If you're not so much interested in using the language you learn in a job, I would recommend learning a proof assistant like Coq (basically Ocaml on steroids) or Lean (newer toy originally by Microsoft). The functional programming is fully there and you can even formally prove things about your program. The type systems these languages use are the richest out there (enough to encode basically any mathematical statement as a type)
I've heard of those, but I don't think I quite know what they are for. Formally proving things about a program sounds cool, but what are some practical applications of that?
For example, making "exhaustive tests" (as if you're able to check every single possible user input), eliminating a lot of security vulnerabilities, provably correct implementations of cryptographic algorithms (DJB is playing with Lean a lot right now and wrote a paper on what it's like), being able to reason better about the program, composing different programs while maintaining these advantages.
The "reason" why people ask "what's the use of that" is that they just assume that writing bug-free and correct software is impossible and not worth trying.
