Dynamic code with clojure

Question

I'm trying to create some dynamic code within clojure. In the function below, the idea is that the conditions for the (and) macro will be dynamically generated.

(defn matching-keys [rec match-feed keys]
  (> (count (clojure.set/select #(and (for [k keys]
                                        (= (% k) (rec k))))
                                (set match-feed)))
     0))

So if it worked!! then this code would produce an (and) something like this when passed keys of [:tag :attrs]:

(and (= (% :tag) (rec :tag))
     (= (% :attrs) (rec :attrs)))

I've been messing around with various `` and~` operators to try to make it work, and am now in a state of confusion. Any guidance is welcome.

Thanks,

Colin

Answer 1

You don't need dynamically generated code for this. Changing the anonymous function to #(every? (fn [k] (= (% k) (rec k))) keys) should do what you want without generating code at runtime.

The ability to use higher-order functions means that you should hardly ever need to dynamically generate code.

Answer 2

You can use eval to evaluate a dynamically built form, e.g.:

(eval '(= 2 3))

Keep in mind that a dynamically evaluated form will have no access to the lexical context. It means that:

(let [a 1 b 2]
  (eval '(+ a b)))

will not work.

However, it is still possible to use a dynamic environment:

(def a nil)
(def b nil)

(binding [a 1 b 2]
  (eval '(+ a b)))