You first concern will be to define just what makes for a functional programming language. Is having first-class functions enough? What about referential tr

You first concern will be to define just what makes for a functional programming language. Is having first-class functions enough? What about referential transparency or immutable state? Does it have to adhere to the Lambda Calculus? You might get some ideas here: http://en.wikipedia.org/wiki/Functional_programming http://en.wikipedia.org/wiki/Functional_programming However, "What's FP?" is ripe for flame wars so you may do better to ask abut languages that offer specific characteristics rather than whether or not people call it a functional programming language. That said, I'd suggest Haskell. I mean, what the hell, might as well go hardcore. Start with Learn You a Haskell For Great Good, and/or Real World Haskell. Both available free online, both available as for-purchase books, both well worth the money. http://learnyouahaskell.com/chapters http://learnyouahaskell.com/chapters http://book.realworldhaskell.org/read/ http://book.realworldhaskell.org/read/