feature - Perl pragma to enable new features
- 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
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.)
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 reset to the default group. To
disable all features (an unusual request!) use no feature ':all'
.
use feature 'say'
tells the compiler to enable the Perl 6 style
say function.
See say for details.
This feature is available starting with Perl 5.10.
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.
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.
use feature 'unicode_strings'
tells the compiler to use Unicode rules
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. It does not change the internal representation of strings, but only how
they are interpreted.
no feature 'unicode_strings'
tells the compiler to use the traditional
Perl rules wherein the native character set rules 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.
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:
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.
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:
evalbytes fixes that to work the way one would expect:
These two features are available starting with Perl 5.16.
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.
This feature supports the legacy $[
variable. See $[ in perlvar and
arybase. It is on by default but disabled under use v5.16
(see
IMPLICIT LOADING, below).
This feature is available under this name starting with Perl 5.16. In previous versions, it was simply on all the time, and this pragma knew nothing about it.
use feature 'fc'
tells the compiler to enable the fc function,
which implements Unicode casefolding.
See fc for details.
This feature is available from Perl 5.16 onwards.
WARNING: This feature is still experimental and the implementation may change in future versions of Perl. For this reason, Perl will warn when you use the feature, unless you have explicitly disabled the warning:
- no warnings "experimental::lexical_subs";
This enables declaration of subroutines via my sub foo
, state sub foo
and our sub foo
syntax. See Lexical Subroutines in perlsub for details.
This feature is available from Perl 5.18 onwards.
WARNING: This feature is still experimental and the implementation may change in future versions of Perl. For this reason, Perl will warn when you use the feature, unless you have explicitly disabled the warning:
- no warnings "experimental::signatures";
This enables unpacking of subroutine arguments into lexical variables by syntax such as
- sub foo ($left, $right) {
- return $left + $right;
- }
See Signatures in perlsub for details.
This feature is available from Perl 5.20 onwards.
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
- :5.18 say state switch unicode_strings
- unicode_eval evalbytes current_sub fc
- :5.20 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.
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:
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
).
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
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.
If the required version is older than Perl 5.10, the ":default" feature bundle is automatically loaded instead.