[Haskell-beginners] error monad and Map lookup

[Daniel Fischer]

On Thursday 22 December 2011, 20:44:26, Dennis Raddle wrote:
> Is there a more elegant way to write 'test' in the following?
> 
> import qualified Data.Map as M
> import Data.Map(Map)
> import Control.Monad.Error
> 
> testMap :: Map Int String
> testMap = M.fromList [(1, "whatever"), (2, "dude")]
> 
> test :: Int -> Either String String
> test x = do
>   y <- case M.lookup x testMap of
>          Nothing -> throwError "not in map"
>          Just z -> return z
>   return y

test = maybe (throwError "not in map") return . flip M.lookup testMap