Like many languages, Perl provides for user-defined subroutines.
These may be located anywhere in the main program, loaded in from
other files via the C<do>, C<require>, or C<use> keywords, or
-generated on the fly using C<eval> or anonymous subroutines (closures).
+generated on the fly using C<eval> or anonymous subroutines.
You can even call a function indirectly using a variable containing
its name or a CODE reference.
}
Notice how this (unprototyped) function doesn't care whether it was
-passed real scalars or arrays. Perl sees all arugments as one big,
+passed real scalars or arrays. Perl sees all arguments as one big,
long, flat parameter list in C<@_>. This is one area where
Perl's simple argument-passing style shines. The C<upcase()>
function would work perfectly well without changing the C<upcase()>
an argument to defined() or undef(). Nor is it optional when you
want to do an indirect subroutine call with a subroutine name or
reference using the C<&$subref()> or C<&{$subref}()> constructs,
-although the C<$subref-E<gt>()> notation solves that problem.
+although the C<< $subref->() >> notation solves that problem.
See L<perlref> for more about all that.
Subroutines may be called recursively. If a subroutine is called
function in all capitals is a loosely-held convention meaning it
will be called indirectly by the run-time system itself, usually
due to a triggered event. Functions that do special, pre-defined
-things include C<BEGIN>, C<END>, C<AUTOLOAD>, and C<DESTROY>--plus
-all functions mentioned in L<perltie>. The 5.005 release adds
-C<INIT> to this list.
+things include C<BEGIN>, C<CHECK>, C<INIT>, C<END>, C<AUTOLOAD>, and
+C<DESTROY>--plus all functions mentioned in L<perltie>.
=head2 Private Variables via my()
then any variable mentioned from there to the end of the enclosing
block must either refer to a lexical variable, be predeclared via
-C<use vars>, or else must be fully qualified with the package name.
+C<our> or C<use vars>, or else must be fully qualified with the package name.
A compilation error results otherwise. An inner block may countermand
this with C<no strict 'vars'>.
A C<my> has both a compile-time and a run-time effect. At compile
-time, the compiler takes notice of it. The principle usefulness
+time, the compiler takes notice of it. The principal usefulness
of this is to quiet C<use strict 'vars'>, but it is also essential
for generation of closures as detailed in L<perlref>. Actual
initialization is delayed until run time, though, so it gets executed
}
See L<perlmod/"Package Constructors and Destructors"> about the
-special triggered functions, C<BEGIN> and C<INIT>.
+special triggered functions, C<BEGIN>, C<CHECK>, C<INIT> and C<END>.
If declared at the outermost scope (the file scope), then lexicals
work somewhat like C's file statics. They are available to all
The behavior of local() on non-existent members of composite
types is subject to change in future.
+=head2 Lvalue subroutines
+
+B<WARNING>: Lvalue subroutines are still experimental and the implementation
+may change in future versions of Perl.
+
+It is possible to return a modifiable value from a subroutine.
+To do this, you have to declare the subroutine to return an lvalue.
+
+ my $val;
+ sub canmod : lvalue {
+ $val;
+ }
+ sub nomod {
+ $val;
+ }
+
+ canmod() = 5; # assigns to $val
+ nomod() = 5; # ERROR
+
+The scalar/list context for the subroutine and for the right-hand
+side of assignment is determined as if the subroutine call is replaced
+by a scalar. For example, consider:
+
+ data(2,3) = get_data(3,4);
+
+Both subroutines here are called in a scalar context, while in:
+
+ (data(2,3)) = get_data(3,4);
+
+and in:
+
+ (data(2),data(3)) = get_data(3,4);
+
+all the subroutines are called in a list context.
+
+The current implementation does not allow arrays and hashes to be
+returned from lvalue subroutines directly. You may return a
+reference instead. This restriction may be lifted in future.
+
=head2 Passing Symbol Table Entries (typeglobs)
B<WARNING>: The mechanism described in this section was originally
subroutine, then it behaves like an old-fashioned subroutine. It
naturally falls out from this rule that prototypes have no influence
on subroutine references like C<\&foo> or on indirect subroutine
-calls like C<&{$subref}> or C<$subref-E<gt>()>.
+calls like C<&{$subref}> or C<< $subref->() >>.
Method calls are not influenced by prototypes either, because the
function to be called is indeterminate at compile time, since
unbackslashed C<@> or C<%> eats all remaining arguments, and forces
list context. An argument represented by C<$> forces scalar context. An
C<&> requires an anonymous subroutine, which, if passed as the first
-argument, does not require the C<sub> keyword or a subsequent comma. A
-C<*> allows the subroutine to accept a bareword, constant, scalar expression,
+argument, does not require the C<sub> keyword or a subsequent comma.
+
+A C<*> allows the subroutine to accept a bareword, constant, scalar expression,
typeglob, or a reference to a typeglob in that slot. The value will be
available to the subroutine either as a simple scalar, or (in the latter
-two cases) as a reference to the typeglob.
+two cases) as a reference to the typeglob. If you wish to always convert
+such arguments to a typeglob reference, use Symbol::qualify_to_ref() as
+follows:
+
+ use Symbol 'qualify_to_ref';
+
+ sub foo (*) {
+ my $fh = qualify_to_ref(shift, caller);
+ ...
+ }
A semicolon separates mandatory arguments from optional arguments.
It is redundant before C<@> or C<%>, which gobble up everything else.
A subroutine declaration or definition may have a list of attributes
associated with it. If such an attribute list is present, it is
-broken up at space or comma boundaries and treated as though a
+broken up at space or colon boundaries and treated as though a
C<use attributes> had been seen. See L<attributes> for details
about what attributes are currently supported.
Unlike the limitation with the obsolescent C<use attrs>, the
Examples of valid syntax (even though the attributes are unknown):
- sub fnord (&\%) : switch(10,foo(7,3)) , , expensive ;
- sub plugh () : Ugly('\(") , Bad ;
+ sub fnord (&\%) : switch(10,foo(7,3)) : expensive ;
+ sub plugh () : Ugly('\(") :Bad ;
sub xyzzy : _5x5 { ... }
Examples of invalid syntax:
sub snoid : Ugly('(') ; # ()-string not balanced
sub xyzzy : 5x5 ; # "5x5" not a valid identifier
sub plugh : Y2::north ; # "Y2::north" not a simple identifier
- sub snurt : foo + bar ; # "+" not a comma or space
+ sub snurt : foo + bar ; # "+" not a colon or space
The attribute list is passed as a list of constant strings to the code
which associates them with the subroutine. In particular, the second example
See L<perlref/"Function Templates"> for more about references and closures.
See L<perlxs> if you'd like to learn about calling C subroutines from Perl.
-See L<perlembed> if you'd like to learn about calling PErl subroutines from C.
+See L<perlembed> if you'd like to learn about calling Perl subroutines from C.
See L<perlmod> to learn about bundling up your functions in separate files.
See L<perlmodlib> to learn what library modules come standard on your system.
See L<perltoot> to learn how to make object method calls.
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