Euler_3.hs

import Data.Maybe
import Data.List

primeFactors :: Int -> [Int]
primeFactors = (filter prime) . divisors

prime :: Int -> Bool
prime a =
        let upper = floor (sqrt (fromIntegral a)) + 1 in
        (a `mod` 2 /= 0) && isNothing (find (isMultiple a) [3,5..upper])

divisors :: Int -> [Int]
divisors a = let upper = floor(fromIntegral(a)/2) in filter (isMultiple a) [2..upper]

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