factor_a_number.hs

-- Factor a number
factor :: Int -> [(Int, Int)]
factor n    | n < 2        = []
            | otherwise    = factor' n (eratosthenes.floor.sqrt.fromIntegral $ n) 0 []
    where
        factor' :: Int -> [Int] -> Int -> [(Int, Int)] -> [(Int, Int)]
        factor' n [] _ res          = res ++ [(n,1)]
        factor' 1 (p:ps) pw res     | pw > 0         = res ++ [(p,pw)]
                                    | otherwise      = res
        factor' n (p:ps) pw res     | n `mod` p == 0 = factor' (n `div` p) (p:ps) (pw + 1) res
                                    | pw > 0         = factor' n ps 0 (res ++ [(p,pw)])
                                    | otherwise      = factor' n ps 0 res