Re: [Haskell-cafe] Instances that shouldn't overlap

On Wed, Nov 26, 2008 at 1:54 PM, Miguel Mitrofanov wrote:

Maybe it'd be more intuitive if written backwards:

AppEq f a <= (Applicative f, Eq a)

or even

AppEq f a => (Applicative f, Eq a)

The first is good, the second isn't. The first says the right thing: if you can prove Applicative f and Eq a, you have a way to prove AppEq f a. The second has the implication the wrong way around. Classes get the implication wrong too: class Eq a => Ord a doesn't say that if you can prove Eq a you can prove Ord a; it says that if you can prove Ord a you can prove Eq a /g -- I am in here