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diff --git a/devdocs/elisp/setting-variables.html b/devdocs/elisp/setting-variables.html new file mode 100644 index 00000000..e44f94d9 --- /dev/null +++ b/devdocs/elisp/setting-variables.html @@ -0,0 +1,55 @@ + <h3 class="section">Setting Variable Values</h3> <p>The usual way to change the value of a variable is with the special form <code>setq</code>. When you need to compute the choice of variable at run time, use the function <code>set</code>. </p> <dl> <dt id="setq">Special Form: <strong>setq</strong> <em>[symbol form]…</em> +</dt> <dd> +<p>This special form is the most common method of changing a variable’s value. Each <var>symbol</var> is given a new value, which is the result of evaluating the corresponding <var>form</var>. The current binding of the symbol is changed. </p> <p><code>setq</code> does not evaluate <var>symbol</var>; it sets the symbol that you write. We say that this argument is <em>automatically quoted</em>. The ‘<samp>q</samp>’ in <code>setq</code> stands for “quoted”. </p> <p>The value of the <code>setq</code> form is the value of the last <var>form</var>. </p> <div class="example"> <pre class="example">(setq x (1+ 2)) + ⇒ 3 +</pre> +<pre class="example">x ; <span class="roman"><code>x</code> now has a global value.</span> + ⇒ 3 +</pre> +<pre class="example">(let ((x 5)) + (setq x 6) ; <span class="roman">The local binding of <code>x</code> is set.</span> + x) + ⇒ 6 +</pre> +<pre class="example">x ; <span class="roman">The global value is unchanged.</span> + ⇒ 3 +</pre> +</div> <p>Note that the first <var>form</var> is evaluated, then the first <var>symbol</var> is set, then the second <var>form</var> is evaluated, then the second <var>symbol</var> is set, and so on: </p> <div class="example"> <pre class="example">(setq x 10 ; <span class="roman">Notice that <code>x</code> is set before</span> + y (1+ x)) ; <span class="roman">the value of <code>y</code> is computed.</span> + ⇒ 11 +</pre> +</div> </dd> +</dl> <dl> <dt id="set">Function: <strong>set</strong> <em>symbol value</em> +</dt> <dd> +<p>This function puts <var>value</var> in the value cell of <var>symbol</var>. Since it is a function rather than a special form, the expression written for <var>symbol</var> is evaluated to obtain the symbol to set. The return value is <var>value</var>. </p> <p>When dynamic variable binding is in effect (the default), <code>set</code> has the same effect as <code>setq</code>, apart from the fact that <code>set</code> evaluates its <var>symbol</var> argument whereas <code>setq</code> does not. But when a variable is lexically bound, <code>set</code> affects its <em>dynamic</em> value, whereas <code>setq</code> affects its current (lexical) value. See <a href="variable-scoping">Variable Scoping</a>. </p> <div class="example"> <pre class="example">(set one 1) +error→ Symbol's value as variable is void: one +</pre> +<pre class="example">(set 'one 1) + ⇒ 1 +</pre> +<pre class="example">(set 'two 'one) + ⇒ one +</pre> +<pre class="example">(set two 2) ; <span class="roman"><code>two</code> evaluates to symbol <code>one</code>.</span> + ⇒ 2 +</pre> +<pre class="example">one ; <span class="roman">So it is <code>one</code> that was set.</span> + ⇒ 2 +(let ((one 1)) ; <span class="roman">This binding of <code>one</code> is set,</span> + (set 'one 3) ; <span class="roman">not the global value.</span> + one) + ⇒ 3 +</pre> +<pre class="example">one + ⇒ 2 +</pre> +</div> <p>If <var>symbol</var> is not actually a symbol, a <code>wrong-type-argument</code> error is signaled. </p> <div class="example"> <pre class="example">(set '(x y) 'z) +error→ Wrong type argument: symbolp, (x y) +</pre> +</div> </dd> +</dl><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/Setting-Variables.html" class="_attribution-link">https://www.gnu.org/software/emacs/manual/html_node/elisp/Setting-Variables.html</a> + </p> +</div> |