[p5sagit/p5-mst-13.2.git] / 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 Foo $var = Foo::->new;
    $var->{foo} = 42;

    # this will generate a compile-time 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.

If a typed lexical variable holding a reference is used to access a
hash element and a package with the same name as the type has declared
class fields using this pragma, then the operation is turned into an
array access at compile time.

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 effect of all this is that you can have objects with named fields
which are as compact and as fast arrays to access.  This only works
as long as the objects are accessed through properly typed variables.
If the objects are not typed, access is only checked at run time.

The following functions are supported:

=over 8

=item new

fields::new() creates and blesses a pseudo-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 Critter::Sounds $self = shift;
	$self = fields::new($self) unless ref $self;
	$self->{cat} = 'meow';				# scalar element
	@$self{'dog','bird'} = ('bark','tweet');	# slice
	return $self;
    }

=item phash

fields::phash() can be used to create and initialize a plain (unblessed)
pseudo-hash.  This function should always be used instead of creating
pseudo-hashes directly.

If the first argument is a reference to an array, the pseudo-hash will
be created with keys from that array.  If a second argument is supplied,
it must also be a reference to an array whose elements will be used as
the values.  If the second array contains less elements than the first,
the trailing elements of the pseudo-hash will not be initialized.
This makes it particularly useful for creating a pseudo-hash from
subroutine arguments:

    sub dogtag {
	my $tag = fields::phash([qw(name rank ser_num)], [@_]);
    }

fields::phash() also accepts a list of key-value pairs that will
be used to construct the pseudo hash.  Examples:

    my $tag = fields::phash(name => "Joe",
			    rank => "captain",
			    ser_num => 42);

    my $pseudohash = fields::phash(%args);

=back

=head1 SEE ALSO

L<base>,
L<perlref/Pseudo-hashes: Using an array as a hash>

=cut

use 5.005_64;
use strict;
no strict 'refs';
use warnings::register;
our(%attr, $VERSION);

$VERSION = "1.01";

# 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 < $fattr->[0]) {
                warnings::warn("Hides field '$f' in base class") 
		    if warnings::enabled();
            } 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 $class;
    return bless [\%{$class . "::FIELDS"}], $class;
}

sub phash {
    my $h;
    my $v;
    if (@_) {
	if (ref $_[0] eq 'ARRAY') {
	    my $a = shift;
	    @$h{@$a} = 1 .. @$a;
	    if (@_) {
		$v = shift;
		unless (! @_ and ref $v eq 'ARRAY') {
		    require Carp;
		    Carp::croak ("Expected at most two array refs\n");
		}
	    }
	}
	else {
	    if (@_ % 2) {
		require Carp;
		Carp::croak ("Odd number of elements initializing pseudo-hash\n");
	    }
	    my $i = 0;
	    @$h{grep ++$i % 2, @_} = 1 .. @_ / 2;
	    $i = 0;
	    $v = [grep $i++ % 2, @_];
	}
    }
    else {
	$h = {};
	$v = [];
    }
    [ $h, @$v ];
}

1;
Commit	Line	Data
458fb581	1	package fields;
458fb581	2
d516a115	3	=head1 NAME
	4
	5	fields - compile-time class fields
	6
	7	=head1 SYNOPSIS
	8
	9	{
	10	package Foo;
479ba383	11	use fields qw(foo bar _Foo_private);
	12	sub new {
	13	my Foo $self = shift;
	14	unless (ref $self) {
	15	$self = fields::new($self);
	16	$self->{_Foo_private} = "this is Foo's secret";
	17	}
	18	$self->{foo} = 10;
	19	$self->{bar} = 20;
	20	return $self;
	21	}
d516a115	22	}
479ba383	23
479ba383	24	my Foo $var = Foo::->new;
d516a115	25	$var->{foo} = 42;
d516a115	26
479ba383	27	# this will generate a compile-time error
d516a115	28	$var->{zap} = 42;
d516a115	29
479ba383	30	# subclassing
f1192cee	31	{
	32	package Bar;
	33	use base 'Foo';
479ba383	34	use fields qw(baz _Bar_private); # not shared with Foo
	35	sub new {
	36	my $class = shift;
	37	my $self = fields::new($class);
	38	$self->SUPER::new(); # init base fields
	39	$self->{baz} = 10; # init own fields
	40	$self->{_Bar_private} = "this is Bar's secret";
	41	return $self;
	42	}
f1192cee	43	}
f1192cee	44
d516a115	45	=head1 DESCRIPTION
d516a115	46
479ba383	47	The C<fields> pragma enables compile-time verified class fields.
	48
	49	NOTE: The current implementation keeps the declared fields in the %FIELDS
	50	hash of the calling package, but this may change in future versions.
	51	Do B<not> update the %FIELDS hash directly, because it must be created
	52	at compile-time for it to be fully useful, as is done by this pragma.
f1192cee	53
f1192cee	54	If a typed lexical variable holding a reference is used to access a
479ba383	55	hash element and a package with the same name as the type has declared
	56	class fields using this pragma, then the operation is turned into an
	57	array access at compile time.
	58
	59	The related C<base> pragma will combine fields from base classes and any
33e06c89	60	fields declared using the C<fields> pragma. This enables field
479ba383	61	inheritance to work properly.
	62
	63	Field names that start with an underscore character are made private to
	64	the class and are not visible to subclasses. Inherited fields can be
51301382	65	overridden but will generate a warning if used together with the C<-w>
51301382	66	switch.
f1192cee	67
f1192cee	68	The effect of all this is that you can have objects with named fields
51301382	69	which are as compact and as fast arrays to access. This only works
f1192cee	70	as long as the objects are accessed through properly typed variables.
479ba383	71	If the objects are not typed, access is only checked at run time.
	72
	73	The following functions are supported:
	74
	75	=over 8
	76
	77	=item new
f1192cee	78
479ba383	79	fields::new() creates and blesses a pseudo-hash comprised of the fields
	80	declared using the C<fields> pragma into the specified class.
	81	This makes it possible to write a constructor like this:
	82
	83	package Critter::Sounds;
	84	use fields qw(cat dog bird);
	85
	86	sub new {
	87	my Critter::Sounds $self = shift;
33e06c89	88	$self = fields::new($self) unless ref $self;
479ba383	89	$self->{cat} = 'meow'; # scalar element
33e06c89	90	@$self{'dog','bird'} = ('bark','tweet'); # slice
479ba383	91	return $self;
	92	}
	93
	94	=item phash
	95
	96	fields::phash() can be used to create and initialize a plain (unblessed)
	97	pseudo-hash. This function should always be used instead of creating
	98	pseudo-hashes directly.
	99
	100	If the first argument is a reference to an array, the pseudo-hash will
	101	be created with keys from that array. If a second argument is supplied,
	102	it must also be a reference to an array whose elements will be used as
	103	the values. If the second array contains less elements than the first,
	104	the trailing elements of the pseudo-hash will not be initialized.
	105	This makes it particularly useful for creating a pseudo-hash from
	106	subroutine arguments:
	107
	108	sub dogtag {
	109	my $tag = fields::phash([qw(name rank ser_num)], [@_]);
	110	}
f1192cee	111
479ba383	112	fields::phash() also accepts a list of key-value pairs that will
	113	be used to construct the pseudo hash. Examples:
	114
	115	my $tag = fields::phash(name => "Joe",
	116	rank => "captain",
	117	ser_num => 42);
	118
	119	my $pseudohash = fields::phash(%args);
	120
	121	=back
f1192cee	122
	123	=head1 SEE ALSO
	124
	125	L<base>,
31a572f1	126	L<perlref/Pseudo-hashes: Using an array as a hash>
d516a115	127
	128	=cut
	129
17f410f9	130	use 5.005_64;
f1192cee	131	use strict;
f1192cee	132	no strict 'refs';
9f1b1f2d	133	use warnings::register;
17f410f9	134	our(%attr, $VERSION);
f1192cee	135
f30a1143	136	$VERSION = "1.01";
f1192cee	137
	138	# some constants
	139	sub _PUBLIC () { 1 }
	140	sub _PRIVATE () { 2 }
f1192cee	141
	142	# The %attr hash holds the attributes of the currently assigned fields
	143	# per class. The hash is indexed by class names and the hash value is
f30a1143	144	# an array reference. The first element in the array is the lowest field
	145	# number not belonging to a base class. The remaining elements' indices
	146	# are the field numbers. The values are integer bit masks, or undef
	147	# in the case of base class private fields (which occupy a slot but are
	148	# otherwise irrelevant to the class).
f1192cee	149
458fb581	150	sub import {
458fb581	151	my $class = shift;
f30a1143	152	return unless @_;
f1192cee	153	my $package = caller(0);
479ba383	154	# avoid possible typo warnings
479ba383	155	%{"$package\::FIELDS"} = () unless %{"$package\::FIELDS"};
458fb581	156	my $fields = \%{"$package\::FIELDS"};
f30a1143	157	my $fattr = ($attr{$package} \|\|= [1]);
f30a1143	158	my $next = @$fattr;
f1192cee	159
f30a1143	160	if ($next > $fattr->[0]
	161	and ($fields->{$_[0]} \|\| 0) >= $fattr->[0])
	162	{
	163	# There are already fields not belonging to base classes.
	164	# Looks like a possible module reload...
	165	$next = $fattr->[0];
	166	}
458fb581	167	foreach my $f (@_) {
f30a1143	168	my $fno = $fields->{$f};
	169
	170	# Allow the module to be reloaded so long as field positions
	171	# have not changed.
	172	if ($fno and $fno != $next) {
458fb581	173	require Carp;
f30a1143	174	if ($fno < $fattr->[0]) {
9f1b1f2d	175	warnings::warn("Hides field '$f' in base class")
9f1b1f2d	176	if warnings::enabled();
f1192cee	177	} else {
	178	Carp::croak("Field name '$f' already in use");
	179	}
458fb581	180	}
f30a1143	181	$fields->{$f} = $next;
	182	$fattr->[$next] = ($f =~ /^_/) ? _PRIVATE : _PUBLIC;
	183	$next += 1;
	184	}
	185	if (@$fattr > $next) {
	186	# Well, we gave them the benefit of the doubt by guessing the
	187	# module was reloaded, but they appear to be declaring fields
	188	# in more than one place. We can't be sure (without some extra
	189	# bookkeeping) that the rest of the fields will be declared or
	190	# have the same positions, so punt.
	191	require Carp;
	192	Carp::croak ("Reloaded module must declare all fields at once");
458fb581	193	}
f1192cee	194	}
f1192cee	195
479ba383	196	sub inherit { # called by base.pm when $base_fields is nonempty
f1192cee	197	my($derived, $base) = @_;
f30a1143	198	my $base_attr = $attr{$base};
f30a1143	199	my $derived_attr = $attr{$derived} \|\|= [];
479ba383	200	# avoid possible typo warnings
	201	%{"$base\::FIELDS"} = () unless %{"$base\::FIELDS"};
	202	%{"$derived\::FIELDS"} = () unless %{"$derived\::FIELDS"};
f30a1143	203	my $base_fields = \%{"$base\::FIELDS"};
	204	my $derived_fields = \%{"$derived\::FIELDS"};
	205
	206	$derived_attr->[0] = $base_attr ? scalar(@$base_attr) : 1;
	207	while (my($k,$v) = each %$base_fields) {
	208	my($fno);
	209	if ($fno = $derived_fields->{$k} and $fno != $v) {
	210	require Carp;
	211	Carp::croak ("Inherited %FIELDS can't override existing %FIELDS");
	212	}
	213	if ($base_attr->[$v] & _PRIVATE) {
	214	$derived_attr->[$v] = undef;
	215	} else {
	216	$derived_attr->[$v] = $base_attr->[$v];
	217	$derived_fields->{$k} = $v;
	218	}
	219	}
f1192cee	220	}
	221
	222	sub _dump # sometimes useful for debugging
	223	{
479ba383	224	for my $pkg (sort keys %attr) {
	225	print "\n$pkg";
	226	if (@{"$pkg\::ISA"}) {
	227	print " (", join(", ", @{"$pkg\::ISA"}), ")";
	228	}
	229	print "\n";
	230	my $fields = \%{"$pkg\::FIELDS"};
	231	for my $f (sort {$fields->{$a} <=> $fields->{$b}} keys %$fields) {
	232	my $no = $fields->{$f};
	233	print " $no: $f";
	234	my $fattr = $attr{$pkg}[$no];
	235	if (defined $fattr) {
	236	my @a;
	237	push(@a, "public") if $fattr & _PUBLIC;
	238	push(@a, "private") if $fattr & _PRIVATE;
	239	push(@a, "inherited") if $no < $attr{$pkg}[0];
	240	print "\t(", join(", ", @a), ")";
	241	}
	242	print "\n";
	243	}
	244	}
	245	}
	246
	247	sub new {
	248	my $class = shift;
	249	$class = ref $class if ref $class;
	250	return bless [\%{$class . "::FIELDS"}], $class;
	251	}
	252
	253	sub phash {
	254	my $h;
	255	my $v;
	256	if (@_) {
	257	if (ref $_[0] eq 'ARRAY') {
	258	my $a = shift;
	259	@$h{@$a} = 1 .. @$a;
	260	if (@_) {
	261	$v = shift;
	262	unless (! @_ and ref $v eq 'ARRAY') {
	263	require Carp;
	264	Carp::croak ("Expected at most two array refs\n");
	265	}
	266	}
	267	}
	268	else {
	269	if (@_ % 2) {
	270	require Carp;
	271	Carp::croak ("Odd number of elements initializing pseudo-hash\n");
	272	}
	273	my $i = 0;
	274	@$h{grep ++$i % 2, @_} = 1 .. @_ / 2;
	275	$i = 0;
	276	$v = [grep $i++ % 2, @_];
	277	}
	278	}
	279	else {
	280	$h = {};
	281	$v = [];
	282	}
	283	[ $h, @$v ];
458fb581	284	}
	285
	286	1;