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wsgac/mixins.lisp

Last active October 21, 2025 20:56
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Commutative CLOS mixins demo
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mixins.lisp
(defvar level)
(defclass setup-mixin () ())
(defmethod run :around ((m setup-mixin))
(let ((level 0))
(call-next-method)))
(defclass mixin-1 () ())
(defmethod run :around ((m mixin-1))
(incf level)
(format t "~%>~v@tmixin-1" (* 4 level))
(call-next-method)
(format t "~%<~v@tmixin-1" (* 4 level))
(decf level))
(defclass mixin-2 () ())
(defmethod run :around ((m mixin-2))
(incf level)
(format t "~%>~v@tmixin-2" (* 4 level))
(call-next-method)
(format t "~%<~v@tmixin-2" (* 4 level))
(decf level))
(defclass mixin-3 () ())
(defmethod run :around ((m mixin-3))
(incf level)
(format t "~%>~v@tmixin-3" (* 4 level))
(call-next-method)
(format t "~%<~v@tmixin-3" (* 4 level))
(decf level))
(defclass main (setup-mixin mixin-1 mixin-2 mixin-3) ())
(defmethod run ((m main))
(format t "~%-~v@tmain" (* 4 (1+ level))))
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wsgac commented Oct 21, 2025 •
edited
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This is a short demonstration of how you can wrap your main method functionality with a series of mixins defining their own :around methods. Since the mixin methods are independent of each other, their order in the main superclass list can be altered, in effect modifying the order of execution.

When main is defined as follows:

(defclass main (setup-mixin mixin-1 mixin-2 mixin-3) ())

we get the following behavior:

CL-USER> (run (make-instance 'main))

>    mixin-1
>        mixin-2
>            mixin-3
-                main
<            mixin-3
<        mixin-2
<    mixin-1

If we rearrange the mixins in main's superclass list:

(defclass main (setup-mixin mixin-3 mixin-2 mixin-1) ())

we get a different order of execution:

CL-USER> (run (make-instance 'main))

>    mixin-3
>        mixin-2
>            mixin-1
-                main
<            mixin-1
<        mixin-2
<    mixin-3

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