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Date: Thu, 10 Aug 2000 13:13:06 -0400
From: Norman Ramsey <nr@eecs.harvard.edu>
Message-Id: <200008101713.NAA18938@labrador.eecs.harvard.edu>
To: caml-list@inria.fr
Subject: help wanted with recursive types across module boundaries
Cc: avi@labrador.eecs.harvard.edu, nr@labrador.eecs.harvard.edu
Sender: weis@pauillac.inria.fr

I'm having trouble breaking a recursion that crosses module
boundaries.

I have a representation of probability distributions.
Ordinarily I would use a monad, declaring type 'a dist, with unit,
bind, and other operations, but I want (among other implementations)
an efficient implementations in terms of sets, so polymorphism is not
indicated.  I therefore have

  module type Dist = sig
    type v (* values *)
    type dist (* probability distribution over values *)
    val unit : v -> dist   
    val bind : dist -> (v -> dist) -> dist
      ... 
  end

Of the values in which I am interested, one kind of value is a
function from values to probability distributions over values.

  module Value = struct
    type 'vdist v
      = Int of int
      | Record of (name * 'vdist v) list
      | Function of ('vdist v -> 'vdist)
  end

Now I want to close the loop.  If I were writing Standard ML, I would
attempt the following:

  module type ValueDist = sig
    include Dist
    type v' = V of dist Value.v
    sharing type v = v'
  end

(I enclose the equivalent Standard ML code below, which compiles.)

I want to write a functor that takes ValueDist as an argument.
How can I do this in OCaml?


Norman

(* type-correct standard ml code follows *)

  signature Dist = sig
    type v (* values *)
    type dist (* probability distribution over values *)
    val unit : v -> dist   
    val bind : dist -> (v -> dist) -> dist
  end

  structure Value = struct
    datatype 'vdist v
      = Int of int
      | Record of (string * 'vdist v) list
      | Function of ('vdist v -> 'vdist)
  end

  signature ValueDist = sig
    include Dist
    datatype v' = V of dist Value.v
    sharing type v = v'
  end