Streams in SML

streams.sml

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

fun from n = Cons(n, fn () => from(n +1))
val naturals = from(1)

fun filter f xs = 
   case xs of
      Empty => Empty
    | Cons(h,t) => if f(h)
                   then Cons(h, fn () => filter f (t()))
                   else filter f (t())

fun take n xs =
    if n = 0
    then Empty
    else case xs of
            Empty => Empty
          | Cons(h,t) => Cons(h, fn() => take (n-1) (t()))

fun toList xs =
    case xs of
        Empty => []
      | Cons(h,t) => h::toList(t())

fun primes n =
    let
	fun sieve xs = 
	    case xs of
		Empty => Empty
	      | Cons(h, t) => Cons(h, fn() => sieve(filter (fn n => n mod h <> 0) (t())))
    in
	toList (take n (sieve(from(2))))
    end


val res = primes 10