Which functional programming language should I learn for personal enrichment?

I would recommend to learn Haskell. It's a very powerful language and it is fairly optimised for a functional language. Also it shows you a lot of concepts of functional programming in a "easy to spot"-way.

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Elixir is a spinoff/written in Erlang, it definitely is a great tool, which will be used to a greater extent later on down the road. I would say learn Elixir based upon these reasons:

1. It essentially is Erlang without some of the quirks, to make it easier to develop on.

2. You will have easier time being exposed to OTP behavior.

3. It will give you a taste of how FP solves web development issues.

The basis for this choice would be based upon attrition in the learning sector. Although learning a language like O'Caml to work with in the financial sector would be optimal, you are probably not going to find a huge job market associated to this. the best cross section for FP would be what JS does in ES6, otherwise you will be learning these languages just to sharpen your current tool set.

If you are good at math: Haskell, Ocaml, Delphi, D

If you like telecommunications: Elixir, Erlang

If you are looking for the biggest market: Small talk (weirdly enough) and Lisp (You need to know ML and NLP concepts)

These languages are used for niche modules in a niche infrastructure. Don't learn the language to get a job with it, but to better understand computing in and of itself. If you are looking for ths next "BIG" thing, then D and Elixir.

Most of these languages require cross domain knowledge to be useful.

O'Caml === Actuarial Math;
Erlang === telecommunication systems;
Haskell === FP for C++ and weirdos alike;

Like I said Elixir is the most fun and opens up greater opportunities. The FP paradigm is in and of itself dependent upon the problem being solved, sometimes a foreach is slower than a for depending on the situation. What it offers is a greater safety towards your data since it maintains a "pass by value" culture. However, all of these things can be implemented by languages such as C++, but you would use a recursive function instead of for loops. A little taste of Erlang is that is does NOT have loop control flows, which means that for loops must be implemented with tail recursion...yea

Biggest tip is just pick one (you can literally put the names in a hat and pick one, srsly) then do the 99 prolog questions, just Google prolog 99 problems. This will walk you through most of the details of the language and will allow you to research and find the best blogs/resources to use for the language. Have fun!

Edit: Oh I'm sorry, wow, erlang is considered "pass by value" but other languages in FP paradigm don't distinguish because you cant reassign variables once compiled. Erlang you can clear a variable and then pattern match something to it during run time (I'm pretty sure).

However I missed two massive languages, Clojure, which runs on the JVM, and is a great language to start with in FP, but Scala is probably the most popular and most lucrative language that maintains functional programming. If you know Java already then you might want to do Scala instead, that is probably the only language that would take precedence just because of how useful it is. If you dont know how to build Java applications, then the language choice doesnt really matter. Now your welcome!

I would go with Scala. It’s the best out there imho. Everything (Java, Haskell, Lisp etc) becomes easy after learning Scala.

Does anyone know any good functional programming problems?

Erlang or Julia.

Scala, it translates to bytecode, so you can run it on any machine with a JVM.

Note that its not a pure functional programming language.