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package feature;
our $VERSION = '1.10';
# (feature name) => (internal name, used in %^H)
my %feature = (
switch => 'feature_switch',
say => "feature_say",
err => "feature_err",
state => "feature_state",
);
my %feature_bundle = (
"5.10.0" => [qw(switch say err state)],
);
# latest version here
# keep it harcoded until we actually bump the version number to 5.10
$feature_bundle{"5.10"} = $feature_bundle{"5.10.0"};
#$feature_bundle{"5.10"} = $feature_bundle{sprintf("%vd",$^V)};
$feature_bundle{"5.9.5"} = $feature_bundle{"5.10.0"};
# TODO:
# - think about versioned features (use feature switch => 2)
=head1 NAME
feature - Perl pragma to enable new syntactic features
=head1 SYNOPSIS
use feature qw(switch say);
given ($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
=head1 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 can be enabled by C<use feature 'foo'>,
and will be parsed only when the appropriate feature pragma is in
scope.
=head2 Lexical effect
Like other pragmas (C<use strict>, for example), features have a lexical
effect. C<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";
=head2 C<no feature>
Features can also be turned off by using C<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.";
C<no feature> with no features specified will turn off all features.
=head2 The 'switch' feature
C<use feature 'switch'> tells the compiler to enable the Perl 6
given/when construct.
See L<perlsyn/"Switch statements"> for details.
=head2 The 'say' feature
C<use feature 'say'> tells the compiler to enable the Perl 6
C<say> function.
See L<perlfunc/say> for details.
=head2 the 'err' feature
C<use feature 'err'> tells the compiler to enable the C<err>
operator.
C<err> is a low-precedence variant of the C<//> operator:
see C<perlop> for details.
=head2 the 'state' feature
C<use feature 'state'> tells the compiler to enable C<state>
variables.
See L<perlsub/"Persistent Private Variables"> for details.
=head1 FEATURE BUNDLES
It's possible to load a whole slew of features in one go, using
a I<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 bundles are C<use feature ":5.10"> and C<use feature ":5.10.0">,
which both are equivalent to C<use feature qw(switch say err state)>.
In the forthcoming 5.10.X perl releases, C<use feature ":5.10"> will be
equivalent to the latest C<use feature ":5.10.X">.
=head1 IMPLICIT LOADING
There are two ways to load the C<feature> pragma implicitly :
=over 4
=item *
By using the C<-E> switch on the command-line instead of C<-e>. It enables
all available features in the main compilation unit (that is, the one-liner.)
=item *
By requiring explicitly a minimal Perl version number for your program, with
the C<use VERSION> construct, and when the version is higher than or equal to
5.9.5. That is,
use 5.9.5;
will do an implicit
use feature ':5.9.5';
and so on.
=back
=cut
sub import {
my $class = shift;
if (@_ == 0) {
croak("No features specified");
}
while (@_) {
my $name = shift(@_);
if (substr($name, 0, 1) eq ":") {
my $v = substr($name, 1);
if (!exists $feature_bundle{$v}) {
unknown_feature_bundle($v);
}
unshift @_, @{$feature_bundle{$v}};
next;
}
if (!exists $feature{$name}) {
unknown_feature($name);
}
$^H{$feature{$name}} = 1;
}
}
sub unimport {
my $class = shift;
# A bare C<no feature> should disable *all* features
if (!@_) {
delete @^H{ values(%feature) };
return;
}
while (@_) {
my $name = shift;
if (substr($name, 0, 1) eq ":") {
my $v = substr($name, 1);
if (!exists $feature_bundle{$v}) {
unknown_feature_bundle($v);
}
unshift @_, @{$feature_bundle{$v}};
next;
}
if (!exists($feature{$name})) {
unknown_feature($name);
}
else {
delete $^H{$feature{$name}};
}
}
}
sub unknown_feature {
my $feature = shift;
croak(sprintf('Feature "%s" is not supported by Perl %vd',
$feature, $^V));
}
sub unknown_feature_bundle {
my $feature = shift;
croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
$feature, $^V));
}
sub croak {
require Carp;
Carp::croak(@_);
}
1;
|
