[Haskell-cafe] Fundeps and overlapping instances
etienne at atnnn.com
Wed May 30 22:31:14 CEST 2012
I disagree with your example.
> 1. Check that an instance is consistent with itself. For example, this
> should be rejected:
> instance C a b
> because it allows C Int Bool and C Int Char which violate the functional
Functional dependencies are not used to pick types, they are used to
class C a b | a → b where
k ∷ a
f ∷ a → Maybe b
The functional dependency allows you to have a method such as k that
doesn't use all the arguments of the class.
I expect to be able to make a instance that works for any b.
instance C Int b where
k = 2
f _ = Nothing
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