Mixins 和介面

Common Lisp 在某些語言（例如 Java）的意義上沒有介面，並且由於 Common Lisp 支援多重繼承和泛型函式，因此對該型別介面的需求較少。但是，使用 mixin 類可以輕鬆實現相同型別的模式。此示例顯示了具有多個相應泛型函式的集合介面的規範。

;; Specification of the COLLECTION "interface"

(defclass collection () ()
  (:documentation "A collection mixin."))

(defgeneric collection-elements (collection)
  (:documentation "Returns a list of the elements in the collection."))

(defgeneric collection-add (collection element)
  (:documentation "Adds an element to the collection."))

(defgeneric collection-remove (collection element)
  (:documentation "Removes the element from the collection, if it is present."))

(defgeneric collection-empty-p (collection)
  (:documentation "Returns whether the collection is empty or not."))

(defmethod collection-empty-p ((c collection))
  "A 'default' implementation of COLLECTION-EMPTY-P that tests
whether the list returned by COLLECTION-ELEMENTS is the empty
list."
  (endp (collection-elements c)))

介面的實現只是一個將 mixin 作為其超類之一的類，以及相應泛型函式的定義。 （此時，請注意，mixin 類實際上僅用於表示類實現介面的意圖。這個示例也適用於一些通用函式和文件，這些函式和文件宣告函式上有方法類。）

;; Implementation of a sorted-set class

(defclass sorted-set (collection)
  ((predicate
    :initarg :predicate
    :reader sorted-set-predicate)
   (test
    :initarg :test
    :initform 'eql
    :reader sorted-set-test)
   (elements
    :initform '()
    :accessor sorted-set-elements
    ;; We can "implement" the COLLECTION-ELEMENTS function, that is,
    ;; define a method on COLLECTION-ELEMENTS, simply by making it
    ;; a reader (or accessor) for the slot.
    :reader collection-elements)))

(defmethod collection-add ((ss sorted-set) element)
  (unless (member element (sorted-set-elements ss)
                  :test (sorted-set-test ss))
    (setf (sorted-set-elements ss)
          (merge 'list
                 (list element)
                 (sorted-set-elements ss)
                 (sorted-set-predicate ss)))))

(defmethod collection-remove ((ss sorted-set) element)
  (setf (sorted-set-elements ss)
        (delete element (sorted-set-elements ss))))

最後，我們可以看到在使用 interface 函式時使用 sorted-set 類的例項是什麼樣的 ：

(let ((ss (make-instance 'sorted-set :predicate '<)))
  (collection-add ss 3)
  (collection-add ss 4)
  (collection-add ss 5)
  (collection-add ss 3)
  (collection-remove ss 5)
  (collection-elements ss))
;; => (3 4)