signature THUNK =
  sig
    (* A 'a thunk is a lazily
     * evaluated expression e of type
     * 'a. *)
    type 'a thunk
    (* make(fn()=>e) creates a thunk
     * for e *)
    val make : (unit->'a) -> 'a thunk
    (* apply(t) is the value of t's expression,
     * which is only evaluated once *)
    val apply : 'a thunk -> 'a
  end

structure Thunk :> THUNK =
  struct
    datatype 'a thunkPart =
      Done of 'a
    | Undone of unit -> 'a

    type 'a thunk =  ('a thunkPart) ref

    fun make (f : unit -> 'a) : 'a thunk =
      ref (Undone f)

    fun apply (th : 'a thunk) : 'a =
      case !th
        of Done x => x
         | Undone f =>
          let
            val ans = f()
          in
            (th := Done ans;
             ans)
          end
  end