I'm think of Haskell, OCaml or (AFAIK) Erlang. From what I read, Clojure has also a lot of emphasis on immutability.
Both clojure and haskell have mutability, just at differing granularities (haskell monads, clojure replace the world object). They can do it with referential transparency though (which is probably your meaning, obviously pure immutability is impossible and not sensical).
> Both clojure and haskell have mutability, just at differing granularities (haskell monads, clojure replace the world object). They can do it with referential transparency though (which is probably your meaning, obviously pure immutability is impossible and not sensical).
Well, you have to have to be able to manipulate state somehow (even if, at least on the surface, the state monad emulates mutability, as opposed to, eg, OCaml's notion of mutation, which is very explicit in what it does). But the point is that it is not the path of least resistance in these languages.
What C# lacks for me in FP, case matching, has little to do with purity. The FP story involves purity but is not dominated by it. List comprehensions still work in C# even though you can theoretically side effect in your select and filter functions.