Be as lazy as possible.

It seems that Haskell solutions will certainly time out, even though the solution is translated from a nice Python version..

It really upset me. But how can I optimize my code?

(I use Data.Vector and expect that it will makes the my code faster.)

{-# OPTIONS_GHC -optc-O3 #-}
module LastDigit (lastDigit) where
import qualified Data.Vector as V
lastDigit :: [Integer] -> Integer
lastDigit xs = (V.foldr' (\x acc -> x ^ (if acc < 4 then acc else acc `mod` 4 + 4)) 1 $ V.fromList xs) `mod` 10

10 is not a prime number.

Thanks a lot! I've understand it now.