feature - Perl pragma to enable new features
- 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
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.
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.
no feature
Features can also be turned off by using no feature "foo"
. This too
has lexical effect.
no feature
with no features specified will turn off all features.
use feature 'switch'
tells the compiler to enable the Perl 6
given/when construct.
See Switch statements in perlsyn for details.
use feature 'say'
tells the compiler to enable the Perl 6
say function.
See say for details.
use feature 'state'
tells the compiler to enable state
variables.
See Persistent Private Variables in perlsub for details.
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.
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.
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), you may prefer:
- use 5.010;
with the same effect.