oftlisp typeclasses

example.lisp

(deftypeclass (indexed 'a)
  (fn ((get 'a) (this @) (index fixnum)))
  (fn ((len fixnum) (this @))))

; An example data structure.
(deftype (append-list 'a)
  (list (list 'a)))

(instance (append-list 'a) (indexed 'a)
  (defn (get this index)
    ...)
  (defn (len this)
    (-> this
	  (map len)
	  sum)))

(defn (main)
  ; I can't find a way to get rid of the need for a type annotation to
  ; transform a built-in data structure into a deftype'd one. The problem is:

  (def x '((1 2 3) (4 5 6) (7 8 9)))
  (def y (as (append-list fixnum) x))

  (assert-eq (len x) 3)
  (assert-eq (len y) 9))

; And as an example of a non-required function on a typeclass:
(defn ((last 'a) (x (indexed 'a)))
  (get x (1- (len x))))