140 lines
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140 lines
6.6 KiB
HTML
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Lambda (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Lambda">
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<div class="nav-panel">
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<p>
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Next: <a href="Primitive-Procedures.html" accesskey="n" rel="next">Primitive Procedures</a>, Up: <a href="Procedures.html" accesskey="u" rel="up">Procedures</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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<hr>
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<h4 class="subsection" id="Lambda_003a-Basic-Procedure-Creation"><span>6.7.1 Lambda: Basic Procedure Creation<a class="copiable-link" href="#Lambda_003a-Basic-Procedure-Creation"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-lambda"></a>
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<p>A <code class="code">lambda</code> expression evaluates to a procedure. The environment
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which is in effect when a <code class="code">lambda</code> expression is evaluated is
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enclosed in the newly created procedure, this is referred to as a
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<em class="dfn">closure</em> (see <a class="pxref" href="About-Closure.html">The Concept of Closure</a>).
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</p>
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<p>When a procedure created by <code class="code">lambda</code> is called with some actual
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arguments, the environment enclosed in the procedure is extended by
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binding the variables named in the formal argument list to new locations
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and storing the actual arguments into these locations. Then the body of
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the <code class="code">lambda</code> expression is evaluated sequentially. The result of
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the last expression in the procedure body is then the result of the
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procedure invocation.
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</p>
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<p>The following examples will show how procedures can be created using
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<code class="code">lambda</code>, and what you can do with these procedures.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(lambda (x) (+ x x)) ⇒ <span class="r">a procedure</span>
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((lambda (x) (+ x x)) 4) ⇒ 8
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</pre></div>
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<p>The fact that the environment in effect when creating a procedure is
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enclosed in the procedure is shown with this example:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define add4
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(let ((x 4))
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(lambda (y) (+ x y))))
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(add4 6) ⇒ 10
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-lambda-1"><span class="category-def">syntax: </span><span><strong class="def-name">lambda</strong> <var class="def-var-arguments">formals body</var><a class="copiable-link" href="#index-lambda-1"> ¶</a></span></dt>
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<dd><p><var class="var">formals</var> should be a formal argument list as described in the
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following table.
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</p>
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<dl class="table">
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<dt><code class="code">(<var class="var">variable1</var> …)</code></dt>
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<dd><p>The procedure takes a fixed number of arguments; when the procedure is
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called, the arguments will be stored into the newly created location for
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the formal variables.
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</p></dd>
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<dt><code class="code"><var class="var">variable</var></code></dt>
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<dd><p>The procedure takes any number of arguments; when the procedure is
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called, the sequence of actual arguments will be converted into a list
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and stored into the newly created location for the formal variable.
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</p></dd>
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<dt><code class="code">(<var class="var">variable1</var> … <var class="var">variablen</var> . <var class="var">variablen+1</var>)</code></dt>
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<dd><p>If a space-delimited period precedes the last variable, then the
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procedure takes <var class="var">n</var> or more variables where <var class="var">n</var> is the number
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of formal arguments before the period. There must be at least one
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argument before the period. The first <var class="var">n</var> actual arguments will be
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stored into the newly allocated locations for the first <var class="var">n</var> formal
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arguments and the sequence of the remaining actual arguments is
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converted into a list and the stored into the location for the last
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formal argument. If there are exactly <var class="var">n</var> actual arguments, the
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empty list is stored into the location of the last formal argument.
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</p></dd>
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</dl>
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<p>The list in <var class="var">variable</var> or <var class="var">variablen+1</var> is always newly
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created and the procedure can modify it if desired. This is the case
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even when the procedure is invoked via <code class="code">apply</code>, the required part
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of the list argument there will be copied (see <a class="pxref" href="Fly-Evaluation.html">Procedures for On the Fly Evaluation</a>).
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</p>
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<p><var class="var">body</var> is a sequence of Scheme expressions which are evaluated in
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order when the procedure is invoked.
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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="Primitive-Procedures.html">Primitive Procedures</a>, Up: <a href="Procedures.html">Procedures</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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