[MLton-devel] phantom types and directed graphs

[Stephen Weeks]

I was thinking of using phantom types to improve the static checking
of directed graphs.  The idea is to parameterize the graph type with a
phantom parameter that will let us give different graphs different
types.  This will help detect errors in confusing nodes from different
graphs.  Each graph transformation will allow a new type of graph to
be produced.

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

      val new: unit -> unit 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

To use it, whenever you want a new graph type, you write down a
datatype and constrain the result of the graph operation.

Any thoughts?  Better ways to do it?


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