[Haskell-cafe] Issues(Bugs?) with GHC Type Families
Manuel M T Chakravarty
chak at cse.unsw.edu.au
Thu Mar 6 19:55:16 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?
I think I don't understand your question fully. This class instance
requires both -XFlexibleInstances and -fallow-undecidable-instances to
If the equality does not hold, you should get a type error because
your program is not type correct. So, what is it that you would like
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