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<h4 class="subsubsection" id="lambda_002a-and-define_002a_002e"><span>6.7.4.1 lambda* and define*.<a class="copiable-link" href="#lambda_002a-and-define_002a_002e"> &para;</a></span></h4>
<p><code class="code">lambda*</code> is like <code class="code">lambda</code>, except with some extensions to
allow optional and keyword arguments.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-lambda_002a"><span class="category-def">library syntax: </span><span><strong class="def-name">lambda*</strong> <var class="def-var-arguments">([var&hellip;] <br> [#:optional vardef&hellip;] <br> [#:key vardef&hellip; [#:allow-other-keys]] <br> [#:rest var | . var]) <br> body1 body2 &hellip;</var><a class="copiable-link" href="#index-lambda_002a"> &para;</a></span></dt>
<dd><br>
<p>Create a procedure which takes optional and/or keyword arguments
specified with <code class="code">#:optional</code> and <code class="code">#:key</code>. For example,
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(lambda* (a b #:optional c d . e) '())
</pre></div>
<p>is a procedure with fixed arguments <var class="var">a</var> and <var class="var">b</var>, optional
arguments <var class="var">c</var> and <var class="var">d</var>, and rest argument <var class="var">e</var>. If the
optional arguments are omitted in a call, the variables for them are
bound to <code class="code">#f</code>.
</p>
<a class="index-entry-id" id="index-define_002a"></a>
<p>Likewise, <code class="code">define*</code> is syntactic sugar for defining procedures
using <code class="code">lambda*</code>.
</p>
<p><code class="code">lambda*</code> can also make procedures with keyword arguments. For
example, a procedure defined like this:
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define* (sir-yes-sir #:key action how-high)
(list action how-high))
</pre></div>
<p>can be called as <code class="code">(sir-yes-sir #:action 'jump)</code>,
<code class="code">(sir-yes-sir #:how-high 13)</code>, <code class="code">(sir-yes-sir #:action
'lay-down #:how-high 0)</code>, or just <code class="code">(sir-yes-sir)</code>. Whichever
arguments are given as keywords are bound to values (and those not
given are <code class="code">#f</code>).
</p>
<p>Optional and keyword arguments can also have default values to take
when not present in a call, by giving a two-element list of variable
name and expression. For example in
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define* (frob foo #:optional (bar 42) #:key (baz 73))
(list foo bar baz))
</pre></div>
<p><var class="var">foo</var> is a fixed argument, <var class="var">bar</var> is an optional argument with
default value 42, and baz is a keyword argument with default value 73.
Default value expressions are not evaluated unless they are needed,
and until the procedure is called.
</p>
<p>Normally it&rsquo;s an error if a call has keywords other than those
specified by <code class="code">#:key</code>, but adding <code class="code">#:allow-other-keys</code> to the
definition (after the keyword argument declarations) will ignore
unknown keywords.
</p>
<p>If a call has a keyword given twice, the last value is used. For
example,
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define* (flips #:key (heads 0) (tails 0))
(display (list heads tails)))
(flips #:heads 37 #:tails 42 #:heads 99)
-| (99 42)
</pre></div>
<p><code class="code">#:rest</code> is a synonym for the dotted syntax rest argument. The
argument lists <code class="code">(a . b)</code> and <code class="code">(a #:rest b)</code> are equivalent
in all respects. This is provided for more similarity to DSSSL,
MIT-Scheme and Kawa among others, as well as for refugees from other
Lisp dialects.
</p>
<p>When <code class="code">#:key</code> is used together with a rest argument, the keyword
parameters in a call all remain in the rest list. This is the same as
Common Lisp. For example,
</p>
<div class="example lisp">
<pre class="lisp-preformatted">((lambda* (#:key (x 0) #:allow-other-keys #:rest r)
(display r))
#:x 123 #:y 456)
-| (#:x 123 #:y 456)
</pre></div>
<p><code class="code">#:optional</code> and <code class="code">#:key</code> establish their bindings
successively, from left to right. This means default expressions can
refer back to prior parameters, for example
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(lambda* (start #:optional (end (+ 10 start)))
(do ((i start (1+ i)))
((&gt; i end))
(display i)))
</pre></div>
<p>The exception to this left-to-right scoping rule is the rest argument.
If there is a rest argument, it is bound after the optional arguments,
but before the keyword arguments.
</p></dd></dl>
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