Re: [Haskell-cafe] translating from fundeps into type families

Chaddaï Fouché:

2008/4/8, Manuel M T Chakravarty :

...

You need to write the instance as

instance (b ~ TheFoo a, Foo a) => Bar (Either a b) where bar (Left a) = foo' a bar (Right b) = foo' (foo b :: a)

If you do that, the program compile, but res still raise a panic in GHC6.8.2 .

Before you sent your solution, I also tried to do

class (TheFoo a ~ b) => Foo a b where ...

instance (Foo a b) => Bar (Either a b) where ...

But it didn't compile, it seems to me both versions should have the same behaviour ?

6.8.2 has many bugs concerning type families that have been removed in the development version of GHC. Manuel