Edd Mann Developer

Exploring Multi-Methods in Clojure

Currently reading through the Joy of Clojure book I have recently been introduced to Clojure’s multi-method support. I decided that it would be interesting to see this feature in action, so I opened up LightTable and codified some example use-cases.


The implementation below carries on from were my article about FizzBuzz in Clojure left off. Using the two documented fizz? and buzz? helpers, followed by the juxt core function we are able to codify the problem in a highly descriptive manner.

(def fizz? #(zero? (mod % 3)))
(def buzz? #(zero? (mod % 5)))

(defmulti fizzbuzz (juxt fizz? buzz?))
(defmethod fizzbuzz [true false] [n] "Fizz")
(defmethod fizzbuzz [false true] [n] "Buzz")
(defmethod fizzbuzz [true true] [n]  "FizzBuzz")
(defmethod fizzbuzz :default [n] (str n))

(->> (range 1 10) (map fizzbuzz)) ; (1 2 "Fizz" 4 "Buzz" "Fizz" 7 8 "Fizz")


We are also able to use multi-methods in a similar manner to pattern-matching found in languages such as Scala and Haskell. The below implementation recursively computes the supplied arguments factorial value, handling the base-case of 0 using the function identity as the dispatch call.

(defmulti factorial identity)
(defmethod factorial 0 [_] 1)
(defmethod factorial :default [n] (* n (factorial (dec n))))

(factorial 5) ; 120


We are able to expand upon the rules based approach laid out in the previous examples by documenting the Rock-Paper-Scissors game in code. Using this multi-method allows us to compute all the possible winning-moves for both players.

(defmulti beats? vector)
(defmethod beats? [:paper :rock] [_ _] true)
(defmethod beats? [:scissors :paper] [_ _] true)
(defmethod beats? [:rock :scissors] [_ _] true)
(defmethod beats? :default [_ _] false)

(def winning-moves
  (for [one [:rock :paper :scissors]
        two [:rock :paper :scissors]
        :let [one-wins (beats? one two)
              two-wins (beats? two one)]
        :when (or one-wins two-wins)]
    (if one-wins ["1" one two] ["2" one two])))
; [2 :rock :paper] [1 :rock :scissors] [1 :paper :rock]
; [2 :paper :scissors] [2 :scissors :rock] [1 :scissors :paper])

Odd or Even

Finally we can use the core odd? function to polymorphically return a string depicting the supplied values odd-even status.

(defmulti odd-even odd?)
(defmethod odd-even true [n] (str n " is Odd"))
(defmethod odd-even false [n] (str n " is Even"))

(odd-even? 5) ; 5 is Odd