Try these 4 languages from 4 corners of Programming (opens in new tab)

Definitely Prolog. I have barely played with Prolog except when it is included in the PL as in picoLisp or Shen, but it's in its own category. I agree about Erlang too, but it made me stop to think. Erlang without the BEAM or OTP is pretty much a functional language like Haskell, no? I looked at Pony once. Yeah, ok, Erlang too! I prefer the Lisp version: LFE!

I started playing with it, and J became a very powerful desktop calculator. Then I started to tackle other programs in it, and I was hooked by the way it made me think about a problem. All of the selling points of functional programming, which didn't resonate when I was learning F# or Haskell or Lisp, became crystal to me when tacit programming in J or solely functions. I am a polyglot, but I like the array as the basic unit of computation (now very popular with GPUs, Pandas, and Tensorflow and all other array-laden technologies). I feel like I am distilling a problem down to a very well thought out distillation, and it brings me joy!

I didn't see your reply before I brought up APL and J here. I've tried k several times, but I always fall back on J, then APL, and then k. I think it's personal bias, since I found J first. I like the symbols of APL as I like mathematical formulas, and now that Dyalog has adopted some J-isms, I've been trying reimplement my J stuff in Dyalog APL. Also Roger Hui of J fame is a Dyalog user too. I thought I would gravitate to k because of my Lisp background, but J is sticking with me.

Never did find the motivation to dive into Joy. I did write a lot of Haskell though, and it left a slightly sour taste.

I feel the same about statically typed functional programming as about Rust, it's a nice theory but in practice you end up squeezing problems into weird shapes to keep the language happy.

Is Joy getting any active development? The only resources I could find with a quick search seem to be almost 20 years old.

Actually no. Familiarity with Haskell can actually make learning Idris more difficult :). It is better if you start from scratch when learning Idris.

There are some pretty good tutorials on Idris which cover just enough Haskell to get you started but yes, maybe it's too much in one go. Dependent Types though <3

Is the original 7 languages worth reading?

Though Clojure in particular probably leans heavily to the functional side.