That's exactly what I was thinking. I've not used Clojure specifically, but in the sitations where OO people use dependency injection I tend to use a free monad

That's exactly what I was thinking. I've not used Clojure specifically, but in the sitations where OO people use dependency injection I tend to use a free monad with multiple interpreters. I'm yet to make serious use of an algebraic effect system, but that would make this even simpler and more composable.