Tom Murphy wrote:
> So it's the "(rigid, skolem)" error you sometimes happen across. The code
> that's causing it, though, is pretty unsuspicious:

Just two clues, pointing towards `b` being monomorphic as one
ontributing factor:

* adding {-# LANGUAGE NoMonoLocalBinds #-} works.
* giving `b` a standalone type signature makes it work, that is:
(...)
I'm not sure how enabling GADTs affects type-checking here.