What evidence do you have that functional is taking off? Since this board was around, every couple of years there's been a flurry of 3 months when a chunk of the community bangs on about functional programming.
First it was Lisp, then Erlang, then Haskell, then F#, then Scheme. I might have got the order wrong.
And it never, ever, ever takes off because while 10% of programmers find functional truly brilliant, the other 90% can't work with it.
For context, in that time Ruby (via Rails), Python (via Django and use in Science), javascript server-side (Node.js), Go, Swift, Rust and now possibly Kotlin have all taken off. Plus various other techs like NoSQL.
In the front end space, functional programming is taking off, I think. Could be wrong, but React + Redux + immutablejs is a popular and very functional approach to UI.
Elixir appears to be on the same trajectory as Rails in it's early days. Anectodally, I know a lot of Rails devs and shops that are now primarily Elixir.
Most mainstream languages have adopted functional styles and best practices. Higher order functions, a general acknowledgement of the benefits of immutability, etc.
Fp may not be mainstream, whatever that means, but it's more popular than I can ever remember it being.
it has dramatically reduced the amount of time i spend on these and many other sorts of problems. honestly it makes programming way more fun. maybe haskell is a bit extreme for most situations but the concepts are not.