[MLton-devel] phantom types and directed graphs

[Stephen Weeks]

> Sounds good, but don't all users of phantom types have to be polymorphic?
> That means that they have to be let-bound.  It could be that that is the case
> any way.

I don't quite understand your question, but the way I envision Graph
being used is like this:

------------------------------------------------------------
signature DIRECTED_GRAPH = 
   sig
      structure Node: 
	 sig
	    type 'a t
	 end
      
      type 'a t

      val new: unit -> 'a t
      val newNode: 'a t -> 'a Node.t
      val nodes: 'a t -> 'a Node.t list
      val transpose: 'a t -> 'b t * {destroy: unit -> unit,
				     newNode: 'a Node.t -> 'b Node.t}
   end

functor F (G: DIRECTED_GRAPH) =
   struct
      datatype t = T
      val g: t G.t = G.new ()
      val n = G.newNode g
      datatype t = T
      val (g': t G.t, {destroy, newNode}) = G.transpose g
      val _ = newNode n  (* notice that newNode (newNode n) fails *)
   end
------------------------------------------------------------

So, you see that whenever I want to distinguish between graph types, I
create new types and constrain the graphs.


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