ML: Meta Language, most usable functional language

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  1. Non pure but highly usable functional language.
  2. Example: Calculate prime numbers by sieve of Eratosthenes. 

    (* Lazy prime number generation in Standard ML *)
datatype 'a seq = Nil
                | Cons of 'a * (unit -> 'a seq);

fun from k = Cons(k, fn()=> from(k + 1));

fun filterq pred Nil      = Nil
  | filterq pred (Cons(x, xf)) =
          if pred x then Cons(x, fn()=> filterq pred (xf()))
                    else filterq pred (xf());

fun sift p = filterq (fn n => n mod p <> 0);

fun sieve (Cons(p, nf)) = Cons(p, fn()=>sieve(sift p (nf())));

fun upto(0, _)               = []
  | upto(_, Nil)             = []
  | upto(n, Cons(x, xf)) =
        if n < x then []
                 else x :: upto(n, xf());

fun primes(n) = upto(n, sieve(from 2));

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