[MLton-devel] phantom types and directed graphs

[Stephen Weeks]

> Right, and my point is that in that world, any function that wants to deal
> with a directed graph MUST be polymorphic since it could never be monomorphic
> and handle a type involving the phantom type in a node.  This means that those
> functions have to be let-bound.

I don't agree completely.  If you want a function that deals with an
arbitrary graph, e.g. transpose, then yes, it is polymorphic.  But if
you can also have a function that is monomorphic and deals with a
specific graph type.  See sameSize and addNode in the following.

functor F (G: DIRECTED_GRAPH) =
   struct

      datatype t = T
      val g: t G.t = G.new ()
      val n = G.newNode g

      datatype u = U
      val (g': u G.t, {destroy, newNode}) = G.transpose g
      val _ = newNode n

      fun sameSize (g: 'a G.t, g': 'b G.t): bool =
	 length (G.nodes g) = length (G.nodes g')

      fun addNode (g: t G.t): unit = (G.newNode g; ())
   end


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