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copied!<p>In Common Lisp, structures are thought as rigid and low-level records. They don't have fancy dynamic features.</p> <p>What you can do with structures is to defined a new structure type which inherits from another one. There is single inheritance available.</p> <p>To handle dynamic extensibility, a typical way is to add a property list slot to a structure. See the answer of Joshua.</p> <p>Then there is the Common Lisp Object System, which provides multiple inheritance and you can change classes at runtime. So you can add a slot to a class and the instances of that class update themselves. You can also change the class of an object and slots may get added or deleted. Still, typically all instances of a class will have the same set of slots. Again, one sees that a slot with a property list can be added and used for extensibility.</p> <p>There are other object systems for Common Lisp, which can easily add slots on a per-instance base. But it is usually too much to use them just for that, since they are quite a bit more powerful.</p> <p>With CLOS and the Meta-object protocol one can try to hide it. Here I am using LispWorks:</p> <p>We define a mixin class for our properties:</p> <pre><code>(defclass property-mixin () ((plist :initform nil)) #+lispworks (:optimize-slot-access nil)) </code></pre> <p>Setting and reading the properties:</p> <pre><code>(defmethod set-property ((object property-mixin) key value) (setf (getf (slot-value object 'plist) key) value)) (defmethod get-property ((object property-mixin) key) (getf (slot-value object 'plist) key)) </code></pre> <p>Now we write methods to make <code>SLOT-VALUE</code> accepting our property names:</p> <pre><code>(defmethod (setf clos:slot-value-using-class) (value (class standard-class) (object property-mixin) slot-name) (declare (ignorable class)) (if (slot-exists-p object slot-name) (call-next-method) (progn (set-property object slot-name value) value))) (defmethod clos:slot-value-using-class ((class standard-class) (object property-mixin) slot-name) (declare (ignorable class)) (if (slot-exists-p object slot-name) (call-next-method) (get-property object slot-name))) </code></pre> <p>Example. We define an automobile class with two slots:</p> <pre><code>(defclass automobile (property-mixin) ((company :initarg :company) (motor :initarg :motor)) #+lispworks (:optimize-slot-access nil)) </code></pre> <p>Now an instance:</p> <pre><code>CL-USER 45 > (setf a6 (make-instance 'automobile :company :audi :motor :v6)) #<AUTOMOBILE 402005B47B> </code></pre> <p>We can get a normal slot value:</p> <pre><code>CL-USER 46 > (slot-value c1 'motor) :V6 </code></pre> <p>Let's write to a slot which does not exist, but will be added to our property list:</p> <pre><code>CL-USER 47 > (setf (slot-value a6 'seats) 4) 4 </code></pre> <p>We can get the value back:</p> <pre><code>CL-USER 48 > (slot-value c1 'seats) 4 </code></pre>

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