NAME

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

SYNOPSIS

          use Object::Pad;
          use Object::Pad::FieldAttr::Isa;

          class ListNode {
             has $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
          has $field :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>