projeuler prob 3 hack

gistfile1.hs

import Control.Monad
import Data.List
import Data.Maybe

factorBy :: Int -> Int -> Maybe (Int, Int)
factorBy n factor = if snd result == 0 then Just (fst result, factor)
                    else Nothing
    where result = n `divMod` factor

firstFactor :: Int -> Maybe (Int, Int)
firstFactor n = join . find isJust . map (factorBy n) $ [2..n]

problem3 :: Int -> Maybe Int
problem3 n = case firstFactor n of
    Nothing -> Nothing
    (Just (1,x)) -> Just x
    (Just (x,y)) -> problem3 x

my version:

import Control.Monad
import Data.List
import Data.Maybe

factorBy :: Int -> Int -> Maybe (Int, Int)
factorBy n factor = guard (s==0) >> return (f, factor)
where (f,s) = n divMod factor

firstFactor :: Int -> Maybe (Int, Int)
firstFactor n = listToMaybe . mapMaybe (factorBy n) $ [2..n]

problem3 :: Int -> Maybe Int
problem3 n = firstFactor n >>= (a,b) ->
if a == 1
then return b
else problem3 a