Porting Haskell code to new version of Data.Map.lookup

Question

I must compile on a recent version of Haskell a software written for a previous version of the standard libraries. The code assumes Data.Map.lookup has type:

lookup :: (Monad m, Ord k) => k -> Map k a -> m a

as it was the case with e.g. GHC 6.8.1/containers 0.1.0.0., but since (at least) GHC 6.10.1/containers 0.2.0.0 Data.Map.lookup has type:

lookup :: Ord k => k -> Map k a -> Maybe a

Since I don't know much of Haskell I'm looking for a workaround or an equivalent function in the current libraries. Can anyone help me?

Answer 1

Just find out what old lookup did in each possible case and replicate the functionality.

What did occur in case the element did not exist in the map? I guess, fail is called?

Then it would be

lookup2 el map =
  case lookup el map of
    Just x -> return x
    Nothing -> fail "Element doesn't exist in the map"