With the niceties of C++11, it's certainly become a lot less painful; lambdas, auto (both variable and return type), variadic templates and r-value references give the language just enough flexibility to allow functional style with far more natural syntax. To be sure, it's not a perfect setup; type traits (e.g., enable_if) in function signatures are ugly and hardly a replacement for concepts.

I've been dabbling in the creation of a functional library that provides a modest subset of Prelude-inspired features from Haskell. https://github.com/jdduke/fpcpp Many of the algorithms are simply wrappers around stl algorithms, hiding some of the syntactic awkwardness attendant with the heavy use of iterators. Some day I'll get around to playing with monadic devices, and integrating the list comprehension work that I've done recently.

You can if you want to. Just make sure you write all your functions in a purely functional way, and avoid all side effects.

You do not really need changes to the language, just iron discipline.

One of my undergraduate mentors wrote a library called FC++.