path: root/lib/feature.pm

package feature;

our $VERSION = '1.19';

# (feature name) => (internal name, used in %^H)
my %feature = (
    switch          => 'feature_switch',
    say             => "feature_say",
    state           => "feature_state",
    unicode_strings => "feature_unicode",
);

# This gets set (for now) in $^H as well as in %^H,
# for runtime speed of the uc/lc/ucfirst/lcfirst functions.
# See HINT_UNI_8_BIT in perl.h.
our $hint_uni8bit = 0x00000800;

# NB. the latest bundle must be loaded by the -E switch (see toke.c)

my %feature_bundle = (
    "5.10" => [qw(switch say state)],
    "5.11" => [qw(switch say state unicode_strings)],
    "5.12" => [qw(switch say state unicode_strings)],
    "5.13" => [qw(switch say state unicode_strings)],
);

# special case
$feature_bundle{"5.9.5"} = $feature_bundle{"5.10"};

# TODO:
# - think about versioned features (use feature switch => 2)

=head1 NAME

feature - Perl pragma to enable new 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, or new semantic meanings to older
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 'state' feature

C<use feature 'state'> tells the compiler to enable C<state>
variables.

See L<perlsub/"Persistent Private Variables"> for details.

=head2 the 'unicode_strings' feature

C<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 C<use locale> or C<use bytes>).  The same applies
to all regular expressions compiled within the scope, even if executed outside
it.

C<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
L<perlunicode/The "Unicode Bug"> for details.)  For this reason, if you are
potentially using Unicode in your program, the
C<use feature 'unicode_strings'> subpragma is B<strongly> recommended.

This subpragma is available starting with Perl 5.11.3, but was not fully
implemented until 5.13.8.

=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 bundle is C<use feature ":5.10"> which is equivalent
to C<use feature qw(switch say state)>.

Specifying sub-versions such as the C<0> in C<5.10.0> in feature bundles has
no effect: feature bundles are guaranteed to be the same for all sub-versions.

=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
5.10.0. 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 L<perlfunc/use>), you may prefer:

    use 5.010;

with the same effect.

=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}) {
		$v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
		if (!exists $feature_bundle{$v}) {
		    unknown_feature_bundle(substr($name, 1));
		}
	    }
	    unshift @_, @{$feature_bundle{$v}};
	    next;
	}
	if (!exists $feature{$name}) {
	    unknown_feature($name);
	}
	$^H{$feature{$name}} = 1;
        $^H |= $hint_uni8bit if $name eq 'unicode_strings';
    }
}

sub unimport {
    my $class = shift;

    # A bare C<no feature> should disable *all* features
    if (!@_) {
	delete @^H{ values(%feature) };
        $^H &= ~ $hint_uni8bit;
	return;
    }

    while (@_) {
	my $name = shift;
	if (substr($name, 0, 1) eq ":") {
	    my $v = substr($name, 1);
	    if (!exists $feature_bundle{$v}) {
		$v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
		if (!exists $feature_bundle{$v}) {
		    unknown_feature_bundle(substr($name, 1));
		}
	    }
	    unshift @_, @{$feature_bundle{$v}};
	    next;
	}
	if (!exists($feature{$name})) {
	    unknown_feature($name);
	}
	else {
	    delete $^H{$feature{$name}};
            $^H &= ~ $hint_uni8bit if $name eq 'unicode_strings';
	}
    }
}

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;