initialize will return the metaclass if it's there, no need to call meta

[gitmo/Moose.git] / lib / Moose / Meta / Attribute.pm

package Moose::Meta::Attribute;

use strict;
use warnings;

use Scalar::Util 'blessed', 'weaken';
use overload     ();

our $VERSION   = '0.73';
our $AUTHORITY = 'cpan:STEVAN';

use Moose::Meta::Method::Accessor;
use Moose::Meta::Method::Delegation;
use Moose::Util ();
use Moose::Util::TypeConstraints ();

use base 'Class::MOP::Attribute';

# options which are not directly used
# but we store them for metadata purposes
__PACKAGE__->meta->add_attribute('isa'  => (reader    => '_isa_metadata'));
__PACKAGE__->meta->add_attribute('does' => (reader    => '_does_metadata'));
__PACKAGE__->meta->add_attribute('is'   => (reader    => '_is_metadata'));

# these are actual options for the attrs
__PACKAGE__->meta->add_attribute('required'   => (reader => 'is_required'      ));
__PACKAGE__->meta->add_attribute('lazy'       => (reader => 'is_lazy'          ));
__PACKAGE__->meta->add_attribute('lazy_build' => (reader => 'is_lazy_build'    ));
__PACKAGE__->meta->add_attribute('coerce'     => (reader => 'should_coerce'    ));
__PACKAGE__->meta->add_attribute('weak_ref'   => (reader => 'is_weak_ref'      ));
__PACKAGE__->meta->add_attribute('auto_deref' => (reader => 'should_auto_deref'));
__PACKAGE__->meta->add_attribute('type_constraint' => (
    reader    => 'type_constraint',
    predicate => 'has_type_constraint',
));
__PACKAGE__->meta->add_attribute('trigger' => (
    reader    => 'trigger',
    predicate => 'has_trigger',
));
__PACKAGE__->meta->add_attribute('handles' => (
    reader    => 'handles',
    predicate => 'has_handles',
));
__PACKAGE__->meta->add_attribute('documentation' => (
    reader    => 'documentation',
    predicate => 'has_documentation',
));
__PACKAGE__->meta->add_attribute('traits' => (
    reader    => 'applied_traits',
    predicate => 'has_applied_traits',
));

# we need to have a ->does method in here to 
# more easily support traits, and the introspection 
# of those traits. We extend the does check to look
# for metatrait aliases.
sub does {
    my ($self, $role_name) = @_;
    my $name = eval {
        Moose::Util::resolve_metatrait_alias(Attribute => $role_name)
    };
    return 0 if !defined($name); # failed to load class
    return $self->Moose::Object::does($name);
}

sub throw_error {
    my $self = shift;
    my $class = ( ref $self && $self->associated_class ) || "Moose::Meta::Class";
    unshift @_, "message" if @_ % 2 == 1;
    unshift @_, attr => $self if ref $self;
    unshift @_, $class;
    my $handler = $class->can("throw_error"); # to avoid incrementing depth by 1
    goto $handler;
}

sub new {
    my ($class, $name, %options) = @_;
    $class->_process_options($name, \%options) unless $options{__hack_no_process_options}; # used from clone()... YECHKKK FIXME ICKY YUCK GROSS
    return $class->SUPER::new($name, %options);
}

sub interpolate_class_and_new {
    my ($class, $name, @args) = @_;

    my ( $new_class, @traits ) = $class->interpolate_class(@args);
    
    $new_class->new($name, @args, ( scalar(@traits) ? ( traits => \@traits ) : () ) );
}

sub interpolate_class {
    my ($class, %options) = @_;

    $class = ref($class) || $class;

    if ( my $metaclass_name = delete $options{metaclass} ) {
        my $new_class = Moose::Util::resolve_metaclass_alias( Attribute => $metaclass_name );
        
        if ( $class ne $new_class ) {
            if ( $new_class->can("interpolate_class") ) {
                return $new_class->interpolate_class(%options);
            } else {
                $class = $new_class;
            }
        }
    }

    my @traits;

    if (my $traits = $options{traits}) {
        my $i = 0;
        while ($i < @$traits) {
            my $trait = $traits->[$i++];
            next if ref($trait); # options to a trait we discarded

            $trait = Moose::Util::resolve_metatrait_alias(Attribute => $trait)
                  || $trait;

            next if $class->does($trait);

            push @traits, $trait;

            # are there options?
            push @traits, $traits->[$i++]
                if $traits->[$i] && ref($traits->[$i]);
        }

        if (@traits) {
            my $anon_class = Moose::Meta::Class->create_anon_class(
                superclasses => [ $class ],
                roles        => [ @traits ],
                cache        => 1,
            );

            $class = $anon_class->name;
        }
    }

    return ( wantarray ? ( $class, @traits ) : $class );
}

# ...

my @legal_options_for_inheritance = qw(
    default coerce required 
    documentation lazy handles 
    builder type_constraint
    definition_context
    lazy_build
);

sub legal_options_for_inheritance { @legal_options_for_inheritance }

# NOTE/TODO
# This method *must* be able to handle 
# Class::MOP::Attribute instances as 
# well. Yes, I know that is wrong, but 
# apparently we didn't realize it was 
# doing that and now we have some code 
# which is dependent on it. The real 
# solution of course is to push this 
# feature back up into Class::MOP::Attribute
# but I not right now, I am too lazy.
# However if you are reading this and 
# looking for something to do,.. please 
# be my guest.
# - stevan
sub clone_and_inherit_options {
    my ($self, %options) = @_;
    
    my %copy = %options;
    
    my %actual_options;
    
    # NOTE:
    # we may want to extends a Class::MOP::Attribute
    # in which case we need to be able to use the 
    # core set of legal options that have always 
    # been here. But we allows Moose::Meta::Attribute
    # instances to changes them.
    # - SL
    my @legal_options = $self->can('legal_options_for_inheritance')
        ? $self->legal_options_for_inheritance
        : @legal_options_for_inheritance;
    
    foreach my $legal_option (@legal_options) {
        if (exists $options{$legal_option}) {
            $actual_options{$legal_option} = $options{$legal_option};
            delete $options{$legal_option};
        }
    }    

    if ($options{isa}) {
        my $type_constraint;
        if (blessed($options{isa}) && $options{isa}->isa('Moose::Meta::TypeConstraint')) {
            $type_constraint = $options{isa};
        }
        else {
            $type_constraint = Moose::Util::TypeConstraints::find_or_create_isa_type_constraint($options{isa});
            (defined $type_constraint)
                || $self->throw_error("Could not find the type constraint '" . $options{isa} . "'", data => $options{isa});
        }

        $actual_options{type_constraint} = $type_constraint;
        delete $options{isa};
    }
    
    if ($options{does}) {
        my $type_constraint;
        if (blessed($options{does}) && $options{does}->isa('Moose::Meta::TypeConstraint')) {
            $type_constraint = $options{does};
        }
        else {
            $type_constraint = Moose::Util::TypeConstraints::find_or_create_does_type_constraint($options{does});
            (defined $type_constraint)
                || $self->throw_error("Could not find the type constraint '" . $options{does} . "'", data => $options{does});
        }

        $actual_options{type_constraint} = $type_constraint;
        delete $options{does};
    }    

    # NOTE:
    # this doesn't apply to Class::MOP::Attributes, 
    # so we can ignore it for them.
    # - SL
    if ($self->can('interpolate_class')) {
        ( $actual_options{metaclass}, my @traits ) = $self->interpolate_class(%options);

        my %seen;
        my @all_traits = grep { $seen{$_}++ } @{ $self->applied_traits || [] }, @traits;
        $actual_options{traits} = \@all_traits if @all_traits;

        delete @options{qw(metaclass traits)};
    }

    (scalar keys %options == 0)
        || $self->throw_error("Illegal inherited options => (" . (join ', ' => keys %options) . ")", data => \%options);


    $self->clone(%actual_options);
}

sub clone {
    my ( $self, %params ) = @_;

    my $class = $params{metaclass} || ref $self;

    my ( @init, @non_init );

    foreach my $attr ( grep { $_->has_value($self) } Class::MOP::class_of($self)->compute_all_applicable_attributes ) {
        push @{ $attr->has_init_arg ? \@init : \@non_init }, $attr;
    }

    my %new_params = ( ( map { $_->init_arg => $_->get_value($self) } @init ), %params );

    my $name = delete $new_params{name};

    my $clone = $class->new($name, %new_params, __hack_no_process_options => 1 );

    foreach my $attr ( @non_init ) {
        $attr->set_value($clone, $attr->get_value($self));
    }

    return $clone;
}

sub _process_options {
    my ($class, $name, $options) = @_;

    if (exists $options->{is}) {

        ### -------------------------
        ## is => ro, writer => _foo    # turns into (reader => foo, writer => _foo) as before
        ## is => rw, writer => _foo    # turns into (reader => foo, writer => _foo)
        ## is => rw, accessor => _foo  # turns into (accessor => _foo)
        ## is => ro, accessor => _foo  # error, accesor is rw
        ### -------------------------
        
        if ($options->{is} eq 'ro') {
            $class->throw_error("Cannot define an accessor name on a read-only attribute, accessors are read/write", data => $options)
                if exists $options->{accessor};
            $options->{reader} ||= $name;
        }
        elsif ($options->{is} eq 'rw') {
            if ($options->{writer}) {
                $options->{reader} ||= $name;
            }
            else {
                $options->{accessor} ||= $name;
            }
        }
        else {
            $class->throw_error("I do not understand this option (is => " . $options->{is} . ") on attribute ($name)", data => $options->{is});
        }
    }

    if (exists $options->{isa}) {
        if (exists $options->{does}) {
            if (eval { $options->{isa}->can('does') }) {
                ($options->{isa}->does($options->{does}))
                    || $class->throw_error("Cannot have an isa option and a does option if the isa does not do the does on attribute ($name)", data => $options);
            }
            else {
                $class->throw_error("Cannot have an isa option which cannot ->does() on attribute ($name)", data => $options);
            }
        }

        # allow for anon-subtypes here ...
        if (blessed($options->{isa}) && $options->{isa}->isa('Moose::Meta::TypeConstraint')) {
            $options->{type_constraint} = $options->{isa};
        }
        else {
            $options->{type_constraint} = Moose::Util::TypeConstraints::find_or_create_isa_type_constraint($options->{isa});
        }
    }
    elsif (exists $options->{does}) {
        # allow for anon-subtypes here ...
        if (blessed($options->{does}) && $options->{does}->isa('Moose::Meta::TypeConstraint')) {
                $options->{type_constraint} = $options->{does};
        }
        else {
            $options->{type_constraint} = Moose::Util::TypeConstraints::find_or_create_does_type_constraint($options->{does});
        }
    }

    if (exists $options->{coerce} && $options->{coerce}) {
        (exists $options->{type_constraint})
            || $class->throw_error("You cannot have coercion without specifying a type constraint on attribute ($name)", data => $options);
        $class->throw_error("You cannot have a weak reference to a coerced value on attribute ($name)", data => $options)
            if $options->{weak_ref};
    }

    if (exists $options->{trigger}) {
        ('CODE' eq ref $options->{trigger})
            || $class->throw_error("Trigger must be a CODE ref on attribute ($name)", data => $options->{trigger});
    }

    if (exists $options->{auto_deref} && $options->{auto_deref}) {
        (exists $options->{type_constraint})
            || $class->throw_error("You cannot auto-dereference without specifying a type constraint on attribute ($name)", data => $options);
        ($options->{type_constraint}->is_a_type_of('ArrayRef') ||
         $options->{type_constraint}->is_a_type_of('HashRef'))
            || $class->throw_error("You cannot auto-dereference anything other than a ArrayRef or HashRef on attribute ($name)", data => $options);
    }

    if (exists $options->{lazy_build} && $options->{lazy_build} == 1) {
        $class->throw_error("You can not use lazy_build and default for the same attribute ($name)", data => $options)
            if exists $options->{default};
        $options->{lazy}      = 1;
        $options->{required}  = 1;
        $options->{builder} ||= "_build_${name}";
        if ($name =~ /^_/) {
            $options->{clearer}   ||= "_clear${name}";
            $options->{predicate} ||= "_has${name}";
        } 
        else {
            $options->{clearer}   ||= "clear_${name}";
            $options->{predicate} ||= "has_${name}";
        }
    }

    if (exists $options->{lazy} && $options->{lazy}) {
        (exists $options->{default} || defined $options->{builder} )
            || $class->throw_error("You cannot have lazy attribute ($name) without specifying a default value for it", data => $options);
    }

    if ( $options->{required} && !( ( !exists $options->{init_arg} || defined $options->{init_arg} ) || exists $options->{default} || defined $options->{builder} ) ) {
        $class->throw_error("You cannot have a required attribute ($name) without a default, builder, or an init_arg", data => $options);
    }

}

sub initialize_instance_slot {
    my ($self, $meta_instance, $instance, $params) = @_;
    my $init_arg = $self->init_arg();
    # try to fetch the init arg from the %params ...

    my $val;
    my $value_is_set;
    if ( defined($init_arg) and exists $params->{$init_arg}) {
        $val = $params->{$init_arg};
        $value_is_set = 1;    
    }
    else {
        # skip it if it's lazy
        return if $self->is_lazy;
        # and die if it's required and doesn't have a default value
        $self->throw_error("Attribute (" . $self->name . ") is required", object => $instance, data => $params)
            if $self->is_required && !$self->has_default && !$self->has_builder;

        # if nothing was in the %params, we can use the
        # attribute's default value (if it has one)
        if ($self->has_default) {
            $val = $self->default($instance);
            $value_is_set = 1;
        } 
        elsif ($self->has_builder) {
            $val = $self->_call_builder($instance);
            $value_is_set = 1;
        }
    }

    return unless $value_is_set;

    $val = $self->_coerce_and_verify( $val, $instance );

    $self->set_initial_value($instance, $val);
    $meta_instance->weaken_slot_value($instance, $self->name)
        if ref $val && $self->is_weak_ref;
}

sub _call_builder {
    my ( $self, $instance ) = @_;

    my $builder = $self->builder();

    return $instance->$builder()
        if $instance->can( $self->builder );

    $self->throw_error(  blessed($instance)
            . " does not support builder method '"
            . $self->builder
            . "' for attribute '"
            . $self->name
            . "'",
            object => $instance,
     );
}

## Slot management

# FIXME:
# this duplicates too much code from 
# Class::MOP::Attribute, we need to 
# refactor these bits eventually.
# - SL
sub _set_initial_slot_value {
    my ($self, $meta_instance, $instance, $value) = @_;

    my $slot_name = $self->name;

    return $meta_instance->set_slot_value($instance, $slot_name, $value)
        unless $self->has_initializer;

    my ($type_constraint, $can_coerce);
    if ($self->has_type_constraint) {
        $type_constraint = $self->type_constraint;
        $can_coerce      = ($self->should_coerce && $type_constraint->has_coercion);
    }

    my $callback = sub {
        my $val = $self->_coerce_and_verify( shift, $instance );;

        $meta_instance->set_slot_value($instance, $slot_name, $val);
    };
    
    my $initializer = $self->initializer;

    # most things will just want to set a value, so make it first arg
    $instance->$initializer($value, $callback, $self);
}

sub set_value {
    my ($self, $instance, @args) = @_;
    my $value = $args[0];

    my $attr_name = $self->name;

    if ($self->is_required and not @args) {
        $self->throw_error("Attribute ($attr_name) is required", object => $instance);
    }

    $value = $self->_coerce_and_verify( $value, $instance );

    my $meta_instance = Class::MOP::Class->initialize(blessed($instance))
                                         ->get_meta_instance;

    $meta_instance->set_slot_value($instance, $attr_name, $value);

    if (ref $value && $self->is_weak_ref) {
        $meta_instance->weaken_slot_value($instance, $attr_name);
    }

    if ($self->has_trigger) {
        $self->trigger->($instance, $value);
    }
}

sub get_value {
    my ($self, $instance) = @_;

    if ($self->is_lazy) {
        unless ($self->has_value($instance)) {
            my $value;
            if ($self->has_default) {
                $value = $self->default($instance);
            } elsif ( $self->has_builder ) {
                $value = $self->_call_builder($instance);
            }

            $value = $self->_coerce_and_verify( $value, $instance );

            $self->set_initial_value($instance, $value);
        }
    }

    if ($self->should_auto_deref) {

        my $type_constraint = $self->type_constraint;

        if ($type_constraint->is_a_type_of('ArrayRef')) {
            my $rv = $self->SUPER::get_value($instance);
            return unless defined $rv;
            return wantarray ? @{ $rv } : $rv;
        }
        elsif ($type_constraint->is_a_type_of('HashRef')) {
            my $rv = $self->SUPER::get_value($instance);
            return unless defined $rv;
            return wantarray ? %{ $rv } : $rv;
        }
        else {
            $self->throw_error("Can not auto de-reference the type constraint '" . $type_constraint->name . "'", object => $instance, type_constraint => $type_constraint);
        }

    }
    else {

        return $self->SUPER::get_value($instance);
    }
}

## installing accessors

sub accessor_metaclass { 'Moose::Meta::Method::Accessor' }

sub install_accessors {
    my $self = shift;
    $self->SUPER::install_accessors(@_);
    $self->install_delegation if $self->has_handles;
    return;
}

sub remove_accessors {
    my $self = shift;
    $self->SUPER::remove_accessors(@_);
    $self->remove_delegation if $self->has_handles;
    return;
}

sub install_delegation {
    my $self = shift;

    # NOTE:
    # Here we canonicalize the 'handles' option
    # this will sort out any details and always
    # return an hash of methods which we want
    # to delagate to, see that method for details
    my %handles = $self->_canonicalize_handles;


    # install the delegation ...
    my $associated_class = $self->associated_class;
    foreach my $handle (keys %handles) {
        my $method_to_call = $handles{$handle};
        my $class_name = $associated_class->name;
        my $name = "${class_name}::${handle}";

            (!$associated_class->has_method($handle))
                || $self->throw_error("You cannot overwrite a locally defined method ($handle) with a delegation", method_name => $handle);

        # NOTE:
        # handles is not allowed to delegate
        # any of these methods, as they will
        # override the ones in your class, which
        # is almost certainly not what you want.

        # FIXME warn when $handle was explicitly specified, but not if the source is a regex or something
        #cluck("Not delegating method '$handle' because it is a core method") and
        next if $class_name->isa("Moose::Object") and $handle =~ /^BUILD|DEMOLISH$/ || Moose::Object->can($handle);

        my $method = $self->_make_delegation_method($handle, $method_to_call);

        $self->associated_class->add_method($method->name, $method);
    }    
}

sub remove_delegation {
    my $self = shift;
    my %handles = $self->_canonicalize_handles;
    my $associated_class = $self->associated_class;
    foreach my $handle (keys %handles) {
        $self->associated_class->remove_method($handle);
    }
}

# private methods to help delegation ...

sub _canonicalize_handles {
    my $self    = shift;
    my $handles = $self->handles;
    if (my $handle_type = ref($handles)) {
        if ($handle_type eq 'HASH') {
            return %{$handles};
        }
        elsif ($handle_type eq 'ARRAY') {
            return map { $_ => $_ } @{$handles};
        }
        elsif ($handle_type eq 'Regexp') {
            ($self->has_type_constraint)
                || $self->throw_error("Cannot delegate methods based on a Regexp without a type constraint (isa)", data => $handles);
            return map  { ($_ => $_) }
                   grep { /$handles/ } $self->_get_delegate_method_list;
        }
        elsif ($handle_type eq 'CODE') {
            return $handles->($self, $self->_find_delegate_metaclass);
        }
        else {
            $self->throw_error("Unable to canonicalize the 'handles' option with $handles", data => $handles);
        }
    }
    else {
        Class::MOP::load_class($handles) 
            unless Class::MOP::is_class_loaded($handles);
            
        my $role_meta = Class::MOP::class_of($handles);

        (blessed $role_meta && $role_meta->isa('Moose::Meta::Role'))
            || $self->throw_error("Unable to canonicalize the 'handles' option with $handles because its metaclass is not a Moose::Meta::Role", data => $handles);
            
        return map { $_ => $_ } (
            $role_meta->get_method_list,
            $role_meta->get_required_method_list
        );
    }
}

sub _find_delegate_metaclass {
    my $self = shift;
    if (my $class = $self->_isa_metadata) {
        # we might be dealing with a non-Moose class,
        # and need to make our own metaclass. if there's
        # already a metaclass, it will be returned
        return Moose::Meta::Class->initialize($class);
    }
    elsif (my $role = $self->_does_metadata) {
        # our role will always have
        # a meta method
        return $role->meta;
    }
    else {
        $self->throw_error("Cannot find delegate metaclass for attribute " . $self->name);
    }
}

sub _get_delegate_method_list {
    my $self = shift;
    my $meta = $self->_find_delegate_metaclass;
    if ($meta->isa('Class::MOP::Class')) {
        return map  { $_->name }  # NOTE: !never! delegate &meta
               grep { $_->package_name ne 'Moose::Object' && $_->name ne 'meta' }
                    $meta->get_all_methods;
    }
    elsif ($meta->isa('Moose::Meta::Role')) {
        return $meta->get_method_list;
    }
    else {
        $self->throw_error("Unable to recognize the delegate metaclass '$meta'", data => $meta);
    }
}

sub delegation_metaclass { 'Moose::Meta::Method::Delegation' }

sub _make_delegation_method {
    my ( $self, $handle_name, $method_to_call ) = @_;

    my $method_body;

    $method_body = $method_to_call
        if 'CODE' eq ref($method_to_call);

    return $self->delegation_metaclass->new(
        name               => $handle_name,
        package_name       => $self->associated_class->name,
        attribute          => $self,
        delegate_to_method => $method_to_call,
    );
}

sub _coerce_and_verify {
    my $self     = shift;
    my $val      = shift;
    my $instance = shift;

    return $val unless $self->has_type_constraint;

    my $type_constraint = $self->type_constraint;
    if ($self->should_coerce && $type_constraint->has_coercion) {
        $val = $type_constraint->coerce($val);
    }

    $self->verify_against_type_constraint($val, instance => $instance);

    return $val;
}

sub verify_against_type_constraint {
    my $self = shift;
    my $val  = shift;

    return 1 if !$self->has_type_constraint;

    my $type_constraint = $self->type_constraint;

    $type_constraint->check($val)
        || $self->throw_error("Attribute ("
                 . $self->name
                 . ") does not pass the type constraint because: "
                 . $type_constraint->get_message($val), data => $val, @_);
}

package Moose::Meta::Attribute::Custom::Moose;
sub register_implementation { 'Moose::Meta::Attribute' }

1;

__END__

=pod

=head1 NAME

Moose::Meta::Attribute - The Moose attribute metaclass

=head1 DESCRIPTION

This class is a subclass of L<Class::MOP::Attribute> that provides
additional Moose-specific functionality.

To really understand this class, you will need to start with the
L<Class::MOP::Attribute> documentation. This class can be understood
as a set of additional features on top of the basic feature provided
by that parent class.

=head1 INHERITANCE

C<Moose::Meta::Attribute> is a subclass of L<Class::MOP::Attribute>.

=head1 METHODS

Many of the documented below override methods in
L<Class::MOP::Attribute> and add Moose specific features.

=head2 Creation

=over 4

=item B<< Moose::Meta::Attribute->new(%options) >>

This method overrides the L<Class::MOP::Attribute> constructor.

Many of the options below are described in more detail in the
L<Moose::Manual::Attributes> document.

It adds the following options to the constructor:

=over 8

=item * is => 'ro' or 'rw'

This provides a shorthand for specifying the C<reader>, C<writer>, or
C<accessor> names. If the attribute is read-only ('ro') then it will
have a C<reader> method with the same attribute as the name.

If it is read-write ('rw') then it will have an C<accessor> method
with the same name. If you provide an explicit C<writer> for a
read-write attribute, then you will have a C<reader> with the same
name as the attribute, and a C<writer> with the name you provided.

=item * isa => $type

This option accepts a type. The type can be a string, which should be
a type name. If the type name is unknown, it is assumed to be a class
name.

This option can also accept a L<Moose::Meta::TypeConstraint> object.

If you I<also> provide a C<does> option, then your C<isa> option must
be a class name, and that class must do the role specified with
C<does>.

=item * does => $role

This is short-hand for saying that the attribute's type must be an
object which does the named role.

=item * coerce => $bool

This option is only valid for objects with a type constraint
(C<isa>). If this is true, then coercions will be applied whenever
this attribute is set.

You can make both this and the C<weak_ref> option true.

=item * trigger => $sub

This option accepts a subroutine reference, which will be called after
the attribute is set.

=item * required => $bool

An attribute which is required must be provided to the constructor. An
attribute which is required can also have a C<default> or C<builder>,
which will satisfy its required-ness.

A required attribute must have a C<default>, C<builder> or a
non-C<undef> C<init_arg>

=item * lazy => $bool

A lazy attribute must have a C<default> or C<builder>. When an
attribute is lazy, the default value will not be calculated until the
attribute is read.

=item * weak_ref => $bool

If this is true, the attribute's value will be stored as a weak
reference.

=item * auto_deref => $bool

If this is true, then the reader will dereference the value when it is
called. The attribute must have a type constraint which defines the
attribute as an array or hash reference.

=item * lazy_build => $bool

Setting this to true makes the attribute lazy and provides a number of
default methods.

  has 'size' => (
      is         => 'ro',
      lazy_build => 1,
  );

is equivalent to this:

  has 'size' => (
      is        => 'ro',
      lazy      => 1,
      builder   => '_build_size',
      clearer   => 'clear_size',
      predicate => 'has_size',
  );

=item * documentation

An arbitrary string that can be retrieved later by calling C<<
$attr->documentation >>.

=back

=item B<< $attr->clone(%options) >>

This creates a new attribute based on attribute being cloned. You must
supply a C<name> option to provide a new name for the attribute.

The C<%options> can only specify options handled by
L<Class::MOP::Attribute>.

=back

=head2 Value management

=over 4

=item B<< $attr->initialize_instance_slot($meta_instance, $instance, $params) >>

This method is used internally to initialize the attribute's slot in
the object C<$instance>.

This overrides the L<Class::MOP::Attribute> method to handle lazy
attributes, weak references, and type constraints.

=item B<get_value>

=item B<set_value>

  eval { $point->meta->get_attribute('x')->set_value($point, 'forty-two') };
  if($@) {
    print "Oops: $@\n";
  }

I<Attribute (x) does not pass the type constraint (Int) with 'forty-two'>

Before setting the value, a check is made on the type constraint of
the attribute, if it has one, to see if the value passes it. If the
value fails to pass, the set operation dies with a L<throw_error>.

Any coercion to convert values is done before checking the type constraint.

To check a value against a type constraint before setting it, fetch the
attribute instance using L<Class::MOP::Class/find_attribute_by_name>,
fetch the type_constraint from the attribute using L<Moose::Meta::Attribute/type_constraint>
and call L<Moose::Meta::TypeConstraint/check>. See L<Moose::Cookbook::Basics::Recipe4>
for an example.

=back

=head2 Attribute Accessor generation

=over 4

=item B<< $attr->install_accessors >>

This method overrides the parent to also install delegation methods.

=item B<< $attr->remove_accessors >>

This method overrides the parent to also remove delegation methods.

=item B<< $attr->install_delegation >>

This method adds its delegation methods to the attribute's associated
class, if it has any to add.

=item B<< $attr->remove_delegation >>

This method remove its delegation methods from the attribute's
associated class.

=item B<< $attr->accessor_metaclass >>

Returns the accessor metaclass name, which defaults to
L<Moose::Meta::Method::Accessor>.

=item B<< $attr->delegation_metaclass >>

Returns the delegation metaclass name, which defaults to
L<Moose::Meta::Method::Delegation>.

=back

=head2 Additional Moose features

These methods are not found in the superclass. They support features
provided by Moose.

=over 4

=item B<< $attr->does($role) >>

This indicates whether the I<attribute itself> does the given
role. The role can be given as a full class name, or as a resolvable
trait name.

Note that this checks the attribute itself, not its type constraint,
so it is checking the attribute's metaclass and any traits applied to
the attribute.

=item B<< Moose::Meta::Class->interpolate_class_and_new($name, %options) >>

This is an alternate constructor that handles the C<metaclass> and
C<traits> options.

Effectively, this method is a factory that finds or creates the
appropriate class for the given C<metaclass> and/or C<traits>.

Once it has the appropriate class, it will call C<< $class->new($name,
%options) >> on that class.

=item B<< $attr->clone_and_inherit_options(%options) >>

This method supports the C<has '+foo'> feature. It does various bits
of processing on the supplied C<%options> before ultimately calling
the C<clone> method.

One of its main tasks is to make sure that the C<%options> provided
only includes the options returned by the
C<legal_options_for_inheritance> method.

=item B<< $attr->legal_options_for_inheritance >>

This returns a whitelist of options that can be overridden in a
subclass's attribute definition.

This exists to allow a custom metaclass to change or add to the list
of options which can be changed.

=item B<< $attr->type_constraint >>

Returns the L<Moose::Meta::TypeConstraint> object for this attribute,
if it has one.

=item B<< $attr->has_type_constraint >>

Returns true if this attribute has a type constraint.

=item B<< $attr->verify_against_type_constraint($value) >>

Given a value, this method returns true if the value is valid for the
attribute's type constraint. If the value is not valid, it throws an
error.

=item B<< $attr->handles >>

This returns the value of the C<handles> option passed to the
constructor.

=item B<< $attr->has_handles >>

Returns true if this attribute performs delegation.

=item B<< $attr->is_weak_ref >>

Returns true if this attribute stores its value as a weak reference.

=item B<< $attr->is_required >>

Returns true if this attribute is required to have a value.

=item B<< $attr->is_lazy >>

Returns true if this attribute is lazy.

=item B<< $attr->is_lazy_build >>

Returns true if the C<lazy_build> option was true when passed to the
constructor.

=item B<< $attr->should_coerce >>

Returns true if the C<coerce> option passed to the constructor was
true.

=item B<< $attr->should_auto_deref >>

Returns true if the C<auto_deref> option passed to the constructor was
true.

=item B<< $attr->trigger >>

This is the subroutine reference that was in the C<trigger> option
passed to the constructor, if any.

=item B<< $attr->has_trigger >>

Returns true if this attribute has a trigger set.

=item B<< $attr->documentation >>

Returns the value that was in the C<documentation> option passed to
the constructor, if any.

=item B<< $attr->has_documentation >>

Returns true if this attribute has any documentation.

=item B<< $attr->applied_traits >>

This returns an array reference of all the traits which were applied
to this attribute. If none were applied, this returns C<undef>.

=item B<< $attr->has_applied_traits >>

Returns true if this attribute has any traits applied.

=back

=head1 BUGS

All complex software has bugs lurking in it, and this module is no
exception. If you find a bug please either email me, or add the bug
to cpan-RT.

=head1 AUTHOR

Stevan Little E<lt>stevan@iinteractive.comE<gt>

Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2006-2009 by Infinity Interactive, Inc.

L<http://www.iinteractive.com>

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.

=cut