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copied!<p>For example you want to name font sizes:</p> <pre><code>(defconstant +large+ 3) (defconstant +medium+ 2) (defconstant +small+ 1) </code></pre> <p>You could write a macro to make that shorter.</p> <p>Above constant definitions are usually written ONLY when these numbers need to be passed to some external non-Lisp code.</p> <p>Otherwise one would just use keyword symbols: :large, :medium and :small.</p> <p>You can test them with EQ and everything that uses some test for equality.</p> <pre><code>(let ((size :medium)) (ecase size (:small ...) (:medium ...) (:large ...))) </code></pre> <p>You can also write methods for it:</p> <pre><code>(defmethod draw-string (message x y (size (eql :large))) ...) </code></pre> <p>As mentioned you could define a set type:</p> <pre><code>(deftype size () '(member :small :medium :large)) </code></pre> <p>Then you can check if something is either of those:</p> <pre><code>(let ((my-size :medium)) (check-type my-size size)) </code></pre> <p>Above would signal an error if my-size is not one of :small, :medium or :large.</p> <p>You can also use the type in a defclass form:</p> <pre><code>(defclass vehicle () ((width :type size :initarg :width))) </code></pre> <p>Now you would create objects like here:</p> <pre><code>(make-instance 'vehicle :width :large) </code></pre> <p>Some Common Lisp implementations will check when you set the slot to some illegal value.</p> <p>If you now create objects of class vehicle, the slots will be one of :large, :medium or :small. If you look at the object in a debugger, inspector or some other tool you will see the symbolic names and not 1, 2 or 3 (or whatever values you would normally use).</p> <p>This is part of the Lisp style: use symbolic names when possible. Use symbols with number values only in interface code to foreign functions (like calling external C code that uses enums).</p>

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