Object::Pad::SlotAttr::Isa - apply class type constraints to Object::Pad slots


   use Object::Pad;
   use Object::Pad::SlotAttr::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");


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

WARNING The ability for Object::Pad to take third-party slot 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.



   has $slot :Isa(CLASSNAME) ...;

Declares that any value assigned to the slot 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 slot value will not be modified.

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


Paul Evans <>