I'm working on a programming language. Just as a hobby, not commercially. :-)

Design-wise it's really just how my ideal language would work: functional, compiled, statically typed(higher-ranked, impredicative, but type-level functions are first-order, and no effects system), mutable refs that are _not_ GCed, first class delimited continuations, equi-recursive _and_ iso-recursive types, implemented in C so the compiler itself has a good performance baseline, etc...

It has been really fun so far and an amazing learning experience. I intentionally didn't read up on type theory, so implementing the type checker was a huge challenge.

Amazing feeling when I finally got it to (correctly) work, though :-).