This sounds like a fit for OCaml with either Async or preferably Lwt. ADTs are natural in OCaml, and you can even have Generalised ADTs which can encode ADTs in

This sounds like a fit for OCaml with either Async or preferably Lwt. ADTs are natural in OCaml, and you can even have Generalised ADTs which can encode ADTs into the type system. Granted, the main downside of OCaml is it still being single threaded. That can be worked around, but it depends on your workload.