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Real World Haskell, Exercise 4, p. 98:

The asInt_fold function uses error, so its callers cannot handle errors. Rewrite it to fix this problem.

-- file: ch04/IntParse.hs
import Data.Char (digitToInt) 

asInt :: String -> Int
asInt ('-':xs) = -1 * asInt xs
asInt xs = foldl step 0 xs
  where step acc x = acc * 10 + safeDigitToInt x

safeDigitToInt :: Char -> Int
safeDigitToInt x =  if x >= '0' && x <= '9'
                      then digitToInt x
                      else error "Non-digit"

asInt_either :: String -> Ei
asInt_either xs = if onlyHasDigits xs
                  then Main.Right (asInt xs)
                  else Main.Left ("non-digit '" ++ ((firstNonDigit xs) : "'"))

onlyHasDigits :: String -> Bool
onlyHasDigits [] = True
onlyHasDigits (x:xs) = if x >= '0' && x <= '9'
                      then onlyHasDigits xs
                      else False

firstNonDigit :: String -> Char
firstNonDigit [] = '\0'
firstNonDigit (x:xs) = if x <= '0' || x >= '9'
                      then x
                      else firstNonDigit xs

data Ei = Right Int
          | Left String
        deriving (Show)

Doubtless this could be much mproved by someone who knows what they’re doing.