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<html><head><title>XLISP macrolet</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>macrolet</h1>
<hr>
<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</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<p><div class="box">
<dl>
<dt>(<b>macrolet</b> ([<i>macro</i> ... ]) <i>expr</i> ... )</dt>
<dd><i>macro</i> - a macro definition binding which is of the form:<br>
<dl><dd>(<i>symbol arg-list body</i>)</dd>
<dl><dd><i>symbol</i> - the symbol specifying the macro name<br>
<i>arg-list</i> - the argument list for the macro<br>
<i>body</i> - the body of the macro</dd></dl></dl>
<i>expr</i> - an expression<br>
returns - the value of the last expression</dd>
</dl>
</div></p>
<h2>Description</h2>
<p>The 'macrolet' special form is basically a local <a
href="block.htm">block</a> construct that allows local 'macro' definitions
followed by a block of code to evaluate. <nobr>The first</nobr> form after
the macrolet is the 'binding' form. <nobr>It contains</nobr> a series of
'macros'. <nobr>The 'macrolet'</nobr> form will sequentially execute the
'exprs' after defining the 'macros'. <nobr>The value</nobr> of the last
'expr' evaluated is returned. When the 'macrolet' is finished execution, the
'symbols' that were defined will no longer exist.</p>
<h2>Examples</h2>
<pre class="example">
> (macrolet ((pls (n1 n2) <font color="#008844">; MACROLET defining a PLS macro</font>
`(+ ,n1 ,n2)))
(pls 4 5))
9
> (pls 4 5) <font color="#008844">; the PLS macro no longer exists</font>
<font color="#AA0000">error: unbound function - PLS</font>
> (macrolet () <font color="#008844">; an empty MACROLET</font>
(print 'a))
A <font color="#008844">; screen output of PRINT</font>
A <font color="#008844">; return value</font>
</pre>
<h2>Known Problems</h2>
<p><b>1.</b> In XLISP, only macros defined by
<a href="defmacro.htm">defmacro</a> [interned in the
<a href="global-obarray.htm">*obarray*</a>] can be used with
<a href="setf.htm">setf</a>:</p>
<pre class="example">
(setq a #(1 2 3))
(defmacro second-array-element (array)
`(aref ,array 1))
(second-array-element a) => 2
(setf (second-array-element a) 'x) => X
a => #(1 X 3)
</pre>
<p>With macros defined by 'macrolet' [stored in the lexical environment],
<a href="setf.htm">setf</a> signals a '<nobr>bad place form</nobr>' error:</p>
<pre class="example">
(macrolet ((second-element (array)
`(aref ,array 1)))
(second-element a)) => X
(macrolet ((second-element (array)
`(aref ,array 1)))
(setf (second-element a) 'y)) => <font color="#AA0000">error: bad place form</font>
</pre>
<p><b>2.</b> In XLISP, the <a href="macroexpand.htm">macroexpand</a> and
<nobr><a href="macroexpand-1.htm">macroexpand-1</a></nobr> functions can
only expand macros defined
<nobr>by <a href="defmacro.htm">defmacro</a>:</nobr></p>
<pre class="example">
> (macroexpand-1 '(second-array-element a))
(AREF A 1)
</pre>
<p>With macros defined by 'macrolet', the macro form is returned
unexpanded:</p>
<pre class="example">
> (macrolet ((second-element (array)
`(aref ,array 1)))
(macroexpand-1 '(second-element a)))
(SECOND-ELEMENT A)
</pre>
<p>In XLISP, the <a href="macroexpand.htm">macroexpand</a> and
<nobr><a href="macroexpand-1.htm">macroexpand-1</a></nobr> functions only
search in the <a href="global-obarray.htm">*obarray*</a> for defined macros,
but not in the lexical environment.</p>
<p><nobr> <a href="#top">Back to Top</nobr></a></p>
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