feature(3pm) Perl Programmers Reference Guide feature(3pm)
NAME
feature - Perl pragma to enable new features
SYNOPSIS
use feature qw(say switch);
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
use v5.10; # implicitly loads :5.10 feature bundle
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. (Nevertheless,
the "CORE::" prefix provides access to all Perl keywords, regardless of
this pragma.)
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 reset to the default
group. To disable all features (an unusual request!) use "no feature
':all'".
AVAILABLE FEATURES
The 'say' feature
"use feature 'say'" tells the compiler to enable the Perl 6 style "say"
function.
See "say" in perlfunc for details.
This feature is available starting with Perl 5.10.
The 'state' feature
"use feature 'state'" tells the compiler to enable "state" variables.
See "Persistent Private Variables" in perlsub for details.
This feature is available starting with Perl 5.10.
The 'switch' feature
"use feature 'switch'" tells the compiler to enable the Perl 6
given/when construct.
See "Switch Statements" in perlsyn for details.
This feature is available starting with Perl 5.10.
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 feature is available starting with Perl 5.12; was almost fully
implemented in Perl 5.14; and extended in Perl 5.16 to cover
"quotemeta".
The 'unicode_eval' and 'evalbytes' features
Under the "unicode_eval" feature, Perl's "eval" function, when passed a
string, will evaluate it as a string of characters, ignoring any "use
utf8" declarations. "use utf8" exists to declare the encoding of the
script, which only makes sense for a stream of bytes, not a string of
characters. Source filters are forbidden, as they also really only
make sense on strings of bytes. Any attempt to activate a source
filter will result in an error.
The "evalbytes" feature enables the "evalbytes" keyword, which
evaluates the argument passed to it as a string of bytes. It dies if
the string contains any characters outside the 8-bit range. Source
filters work within "evalbytes": they apply to the contents of the
string being evaluated.
Together, these two features are intended to replace the historical
"eval" function, which has (at least) two bugs in it, that cannot
easily be fixed without breaking existing programs:
o "eval" behaves differently depending on the internal encoding of
the string, sometimes treating its argument as a string of bytes,
and sometimes as a string of characters.
o Source filters activated within "eval" leak out into whichever file
scope is currently being compiled. To give an example with the
CPAN module Semi::Semicolons:
BEGIN { eval "use Semi::Semicolons; # not filtered here " }
# filtered here!
"evalbytes" fixes that to work the way one would expect:
use feature "evalbytes";
BEGIN { evalbytes "use Semi::Semicolons; # filtered " }
# not filtered
These two features are available starting with Perl 5.16.
The 'current_sub' feature
This provides the "__SUB__" token that returns a reference to the
current subroutine or "undef" outside of a subroutine.
This feature is available starting with Perl 5.16.
The 'array_base' feature
This feature supports the legacy $[ variable. See "$[" in perlvar and
arybase. It is on by default but disabled under "use v5.16" (see
"IMPLICIT LOADING", below).
This feature is available under this name starting with Perl 5.16. In
previous versions, it was simply on all the time, and this pragma knew
nothing about it.
The 'fc' feature
"use feature 'fc'" tells the compiler to enable the "fc" function,
which implements Unicode casefolding.
See "fc" in perlfunc for details.
This feature is available from Perl 5.16 onwards.
FEATURE BUNDLES
It's possible to load multiple features together, using a feature
bundle. The name of a feature bundle is prefixed with a colon, to
distinguish it from an actual feature.
use feature ":5.10";
The following feature bundles are available:
bundle features included
--------- -----------------
:default array_base
:5.10 say state switch array_base
:5.12 say state switch unicode_strings array_base
:5.14 say state switch unicode_strings array_base
:5.16 say state switch unicode_strings
unicode_eval evalbytes current_sub fc
The ":default" bundle represents the feature set that is enabled before
any "use feature" or "no feature" declaration.
Specifying sub-versions such as the 0 in 5.14.0 in feature bundles has
no effect. Feature bundles are guaranteed to be the same for all sub-
versions.
use feature ":5.14.0"; # same as ":5.14"
use feature ":5.14.1"; # same as ":5.14"
IMPLICIT LOADING
Instead of loading feature bundles by name, it is easier to let Perl do
implicit loading of a feature bundle for you.
There are two ways to load the "feature" pragma implicitly:
o By using the "-E" switch on the Perl command-line instead of "-e".
That will enable the feature bundle for that version of Perl in the
main compilation unit (that is, the one-liner that follows "-E").
o By explicitly requiring a minimum Perl version number for your
program, with the "use VERSION" construct. That is,
use v5.10.0;
will do an implicit
no feature ':all';
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.
If the required version is older than Perl 5.10, the ":default"
feature bundle is automatically loaded instead.
perl v5.16.3 2013-03-04 feature(3pm)
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