datatype 'a stream = Cons of 'a * (unit -> 'a stream)

fun force s = s ()

fun chop 0 _ = []
  | chop n (Cons (x, s)) = x :: chop (n-1) (force s)

fun repeat 0 x s = force s
  | repeat n x s = Cons (x, fn () => repeat (n-1) x s)

fun decode (Cons ((x, n), s)) =
    repeat n x (fn () => decode (force s))

fun encode (Cons (x, s)) =
    let fun accumulate x n (Cons (y, s)) =
            if x = y then
                accumulate x (n+1) (force s)
            else
                Cons ((x, n), fn () => accumulate y 1 (force s))
    in
        accumulate x 1 (force s)
    end