Are you specifically looking for an introduction to functional programming, or to more generally modern language features? The strong static typing of Haskell o

Are you specifically looking for an introduction to functional programming, or to more generally modern language features? The strong static typing of Haskell or OCaml is orthogonal to their functional properties, but they tend to go together because they were designed by academic types in the last 20-ish years. Assuming you're interested in all of the above: For immediate productivity, I would recommend Scala - it's run on the JVM, and lets you fall back to a fairly Java-like subset. It's ugly in some ways, but has a more mature web programming ecosystem than a lot of other functional programming languages - a decent backend framework in Play and Scala.js for maybe writing a few critical browser-side libraries - and has decent and growing adoption. If you're not looking for quick productivity in web development, and don't care all that much about learning a language that's immediately applicable to existing job opportunities, I'd probably recommend the more "pure" modern languages like Haskell or OCaml; they're niche, but it's a stable niche, especially in more academic circles, and will give you a cleaner mental model of all the features that are hackily being tacked on in JS and Scala. I've heard good things about Clojure (think LISP on the JVM), but haven't personally tried it and don't know how vibrant its community is. Having access to JVM libraries, as with Scala, is a great plus for immediate productivity.