multiaritytest.clj

(ns multiaritytest)

; This works great.
(defprotocol P
  (foo [this a] [this a b]))

(deftype R []
  P
  (foo [this a] (println "hi"))
  (foo [this a b] (println "hi2"))) 

(def r (R.)) 

(foo r :a) 
(foo r :a :b) 

;=> hi
;=> hi2

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; This does not.
(defprotocol P
  (foo [this a] [this a b]))

(defrecord R []) 

(extend-protocol P R
  (foo [this a] (println "hi"))
  (foo [this a b] (println "hi2"))) 

(def r (R.)) 

(foo r :a) 
(foo r :a :b) 

;=> java.lang.IllegalArgumentException: Wrong number of args passed to: multiaritytest$eval--84$fn (multiaritytest.clj:0)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; This works great also. (thanks Kotarak!)
(defprotocol P
  (foo [this a] [this a b]))

(defrecord R []) 

(extend R P
  {:foo (fn
          ([this a] (println "hi"))
          ([this a b] (println "hi2")))})

(def r (R.)) 

(foo r :a) 
(foo r :a :b) 

;=> hi
;=> hi2

Try (extend R P {:foo (fn ([this a] 1) ([this a b] 2))}).

Awesome, thanks Kotarak, I updated the gist with that example.