"I’ve been fiddling around with top-down (mock-style) TDD of functional programs off-and-on for a few months. I’ve gotten obsessed with deferring the choice of data structures as long as possible. That seems appropriate in a functional language, where we should be talking about functions more than data. (And especially appropriate in Clojure, my language of choice, since Clojure lets you treat maps/dictionaries as if they were functions from keys to values.)"