Maybeならguardで例外送出がかけるけどEitherならどうするんだろう？

maybe.hs

import Data.Either
import Control.Monad

failable :: Int -> Maybe Int
failable x = do
  guard $ x /= 0
  guard $ x /= 10
  return $ x + 10

doFailable :: Int -> IO ()
doFailable x = do
  let y = failable x
  case y of
    Nothing -> putStrLn "Error!"
    Just z  -> print z

main :: IO ()
main = do
  doFailable 10
  doFailable 200