Clojure: defmulti on different class types

Question

Quick clojure question, I think this is mostly syntax related. How do I dispatch a multimethod based on specific type signatures of the arguments, for example:

(defn foo 
     ([String a String b] (println a b))
     ([Long a Long b] (println (+ a b))
     ([String a Long b] (println a (str b))))

I want to extend this to arbitrary stuff, eg two strings followed by a map, to maps followed by a double, two doubles followed by an IFn etc...

Answer 1

(defn class2 [x y]
  [(class x) (class y)])

(defmulti foo class2)

(defmethod foo [String String] [a b]
  (println a b))

(defmethod foo [Long Long] [a b]
  (println (+ a b)))

From the REPL:

user=> (foo "bar" "baz")
bar baz
nil
user=> (foo 1 2)
3
nil

You could also consider using type instead of class; type returns :type metadata, delegating to class if there is none.

Also, class2 does not have to be defined at top level; passing (fn [x y] ...) as the dispatch function to defmulti is fine too.

Answer 2

If you use type instead of class, the code will also work in ClojureScript.