Dynamic function-binding on runtime with Clojure

Question

I started playing with Clojure today and stumbled upon the statement that one could change functions dynamically during runtime. That sounds pretty cool so I wrote a little piece of code using this feature.

(defn ^:dynamic state [x]
   (odd x))

(defn even [x]
  (if (= x 0)
    (println "even")
    (binding [state odd] (parity x))))

(defn odd [x]
  (if (= x 0)
    (println "odd")
    (binding [state even](parity x))))

(defn parity [x]
    (state (dec x)))

It works out fine, but since I am completly new to Clojure I don't know whether this is
a) clean functional code (since odd and even seem to have sideeffects?)
b) the way changing functions on runtime is supposed to be done

I would appreciate any kind of advice on that! :) -Zakum

Answer 1

Use of dynamic bindings is mostly a question of taste, but there are a few considerations:

Dynamic bindings are pretty much a shortcut for explicitly passing values on the call stack. There are only a few situations where doing that is a totally obvious win; mostly things like passing "global" configuration settings/arguments "through" APIs that don't support them.

An API that relies on dynamic bindings is hard to wrap into something more explicit, while the other way around is much easier (and can usually be done semi-automatically).

Dynamic bindings do not play nice with lazy sequences or anything else that evaluates outside of the current call stack (like other threads).

All in all, I think the "cleaner" functional solution would be to pass state as an argument to parity, but arguments can be made either way.