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<html><head><title>XLISP backquote</title>
<link rel="stylesheet" type="text/css" href="reference.css">
</head>
<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="../tutorials/tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
<hr>
<h1>backquote</h1>
<hr>
<p>backquote:</p>
<p><table cellpadding="0" cellspacing="0" style="margin-left:10px"><tbody>
<tr valign="top">
<td><nobr>Type:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>special form (fsubr)</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xlcont.c, xlread.c</nobr></td>
</tr>
</tbody></table></p>
<p>comma, comma-at:</p>
<p><table cellpadding="0" cellspacing="0" style="margin-left:10px"><tbody>
<tr valign="top">
<td><nobr>Type:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>reader expansion</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xlcont.c, xlread.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<p><div class="box">
<dl>
<dt>(<b>backquote</b> expr)</dt>
<dd><i>expr</i> - an expression which is not evaluated except for 'comma' and 'comma-at' portions<br>
<i>returns</i> - a copy of the template with 'comma' and 'comma-at' expressions expanded</dd>
</dl>
<dl>
<dt>(<b>comma</b> <i>expr</i>)</dt>
<dd><i>expr</i> - an expression which is evaluated within a backquoted expression</dd>
</dl>
<dl>
<dt>(<b>comma-at</b> <i>expr</i>)</dt>
<dd><i>expr</i> - an expression which is evaluated within a backquoted expression</dd>
</dl>
</div></p>
<h2>Description</h2>
<p>The 'backquote' special form returns 'expr' unevaluated, like
<a href="quote.htm">quote</a>. The difference is that portions of the
expression may be evaluated when they are preceeded by a 'comma' or
'<nobr>comma-at</nobr>'.</p>
<ul>
<li><p><b>comma</b> will evaluate the portion of the expression the comma
preceeds. <nobr>If the</nobr> portion is an atom or a list, it is placed as
is within the expression.</p></li>
<li><p><nobr><b>comma-at</b></nobr> will evaluate the portion of the
expression that the <nobr>'comma-at</nobr>' preceeds. <nobr>The
portion</nobr> needs to be a list. <nobr>The list</nobr> is spliced into the
expression. <nobr>If the</nobr> portion is not a list,
'<nobr>comma-at</nobr>' will splice in nothing.</p></li>
</ul>
<p>XLISP supports the following read macros:</p>
<p><table cellpadding="0" cellspacing="0"><tbody>
<tr>
<td><nobr><code> </code></nobr></td>
<td class="button" align="right"><nobr><code>`<font color="#0000CC">expression</font></code></nobr></td>
<td><nobr> → </nobr></td>
<td class="button" align="right"><nobr><code>(backquote <font color="#0000CC">expression</font>)</code></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr>
<td><nobr><code> </code></nobr></td>
<td class="button" align="right"><nobr><code>,<font color="#0000CC">expression</font></code></nobr></td>
<td><nobr> → </nobr></td>
<td class="button" align="right"><nobr><code>(comma <font color="#0000CC">expression</font>)</code></nobr></td>
</tr>
<tr>
<td height="2px"></td>
</tr>
<tr>
<td><nobr><code> </code></nobr></td>
<td class="button" align="right"><nobr><code>,@<font color="#0000CC">expression</font></code></nobr></td>
<td><nobr> → </nobr></td>
<td class="button" align="right"><nobr><code>(comma-at <font color="#0000CC">expression</font>)</code></nobr></td>
</tr>
</tbody></table></p>
<h2>Examples</h2>
<pre class="example">
(setq box 'stuff-inside) => STUFF-INSIDE <font color="#008844">; BOX contains STUFF-INSIDE</font>
(print box) => STUFF-INSIDE
'(i have the box) ≡ (quote (i have the box)) => (I HAVE THE BOX)
`(i have the box) ≡ (backquote (i have the box)) => (I HAVE THE BOX)
`(i have the ,box) ≡ (backquote (i have the (comma box))) => (I HAVE THE STUFF-INSIDE)
`(i have the ,@box) ≡ (backquote (I have the (comma-at box))) => (I HAVE THE) <font color="#008844">; STUFF-INSIDE is not a list</font>
(setq automobile '(a van)) => (A VAN) <font color="#008844">; AUTOMOBILE is a VAN</font>
(print automobile) => (A VAN)
'(I have automobile) ≡ (quote (I have automobile)) => (I HAVE AUTOMOBILE)
`(I have automobile) ≡ (backquote (I have automobile)) => (I HAVE AUTOMOBILE)
`(I have ,automobile) ≡ (backquote (I have (comma automobile))) => (I HAVE (A VAN))
`(I have ,@automobile) ≡ (backquote (I have (comma-at automobile))) => (I HAVE A VAN)
</pre>
<p>Common errors:</p>
<pre class="example">
`(,@(i am a list)) => <font color="#AA0000">error: bad function - I</font>
`(,@'(i am a list)) => (I AM A LIST)
</pre>
<p><b>Note:</b> 'backquote', 'comma', and 'comma-at' are very useful in
defining macros via <a href="defmacro.htm">defmacro</a>.</p>
<p>See also:</p>
<ul>
<li><nobr>Contents → <a href="../manual/contents.htm#evaluation-functions">Evaluation Functions</a></nobr></li>
<li><nobr>XLISP 2.0 Manual → <a href="../manual/xlisp.htm#the-readtable">Readtable</a></nobr></li>
</ul>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
<hr>
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<a href="../manual/contents.htm">Contents</a> |
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