Am Freitag, 28. März 2008 02:12 schrieb Henning Günther:
Hi,
suppose there are two (identical) classes:
class Res a b | a -> b where getRes :: a -> b
and
class Res2 t where type Member t getRes2 :: t -> Member t
It is easy to automatically make every instance of Res2 an instance of
res:
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 families.
Regards, Henning
Hello Henning, I also came across this problem half a year ago, tried to find a solution and came to the conclusion that there is none. It seems as if functional dependencies are a trap: Once you’ve started using them it is impossible to escape them by switching to type families. So you’ll probably have your own code use functional dependencies too (This was the “solution” I used in my case.) or you have to rewrite the library to use type families. Best wishes, Wolfgang