[MLton] Mlton style question concerning modules

[Jesper Louis Andersen]

The style guide specifies that signatures should be independent of the
implementation. I liked it so much I chose to use it for my own project
this time. But - I ran into a problem which I sketch below:

A graph is represented by both a (triangular) bit matrix and a 
array/list structure. I have:

signature GRAPH_ARRAY_STRUCTS =
sig

  structure Vertex: VERTEX

end

signature GRAPH_ARRAY =
sig

  type t

  (* ... various uses of Vertex.t *)

end

GRAPH_MATRIX_STRUCTS and GRAPH_MATRIX are similiar.

I drive these 2 structures in parallel, using a joining
Graph structure to query the representation giving the
best running time:

signature GRAPH_STRUCTS = 
sig

  structure Vertex: VERTEX

end

signature GRAPH = 
sig

  type t 


  (* ... various uses of Vertex.t *)

end

Now, I have:

functor GraphArray (S: GRAPH_ARRAY_STRUCTS):> GRAPH_ARRAY = ...
functor GraphMatrix (S: GRAPH_MATRIX_STRUCTS):> GRAPH_MATRIX = ...

and 

functor Graph(S: GRAPH_STRUCTS):> GRAPH =
struct

open S

structure GraphArray = GraphArray(structure Vertex = Vertex)
structure GraphMatrix = GraphMatrix(structure Vertex = Vertex)

(* ... *)

end

but this obviously doesn't work since the Array and Matrix 
representations need to have a sharing constraint so the
Vertex structure is shared amongst them. I do not want to
put them into the GRAPH_STRUCTS signature, since they are
local to the Graph structure. 

How is this accomplished in a neat way? The best thing I
can think of now is something along the lines of:

structure Graphs:
  sig
      structure GraphArray:> GRAPH_ARRAY
      structrue GraphMatrix:> GRAPH_MATRIX
	sharing GraphArray.Vertex = GraphMatrix.Vertex
  end =
  struct
      structure GraphArray = GraphArray(structure Vertex = Vertex)
      strucutre GraphMatrix = GrapmMatrix(strucutre Vertex = Vertex)
  end

and then put this inside the Graph structure and refer to these
now identifiers. But I would like to ask what the MLton style is.

-- 
j.