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ActivePerl 5.14 Documentation

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feature - Perl pragma to enable new features


NAME

feature - Perl pragma to enable new 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, 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.

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 'switch' feature

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

See Switch statements in the perlsyn manpage for details.

The 'say' feature

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

See say in the perlfunc manpage for details.

the 'state' feature

use feature 'state' tells the compiler to enable state variables.

See Persistent Private Variables in the perlsub manpage 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 the perlunicode manpage 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 bundle is use feature ":5.10" which is equivalent to use feature qw(switch say state).

Specifying sub-versions such as the 0 in 5.10.0 in feature bundles has no effect: feature bundles are guaranteed to be the same for all sub-versions.


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 the perlfunc manpage), you may prefer:

        use 5.010;
    

    with the same effect.