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This is an interesting library which allows to add and remove mixin classes to the CLOS objects on the fly!

Common Lisp allows to change object's class, but this library goes further. It keeps track which mixins were already added to the object and allows to add new or to remove existing!

To demonstrate, how this works, let's pretend we have a graphics system where each figure can be filled with color and/or can have rounded corners. And we can have generic methods behave differently depending on traits of the figure:

POFTHEDAY> (defclass figure () ())

POFTHEDAY> (defclass box (figure) ())

POFTHEDAY> (defclass filled ()
             ((fill-color :initarg :fill-color)))

POFTHEDAY> (defclass rounded ()
             ((border-radius :initarg :border-radius)))

POFTHEDAY> (defmethod describe-object ((obj box) stream)
             (format stream "This is the box.~%"))

POFTHEDAY> (defmethod describe-object :after ((obj rounded) stream)
             (format stream "It has round corners.~%"))

POFTHEDAY> (defmethod describe-object :after ((obj filled) stream)
             (format stream "It filled with color.~%"))

Now we can construct the box object and simulate how it evolves over time when the user decides to make it's corner smoother and to fill it with a color:

POFTHEDAY> (defparameter *obj* (make-instance 'box))

#<BOX {10016F64A3}>

POFTHEDAY> (describe *obj*)
This is the box.

;; Now we'll add a trait to our object:
POFTHEDAY> (dynamic-mixins:ensure-mix *obj* 'rounded)


POFTHEDAY> (describe *obj*)
This is the box.
It has round corners.

;; And yet another trait!
POFTHEDAY> (dynamic-mixins:ensure-mix *obj* 'filled)

POFTHEDAY> (describe *obj*)
This is the box.
It has round corners.
It filled with color.

;; We also can remove a mixin:
POFTHEDAY> (dynamic-mixins:delete-from-mix *obj* 'rounded)

POFTHEDAY> (describe *obj*)
This is the box.
It filled with color.

The only problem I found is that it is impossible to pass initargs to the ensure-mix function. Because of that, slots which we added along with the mixin, remain unbound.

But I found the solution to this problem:

POFTHEDAY> (defun add-mixin (object mixin-class &rest initargs)
             (let ((new-class (dynamic-mixins::ensure-mixin
                               (funcall #'dynamic-mixins::%mix
                                        object mixin-class))))
               (apply #'change-class object new-class initargs)))

POFTHEDAY> (slot-boundp *obj* 'fill-color)

;; Now we'll remove and add this mixin again:
POFTHEDAY> (dynamic-mixins:delete-from-mix *obj* 'filled)

POFTHEDAY> (add-mixin *obj* 'filled
                      :fill-color "#FF7F00")

POFTHEDAY> (slot-boundp *obj* 'fill-color)

POFTHEDAY> (slot-value *obj* 'fill-color)

Hope, Ryan Pavlik will incorporate my pull request with this additional function!

If you are found this post interesting, then you also might like a post about dynamic-classes system.

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