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    <p>In Common Lisp, all the data (such as primitive variable references, lists, arrays and hash tables) are mutable by default, so that nothing can prevent someone from changing the value of a variable, an element of a sequence or a field of a structure. </p> <p>However, if we're not talking about primitive data types but about user-defined data, that is about the CLOS structures and classes, their slots can be made read-only. For example, for structures:</p> <pre><code>(defstruct person (name nil :type string :read-only t) (age nil :type (integer 0 100))) (let ((john (make-person :name "John" :age 30))) (princ john) ;; * `age' is mutable: (incf (person-age john)) (princ john) ;; * `name' is not: ;; (setf (person-name john) "garbage name") ;; ^ you can't do this because the `defstruct' macro just don't emit SETFer ;; for the `name' slot as you made it read-only. ) </code></pre> <p>Classes provide even more access control for the slots (this is similar to the mechanism of the <code>const</code> qualifier in C, the difference is that in Common Lisp it is not a compile-time guarantee, but an exceptions which can be handled in a restarts), you can give them read and write, read-only, write-only, or no access.</p> <hr> <p>See the following links for more information:</p> <ul> <li><p><a href="http://psg.com/~dlamkins/sl/chapter07.html" rel="nofollow noreferrer">http://psg.com/~dlamkins/sl/chapter07.html</a></p></li> <li><p><a href="http://www.gigamonkeys.com/book/object-reorientation-classes.html" rel="nofollow noreferrer">http://www.gigamonkeys.com/book/object-reorientation-classes.html</a></p></li> </ul>
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    1. POIs all data in Lisp immutable?
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    1. CONote: if you uncomment the call to `person-name' then you get a STYLE-WARNING about undefined function (e.g., in SBCL), so this is a weak form of the compile-time guarantee that prohibits write access to a certain slot.
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