largestPrimeFactor.hs

isPrime :: Int -> Bool
isPrime n = null $ dropWhile (not . (n `isDivisableWith`)) [2..n-1]

isDivisableWith :: Int -> Int -> Bool
isDivisableWith a b = a `mod` b == 0

primes = filter isPrime [1..]

largestPrimeFactor :: Int -> Int
largestPrimeFactor 1 = 1
largestPrimeFactor n = helper n (tail primes)
    where helper 1 (prime:_) = prime
          helper n (prime:primes)
            | n `isDivisableWith` prime = helper (n `div` prime) (prime:primes)
            | otherwise                 = helper n primes