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> an empty list. See
+everything in C<@a> and made C<@b> empty. See
L<Pass by Reference> for alternatives.
A subroutine may be called using an explicit C<&> prefix. The
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<CHECK>, C<INIT>, C<END>, C<AUTOLOAD>, and
-C<DESTROY>--plus all functions mentioned in L<perltie>.
+things include C<BEGIN>, C<CHECK>, C<INIT>, C<END>, C<AUTOLOAD>,
+C<CLONE> and C<DESTROY>--plus all functions mentioned in L<perltie>.
=head2 Private Variables via my()
my @oof = @bar; # declare @oof lexical, and init it
my $x : Foo = $y; # similar, with an attribute applied
-B<WARNING>: The use of attribute lists on C<my> declarations is
-experimental. This feature should not be relied upon. It may
-change or disappear in future releases of Perl. See L<attributes>.
+B<WARNING>: The use of attribute lists on C<my> declarations is still
+evolving. The current semantics and interface are subject to change.
+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>),
of that conditional, including any C<elsif> and C<else> clauses,
but not beyond it.
-None of the foregoing text applies to C<if/unless> or C<while/until>
-modifiers appended to simple statements. Such modifiers are not
-control structures and have no effect on scoping.
+B<NOTE:> The behaviour of a C<my> statement modified with a statement
+modifier conditional or loop construct (e.g. C<my $x if ...>) is
+B<undefined>. The value of the C<my> variable may be C<undef>, any
+previously assigned value, or possibly anything else. Don't rely on
+it. Future versions of perl might do something different from the
+version of perl you try it out on. Here be dragons.
The C<foreach> loop defaults to scoping its index variable dynamically
in the manner of C<local>. However, if the index variable is
}
[..%ahash back to its initial tied self again..]
+B<WARNING> The code example above does not currently work as described.
+This will be fixed in a future release of Perl; in the meantime, avoid
+code that relies on any particular behaviour of localising tied arrays
+or hashes (localising individual elements is still okay).
+See L<perldelta/"Localising Tied Arrays and Hashes Is Broken"> for more
+details.
+
As another example, a custom implementation of C<%ENV> might look
like this:
=head2 Lvalue subroutines
-B<WARNING>: Lvalue subroutines are still experimental and the implementation
-may change in future versions of Perl.
+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 {
+ # return $val; this doesn't work, don't say "return"
$val;
}
sub nomod {
all the subroutines are called in a list context.
+=over 4
+
+=item Lvalue subroutines are EXPERIMENTAL
+
+They appear to be convenient, but there are several reasons to be
+circumspect.
+
+You can't use the return keyword, you must pass out the value before
+falling out of subroutine scope. (see comment in example above). This
+is usually not a problem, but it disallows an explicit return out of a
+deeply nested loop, which is sometimes a nice way out.
+
+They violate encapsulation. A normal mutator can check the supplied
+argument before setting the attribute it is protecting, an lvalue
+subroutine never gets that chance. Consider;
+
+ my $some_array_ref = []; # protected by mutators ??
+
+ sub set_arr { # normal mutator
+ my $val = shift;
+ die("expected array, you supplied ", ref $val)
+ unless ref $val eq 'ARRAY';
+ $some_array_ref = $val;
+ }
+ sub set_arr_lv : lvalue { # lvalue mutator
+ $some_array_ref;
+ }
+
+ # set_arr_lv cannot stop this !
+ set_arr_lv() = { a => 1 };
+
+=back
+
=head2 Passing Symbol Table Entries (typeglobs)
B<WARNING>: The mechanism described in this section was originally
sub ioqueue {
local (*READER, *WRITER); # not my!
- pipe (READER, WRITER); or die "pipe: $!";
+ pipe (READER, WRITER) or die "pipe: $!";
return (*READER, *WRITER);
}
($head, $tail) = ioqueue();
as part of C<@_> will be a reference to the actual argument given
in the subroutine call, obtained by applying C<\> to that argument.
+You can also backslash several argument types simultaneously by using
+the C<\[]> notation:
+
+ sub myref (\[$@%&*])
+
+will allow calling myref() as
+
+ myref $var
+ myref @array
+ myref %hash
+ myref &sub
+ myref *glob
+
+and the first argument of myref() will be a reference to
+a scalar, an array, a hash, a code, or a glob.
+
Unbackslashed prototype characters have special meanings. Any
unbackslashed C<@> or C<%> eats all remaining arguments, and forces
list context. An argument represented by C<$> forces scalar context. An
module, nor make it harder to read. The line noise is visually
encapsulated into a small pill that's easy to swallow.
+If you try to use an alphanumeric sequence in a prototype you will
+generate an optional warning - "Illegal character in prototype...".
+Unfortunately earlier versions of Perl allowed the prototype to be
+used as long as its prefix was a valid prototype. The warning may be
+upgraded to a fatal error in a future version of Perl once the
+majority of offending code is fixed.
+
It's probably best to prototype new functions, not retrofit prototyping
into older ones. That's because you must be especially careful about
silent impositions of differing list versus scalar contexts. For example,
C<glob>, study the implementation of C<File::DosGlob> in the standard
library.
+When you override a built-in, your replacement should be consistent (if
+possible) with the built-in native syntax. You can achieve this by using
+a suitable prototype. To get the prototype of an overridable built-in,
+use the C<prototype> function with an argument of C<"CORE::builtin_name">
+(see L<perlfunc/prototype>).
+
+Note however that some built-ins can't have their syntax expressed by a
+prototype (such as C<system> or C<chomp>). If you override them you won't
+be able to fully mimic their original syntax.
+
+The built-ins C<do>, C<require> and C<glob> can also be overridden, but due
+to special magic, their original syntax is preserved, and you don't have
+to define a prototype for their replacements. (You can't override the
+C<do BLOCK> syntax, though).
+
+C<require> has special additional dark magic: if you invoke your
+C<require> replacement as C<require Foo::Bar>, it will actually receive
+the argument C<"Foo/Bar.pm"> in @_. See L<perlfunc/require>.
+
+And, as you'll have noticed from the previous example, if you override
+C<glob>, the C<E<lt>*E<gt>> glob operator is overridden as well.
+
+In a similar fashion, overriding the C<readline> function also overrides
+the equivalent I/O operator C<< <FILEHANDLE> >>.
+
+Finally, some built-ins (e.g. C<exists> or C<grep>) can't be overridden.
+
=head2 Autoloading
If you call a subroutine that is undefined, you would ordinarily
use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';
For further details on attribute lists and their manipulation,
-see L<attributes>.
+see L<attributes> and L<Attribute::Handlers>.
=head1 SEE ALSO
projects / p5sagit/p5-mst-13.2.git / blobdiff