This example is completely contrived, of course: (let [x 1] (defn make-adder [y] (fn [] (+ x y))) (defn make-multiplier [z] (fn [] (* x z

This example is completely contrived, of course: (let [x 1] (defn make-adder [y] (fn [] (+ x y))) (defn make-multiplier [z] (fn [] (* x z))) (defn mutate! [new-x] (set! x new-x))) Now for (make-adder 2) we get a function whose environment includes x and y, whereas the environment for (make-multiplier 2) includes x and z. Assuming you have shallow environments, you'll have one environment [x' y] and another one [x'' z], whereas with deep environments (like in ClojureC) you'd have [[x] y] and [[x] z] where the [x] is shared between them. Now, if you call (mutate! 3) with deep environments you only need to store the 3 once (to x), whereas with shallow environments you'll need to store to x' and x''.