168 lines
7.4 KiB
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168 lines
7.4 KiB
HTML
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>lambda* and define* (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="lambda_002a-and-define_002a">
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<p>
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Next: <a href="ice_002d9-optargs.html" accesskey="n" rel="next">(ice-9 optargs)</a>, Up: <a href="Optional-Arguments.html" accesskey="u" rel="up">Optional Arguments</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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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"> ¶</a></span></h4>
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<p><code class="code">lambda*</code> is like <code class="code">lambda</code>, except with some extensions to
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allow optional and keyword arguments.
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</p>
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<dl class="first-deffn">
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<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…] <br> [#:optional vardef…] <br> [#:key vardef… [#:allow-other-keys]] <br> [#:rest var | . var]) <br> body1 body2 …</var><a class="copiable-link" href="#index-lambda_002a"> ¶</a></span></dt>
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<dd><br>
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<p>Create a procedure which takes optional and/or keyword arguments
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specified with <code class="code">#:optional</code> and <code class="code">#:key</code>. For example,
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(lambda* (a b #:optional c d . e) '())
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</pre></div>
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<p>is a procedure with fixed arguments <var class="var">a</var> and <var class="var">b</var>, optional
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arguments <var class="var">c</var> and <var class="var">d</var>, and rest argument <var class="var">e</var>. If the
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optional arguments are omitted in a call, the variables for them are
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bound to <code class="code">#f</code>.
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</p>
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<a class="index-entry-id" id="index-define_002a"></a>
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<p>Likewise, <code class="code">define*</code> is syntactic sugar for defining procedures
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using <code class="code">lambda*</code>.
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</p>
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<p><code class="code">lambda*</code> can also make procedures with keyword arguments. For
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example, a procedure defined like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define* (sir-yes-sir #:key action how-high)
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(list action how-high))
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</pre></div>
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<p>can be called as <code class="code">(sir-yes-sir #:action 'jump)</code>,
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<code class="code">(sir-yes-sir #:how-high 13)</code>, <code class="code">(sir-yes-sir #:action
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'lay-down #:how-high 0)</code>, or just <code class="code">(sir-yes-sir)</code>. Whichever
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arguments are given as keywords are bound to values (and those not
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given are <code class="code">#f</code>).
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</p>
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<p>Optional and keyword arguments can also have default values to take
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when not present in a call, by giving a two-element list of variable
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name and expression. For example in
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define* (frob foo #:optional (bar 42) #:key (baz 73))
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(list foo bar baz))
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</pre></div>
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<p><var class="var">foo</var> is a fixed argument, <var class="var">bar</var> is an optional argument with
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default value 42, and baz is a keyword argument with default value 73.
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Default value expressions are not evaluated unless they are needed,
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and until the procedure is called.
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</p>
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<p>Normally it’s an error if a call has keywords other than those
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specified by <code class="code">#:key</code>, but adding <code class="code">#:allow-other-keys</code> to the
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definition (after the keyword argument declarations) will ignore
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unknown keywords.
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</p>
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<p>If a call has a keyword given twice, the last value is used. For
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example,
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define* (flips #:key (heads 0) (tails 0))
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(display (list heads tails)))
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(flips #:heads 37 #:tails 42 #:heads 99)
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-| (99 42)
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</pre></div>
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<p><code class="code">#:rest</code> is a synonym for the dotted syntax rest argument. The
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argument lists <code class="code">(a . b)</code> and <code class="code">(a #:rest b)</code> are equivalent
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in all respects. This is provided for more similarity to DSSSL,
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MIT-Scheme and Kawa among others, as well as for refugees from other
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Lisp dialects.
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</p>
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<p>When <code class="code">#:key</code> is used together with a rest argument, the keyword
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parameters in a call all remain in the rest list. This is the same as
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Common Lisp. For example,
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">((lambda* (#:key (x 0) #:allow-other-keys #:rest r)
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(display r))
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#:x 123 #:y 456)
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-| (#:x 123 #:y 456)
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</pre></div>
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<p><code class="code">#:optional</code> and <code class="code">#:key</code> establish their bindings
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successively, from left to right. This means default expressions can
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refer back to prior parameters, for example
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(lambda* (start #:optional (end (+ 10 start)))
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(do ((i start (1+ i)))
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((> i end))
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(display i)))
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</pre></div>
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<p>The exception to this left-to-right scoping rule is the rest argument.
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If there is a rest argument, it is bound after the optional arguments,
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but before the keyword arguments.
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</p></dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="ice_002d9-optargs.html">(ice-9 optargs)</a>, Up: <a href="Optional-Arguments.html">Optional Arguments</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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