gistfile1.clj

(ns example.core
  (:import clojure.lang.ILookup))

(deftype MyType [a b]
  ILookup
  (valAt [this key]
     (cond
        (= key :a) a
        (= key :b) (+ b 100500)
        :else nil)
  (valAt [this key noobj]
     (if-let [ret (key this)] ret noobj)))

gistfile2.clj

user=> (use 'example.core)
nil
user=> (in-ns 'example.core)
#<Namespace example.core>
example.core=> (def mt (MyType. 1 2))
#'example.core/mt
example.core=> (:a mt)
1
example.core=> (:b mt)
100502
example.core=> (:c mt)
nil
example.core=> (:c mt 15)
15