[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

Hugo Pacheco:
> 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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