Haskell: Calculate sum of digits of a number

sumOfDigits.hs

sumOfDigits :: Integer -> Integer
sumOfDigits 0 = 0
sumOfDigits num
    | (num < 0) = sumOfDigits ((-1) * num)
    | otherwise = (getLastDigit num) + (sumOfDigits (num `div` 10))

{- Return the last digit of a number -}
getLastDigit :: Integer -> Integer
getLastDigit num
    | (num < 0) = ((-1) * num) `rem` 10
    | otherwise = num `rem` 10

*Main> :load sumOfDigits.hs
[1 of 1] Compiling Main             ( sumOfDigits.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> sumOfDigits (-1234)
10
*Main> sumOfDigits 1234
10
*Main> sumOfDigits 1
1
*Main> sumOfDigits 0
0
*Main>

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