Hugo Pacheco:
>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
>different?
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.
Manuel
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