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

23 Dec 2011


      On Thu, 22 Dec 2011 20:44:26 +0100, 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
If you run hlint (available on Hackage), you get the following message:

   MonadicCase.hs:11:10: Error: Redundant return
   Found:
     do y <- case M.lookup x testMap of
                 Nothing -> throwError "not in map"
                 Just z -> return z
        return y
   Why not:
     do case M.lookup x testMap of
            Nothing -> throwError "not in map"
            Just z -> return z


And of course, you can leave out the 'do'; the test function then becomes:
   test x =
     case M.lookup x testMap of
       Nothing -> throwError "not in map"
       Just z  -> return z


Regards,
Henk-Jan van Tuyl


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