Not only does the C<&> form make the argument list optional, it also
disables any prototype checking on arguments you do provide. This
is partly for historical reasons, and partly for having a convenient way
-to cheat if you know what you're doing. See L<Prototypes> below.
+to cheat if you know what you're doing. See L</Prototypes> below.
X<&>
Subroutines whose names are in all upper case are reserved to the Perl
the scope of $answer extends from its declaration through the rest
of that conditional, including any C<elsif> and C<else> clauses,
-but not beyond it. See L<perlsyn/"Simple statements"> for information
+but not beyond it. See L<perlsyn/"Simple Statements"> for information
on the scope of variables in statements with modifiers.
The C<foreach> loop defaults to scoping its index variable dynamically
=head3 Persistent variables via state()
-Beginning with perl 5.9.4, you can declare variables with the C<state>
-keyword in place of C<my>. For that to work, though, you must have
+Beginning with Perl 5.9.4, you can declare variables with the C<state>
+keyword in place of C<my>. For that to work, though, you must have
enabled that feature beforehand, either by using the C<feature> pragma, or
-by using C<-E> on one-liners. (see L<feature>)
+by using C<-E> on one-liners (see L<feature>). Beginning with Perl 5.16,
+you can also write it as C<CORE::state>, which does not require the
+C<feature> pragma.
For example, the following code maintains a private counter, incremented
each time the gimme_another() function is called:
a local variable. This is known as dynamic scoping. Lexical scoping
is done with C<my>, which works more like C's auto declarations.
-Some types of lvalues can be localized as well : hash and array elements
+Some types of lvalues can be localized as well: hash and array elements
and slices, conditionals (provided that their result is always
localizable), and symbolic references. As for simple variables, this
creates new, dynamically scoped values.
local $1 = 2;
-Similarly, but in a way more difficult to spot, the following snippet will
-die in perl 5.9.0 :
-
- sub f { local $_ = "foo"; print }
- for ($1) {
- # now $_ is aliased to $1, thus is magic and readonly
- f();
- }
-
-See next section for an alternative to this situation.
+One exception is the default scalar variable: starting with perl 5.14
+C<local($_)> will always strip all magic from $_, to make it possible
+to safely reuse $_ in a subroutine.
B<WARNING>: Localization of tied arrays and hashes does not currently
work as described.
will not have any effect on the internal value of the input record
separator.
-Notably, if you want to work with a brand new value of the default scalar
-$_, and avoid the potential problem listed above about $_ previously
-carrying a magic value, you should use C<local *_> instead of C<local $_>.
-As of perl 5.9.1, you can also use the lexical form of C<$_> (declaring it
-with C<my $_>), which avoids completely this problem.
-
=head3 Localization of elements of composite types
X<local, composite type element> X<local, array element> X<local, hash element>
my $val;
sub canmod : lvalue {
- # return $val; this doesn't work, don't say "return"
- $val;
+ $val; # or: return $val;
}
sub nomod {
$val;
=item Lvalue subroutines are EXPERIMENTAL
-They appear to be convenient, but there are several reasons to be
+They appear to be convenient, but there is at least one reason to be
circumspect.
-You can't use the return keyword, you must pass out the value before
-falling out of subroutine scope. (see comment in example above). This
-is usually not a problem, but it disallows an explicit return out of a
-deeply nested loop, which is sometimes a nice way out.
-
They violate encapsulation. A normal mutator can check the supplied
argument before setting the attribute it is protecting, an lvalue
subroutine never gets that chance. Consider;
a local alias.
{
- local *grow = \&shrink; # only until this block exists
+ local *grow = \&shrink; # only until this block exits
grow(); # really calls shrink()
move(); # if move() grow()s, it shrink()s too
}
Any backslashed prototype character represents an actual argument
that must start with that character (optionally preceded by C<my>,
-C<our> or C<local>), with the exception of C<$>, which will accept a
-hash or array element even without a dollar sign, such as
+C<our> or C<local>), with the exception of C<$>, which will
+accept any scalar lvalue expression, such as C<$foo = 7> or
C<< my_function()->[0] >>. The value passed as part of C<@_> will be a
reference to the actual argument given in the subroutine call,
obtained by applying C<\> to that argument.
mytime +2;
you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
-without a prototype.
+without a prototype. If you want to force a unary function to have the
+same precedence as a list operator, add C<;> to the end of the prototype:
+
+ sub mygetprotobynumber($;);
+ mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)
The interesting thing about C<&> is that you can generate new syntax with it,
provided it's in the initial position:
saying C<CORE::open()> always refers to the built-in C<open()>, even
if the current package has imported some other subroutine called
C<&open()> from elsewhere. Even though it looks like a regular
-function call, it isn't: you can't take a reference to it, such as
-the incorrect C<\&CORE::open> might appear to produce.
+function call, it isn't: the CORE:: prefix in that case is part of Perl's
+syntax, and works for any keyword, regardless of what is in the CORE
+package. Taking a reference to it, that is, C<\&CORE::open>, only works
+for some keywords. See L<CORE>.
Library modules should not in general export built-in names like C<open>
or C<chdir> as part of their default C<@EXPORT> list, because these may
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.
+See L<perlootut> to learn how to make object method calls.