My approach to modularizing from the start is to do bottom up development, and to treat each namespace as a little library. Ultimately, pretty much all problems

My approach to modularizing from the start is to do bottom up development, and to treat each namespace as a little library. Ultimately, pretty much all problems can be viewed as data transformation pipelines, and those naturally map to independent components doing different kinds of transformations. I find that the APIs tend to form naturally around domain boundaries as you move data from one context to another. The REPL is indeed a major component of doing the Clojure workflow where you literally run each function as you're writing it. This approach also encourages refactoring as you go since you can make changes and see the results immediately.