[jxub]

By most used: maybe F#, Erlang, Clojure and Haskell. However mindshare in any of them is still pretty small compared to the object-oriented or procedural ones. Also, JavaScript and other mainstream languages also lend themselves quite well to FP-style programming.

[atlasM]

I’ve taken a look at Clojure and Haskell and I’m really liking them. How to F# and Erlang compare in terms of use case?

[jxub]

Erlang is really good for any messaging and/or distributed system, but fails short of power for number-crunching, however you could write any compute-intensive parts in C/Rust and make a NIF link to them.

I think that F# it's the go-to functional language for interfacing with rest of the Microsoft stack, and with the .NET platform. It's also pretty widely used in the trading systems niche, but it hasn't exploded yet in popularity.