signature MLTON_VECTOR =
   sig
      val create: int -> {done: unit -> 'a vector,
                          sub: int -> 'a,
                          update: int * 'a -> unit}
      val unfoldi: int * 'b * (int * 'b -> 'a * 'b) -> 'a vector * 'b
   end

• create n
  initiates the construction a vector v of length n, returning functions to manipulate the vector. The done function may be called to return the created vector; it is an error to call done before all entries have been initialized; it is an error to call done after having called done. The sub function may be called to return an initialized vector entry; it is not an error to call sub after having called done. The update function may be called to initialize a vector entry; it is an error to call update after having called done. One must initialize vector entries in order from lowest to highest; that is, before calling update (i, x), one must have already called update (j, x) for all j in [0, i). The done, sub, and update functions are all constant-time operations.

• unfoldi (n, b, f)
  constructs a vector v of length n, whose elements v_i are determined by the equations b₀ = b and (v_i, b_i+1) = f (i, b_i).