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Date: -- (:)
From: Matthew O'Connor <angagon@e...>
Subject: Object typing
Hello all, I'm in the process of writing a cellular agent-based immune
system simulator.  It uses objects as the agents/entities.  I have a
hierarchy of different entity types (ie, entity, cell, bacteria, virus,
antigen, etc.).  I want to be able to determine, at run time, if a given
entity is of a given type.

My current solution is to use functors to instantiate a weak set for all
the types I care about and then have that type's initializer add itself
to the set.  Then given an object it is simple to determine if it is a
member of that set.  This solution works, but it seems to be rather
slow.  I want to speed this process up.

Here is a version of what I would like to be able to do, but can't or
don't know how.  I would like to use polymorphic variants to enable
easier extensibility of the hierarchy.

(* test.ml *)
class type ientity =
object
  method is_a : [> ] -> bool
end

class entity : ientity =
object
  method is_a v =
    if v = `Entity then true
    else false
end

class predator =
object
  inherit entity as super
  method is_a v =
    if v = `Predator then true
    else super#is_a v
end

let ent = new predator;;
ent#is_a `Predator;; (* returns true *)
ent#is_a `Entity;; (* returns true *)
ent#is_a `Chicken;; (* returns false *)

(* ocaml test.ml *)
File "test.ml", line 7, characters 0-72:
The class type object method is_a : [> `Entity ] -> bool end
is not matched by the class type ientity
The class type object method is_a : [> `Entity ] -> bool end
is not matched by the class type object method is_a : [>  ] -> bool end
The method is_a has type [> `Entity ] -> bool but is expected to have type
  'a. ([>  ] as 'a) -> bool

I'm at a loss as how to proceed.  Any other suggestions or ideas or
better solutions would be greatly appreciated.  Thank you.


Matt