feature(3pm) Perl Programmers Reference Guide feature(3pm)NAMEfeature - 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 featureqw(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 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.
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
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 perlfunc), you may
prefer:
use 5.010;
with the same effect.
perl v5.14.4 2012-12-19 feature(3pm)
