# Pastebin fEhcPysw
isPrime :: Integer -> Bool
isPrime n
  | even n = False
  | any factor [2..isqrt n] = False
  | otherwise = True
  where
    -- The type signatures on `factor` and `isqrt`
    -- are not really required here, but might make
    -- it easier to read
    factor :: Integer -> Bool
    factor x = n `mod` x == 0

    isqrt :: Integer -> Integer
    isqrt x = ceiling (sqrt (fromIntegral x :: Double))