(* Memoized promises *)

module PromiseMemoized = struct

  type 'a promise = unit -> 'a

  let force promise = promise () 

  let makePromise thunk = 
    let memo = ref None in 
      fun () -> 
	match ! memo with 
	  Some v -> v
	| None -> 
	    let result = thunk () in
	    let _ = memo := Some result in 
	    result

end