What you really want or mean when you use the classic syntax with existential quantification is
data Foo = Foo (exists a . (Show a) => a)
Having that would make a lot more sense, and would fit well together with the intuition of the classic syntax.
How would you then define
data Foo :: * where Foo :: forall a. a -> a -> Foo
in which the scope of existentially quantified type variable spans more than one field?
Good point, and one I admit I hadn't considered. Using GADT style syntax? ;-) However, your argument certainly speaks against the style using exists, but it doesn't do much to persuade me that the style we now have is any less of a wart. To me it's just another point in favor of deprecating it with the classic syntax completely. Cheers, /Niklas