[Haskell-cafe] Issues(Bugs?) with GHC Type Families
Manuel M T Chakravarty
chak at cse.unsw.edu.au
Sun Mar 9 23:14:53 EDT 2008
> >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
> I would simply like the compiler not to use that instance if the
> equality constraint does not hold, like some another instance
> dependency constraint, but I assume that is not possible.
This is independent of type families. The selection of type class
instances does *never* dependent on the instance context, only on the
instance head. I know that this is sometimes annoying, but type
checking would be a a lot more complicated/expensive without that rule.
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