I think that MLs “feel” a lot like Lisp, they are missing the obvious macro systems, but have few syntactic constructs and control flow is mostly function application. Namely SML, OCaml, and Haskell. I haven’t touched any others.
Others have said Elixir and to some extent I agree, but maybe the Ruby syntax throws me off.
I tried to setup a keyboard like that where it was programmed through QMK to do have taps be one key and hold do another etc, but I couldn’t stand the latency. So good on you for making it work