[Haskell-cafe] Issues(Bugs?) with GHC Type Families

Hugo Pacheco hpacheco at gmail.com
Wed Mar 5 20:27:47 EST 2008

Just something I have been wondering.

I would like to implement somehting like:

type family F a :: * -> *
class C a b where ...
instance (F a ~ F b) => C a b where ...

But apparently type equality coercions can not be used as a single context.
If I enable -fallow-undecidable-instances, whenever the equality does not
hold, the instance returns a compile error, what does make sense.

Is there any way I could overcome this?

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