gistfile1.txt

test :: Prism Integer [Digit]
test = prism' (snd . digitsToInt) intToDigits
  where
    digitsToInt :: [Digit] -> (Int, Int)
    digitsToInt [] = (0, 0)
    digitsToInt (d:ds) =
      let (count, res) = digitsToInt ds
      in (count+1, 10 ^ count * digitToInt d)

    intToDigits :: Int -> Maybe [Digit]
    intToDigits n
      | n < 0 = Nothing
      | otherwise =
        liftA2 (++)
          (intToDigits $ n `div` 10)
          [unsafeIntToDigit $ n `rem` 10]

    -- Single digit Int to Digit
    unsafeIntToDigit :: Int -> Digit
    unsafeIntToDigit = undefined