chris.danx wrote:

> type childNode = LeafNode of sgLeaf | IntNode of sgInternal
> and
> class sgLeaf
   .... etc. ...

> I've not programmed in O'Caml for a while and am not yet fully 
> comfortable with ocamls object system anyway, so I may be being silly. 
> Can anyone shed some light on a solution to this?

This seems to me like a good case for class types. How about something like:

class type leafType =
   object
     method draw : unit -> unit
   end

class type ['a] internalType =
   object
     method addChild : 'a -> unit
     method getChildren : unit -> 'a list
     method draw : unit -> unit
   end

type childNode =
     LeafNode of leafType
   | IntNode of childNode internalType

Then, of course, you'll need to define concrete classes that fit the 
class type signatures and/or coerce objects to those types.

I'm not sure this is the most elegant solution, but it is at least 
syntactically valid, and seems to do what you want.

--
Matt Gushee
Englewood, CO, USA