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<html><head>
<title>Macros</title>
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<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.htm">Examples</a> |
<a href="../reference/reference-index.htm">Reference</a>
<hr>
<h1>Macro Programming</h1>
<hr>
<ul>
<li><nobr><a href="#with-unique-names">with-unique-names</a> - create local <a href="../reference/gensym.htm">gensym</a> variables</nobr></li>
</ul>
<p><div class="box">
<p><b>Note:</b> The best book for Lisp macro programming is Paul
Graham's '<nobr>On Lisp</nobr>', available for free under:</p>
<ul>
<li><nobr><a href="http://www.paulgraham.com/onlisp.html"
>http://www.paulgraham.com/onlisp.html</a></nobr></li>
</ul>
</div></p>
<a name="with-unique-names"></a>
<hr>
<h2>with-unique-names</h2>
<hr>
<p>See <a href="http://www.cliki.net/WITH-UNIQUE-NAMES"
>http://www.cliki.net/WITH-UNIQUE-NAMES</a>. This macro also appears in
<nobr>Chapter 11</nobr> of Paul Graham's
<nobr><a href="http://www.paulgraham.com/onlisp.html">On Lisp</a></nobr>
under the name '<nobr>with-gensyms</nobr>'.</p>
<p><div class="box">
<dl>
<dt><nobr>(<b>with-unique-names</b> (<i>symbols</i>) <i>body</i>)</nobr></dt>
<dd><i>symbols</i> - a list of Lisp symbols, representing variable names<br>
<i>body</i> - some Lisp code to execute<br>
returns - the <i>body</i> with all <i>symbols</i> bound to different
<a href="../reference/gensym.htm">gensym</a>s</dd>
</dl>
</div></p>
<p>The '<nobr>with-unique-names</nobr>' macro helps to avoid name clashes in
Lisp macros.</p>
<pre class="example">
(defmacro <font color="#0000CC">with-unique-names</font> (symbols &rest body)
`(let ,(mapcar #'(lambda (x) `(,x (gensym))) symbols) ,@body))
</pre>
<p>The '<nobr>with-unique-names</nobr>' macro belongs to the category of
<nobr>write-only</nobr> code. <nobr>No matter</nobr> how you write it, it's
nearly impossible to understand its meaning by reading the macro definition.
<nobr>It's easier</nobr> to understand if you look at the macro
expansion:</p>
<pre class="example">
> (macroexpand-1 '(with-unique-names (a b c)
`(let ((,a 1) (,b 2) (,c 3))
(list ,a ,b ,c))))
(let ((a (gensym)) (b (gensym)) (c (gensym)))
`(let ((,a 1) (,b 2) (,c 3))
(list ,a ,b ,c)))
</pre>
<p>This translates in practice to the following idea:</p>
<pre class="example">
(let ((a (gensym)) (b (gensym)) (c (gensym))) <font color="#008844">; outside the expansion</font>
`(let ((<font color="#AA5500">gensym1</font> 1) (<font color="#AA5500">gensym2</font> 2) (<font color="#AA5500">gensym3</font> 3)) <font color="#008844">; inside the expansion</font>
(list <font color="#AA5500">gensym1 gensym2 gensym3</font>)))
</pre>
<p>The variable names 'a', 'b', and 'c' have been replaced inside the macro
expansion by three <a href="../reference/gensym.htm">gensym</a>s. This way a
variable name inside the macro expansion cannot accidentally collide with a
variable of the same name in the environment of the macro's expansion like
shown here:</p>
<pre class="example">
(defmacro <font color="#0000CC">print-macro</font> (x) <font color="#008844">; bad example</font>
`(let ((<font color="#AA0000">macro-var</font> 'macro))
(print ,x)))
> (let ((<font color="#AA0000">local-var</font> 'let)) <font color="#008844">; this works</font>
(print local-var)
(print-macro local-var))
LET <font color="#008844">; printed by PRINT</font>
LET <font color="#008844">; printed by PRINT-MACRO</font>
> (let ((<font color="#AA0000">macro-var</font> 'let)) <font color="#008844">; this doesn't</font>
(print macro-var)
(print-macro macro-var))
LET <font color="#008844">; printed by PRINT</font>
MACRO <font color="#008844">; printed by PRINT-MACRO</font>
</pre>
<p>The reason for this behaviour is that the '<nobr>print-macro</nobr>'
<nobr>expands to:</nobr></p>
<pre class="example">
> (let ((<font color="#AA0000">local-var</font> 'let)) <font color="#008844">; this works</font>
(print local-var)
(let ((<font color="#AA0000">macro-var</font> 'macro))
(print local-var)))
LET <font color="#008844">; LOCAL-VAR inside the first LET</font>
LET <font color="#008844">; LOCAL-VAR inside the second LET</font>
> (let ((<font color="#AA0000">macro-var</font> 'let)) <font color="#008844">; this doesn't</font>
(print macro-var)
(let ((<font color="#AA0000">macro-var</font> 'macro))
(print macro-var)))
LET <font color="#008844">; MACRO-VAR inside the first LET</font>
MACRO <font color="#008844">; MACRO-VAR inside the second LET</font>
</pre>
<p>Now the same example with unique names. Note the
<a href="..reference/backquote.htm">comma</a> before the
'<nobr>macro-var</nobr>' inside the
<nobr><a href="../reference/let.htm">let</a> form of the macro
definition:</nobr></p>
<pre class="example">
(defmacro <font color="#0000CC">print-macro</font> (x) <font color="#008844">; good example</font>
(with-unique-names (<font color="#AA0000">macro-var</font>)
`(let ((,<font color="#AA0000">macro-var</font> 'macro))
(print ,x))))
> (let ((<font color="#AA0000">macro-var</font> 'let)) <font color="#008844">; now it works</font>
(print macro-var)
(print-macro macro-var))
LET <font color="#008844">; printed by PRINT</font>
LET <font color="#008844">; printed by PRINT-MACRO</font>
</pre>
<p>The reason why it works is that the '<nobr>print-macro</nobr>' now
<nobr>expands to:</nobr></p>
<pre class="example">
> (let ((<font color="#AA0000">macro-var</font> 'let)) <font color="#008844">; works</font>
(print macro-var)
(let ((<font color="#AA5500">gensym</font> 'macro))
(print macro-var)))
LET <font color="#008844">; MACRO-VAR inside the first LET</font>
LET <font color="#008844">; MACRO-VAR inside the second LET</font>
</pre>
<nobr>Now '<nobr>macro-var</nobr>' can even be used as a variable name
inside the macro definition without colliding with the
'<nobr>macro-var</nobr>' bound
<nobr>by <a href="../reference/let.htm">let</a>:</nobr></nobr>
<pre class="example">
(defmacro <font color="#0000CC">print-macro</font> (x) <font color="#008844">; good example</font>
(with-unique-names (<font color="#AA0000">macro-var</font>)
`(let ((,<font color="#AA0000">macro-var</font> 'macro))
(print ,<font color="#AA0000">macro-var</font>)
(print ,x))))
> (let ((<font color="#AA0000">macro-var</font> 'let)) <font color="#008844">; works</font>
(print macro-var)
(print-macro macro-var))
LET <font color="#008844">; MACRO-VAR printed inside LET</font>
MACRO <font color="#008844">; GENSYMed MACRO-VAR, printed inside PRINT-MACRO</font>
LET <font color="#008844">; MACRO-VAR bound by LET, printed inside PRINT-MACRO</font>
</pre>
<p>The expansion of the '<nobr>print-macro</nobr>' shows why this works:</p>
<pre class="example">
> (let ((<font color="#AA0000">macro-var</font> 'let)) <font color="#008844">; works</font>
(print macro-var)
(let ((<font color="#AA5500">gensym</font> 'macro))
(print <font color="#AA5500">gensym</font>)
(print macro-var)))
LET <font color="#008844">; MACRO-VAR printed inside LET</font>
MACRO <font color="#008844">; GENSYMed MACRO-VAR printed inside PRINT-MACRO</font>
LET <font color="#008844">; MACRO-VAR bound by LET, printed inside PRINT-MACRO</font>
</pre>
<p>You can give as many variable names as you like to
'<nobr>with-unique-names</nobr>', the
<a href="../reference/gensym.htm">gensym</a> management is done
automatically:</p>
<pre class="example">
(defmacro <font color="#0000CC">print-macro</font> (x y z)
(with-unique-names (a b c)
`(let ((,a 1) (,b 2) (,c 3))
(format t <font color="#880000">"outside: a: ~a b: ~a c: ~a~%"</font> ,x ,y ,z)
(format t <font color="#880000">" inside: a: ~a b: ~a c: ~a~%"</font> ,a ,b ,c))))
> (let ((a 'a) (b 'b) (c 'c))
(print-macro a b c))
outside: a: A b: B c: C
inside: a: 1 b: 2 c: 3
</pre>
<p>Two things you still have to care about:</p>
<ol>
<li><p>The 'unique names' should not use the same smbol names as the
parameter variables of the macro, otherwise you will have the same
'shadowing' effect like in ordinary Lisp functions. This is not a real
problem because when writing a macro you can see the parameter names before
your eyes, while you usually cannot see the variable names of the
environment, where the macro will be expanded. <nobr>You also</nobr> do not
have to care which variable names had been used in a macro if you call the
macro from arbitrary Lisp code, where you usually cannot see the code of the
macro definition.</p></li>
<li><p>The local <a href="../reference/gensym.htm">gensym</a>ed variables
now themselves must be expanded by writing a
<a href="../reference/backquote.htm">comma</a> in front of each when they appear
inside a <a href="../reference/backquote.htm">backquote</a> scope.
This sometimes can lead to tricky situations, because the
<a href="../reference/backquote.htm">comma</a> expansion of the symbol does not
produce the variable's value, instead it produces the name of the
<a href="../reference/gensym.htm">gensym</a>, which holds the value.
<nobr>But this</nobr> is a general phenomenon of
<a href="../reference/gensym.htm">gensym</a>s in Lisp macro programming
and not a bug of the '<nobr>with-unique-names</nobr>' macro.</p></li>
</ol>
<p>The alternative would be writing:</p>
<pre class="example">
(defmacro <font color="#0000CC">print-macro</font> (x y z)
(let ((a (gensym)) (b (gensym)) (c (gensym)))
`(let ((,a 1) (,b 2) (,c 3))
(format t <font color="#880000">"outside: a: ~a b: ~a c: ~a~%"</font> ,x ,y ,z)
(format t <font color="#880000">" inside: a: ~a b: ~a c: ~a~%"</font> ,a ,b ,c))))
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
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