devdocs/elisp/generic-commands.html


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 <h4 class="subsection">Select among Command Alternatives</h4>   <p>The macro <code>define-alternatives</code> can be used to define <em>generic commands</em>. These are interactive functions whose implementation can be selected from several alternatives, as a matter of user preference. </p> <dl> <dt id="define-alternatives">Macro: <strong>define-alternatives</strong> <em>command &amp;rest customizations</em>
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<p>Define the new command <var>command</var>, a symbol. </p> <p>When a user runs <kbd>M-x <var>command</var> <span class="key">RET</span></kbd> for the first time, Emacs prompts for which real form of the command to use, and records the selection by way of a custom variable. Using a prefix argument repeats this process of choosing an alternative. </p> <p>The variable <code><var>command</var>-alternatives</code> should contain an alist with alternative implementations of <var>command</var>. Until this variable is set, <code>define-alternatives</code> has no effect. </p> <p>If <var>customizations</var> is non-<code>nil</code>, it should consist of alternating <code>defcustom</code> keywords (typically <code>:group</code> and <code>:version</code>) and values to add to the declaration of <code><var>command</var>-alternatives</code>. </p>
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