On 5/28/08, Evan Laforge
I have two related questions:
#1
I'm getting some annoying type errors that I don't fully understand, and wind up having to do a workaround that I don't totally like. Here's a simplified version of my situation:
data Ambi m = Ambi { ambi_monad :: m Int , ambi_int :: Int }
some_ambi :: Monad m => Ambi m some_ambi = Ambi (return 5) 10
ambi_table :: Monad m => [(String, Ambi m)] ambi_table = [("default", some_ambi)]
get_int :: String -> Maybe Int get_int sym = fmap ambi_int (lookup sym ambi_table)
-----------
get_int produces: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `ambi_table' at ambi.hs:13:40-49
You can let the caller choose "m": get_int :: Monad m => m () -> String -> Maybe Int get_int _ sym = fmap ambi_int (lookup sym ambi_table) which can be called like so: get_int (error "shouldn't evaluate" :: IO ()) "test"