Disclaimer: I use Scala since 2012 and am author of doobie-typesafe and yantl libraries and a contributor to Laminar and Mill build tool. This is also generated by GPT 5 with a prompt tailored from my instructions and reviewed by me.
Short answer: given what you already do (C#, TypeScript, Rust, React, .NET/Express/Axum), I’d seriously pick Scala 3 as your “functional programming language,” not Haskell/Elm/PureScript, and lean on the modern FP ecosystem on top of it.
You get:
- Pure FP when you want it (Cats Effect / ZIO / Kyo).
- A “real” industrial ecosystem for web APIs, CLIs, and UIs.