Sum all Armstrong numbers within a range

armstrong.hs

digits :: (Integral n) => n -> [n]
digits = reverse . go
    where go n
            | n < 10 = [n]
            | otherwise = r : digits q
                where (q, r) = n `quotRem` 10

numberLength :: Int -> Int
numberLength = length . digits

powerNumber :: Int -> [Int]
powerNumber n = [d ^ exponent | d <- digits n]
    where exponent = numberLength n

isArmstrong :: Int -> Bool
isArmstrong a = a == sum (powerNumber a)

sortEnds :: Int -> Int -> (Int, Int)
sortEnds a b = if a < b then (a, b) else (b, a)

sumAllArmstrongNumber :: Int -> Int -> Int
sumAllArmstrongNumber a b = sum [x | x <- [start..end], isArmstrong x]
    where (start, end) = sortEnds a b