haskell error

error.hs

import Test.HUnit
import Control.Monad.Except

-- instance (Error e) => Monad (Either e) where
--   return x = Right x
--   Right x >>= f = f x
--   Left err >>= f = Left err
--   fail msg = Left (strMsg msg)

tests = ["Either1" ~: (Left "boom" >>= (\ x -> return . succ $ x)) ~?= (Left "boom" :: Either String Int),
         "Either2" ~: (Left "boom" >>= \ x -> (Left "no way!" :: Either String Int) ) ~?= Left "boom",
         "Either3" ~: (Right 100 >>= \ x -> (Left "no way" :: Either String Int)) ~?= Left "no way"]

runTests = do
  runTestTT $ TestList tests