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

       Paul Evans <leonerd@leonerd.org.uk>

perl v5.40.0                                       2024-10-20                   Object::Pad::FieldAttr::Isa(3pm)

       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.

: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.

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

          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");