How to associate an iterator to a collection in OCaml

Question

I have these two classes in OCaml

class type ['a] collection =
  object
    method add : 'a -> unit
    method clear : unit -> unit
    method iterator : unit -> 'a iterator
    method remove : 'a -> unit
  end

class type ['a] iterator =
  object 
    method hasNext : unit -> bool 
    method next : unit -> 'a 
  end

And I need to create two concrete classes ['a] queue subtype of collection and ['a] iterator_queue a subtype of iterator.

I want mainly to know how to define the method iterator : unit -> 'a iterator because I don't see how the two types can be connected, Does the ['a] iterator_queue has to be inherited from both the abstract ones? or should I proceed differently.

Answer 1

Probably the easiest way to do this is to define the iterator as an object within the scope of the definition of the queue (in Java, this would be called an "inner class"). For example:

class ['a] queue : ['a] collection = 
  object
    val q = ref []

    (* definitions of add, clear, remove *)

    method iterator () : 'a iterator =
      object
        val lst = ref !q

        (* definitions of hasNext and next *)

      end
  end

Note that lst is a reference to the (immutable) value of q at the time that iterator is called. Subsequent changes to the queue won't be reflected in the iterator.

Answer 2

I suspect this might simply be a test of mutually recursive class definitions.

class ['a] queue = 
  object
    inherit 'a container
    method iterator = new iterator_queue args
    ...
  end
and ['a] iterator_queue args = 
  object
    ...
  end