Re: [Haskell-beginners] error monad and Map lookup

22 Dec 2011


      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

Re: [Haskell-beginners] error monad and Map lookup

Daniel Fischer