test.clj

(defprotocol Arithmetic
  "Basic arithmetic operations"
  (add [a b] "Addition")
  (sub [a b] "Subtraction")
  (mul [a b] "Multiplication")
  (div [a b] "Division"))

;; Number is the class java.lang.Number
(extend Number Arithmetic
        {:add +, :sub -, :mul *, :div /})

(defrecord Complex [#^double real #^double imaginary]
  Arithmetic
  (add [a b] (Complex. (+ (:real a) (:real b))
                      (+ (:imaginary a) (:imaginary b))))
  (sub [a b] (Complex. (- (:real a) (:real b))
                      (- (:imaginary a) (:imaginary b))))
  (mul [a b] (Complex. (- (* (:real a) (:real b))
                         (* (:imaginary a) (:imaginary b)))
                      (+ (* (:real a) (:imaginary b))
                         (* (:imaginary a) (:real b))))))