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<html><head><title>XLISP mapcar</title>
<link rel="stylesheet" type="text/css" href="reference.css">
</head>
<body>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
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<a href="../examples/examples.htm">Examples</a> |
<a href="reference-index.htm">Reference</a>
<hr>
<h1>mapcar</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>function (subr)</nobr></td>
</tr>
<tr valign="top">
<td><nobr>Source:</nobr></td>
<td><nobr> - </nobr></td>
<td width="100%"><nobr>xllist.c</nobr></td>
</tr>
</tbody></table></p>
<h2>Syntax</h2>
<dl>
<dt>(mapcar <i>function list1</i> [<i>list2</i> ... ])</dt>
<dd><i>function</i> - a function definition like a
<a href="lambda.htm">lambda</a> or a function name<br>
<i>listN</i> - a list or list expression<br>
returns - a list that is constructed from the results of the <i>function</i> applications</dd>
</dl>
<h2>Description</h2>
<p>The mapcan function 'mapcar' applies the 'function' to the succesive
<a href="car.htm">car</a>s of each of the lists 'listN'. Each of
the lists supplies one of the arguments to 'function'. The 'mapcar' function
returns a list that is constructed from the results of the 'function'
applications. If the lists are of different lengths, the shortest list will
determine the number of applications of 'function'.</p>
<h2>Examples</h2>
<pre class="example">
> (mapcar #'+ '(1 2 3) '(1 2 3))
(2 4 6)
> (mapcar #'princ '(1 2 3))
123 <font color="#008844">; screen output</font>
(1 2 3) <font color="#008844">; return value</font>
> (mapcar #'+ '(1 2 3) '(1 2 3 4 5 6) <font color="#008844">; different length lists</font>
(2 4 6)
</pre>
<p><b>Note:</b> The use of the 'function' will work properly when it is a
<nobr>sharp-quoted</nobr> symbol, <nobr>a quoted</nobr> symbol [must be the
name of a function], <nobr>an unquoted</nobr> symbol whose value is a
function, or a closure object like a <a href="lambda.htm">lambda</a>
form.</p>
<p><div class="box">
<p><b>Bug:</b> The proper syntax for 'function' when 'function' is a
<a href="lambda.htm">lambda</a> expression is, for example:</p>
<pre class="example">
(mapcar #'(lambda (arg1 arg2) (+ arg1 arg2)) '(1 2))
</pre>
<p>and not:</p>
<pre class="example">
(mapcar '(lambda (arg1 arg2) (+ arg1 arg2)) '(1 2))
</pre>
<p>That is, the #' [<a href="function.htm">function</a>] read macro must be
present. This error should be caught by the XLISP interpreter, but it is
not, with the result that very obscure garbage collection bugs occur.
<nobr>[I still</nobr> haven't tested Nyquist for possible garbage collection
bugs caused by this.]</p>
</div></p>
<h2>Notes</h2>
<p>In XLISP, a '<nobr>special form</nobr>' of type FSUBR is not a function.
This means that 'mapcar' only works with functions of type SUBR
<nobr>[built-in</nobr> function] or CLOSURE [function defined by
<a href="defun.htm">defun</a>, <a href="flet.htm">flet</a>,
<a href="labels.htm">labels</a>, or <a href="lambda.htm">lambda</a>], but
with special forms of <nobr>type FSUBR</nobr> a 'bad function' error is
signalled. Here is an example how to work around this behaviour:</p>
<pre class="example">
(defmacro <font color="#0000CC">mapcar*</font> (function &rest args)
(if (eq (type-of (symbol-function (second function))) 'fsubr)
(let ((rest (gensym)))
`(mapcar #'(lambda (&rest ,rest)
(eval (cons ,function ,rest)))
,@args))
`(mapcar ,function ,@args)))
</pre>
<p>Examples:</p>
<pre class="example">
(type-of #'eql) => SUBR <font color="#008844">; built-in function</font>
(type-of #'and) => FSUBR <font color="#008844">; built-in special form</font>
> (macroexpand-1 '(mapcar* #'eql '(1 2 3) '(t nil 3)))
(MAPCAR (FUNCTION EQL)
(QUOTE (1 2 3))
(QUOTE (T NIL 3)))
> (macroexpand-1 '(mapcar* #'and '(1 2 3) '(t nil 3)))
(MAPCAR (FUNCTION (LAMBDA (&REST G7)
(EVAL (CONS (FUNCTION AND) G7))))
(QUOTE (1 2 3))
(QUOTE (T NIL 3)))
(mapcar #'eql '(1 2 3) '(t nil 3))) => (NIL NIL T)
(mapcar* #'eql '(1 2 3) '(t nil 3))) => (NIL NIL T)
(mapcar #'and '(1 2 3) '(t nil 3))) => <font color="#AA0000">error: bad function</font>
(mapcar* #'and '(1 2 3) '(t nil 3))) => (T NIL 3)
</pre>
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