What is the story behind the type of undefined?

[Richard Eisenberg]

On Feb 1, 2015, at 2:07 PM, Adam Gundry <adam at well-typed.com> wrote:

> (There is a plan to get rid of this subkinding in favour of normal
> polymorphism, but it hasn't been implemented yet. See
> https://ghc.haskell.org/trac/ghc/wiki/NoSubKinds for more details.)

The NoSubKinds plan *is* implemented, in my long-running branch. (It would be hard to implement in normal GHC, as there's an assumption that all kind variables have sort BOX, which NoSubKinds violates.) My branch doesn't have a mechanism for user-declared levity-polymorphic things, but that feature could easily be designed and added. Indeed, I believe user-availability is a goal of NoSubKinds. It just isn't on my critical path, so I didn't do it (yet).

Richard