signature DELAY =
sig
    type 'a T                          (* type of delayed computations *)

    val delay   : (unit -> 'a) -> 'a T (* create delayed computation *)
    val force   : 'a T -> 'a           (* execute and memoize computation *)

    val trivial : 'a -> 'a T           (* create trivial delayed computation *)
end


structure Delay : DELAY =
    (* Facilities for delayed, memoized computations are built into    *)
    (* SML/NJ in the structure System.Unsafe.Susp.  (There is nothing  *)
    (* unsafe about these operations; they appear in System.Unsafe for *)
    (* historical reasons.)  The advantage to using the built-in       *)
    (* functions instead of implementing delay and force in terms of   *)
    (* ref cells is that we can avoid weak polymorphism.               *)
struct
    type 'a T = 'a System.Unsafe.Susp.susp

    val delay = System.Unsafe.Susp.delay
    val force = System.Unsafe.Susp.force

    fun trivial x = delay (fn () => x)
end