NAME

    Object::Pad::FieldAttr::Isa - apply class type constraints to
    Object::Pad fields

SYNOPSIS

       use Object::Pad;
       use Object::Pad::FieldAttr::Isa;
    
       class ListNode {
          field $next :param :reader :writer :Isa(ListNode) = undef;
       }
    
       my $first = ListNode->new();
       my $second = ListNode->new(next => $first);
    
       # This will fail
       my $third = ListNode->new(next => "something else");
    
       # This will fail
       $second->set_next("another thing");

DESCRIPTION

    This module provides a third-party field attribute for
    Object::Pad-based classes, which declares that values assigned to the
    field must conform to a given object type.

    WARNING The ability for Object::Pad to take third-party field
    attributes is still new and highly experimental, and subject to much
    API change in future. As a result, this module should be considered
    equally experimental.

FIELD ATTRIBUTES

 :Isa

       field $name :Isa(CLASSNAME) ...;

    Declares that any value assigned to the field must be an object
    reference, and must be derived from the named class. Attempts to assign
    a non-conforming value, such as a non-reference, or reference to a
    class not derived from that named, will throw an exception, and the
    field value will not be modified.

    This type constraint is applied whenever the field itself is assigned
    to, whether that is from :param initialisation, invoking a :writer or
    :mutator accessor, or direct assignment into the field variable by
    method code within the class.

AUTHOR

    Paul Evans <leonerd@leonerd.org.uk>

