Understanding Clojure functions...

core.clj

(defn foo
  ([a b] (println "bar"))
  ([a b c] (println "foobar")))
;#'user/foo
(foo 1 2)
;bar
;nil
(foo 1 2 3)
;foobar
;nil
(foo 1)
;ArityException Wrong number of args (1) passed to: core$foo  clojure.lang.AFn.throwArity (AFn.java:437)

(foo 1 2 3 4)
;ArityException Wrong number of args (4) passed to: core$foo  clojure.lang.AFn.throwArity (AFn.java:437)

(if false (foo 1 2) (foo 1))
;ArityException Wrong number of args (1) passed to: core$foo  clojure.lang.AFn.throwArity (AFn.java:437)

(defn bar [runme] (if runme (foo 1 2) (foo 1)))
;#'user/bar
(bar true)
;bar
;nil
(bar false)
;ArityException Wrong number of args (1) passed to: core$foo  clojure.lang.AFn.throwArity (AFn.java:437)