That's a contrived example and not "real source code." Furthermore you are leaving so many symbols undefined that it is hard to see what is going on. Where does 'traverse' come from? Nevertheless, here is how you would do it in python: https://gist.github.com/bjourne/6219037
It's extracted, not contrived. Updating nested attributes on a tree of objects as a nice one-liner. The most contrived bits were that I didn't use the built in tree type so I had to define more stuff explicitly.
Traverse comes from Data.Traverable but is exported with lens as lens can be seen as a generalization of traverse.
