first steps :)

firstSatisfying :: (a -> Bool) -> [a] -> a
firstSatisfying f l
    | f (head l) == True = head l
    | otherwise          = firstSatisfying f (tail l)

primeFactorisation :: (Integral a) => a -> [a]
primeFactorisation x
    | x <= 0    = error "Wrong number"
    | x == 1    = []
    | otherwise = [firstDivisor] ++ primeFactorisation (x `div` firstDivisor)
    where
        divisorList = [2..x]
        firstDivisor = firstSatisfying (\y -> x `mod` y == 0) divisorList

main = putStrLn (show (primeFactorisation (2 * 9 * 7 * 100500)))