Hamming's problem (enumerate all positive integers divisible by no primes other than 2, 3, or 5)

hamming.hs

hamming :: [Integer]
hamming =
    1 : merge3
            (map (*2) hamming)
            (map (*3) hamming)
            (map (*5) hamming)
    where
        merge (x:xs) (y:ys) =
            case compare x y of
              LT -> x : merge xs (y:ys)
              EQ -> x : merge xs ys
              GT -> y : merge (x:xs) ys
              
        merge3 xs ys zs =
            merge xs (merge ys zs)