Pass in requests. Get responses back. Same input, same output, except for potentially dirtied DB hits. This is somewhat of a good fit for a functional language program that just handles request after request. For API's Haskell might be a great fit.

For server-side rendering of fat web clients, I don't see any way around node.js and my resentment of javascript without doing some impressive design of the client to be able to reinitialize itself from a JSON that pops out of the client state in the host language. It's possible, but I'm too busy with Kivy to mess with web development atm.