Programming

Your crossposted question was a choice between Clojure and Common Lisp. Between the two I would say Clojure is more functional, but it comes with the baggage of the JVM. Common Lisp on the other hand is more of a 1980s language where you can use a functional style some of the time, and with some pain.

If you want a Lisp-like language, the usual starting point is Scheme, and if you want something with more creature comforts, try Racket. Either way, you'd start by reading SICP (fulltext here). But I think that whole approach misses out on an important aspect of FP, which is how type systems classify values.

So I'd say go with a typed functional language. OCaml is something like what you are used to, while Haskell is more "drinking from the fire hose" (steeper learning curve, but I think you will get more from it).

For Haskell, learnyouahaskell.com is a good place to start. I don't know if there is something similar for OCaml. Haskell can be seen as a gateway drug to even more pointy headed languages like Idris.

Yet another thing to look at as a possible migration point from Ruby is Elixir. It's not really so FP, but it's very practical if you're mostly interested in web development rather than programming languages per se. It's dynamically typed like Ruby and uses Ruby-like syntax, so you should be able to switch to it fairly easily.

I want to learn / use a functional programming language.

That's a good idea.

With over 20 years of programming in languages like C, C++, Python, Go, Pascal, Forth and so on, I think functional languages are often easier - in the same way that programming without lots of global variables is usually and objectively better. There are of course a few domains where the OOP approach works better, like defining data structures with tight invariants, Linux hardware drivers, or simulating ships.

Or sometimes, you need a bit of imperative code for a number-crunching core.

But most code can advantageously be represented and structured as:

input -> transformation -> output

and the transformation part, which is almost everything which is really interesting, is where the functional style shines.

Now if you want something beginner-friendly and practical, it depends on what you want to do. Here my ten cents:

For systems programming, signal processing and embedded stuff, I'd totally recommend Rust. It has certainly a bit of Algol syntax but that's rhe proverbial wolf in sheeps clothing: Many concepts actually come from OCaml, as well as the first Rust compiler. This would also answer the question whether functional languages are relevant and useful in industry.

If you want to do concurrent server-side programming, I believe there is nothing better than Clojure - perhaps with Babashka for scripting and ClojureScript for the Browser. Clojure is also incredibly elegant and concise, and at the same time it can use Java libraries, so it has huge library support, a battery powerhouse included.

If you learn both, you will see that Clojure and Rust have actually a lot in common in their approach to concurrency, which is preventing shared mutable state. It is great to learn both.

If you want a very practical, well-documented, and beginner-friendly language that also has great GUI support, I'd recommend Racket, a Scheme descendant. Like Clojure, it supports gradual typing.

If you want to program close to the Unix/Linux environment and want a simple as well as powerful language, a Scheme version could be a good choice. Personally, I love Guile, and use it where I previously used Python. Guile is very easy to use as a extension library for C and C++ programs or to control low-level code written in C. It also has a quite good REPL and debugger support and thus, interactive experience.

Speaking of Python, it has a great vector library which is heavily influenced by APL and functional designs, named Numpy. A C++ analogon would be Eigen. You see that functional programmers subversively introduce their filth everywhere!!

For data analysis, Scala could be an option. It is like OCaml on the JVM. Similarly, F# for .NET (but who wants to get entangled in Windows?).

If you want to keep it more multi-paradigm, you could chose Common Lisp - it will let you program in many styles and its SBCL implementation probably has the best performance after Rust. But Schemes like Guile or Racket are more geared toward a functional-preferring style. And if you want Lisp with stricter typing, there is Coalton - but I have not tried that last one.

You don't actually need to know any of the functional programming languages to work in the AI sphere. Moreover, codebase in pure funcional languages is hard to understand and maintain, that's why they are rarely used in production. Of course you can learn any language for funsies, but I'd recommend Kotlin as a modern hybrid OOP language with a solid functional toolkit

I recommend picking up Graham Hutton's short text Programming in Haskell, Second Edition. Even if you don't end up using Haskell in "real work" (and you might!) it will teach you a remarkable number of things about how functional programming works.

Most ML development is done in Python to my knowledge. The reason is mostly the readily available side packages like torch, scikit-learn, etc. And Python, although not constructed as such, does indeed support a functional style. A quick search revealed a HowTo:

https://docs.python.org/3/howto/functional.html

For your goals, I would stick with Python unless you want to learn another language. There's not much value to switch away when all the tools you need are primarily designed for Python.

As far as functional programming goes, with AI stuff, my experience is that you generally are more interested in orchestrating services than FP. For example, run input through model #1, then based on the output, run one of these other 3 models (or multiple of them in parallel), then eventually pass it all back into another service/function to aggregate and format the outputs. You can think of each of these as being "functions", but they're much higher level than what you'd traditionally consider functions in FP and more along the lines of microservices.

Elixir is a nice functional language. Not as purely functional as Haskel mind.

It is based off of Erlang and runs on the Erlang VM. Syntax is Ruby inspired. It is not a high performant language (like C, Rust, etc), but it excels at distributed computing and fault tolerance. There is excellent documentation and tooling for it as well.