Scheme's mcons in Clojure. (Feels kind of dirty to build a mutable data structure, but fun nonetheless)

mcons.clj

;; http://docs.racket-lang.org/reference/mpairs.html

(defprotocol IMCons
  (mcar [p])
  (mcdr [p])
  (set-mcar! [p val])
  (set-mcdr! [p val]))

(deftype MCons [^{:volatile-mutable true} car ^{:volatile-mutable true} cdr]
  IMCons
  (mcar [this] car)
  (mcdr [this] cdr)
  (set-mcar! [this val]
    (set! car val))
  (set-mcdr! [this val]
    (set! cdr val)))

(defn mcons [a b]
  (MCons. a b))
(defn mcar [p] (.mcar p))
(defn mcdr [p] (.mcdr p))
(defn set-mcar! [p v] (.set-mcar! p v))
(defn set-mcdr! [p v] (.set-mcdr! p v))

;; Example
(def *m* (mcons 1 2))
(set-mcar! *m* 4)
(mcar *m*) ;; => 4