keyword args in clojure

Wed, 06 May 2009

I like the keyword args support in Common Lisp. So, when I learned that clojure didn't support keyword args in the base language, I was a bit disappointed. But, with clojure's support for destructuring and macros, it should be easy to build keyword args … right?

Rich Hickey posted a simple example on the clojure mailing list, which I adapted slightly to my taste:

(defmacro defun [sym args & body]
  (let [[pargs [_ & kargs]] (split-with (fn [x] (not (= x '&key))) args)
	gkeys (gensym "gkeys__")
	letk (fn [k]
	       (let [[nm val] (if (vector? k) k [k])
		     kname (keyword (name nm))]
		 `(~nm (or (~gkeys ~kname) ~val))))]
    `(defn ~sym [~@pargs & k#]
       (let [~gkeys (apply hash-map k#)
	     ~@(apply concat (map letk kargs))]
	 ~@body))))

After which:

(defun foo [a b &key c [d 5]]
  [a b c d])

user> (foo 1 2 :c 10)
[1 2 10 5]

Joy.