{-# OPTIONS_GHC -Wall #-}

sumDigits :: [Integer] -> Integer

sumDigits []     = 0

sumDigits (n:ns)

  | n < 10       = n + sumDigits ns

  | n >= 10      = r + sumDigits (q : ns)

  where (q, r)   = n `quotRem` 10

sumList :: [Integer] -> Integer

sumList []     = 0

sumList (n:ns) = n + sumList ns