extend.clj

(ns test-app.core)

(defprotocol Fooable
  (foo [this f])
  (bar [this b]))

(deftype FooPrinter []
  Fooable
  (foo [this f] (println "foo: " f))
  (bar [this b] (println "bar: " b)))

(foo (FooPrinter.) 3)

(extend java.lang.String
  Fooable
  {:foo (fn [this f] (str this " foo: " f))
   :bar (fn [this b] (str this " bar: " b))})

(println (foo "a string" 3))

(extend clojure.lang.PersistentArrayMap
  Fooable
  {:foo (fn [this f] (assoc this :foo f))
   :bar (fn [this b] (assoc this :bar b))})

(println (foo {:bar "bar" :baz "baz"} "foozle"))