Actually, there are two views of functional programming: 1) Default to functional ideas: Variables are immutable, recursion is encouraged, etc; but you can get

Actually, there are two views of functional programming: 1) Default to functional ideas: Variables are immutable, recursion is encouraged, etc; but you can get access to imperative code when needed (Call this the Algoritmically functional languages) 2) Functional is the norm: You can't write non-pure functions with side-effects at all. These are the Pure Functional languages. I am leaning towards 1 more than 2 personally, but many people would disagree with me on that one. The question is: Do you want a usable program written in class 1 or 2? Because there is a hell of a difference between them and how you build software for the two classes. Interesting class 1 projects, where you can get the source code, are: ejabberd, mldonkey, unison, harmony/boomerang, coq, twelf, isabelle, coccinelle, and mlton. Interesting class 2 projects could be xmonad, darcs, and GHC. Of these projects, some would be easily implementible in imperative languages, some would be hard but doable and some would be insanely impossible to do. Please note that all things on the list are useful and elegant. But they might not have a user-base as big as other projects.