listadt.clj

(ns user)

;; loosely inspired by http://book.realworldhaskell.org/read/defining-types-streamlining-functions.html

(defprotocol Cons
  (car [x])
  (cdr [x]))

(defprotocol List
  (head [x])
  (tail [x])
  (size [x])
  (cadr [x])
  (cddr [x]))

(deftype TCons [_car _cdr] :as this
  Cons
  (car [] _car)
  (cdr [] _cdr)
  List
  (head [] (car this))
  (tail [] (cdr this))
  (size [] (if (cdr this) (inc (size (cdr this))) 1))
  (cadr [] (car (cdr this)))
  (cddr [] (cdr (cdr this))))

(def c1 (TCons 1 nil))
(car c1)  ;; 1
(cdr c1)  ;; nil

(def l1 (TCons 95 (TCons 96 (TCons 97 (TCons 98 nil)))))
(head l1)  ;; 95
(tail l1)  ;; #:TCons{:_car 96, :_cdr #:TCons{:_car 97, :_cdr #:TCons{:_car 98, :_cdr nil}}}
(size l1)  ;; 4
(cadr l1)  ;; 96
(cddr l1)  ;; #:TCons{:_car 97, :_cdr #:TCons{:_car 98, :_cdr nil}}