/ 
perl-5.15.2
River stage five • 11849 direct dependents • 33721 total dependents
/ feature

NAME

feature - Perl pragma to enable new features

SYNOPSIS

    use feature qw(say switch);
    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, or new semantic meanings to older constructs, can be enabled by use feature 'foo', and will be parsed only when the appropriate feature pragma is in scope. (Nevertheless, the CORE:: prefix provides access to all Perl keywords, regardless of this pragma.)

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";

no feature

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 '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.

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 'unicode_strings' feature

use feature 'unicode_strings' tells the compiler to use Unicode semantics in all string operations executed within its scope (unless they are also within the scope of either use locale or use bytes). The same applies to all regular expressions compiled within the scope, even if executed outside it.

no feature 'unicode_strings' tells the compiler to use the traditional Perl semantics wherein the native character set semantics is used unless it is clear to Perl that Unicode is desired. This can lead to some surprises when the behavior suddenly changes. (See "The "Unicode Bug"" in perlunicode for details.) For this reason, if you are potentially using Unicode in your program, the use feature 'unicode_strings' subpragma is strongly recommended.

This subpragma is available starting with Perl 5.11.3, but was not fully implemented until 5.13.8.

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 correspond to Perl releases, e.g. use feature ":5.10" which is equivalent to use feature qw(switch say state).

By convention, the feature bundle for any given Perl release includes the features of previous releases, down to and including 5.10, the first official release to provide this facility. Since Perl 5.12 only provides one new feature, unicode_strings, and Perl 5.14 provides none, use feature ":5.14" is equivalent to use feature qw(switch say state unicode_strings).

Specifying sub-versions such as the 0 in 5.14.0 in feature bundles has no effect: feature bundles are guaranteed to be the same for all sub-versions.

Note that instead of using release-based feature bundles it is usually better, and shorter, to use implicit loading as described below.

IMPLICIT LOADING

There are two ways to load the feature pragma implicitly :

  • 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.)

  • 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';

    and so on. Note how the trailing sub-version is automatically stripped from the version.

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

    use 5.010;

    with the same effect.