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dynamic-mixins

Dynamic-mixins is for simple, dynamic class combination:

(in-package :dynamic-mixins)

(defclass a () ())
(defclass b () ())
(defclass c () ())

(make-instance (mix 'a 'b)) ;; => #<MIXIN-OBJECT (A B)>

(let ((a (make-instance 'a)))
  (ensure-mix a 'b 'c)      ;; => #<MIXIN-OBJECT (B C A)>
  (delete-from-mix a 'a)    ;; => #<MIXIN-OBJECT (B C)>
  (delete-from-mix a 'c))   ;; => #<B>

This allows objects to be mixed and updated without manually defining many permutations.

Dictionary

• MIX &rest classes: This produces a "mix list", which is generally only useful for passing to MAKE-INSTANCE. Note: Order matters! This determines class precedence.

• ENSURE-MIX object &rest name-or-class: Ensure that classes listed in name-or-class are part of object. This will create a new class and CHANGE-CLASS object if necessary. Note: Order matters! This determines class precedence.

• DELETE-FROM-MIX object &rest name-or-class: Remove classes listed in name-or-class from the object's class. This will create a new class and CHANGE-CLASS object if necessary. However, object must be a MIXIN-OBJECT created by (MAKE-INSTANCE (MIX ...) ...) or ENSURE-MIX. Otherwise, nothing will be changed.

Notes

Order and Precedence

Order matters; you are defining a new class which has the specified classes as direct superclasses.

ENSURE-MIX prepends classes in the order specified. (Originally, it appended classes.) This is simply more useful in practice:

(defclass general-object () ())
(defclass specializing-mixin () ())

(defgeneric some-operation (x))

(defmethod some-operation (x)
   "Handle the general case"
   ...)

(defmethod some-operation ((x specializing-mixin))
   "Handle the case for SPECIALIZING-MIXIN"
   ...)

(let ((x (make-instance 'general-object)))
  (ensure-mix x 'specializing-mixin)
  (some-operation x))

If SPECIALIZING-MIXIN were appended, the method which specialized on it would never be called. In practice, this defeats the point. Therefore, mixins now get precedence.

Errors

Errors regarding precendence and circularity are now handled, or rather, causing such an error will not produce a nearly-unrecoverable situation. Now you will just get an error.