Re: [Haskell-beginners] several questions: multi-param typeclasses, etc.

On Sun, Nov 1, 2009 at 5:32 PM, Michael Mossey wrote:

On the other hand, if you added

instance (Error e) => MonadError String (Either e)

and didn't include the functional dependency, the compiler would still run into a problem with overlapping instances and have no way to decide, which I presume is still an error.

Right, in this case it is true, but supposing the MonadError instance for Either was rather :

instance (Error e) => MonadError (Maybe String) (Either e)

There would be nothing a priori that would prevent you from writing another instance :

instance (Error e) => MonadError String (Either e)

There are a certain number of case where this functional constraint is thus useful. -- Jedaï