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diff --git a/devdocs/elisp/void-variables.html b/devdocs/elisp/void-variables.html new file mode 100644 index 00000000..dfcd140d --- /dev/null +++ b/devdocs/elisp/void-variables.html @@ -0,0 +1,51 @@ + <h3 class="section">When a Variable is Void</h3> <p>We say that a variable is void if its symbol has an unassigned value cell (see <a href="symbol-components">Symbol Components</a>). </p> <p>Under Emacs Lisp’s default dynamic scoping rule (see <a href="variable-scoping">Variable Scoping</a>), the value cell stores the variable’s current (local or global) value. Note that an unassigned value cell is <em>not</em> the same as having <code>nil</code> in the value cell. The symbol <code>nil</code> is a Lisp object and can be the value of a variable, just as any other object can be; but it is still a value. If a variable is void, trying to evaluate the variable signals a <code>void-variable</code> error, instead of returning a value. </p> <p>Under the optional lexical scoping rule, the value cell only holds the variable’s global value—the value outside of any lexical binding construct. When a variable is lexically bound, the local value is determined by the lexical environment; hence, variables can have local values even if their symbols’ value cells are unassigned. </p> <dl> <dt id="makunbound">Function: <strong>makunbound</strong> <em>symbol</em> +</dt> <dd> +<p>This function empties out the value cell of <var>symbol</var>, making the variable void. It returns <var>symbol</var>. </p> <p>If <var>symbol</var> has a dynamic local binding, <code>makunbound</code> voids the current binding, and this voidness lasts only as long as the local binding is in effect. Afterwards, the previously shadowed local or global binding is reexposed; then the variable will no longer be void, unless the reexposed binding is void too. </p> <p>Here are some examples (assuming dynamic binding is in effect): </p> <div class="example"> <pre class="example">(setq x 1) ; <span class="roman">Put a value in the global binding.</span> + ⇒ 1 +(let ((x 2)) ; <span class="roman">Locally bind it.</span> + (makunbound 'x) ; <span class="roman">Void the local binding.</span> + x) +error→ Symbol's value as variable is void: x +</pre> +<pre class="example">x ; <span class="roman">The global binding is unchanged.</span> + ⇒ 1 + +(let ((x 2)) ; <span class="roman">Locally bind it.</span> + (let ((x 3)) ; <span class="roman">And again.</span> + (makunbound 'x) ; <span class="roman">Void the innermost-local binding.</span> + x)) ; <span class="roman">And refer: it’s void.</span> +error→ Symbol's value as variable is void: x +</pre> + +<pre class="example">(let ((x 2)) + (let ((x 3)) + (makunbound 'x)) ; <span class="roman">Void inner binding, then remove it.</span> + x) ; <span class="roman">Now outer <code>let</code> binding is visible.</span> + ⇒ 2 +</pre> +</div> </dd> +</dl> <dl> <dt id="boundp">Function: <strong>boundp</strong> <em>variable</em> +</dt> <dd> +<p>This function returns <code>t</code> if <var>variable</var> (a symbol) is not void, and <code>nil</code> if it is void. </p> <p>Here are some examples (assuming dynamic binding is in effect): </p> <div class="example"> <pre class="example">(boundp 'abracadabra) ; <span class="roman">Starts out void.</span> + ⇒ nil +</pre> +<pre class="example">(let ((abracadabra 5)) ; <span class="roman">Locally bind it.</span> + (boundp 'abracadabra)) + ⇒ t +</pre> +<pre class="example">(boundp 'abracadabra) ; <span class="roman">Still globally void.</span> + ⇒ nil +</pre> +<pre class="example">(setq abracadabra 5) ; <span class="roman">Make it globally nonvoid.</span> + ⇒ 5 +</pre> +<pre class="example">(boundp 'abracadabra) + ⇒ t +</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/Void-Variables.html" class="_attribution-link">https://www.gnu.org/software/emacs/manual/html_node/elisp/Void-Variables.html</a> + </p> +</div> |