[tomp]

Which language? C++? Java? Both now have closures. Closures aren't the indicator that a programming language is functional; I need something more - though I admit that I'm mostly used to static functional languages, I guess ADTs are not so useful in a dynamic language.

[dragonwriter]

As with any programming paradigm, its not really useful to think of "functional" as a binary feature of programming languages, but rather an approach for which programming languages have varying degrees of support.

[pjmlp]

At very least it needs to be able to have directly support for lambda calculus.