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<h3 class="section">Backquote</h3> <p><em>Backquote constructs</em> allow you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form <code>quote</code> (described in the previous section; see <a href="quoting">Quoting</a>). For example, these two forms yield identical results: </p> <div class="example"> <pre class="example">`(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
</pre>
<pre class="example">'(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
</pre>
</div> <p>The special marker ‘<samp>,</samp>’ inside of the argument to backquote indicates a value that isn’t constant. The Emacs Lisp evaluator evaluates the argument of ‘<samp>,</samp>’, and puts the value in the list structure: </p> <div class="example"> <pre class="example">`(a list of ,(+ 2 3) elements)
⇒ (a list of 5 elements)
</pre>
</div> <p>Substitution with ‘<samp>,</samp>’ is allowed at deeper levels of the list structure also. For example: </p> <div class="example"> <pre class="example">`(1 2 (3 ,(+ 4 5)))
⇒ (1 2 (3 9))
</pre>
</div> <p>You can also <em>splice</em> an evaluated value into the resulting list, using the special marker ‘<samp>,@</samp>’. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ‘<samp>`</samp>’ is often unreadable. Here are some examples: </p> <div class="example"> <pre class="example">(setq some-list '(2 3))
⇒ (2 3)
</pre>
<pre class="example">(cons 1 (append some-list '(4) some-list))
⇒ (1 2 3 4 2 3)
</pre>
<pre class="example">`(1 ,@some-list 4 ,@some-list)
⇒ (1 2 3 4 2 3)
</pre>
<pre class="example">(setq list '(hack foo bar))
⇒ (hack foo bar)
</pre>
<pre class="example">(cons 'use
(cons 'the
(cons 'words (append (cdr list) '(as elements)))))
⇒ (use the words foo bar as elements)
</pre>
<pre class="example">`(use the words ,@(cdr list) as elements)
⇒ (use the words foo bar as elements)
</pre>
</div> <p>If a subexpression of a backquote construct has no substitutions or splices, it acts like <code>quote</code> in that it yields conses, vectors and strings that might be shared and should not be modified. See <a href="self_002devaluating-forms">Self-Evaluating Forms</a>. </p><div class="_attribution">
<p class="_attribution-p">
Copyright © 1990-1996, 1998-2022 Free Software Foundation, Inc. <br>Licensed under the GNU GPL license.<br>
<a href="https://www.gnu.org/software/emacs/manual/html_node/elisp/Backquote.html" class="_attribution-link">https://www.gnu.org/software/emacs/manual/html_node/elisp/Backquote.html</a>
</p>
</div>
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