130 lines
5.8 KiB
HTML
130 lines
5.8 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 Alternatives (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="About-Procedures.html" rel="up" title="About Procedures">
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<div class="subsection-level-extent" id="Lambda-Alternatives">
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<p>
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Previous: <a href="Creating-a-Procedure.html" accesskey="p" rel="prev">Creating and Using a New Procedure</a>, Up: <a href="About-Procedures.html" accesskey="u" rel="up">The Representation and Use of 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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<h4 class="subsection" id="Lambda-Alternatives-1"><span>3.2.4 Lambda Alternatives<a class="copiable-link" href="#Lambda-Alternatives-1"> ¶</a></span></h4>
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<p>Since it is so common in Scheme programs to want to create a procedure
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and then store it in a variable, there is an alternative form of the
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<code class="code">define</code> syntax that allows you to do just that.
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</p>
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<p>A <code class="code">define</code> expression of the form
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (<var class="var">name</var> [<var class="var">arg1</var> [<var class="var">arg2</var> ...]])
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<var class="var">body</var> ...)
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</pre></div>
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<p>is exactly equivalent to the longer form
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define <var class="var">name</var>
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(lambda ([<var class="var">arg1</var> [<var class="var">arg2</var> ...]])
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<var class="var">body</var> ...))
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</pre></div>
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<p>So, for example, the definition of <code class="code">make-combined-string</code> in the
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previous subsection could equally be written:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (make-combined-string name address)
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(string-append "Name=" name ":Address=" address))
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</pre></div>
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<p>This kind of procedure definition creates a procedure that requires
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exactly the expected number of arguments. There are two further forms
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of the <code class="code">lambda</code> expression, which create a procedure that can
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accept a variable number of arguments:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(lambda (<var class="var">arg1</var> ... . <var class="var">args</var>) <var class="var">body</var> ...)
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(lambda <var class="var">args</var> <var class="var">body</var> ...)
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</pre></div>
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<p>The corresponding forms of the alternative <code class="code">define</code> syntax are:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define (<var class="var">name</var> <var class="var">arg1</var> ... . <var class="var">args</var>) <var class="var">body</var> ...)
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(define (<var class="var">name</var> . <var class="var">args</var>) <var class="var">body</var> ...)
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</pre></div>
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<p>For details on how these forms work, see See <a class="xref" href="Lambda.html">Lambda: Basic Procedure Creation</a>.
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</p>
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<p>Prior to Guile 2.0, Guile provided an extension to <code class="code">define</code> syntax
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that allowed you to nest the previous extension up to an arbitrary
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depth. These are no longer provided by default, and instead have been
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moved to <a class="ref" href="Curried-Definitions.html">Curried Definitions</a>.
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</p>
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<p>(It could be argued that the alternative <code class="code">define</code> forms are rather
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confusing, especially for newcomers to the Scheme language, as they hide
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both the role of <code class="code">lambda</code> and the fact that procedures are values
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that are stored in variables in the same way as any other kind of value.
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On the other hand, they are very convenient, and they are also a good
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example of another of Scheme’s powerful features: the ability to specify
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arbitrary syntactic transformations at run time, which can be applied to
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subsequently read input.)
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</p>
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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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Previous: <a href="Creating-a-Procedure.html">Creating and Using a New Procedure</a>, Up: <a href="About-Procedures.html">The Representation and Use of 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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