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 featurewith no features specified will turn off all features.
The 'switch' feature
use feature switchtells the compiler to enable the Perl 6 given/when construct.
See Switch statements in perlsyn for details.
The 'say' feature
use feature saytells the compiler to enable the Perl 6
sayfunction.
See say in perlfunc for details.
the 'state' feature
use feature statetells the compiler to enable
statevariables.
See Persistent Private Variables in perlsub for details.
FEATURE BUNDLES
Its 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
featurepragma implicitly :
o |
By using the -Eswitch 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 VERSIONconstruct, 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. |