[Haskell-cafe] Functional dependencies with Type Classes

[Wolfgang Jeltsch]

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