(* stream.sml *)

(* implementation of streams *)

structure Stream :> STREAM =
struct

datatype 'a stream = Stream of ('a * 'a stream)Susp.susp

fun make(state, f) =
      let fun g state =
                Stream
                (Susp.delay
                 (fn () =>
                       let val (x, state) = f state
                       in (x, g state) end))
      in g state end

fun get(Stream x) = Susp.force x

fun takeToList(stm, n) =
      let fun tke(stm, n, xs) =
                if n <= 0
                then rev xs
                else let val (x, stm) = get stm
                     in tke(stm, n - 1, x :: xs) end
      in tke(stm, n, nil) end

fun drop(stm, n) =
      if n <= 0
      then stm
      else let val (_, stm) = get stm
           in drop(stm, n - 1) end

fun rangeToList(stm, n, m) =
      if n <= 0 orelse m < n
      then nil
      else takeToList(drop(stm, n - 1), m - n + 1)

end;