Existential types: want better syntactic support (autoboxing?)

[Ashley Yakeley]

John Meacham wrote:

> every single parameter type class whole parameter is of kind * 
> class Foo a where
> 
> automatically declares a data type defined as
> 
> data Foo = exists a . Foo a => Foo_ a   
> (where Foo_ is some internal, non user accessable name)
> 
> and an instance
> 
> instance Foo Foo where
>         method (Foo_ x) = method x 
>         ...

I don't much like this, it seems like unnecessary sugar to me. My first 
preference would be not to do this at all.

My second preference would be to do this a bit more generally:

every type class
class Foo a b c where

automatically declares a data type defined as

data Foo t = exists a b c . Foo a b c => Foo_ (t a b c)
(where Foo_ is some internal, non user accessable name)

every single parameter type class whole parameter is of kind *
declares an instance

instance Foo (Foo Identity) where
          method (Foo_ x) = method x
          ...

It's still pretty ugly. I don't like special-casing classes that happen 
to have a particular kind signature.

-- 
Ashley Yakeley