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 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.