NAME


feature - Perl pragma to enable new syntactic features

SYNOPSIS



    use feature qw(switch say);
    given ($foo) {
        when (1)          { say "\$foo == 1" }
        when ([2,3])      { say "\$foo == 2 || \$foo == 3" }
        when (/^a[bc]d$/) { say "\$foo eq abd || \$foo eq acd" }
        when ($_ > 100)   { say "\$foo > 100" }
        default           { say "None of the above" }
    }

    use feature :5.10; # loads all features available in perl 5.10

DESCRIPTION


It is usually impossible to add new syntax to Perl without breaking some existing programs. This pragma provides a way to minimize that risk. New syntactic constructs can be enabled by

use feature foo
, and will be parsed only when the appropriate feature pragma is in scope.

Lexical effect


Like other pragmas (

use strict
, for example), features have a lexical effect.
use feature qw(foo)
will only make the feature foo available from that point to the end of the enclosing block.


    {
        use feature say;
        say "say is available here";
    }
    print "But not here.\n";

\f(CWno feature\fP


Features can also be turned off by using

no feature "foo"
. This too has lexical effect.


    use feature say;
    say "say is available here";
    {
        no feature say;
        print "But not here.\n";
    }
    say "Yet it is here.";

no feature
with no features specified will turn off all features.

The 'switch' feature


use feature switch
tells the compiler to enable the Perl 6 given/when construct.

See Switch statements in perlsyn for details.

The 'say' feature


use feature say
tells the compiler to enable the Perl 6
say
function.

See say in perlfunc for details.

the 'state' feature


use feature state
tells the compiler to enable
state
variables.

See Persistent Private Variables in perlsub for details.

FEATURE BUNDLES


It’s possible to load a whole slew of features in one go, using a feature bundle. The name of a feature bundle is prefixed with a colon, to distinguish it from an actual feature. At present, the only feature bundles are

use feature ":5.10"
and
use feature ":5.10.0"
, which both are equivalent to
use feature qw(switch say state)
.

In the forthcoming 5.10.X perl releases,

use feature ":5.10"
will be equivalent to the latest
use feature ":5.10.X"
.

IMPLICIT LOADING


There are two ways to load the

feature
pragma implicitly :
o By using the
-E
switch on the command-line instead of
-e
. It enables all available features in the main compilation unit (that is, the one-liner.)
o By requiring explicitly a minimal Perl version number for your program, with the
use VERSION
construct, and when the version is higher than or equal to 5.10.0. That is,


    use 5.10.0;

will do an implicit


    use feature :5.10.0;

and so on.

But to avoid portability warnings (see use in perlfunc), you may prefer:


    use 5.010;

with the same effect.

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