On Thu, Jun 7, 2012 at 2:46 AM, AntC <anthony_clayden@clear.net.nz> wrote:

What does the `ArgKind' message mean?

`ArgKind` and `OpenKind` is what previously was called `?` and `??` (or the other
way around; I can't remember).
http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/TypeType#Kindsubtyping

You might also want to have a look at Richard and Stephanie's latest paper draft, about
singletons, which is related to what you are trying in your example:
http://www.cis.upenn.edu/~eir/papers/2012/singletons/paper.pdf