I've had experience with both Scala and OCaml (both which allow non-functional programming also). F# is the Microsoft version of OCaml. Scala is easier to lear

I've had experience with both Scala and OCaml (both which allow non-functional programming also). F# is the Microsoft version of OCaml. Scala is easier to learn if you know Java, and has excellent IDE support in both Eclipse and Intellij IDEA. "Programming in Scala" is a good introduction and reference for Scala (http://www.artima.com/shop/programming_in_scala http://www.artima.com/shop/programming_in_scala), but doesn't cover the new features in Scala 2.8. I had good experiences with OCaml also (which compiles down to machine code, so has the potential to be faster than Scala if performance is critical). I believe it has more advanced tail-recursion optimizations. It seemed that floating point arithmetic was slow because each number had to be stored on the heap separately.