path: root/lib/fields.pm

package fields;

=head1 NAME

fields - compile-time class fields

=head1 SYNOPSIS

    {
        package Foo;
        use fields qw(foo bar _Foo_private);
	sub new {
	    my Foo $self = shift;
	    unless (ref $self) {
		$self = fields::new($self);
		$self->{_Foo_private} = "this is Foo's secret";
	    }
	    $self->{foo} = 10;
	    $self->{bar} = 20;
	    return $self;
	}
    }

    my $var = Foo->new;
    $var->{foo} = 42;

    # this will generate an error
    $var->{zap} = 42;

    # subclassing
    {
        package Bar;
        use base 'Foo';
        use fields qw(baz _Bar_private);	# not shared with Foo
	sub new {
	    my $class = shift;
	    my $self = fields::new($class);
	    $self->SUPER::new();		# init base fields
	    $self->{baz} = 10;			# init own fields
	    $self->{_Bar_private} = "this is Bar's secret";
	    return $self;
	}
    }

=head1 DESCRIPTION

The C<fields> pragma enables compile-time verified class fields.

NOTE: The current implementation keeps the declared fields in the %FIELDS
hash of the calling package, but this may change in future versions.
Do B<not> update the %FIELDS hash directly, because it must be created
at compile-time for it to be fully useful, as is done by this pragma.

The related C<base> pragma will combine fields from base classes and any
fields declared using the C<fields> pragma.  This enables field
inheritance to work properly.

Field names that start with an underscore character are made private to
the class and are not visible to subclasses.  Inherited fields can be
overridden but will generate a warning if used together with the C<-w>
switch.

The following functions are supported:

=over 8

=item new

fields::new() creates and blesses a restricted-hash comprised of the
fields declared using the C<fields> pragma into the specified class.
This makes it possible to write a constructor like this:

    package Critter::Sounds;
    use fields qw(cat dog bird);

    sub new {
	my $self = shift;
	$self = fields::new($self) unless ref $self;
	$self->{cat} = 'meow';				# scalar element
	@$self{'dog','bird'} = ('bark','tweet');	# slice
	return $self;
    }

=item phash

Pseudo-hashes have been removed from Perl as of 5.10.  Consider using
restricted hashes instead.  Using fields::phash() will cause an error.

=back

=head1 SEE ALSO

L<base>,

=cut

use 5.006_001;
use strict;
no strict 'refs';
use warnings::register;
our(%attr, $VERSION);

$VERSION = "1.02";

use Hash::Util qw(lock_keys);

# some constants
sub _PUBLIC    () { 1 }
sub _PRIVATE   () { 2 }

# The %attr hash holds the attributes of the currently assigned fields
# per class.  The hash is indexed by class names and the hash value is
# an array reference.  The first element in the array is the lowest field
# number not belonging to a base class.  The remaining elements' indices
# are the field numbers.  The values are integer bit masks, or undef
# in the case of base class private fields (which occupy a slot but are
# otherwise irrelevant to the class).

sub import {
    my $class = shift;
    return unless @_;
    my $package = caller(0);
    # avoid possible typo warnings
    %{"$package\::FIELDS"} = () unless %{"$package\::FIELDS"};
    my $fields = \%{"$package\::FIELDS"};
    my $fattr = ($attr{$package} ||= [1]);
    my $next = @$fattr;

    if ($next > $fattr->[0]
	and ($fields->{$_[0]} || 0) >= $fattr->[0])
    {
	# There are already fields not belonging to base classes.
	# Looks like a possible module reload...
	$next = $fattr->[0];
    }
    foreach my $f (@_) {
	my $fno = $fields->{$f};

	# Allow the module to be reloaded so long as field positions
	# have not changed.
	if ($fno and $fno != $next) {
	    require Carp;
            if ($fno < $fattr->[0]) {
                warnings::warnif("Hides field '$f' in base class") ;
            } else {
                Carp::croak("Field name '$f' already in use");
            }
	}
	$fields->{$f} = $next;
        $fattr->[$next] = ($f =~ /^_/) ? _PRIVATE : _PUBLIC;
	$next += 1;
    }
    if (@$fattr > $next) {
	# Well, we gave them the benefit of the doubt by guessing the
	# module was reloaded, but they appear to be declaring fields
	# in more than one place.  We can't be sure (without some extra
	# bookkeeping) that the rest of the fields will be declared or
	# have the same positions, so punt.
	require Carp;
	Carp::croak ("Reloaded module must declare all fields at once");
    }
}

sub inherit  { # called by base.pm when $base_fields is nonempty
    my($derived, $base) = @_;
    my $base_attr = $attr{$base};
    my $derived_attr = $attr{$derived} ||= [];
    # avoid possible typo warnings
    %{"$base\::FIELDS"} = () unless %{"$base\::FIELDS"};
    %{"$derived\::FIELDS"} = () unless %{"$derived\::FIELDS"};
    my $base_fields    = \%{"$base\::FIELDS"};
    my $derived_fields = \%{"$derived\::FIELDS"};

    $derived_attr->[0] = $base_attr ? scalar(@$base_attr) : 1;
    while (my($k,$v) = each %$base_fields) {
	my($fno);
	if ($fno = $derived_fields->{$k} and $fno != $v) {
	    require Carp;
	    Carp::croak ("Inherited %FIELDS can't override existing %FIELDS");
	}
	if ($base_attr->[$v] & _PRIVATE) {
	    $derived_attr->[$v] = undef;
	} else {
	    $derived_attr->[$v] = $base_attr->[$v];
	    $derived_fields->{$k} = $v;
	}
     }
}

sub _dump  # sometimes useful for debugging
{
    for my $pkg (sort keys %attr) {
	print "\n$pkg";
	if (@{"$pkg\::ISA"}) {
	    print " (", join(", ", @{"$pkg\::ISA"}), ")";
	}
	print "\n";
	my $fields = \%{"$pkg\::FIELDS"};
	for my $f (sort {$fields->{$a} <=> $fields->{$b}} keys %$fields) {
	    my $no = $fields->{$f};
	    print "   $no: $f";
	    my $fattr = $attr{$pkg}[$no];
	    if (defined $fattr) {
		my @a;
		push(@a, "public")    if $fattr & _PUBLIC;
		push(@a, "private")   if $fattr & _PRIVATE;
		push(@a, "inherited") if $no < $attr{$pkg}[0];
		print "\t(", join(", ", @a), ")";
	    }
	    print "\n";
	}
    }
}

sub new {
    my $class = shift;
    $class = ref $class if ref $class;
    my $self = bless {}, $class;
    lock_keys(%$self, keys %{$class.'::FIELDS'});
    return $self;
}

sub phash {
    die "Pseudo-hashes have been removed from Perl";
}

1;