Since you explicitly mentioned C# and F#, I'd suggest OCaml. There's a free translation of a French O'Reilly book, _Developing applications with Objective Caml_

Since you explicitly mentioned C# and F#, I'd suggest OCaml. There's a free translation of a French O'Reilly book, _Developing applications with Objective Caml_, at http://caml.inria.fr/pub/docs/oreilly-book/ http://caml.inria.fr/pub/docs/oreilly-book/ (the site seems to be down for me at the moment). The first few chapters focus on functional programming, then imperative programming, then compare their strengths, weaknesses, and when using each is most appropriate. (OOP and others are covered later in the book.) It's a pretty good intro to FP, imho. Once you know FP techniques, you can apply them in many languages people don't tend to think of as functional (though tail-call optimization helps, and support for higher-order functions is all but necessary); I write strongly FP-influenced code in Lua, for example. Scheme is also an utterly fantastic language, but I'm going to defer recommendations there to others. (Except: I really like Chicken - http://www.call-with-current-continuation.org/ http://www.call-with-current-continuation.org/)