Data commit

Task/Delegates/Common-Lisp/delegates.lisp

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+(defgeneric thing (object)
+  (:documentation "Thing the object."))
+
+(defmethod thing (object)
+  "default implementation")
+
+(defclass delegator ()
+  ((delegate
+    :initarg :delegate
+    :reader delegator-delegate)))
+
+(defmethod thing ((delegator delegator))
+  "If delegator has a delegate, invoke thing on the delegate,
+otherwise return \"no delegate\"."
+  (if (slot-boundp delegator 'delegate)
+    (thing (delegator-delegate delegator))
+    "no delegate"))
+
+(defclass delegate () ())
+
+(defmethod thing ((delegate delegate))
+  "delegate implementation")
+
+(let ((d1 (make-instance 'delegator))
+      (d2 (make-instance 'delegator :delegate nil))
+      (d3 (make-instance 'delegator :delegate (make-instance 'delegate))))
+  (assert (string= "no delegate" (thing d1)))
+  (assert (string= "default implementation" (thing d2)))
+  (assert (string= "delegate implementation" (thing d3))))