Either

example code

main :: IO ()
main =
    do
        print $ f ""
        print $ f "2"
        print $ f "23"
        print $ f "235"
        print $ f "2345"
        print $ f "2345a"
        print $ f "a2345"
        print $ f "a"
        print $ f "1a"
        print $ f "a1"
 
f :: String -> Either String Int
f lch = f' lch 0
    where
        f' :: String -> Int -> Either String Int
        f' [] niAccu = Left "empty"
        f' [ch] niAccu = f'' ch >>= (\ni -> return ((niAccu * 10) + ni))
        f' (ch0:ch1:lrch) niAccu = f'' ch0 >>= (\ni0 -> f' (ch1:lrch) ((niAccu * 10) + ni0))
        f'' :: Char -> Either String Int
        f'' '0' = Right 0
        f'' '1' = Right 1
        f'' '2' = Right 2
        f'' '3' = Right 3
        f'' '4' = Right 4
        f'' '5' = Right 5
        f'' '6' = Right 6
        f'' '7' = Right 7
        f'' '8' = Right 8
        f'' '9' = Right 9
        f'' _ = Left "not a number"

output:

Left "empty"
Right 2
Right 23
Right 235
Right 2345
Left "not a number"
Left "not a number"
Left "not a number"
Left "not a number"
Left "not a number"