diff options
Diffstat (limited to 'devdocs/elisp/constant-variables.html')
| -rw-r--r-- | devdocs/elisp/constant-variables.html | 14 |
1 files changed, 14 insertions, 0 deletions
diff --git a/devdocs/elisp/constant-variables.html b/devdocs/elisp/constant-variables.html new file mode 100644 index 00000000..fd0538ba --- /dev/null +++ b/devdocs/elisp/constant-variables.html @@ -0,0 +1,14 @@ + <h3 class="section">Variables that Never Change</h3> <p>In Emacs Lisp, certain symbols normally evaluate to themselves. These include <code>nil</code> and <code>t</code>, as well as any symbol whose name starts with ‘<samp>:</samp>’ (these are called <em>keywords</em>). These symbols cannot be rebound, nor can their values be changed. Any attempt to set or bind <code>nil</code> or <code>t</code> signals a <code>setting-constant</code> error. The same is true for a keyword (a symbol whose name starts with ‘<samp>:</samp>’), if it is interned in the standard obarray, except that setting such a symbol to itself is not an error. </p> <div class="example"> <pre class="example">nil ≡ 'nil + ⇒ nil +</pre> +<pre class="example">(setq nil 500) +error→ Attempt to set constant symbol: nil +</pre> +</div> <dl> <dt id="keywordp">Function: <strong>keywordp</strong> <em>object</em> +</dt> <dd><p>function returns <code>t</code> if <var>object</var> is a symbol whose name starts with ‘<samp>:</samp>’, interned in the standard obarray, and returns <code>nil</code> otherwise. </p></dd> +</dl> <p>These constants are fundamentally different from the constants defined using the <code>defconst</code> special form (see <a href="defining-variables">Defining Variables</a>). A <code>defconst</code> form serves to inform human readers that you do not intend to change the value of a variable, but Emacs does not raise an error if you actually change it. </p> <p>A small number of additional symbols are made read-only for various practical reasons. These include <code>enable-multibyte-characters</code>, <code>most-positive-fixnum</code>, <code>most-negative-fixnum</code>, and a few others. Any attempt to set or bind these also signals a <code>setting-constant</code> error. </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/Constant-Variables.html" class="_attribution-link">https://www.gnu.org/software/emacs/manual/html_node/elisp/Constant-Variables.html</a> + </p> +</div> |