[Haskell-cafe] Functional dependencies with Type Classes

Henning Günther der_eq at freenet.de
Thu Mar 27 21:12:28 EDT 2008


suppose there are two (identical) classes:

> class Res a b | a -> b  where
>	getRes :: a -> b


> class Res2 t where
>	type Member t
>	getRes2 :: t -> Member t

It is easy to automatically make every instance of Res2 an instance of

> instance Res2 a => Res a (Member a) where
>	getRes x = getRes2 x

However, declaring every instance of Res an instance of Res2 seems
impossible, as the following doesn't compile

> instance Res a b => Res2 a where
>	type Member a = b
>	getRes2 x = getRes x

Question is: How to do this? The reason I need it is because I use a
library which uses functional dependencies, but my classes shall be type


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