I've used Opa sporadically since before the new JS syntax/backend. It's just flat out fun to program with, as the feedback loop is so tight. The only complaint

I've used Opa sporadically since before the new JS syntax/backend. It's just flat out fun to program with, as the feedback loop is so tight. The only complaint I had was the compiler was a bit ... French -- liable to take an extended lunch break just when you ask it to do some work. It might be faster now. Use a structural type system gives the language a very different feel to other statically typed systems. tl;dr: Give Opa a try. It's fun and enlightening.