Hello everyone, I was wondering if you could help me! So, I have a typeclass "Action" which defines method "run": class Action a where run :: a -> Int and two data types that are instances of this typeclass: data A = A Int deriving (Read, Show) instance Action A where run (A n) = n data B = B Int deriving (Read, Show) instance Action B where run (B n) = n Now, I want to parse either "A" or "B" from a String. I was thinking about something like this... parseAction :: (Action a, Read a) => String -> a parseAction str | "(A " `isPrefixOf` str = (read :: String -> A) str | "(B " `isPrefixOf` str = (read :: String -> B) str The problem is that when calling "parseAction" I get "ambiguous type constraints". How to implement a parse function for two distinct types that share the same typeclass "Action". Because after calling "parseAction" I don't whether "A" or "B" was returned: I only care that they are "Action" instances so I can call "run". Best regards, José