[Haskell-cafe] Eta Reduction

[Niklas Haas]

On Wed, 2 Apr 2014 09:51:46 -0400, Edward Kmett <ekmett at gmail.com> wrote:
> We're not storing the instance as a slot in the constructor, so this isn't.
> 
> data Foo a where
>    Foo :: Seq a => Int -> a -> Foo a
> 
> or equivalently
> 
> data Foo a = Seq a => Foo !Int !a

Why not? Say we define Seq to something like

> type family Seq (a :: *) :: Constraint where
>   Seq (a -> b) = 1 ~ 0
>   Seq t = ()

What would be the drawback in this scenario?