On Fri, Nov 13, 2009 at 3:26 PM, Andy Gimblett <haskell@gimbo.org.uk> wrote:
> First a type family where the type Y is functionally dependent on
> the type X, and we have a function from Y to ().
>
>> class X a where
>> type Y a
>> enact :: Y a -> ()

This is ambiguous. Type families are not injective (that is, Y a ~ Y b
does not imply a ~ b), so there's no way for the compiler to figure
out which instance of X is being used when it encounters enact.

Note: that if you need this injectivity you can use a data family instead.