I have been playing around with Erlang, Scala and Go more and more, to the point where I'm able to draw conclusions about them as production languages.  I don't consider myself expert in any of them, but like most people in my line of work I have to be able to make judgments in advance of deep understanding. I've also spent time to a lesser extent with Rust, not enough to decide anything much. I've been foostering with Erlang and Scala on and off for years, Go and Rust are more recent.