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+-- The asInt_fold function uses error, so its callers cannot handle
+-- errors. Rewrite it to fix this problem:
+--
+--   -- file: ch04/ch04.exercises.hs
+--   type ErrorMessage = String
+--   asInt_either :: String -> Either String Int
+--
+--   ghci> asInt_either "33"
+--   Right 33
+--   ghci> asInt_either "foo"
+--   Left "non-digit 'o'"
+
+-- This exercise is based on the previous one (4_b_3.hs). There was only one
+-- call to error, so only that call is replaced by Either in this solution.
+
+-- The example for argument "foo" in the assignment returns 'o' as the first
+-- non-digit character. This seems incorrect as the 'f' is the first
+-- non-digit. This implementation checks every character other than the minus
+-- sign when checking for non-digits.
+
+import Data.Char (digitToInt, isDigit)
+
+asInt_either :: String -> Either String Int
+asInt_either "" = Right 0
+asInt_either "-" = Right 0
+asInt_either ('-':xs) = propagateEither int
+  where int = asInt_either' xs
+        propagateEither (Right x) = Right (-1 * x)
+        propagateEither left = left
+asInt_either xs = asInt_either' xs
+
+asInt_either' :: String -> Either String Int
+asInt_either' xs = foldl nextDigit (Right 0) xs
+  where nextDigit (Right acc) digit
+          | not (isDigit digit) = Left ("non-digit '" ++ [digit] ++ "'")
+          | otherwise = Right ((acc * 10) + (digitToInt digit))
+        nextDigit left _ = left
+
+-- ghci> :l 4_b_4.hs
+-- [1 of 1] Compiling Main             ( 4_b_4.hs, interpreted )
+-- Ok, one module loaded.
+-- ghci> asInt_either "33"
+-- Right 33
+-- ghci> asInt_either "foo"
+-- Left "non-digit 'f'"
+-- ghci> asInt_either "-33"
+-- Right (-33)