo(); # calls the package/global subroutine
+ foo(); # calls the package/global subroutine
state sub foo {
- foo(); # also calls the package subroutine
+ foo(); # also calls the package subroutine
}
- foo(); # calls "state" sub
- my $ref = \&foo; # take a reference to "state" sub
+ foo(); # calls "state" sub
+ my $ref = \&foo; # take a reference to "state" sub
my sub bar { ... }
- bar(); # calls "my" sub
+ bar(); # calls "my" sub
-To use a lexical subroutine from inside the subroutine itself, you must
-predeclare it. The C<sub foo {...}> subroutine definition syntax respects
-any previous C<my sub;> or C<state sub;> declaration.
+You can't (directly) write a recursive lexical subroutine:
- my sub baz; # predeclaration
- sub baz { # define the "my" sub
- baz(); # recursive call
+ # WRONG
+ my sub baz {
+ baz();
}
+This example fails because C<baz()> refers to the package/global subroutine
+C<baz>, not the lexical subroutine currently being defined.
+
+The solution is to use L<C<__SUB__>|perlfunc/__SUB__>:
+
+ my sub baz {
+ __SUB__->(); # calls itself
+ }
+
+It is possible to predeclare a lexical subroutine. The C<sub foo {...}>
+subroutine definition syntax respects any previous C<my sub;> or C<state sub;>
+declaration. Using this to define recursive subroutines is a bad idea,
+however:
+
+ my sub baz; # predeclaration
+ sub baz { # define the "my" sub
+ baz(); # WRONG: calls itself, but leaks memory
+ }
+
+Just like C<< my $f; $f = sub { $f->() } >>, this example leaks memory. The
+name C<baz> is a reference to the subroutine, and the subroutine uses the name
+C<baz>; they keep each other alive (see L<perlref/Circular References>).
+
=head3 C<state sub> vs C<my sub>
What is the difference between "state" subs and "my" subs? Each time that
So, in general, "state" subroutines are faster. But "my" subs are
necessary if you want to create closures:
- no warnings "experimental::lexical_subs";
- use feature 'lexical_subs';
-
sub whatever {
my $x = shift;
my sub inner {
The two main uses for this are to switch back to using the package sub
inside an inner scope:
- no warnings "experimental::lexical_subs";
- use feature 'lexical_subs';
-
sub foo { ... }
sub bar {
package MySneakyModule;
- no warnings "experimental::lexical_subs";
- use feature 'lexical_subs';
-
our sub do_something { ... }
sub do_something_with_caller {
sub myjoin ($@) myjoin ":", $a, $b, $c
sub mypop (\@) mypop @array
sub mysplice (\@$$@) mysplice @array, 0, 2, @pushme
- sub mykeys (\[%@]) mykeys %{$hashref}
+ sub mykeys (\[%@]) mykeys $hashref->%*
sub myopen (*;$) myopen HANDLE, $name
sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
sub mygrep (&@) mygrep { /foo/ } $a, $b, $c
returning a list:
func(@foo);
- func( split /:/ );
+ func( $text =~ /\w+/g );
Then you've just supplied an automatic C<scalar> in front of their
argument, which can be more than a bit surprising. The old C<@foo>
which used to hold one thing doesn't get passed in. Instead,
C<func()> now gets passed in a C<1>; that is, the number of elements
-in C<@foo>. And the C<split> gets called in scalar context so it
-starts scribbling on your C<@_> parameter list. Ouch!
+in C<@foo>. And the C<m//g> gets called in scalar context so instead of a
+list of words it returns a boolean result and advances C<pos($text)>. Ouch!
If a sub has both a PROTO and a BLOCK, the prototype is not applied
until after the BLOCK is completely defined. This means that a recursive
is either a constant or a lexically-scoped scalar which has no other
references, then it will be used in place of function calls made
without C<&>. Calls made using C<&> are never inlined. (See
-F<constant.pm> for an easy way to declare most constants.)
+L<constant> for an easy way to declare most constants.)
The following functions would all be inlined:
C<system> with those arguments. All you'd do is:
sub AUTOLOAD {
- my $program = $AUTOLOAD;
- $program =~ s/.*:://;
- system($program, @_);
+ our $AUTOLOAD; # keep 'use strict' happy
+ my $program = $AUTOLOAD;
+ $program =~ s/.*:://;
+ system($program, @_);
}
date();
- who('am', 'i');
+ who();
ls('-l');
In fact, if you predeclare functions you want to call that way, you don't
use subs qw(date who ls);
date;
- who "am", "i";
+ who;
ls '-l';
A more complete example of this is the Shell module on CPAN, which
https://perl5.git.perl.org / perl5.git / blobdiff