devdocs/elisp/self_002devaluating-forms.html


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 <h4 class="subsection">Self-Evaluating Forms</h4>     <p>A <em>self-evaluating form</em> is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated. Thus, the number 25 evaluates to 25, and the string <code>"foo"</code> evaluates to the string <code>"foo"</code>. Likewise, evaluating a vector does not cause evaluation of the elements of the vector—it returns the same vector with its contents unchanged. </p> <div class="example"> <pre class="example">'123               ; <span class="roman">A number, shown without evaluation.</span>
     ⇒ 123
</pre>
<pre class="example">123                ; <span class="roman">Evaluated as usual—result is the same.</span>
     ⇒ 123
</pre>
<pre class="example">(eval '123)        ; <span class="roman">Evaluated "by hand"—result is the same.</span>
     ⇒ 123
</pre>
<pre class="example">(eval (eval '123)) ; <span class="roman">Evaluating twice changes nothing.</span>
     ⇒ 123
</pre>
</div> <p>A self-evaluating form yields a value that becomes part of the program, and you should not try to modify it via <code>setcar</code>, <code>aset</code> or similar operations. The Lisp interpreter might unify the constants yielded by your program’s self-evaluating forms, so that these constants might share structure. See <a href="mutability">Mutability</a>. </p> <p>It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there’s no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example: </p> <div class="example"> <pre class="example">;; <span class="roman">Build an expression containing a buffer object.</span>
(setq print-exp (list 'print (current-buffer)))
     ⇒ (print #&lt;buffer eval.texi&gt;)
</pre>
<pre class="example">;; <span class="roman">Evaluate it.</span>
(eval print-exp)
     -| #&lt;buffer eval.texi&gt;
     ⇒ #&lt;buffer eval.texi&gt;
</pre>
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