- NAME
- SYNOPSIS
- DESCRIPTION
-
AVAILABLE FEATURES
- The 'say' feature
- The 'state' feature
- The 'switch' feature
- The 'unicode_strings' feature
- The 'unicode_eval' and 'evalbytes' features
- The 'current_sub' feature
- The 'array_base' feature
- The 'fc' feature
- The 'lexical_subs' feature
- The 'postderef' and 'postderef_qq' features
- The 'signatures' feature
- The 'refaliasing' feature
- The 'bitwise' feature
- FEATURE BUNDLES
- IMPLICIT LOADING
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 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
WARNING: Because the smartmatch operator is experimental, Perl will warn when you use this feature, unless you have explicitly disabled the warning:
no warnings "experimental::smartmatch";
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 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
.
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:
-
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: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 for details.
This feature is available from Perl 5.16 onwards.
The 'lexical_subs' feature
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.
The 'postderef' and 'postderef_qq' features
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::postderef";
The 'postderef' feature allows the use of postfix dereference syntax. For example, it will make the following two statements equivalent:
my @x = @{ $h->{a} }; my @x = $h->{a}->@*;
The 'postderef_qq' feature extends this, for array and scalar dereference, to working inside of double-quotish interpolations.
This feature is available from Perl 5.20 onwards.
The 'signatures' feature
WARNING: This feature is still experimental and the implementation may change in future versions of Perl. For this reason, Perl will warn when you use the feature, unless you have explicitly disabled the warning:
no warnings "experimental::signatures";
This enables unpacking of subroutine arguments into lexical variables by syntax such as
sub foo ($left, $right) { return $left + $right; }
See Signatures in perlsub for details.
This feature is available from Perl 5.20 onwards.
The 'refaliasing' feature
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::refaliasing";
This enables aliasing via assignment to references:
\$a = \$b; # $a and $b now point to the same scalar \@a = \@b; # to the same array \%a = \%b; \&a = \&b; foreach \%hash (@array_of_hash_refs) { ... }
See Assigning to References in perlref for details.
This feature is available from Perl 5.22 onwards.
The 'bitwise' feature
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::bitwise";
This makes the four standard bitwise operators (& | ^ ~
) treat their operands consistently as numbers, and introduces four new dotted operators (&. |. ^. ~.
) that treat their operands consistently as strings. The same applies to the assignment variants (&= |= ^= &.= |.= ^.=
).
See Bitwise String Operators in perlop for details.
This feature is available from Perl 5.22 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 :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 :5.22 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:
-
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
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), 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.
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