Primes a la Yves Bertot

primes.hs

---- Prime sieve based on co-inductive streams
---- Taken from "Filters on coinductive streams, an application to
---- Eratosthenes’ sieve" by Yves Bertot

-- a 'stateful' filter
fm p n (x:xs) | (n < x)  = fm p (n+p) (x:xs)
fm p n (x:xs) | (n == x) = fm p (n+p) xs
fm p n (x:xs) | (x < n)  = x : (fm p n xs)

primes = sieve [2..] where
    sieve (p:xs) = p : (sieve $ fm p (2*p) xs)