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142 lines
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HTML
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Copyright (C) 2024 Tomas Volf
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<title>Creating a Procedure (Guile Reference Manual)</title>
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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="Creating-a-Procedure">
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<p>
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Next: <a href="Lambda-Alternatives.html" accesskey="n" rel="next">Lambda Alternatives</a>, Previous: <a href="Simple-Invocation.html" accesskey="p" rel="prev">Simple Procedure Invocation</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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<hr>
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<h4 class="subsection" id="Creating-and-Using-a-New-Procedure"><span>3.2.3 Creating and Using a New Procedure<a class="copiable-link" href="#Creating-and-Using-a-New-Procedure"> ¶</a></span></h4>
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<p>Scheme has lots of standard procedures, and Guile provides all of these
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via predefined top level variables. All of these standard procedures
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are documented in the later chapters of this reference manual.
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</p>
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<p>Before very long, though, you will want to create new procedures that
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encapsulate aspects of your own applications’ functionality. To do
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this, you can use the famous <code class="code">lambda</code> syntax.
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</p>
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<p>For example, the value of the following Scheme expression
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(lambda (name address) <var class="var">body</var> ...)
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</pre></div>
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<p>is a newly created procedure that takes two arguments: <code class="code">name</code> and
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<code class="code">address</code>. The behavior of the new procedure is determined by the
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sequence of expressions and definitions in the <var class="var">body</var> of the
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procedure definition. (Typically, <var class="var">body</var> would use the arguments in
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some way, or else there wouldn’t be any point in giving them to the
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procedure.) When invoked, the new procedure returns a value that is the
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value of the last expression in the <var class="var">body</var>.
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</p>
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<p>To make things more concrete, let’s suppose that the two arguments are
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both strings, and that the purpose of this procedure is to form a
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combined string that includes these arguments. Then the full lambda
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expression might look like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(lambda (name address)
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(string-append "Name=" name ":Address=" address))
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</pre></div>
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<p>We noted in the previous subsection that the <var class="var">procedure</var> part of a
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procedure invocation expression can be any Scheme expression whose value
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is a procedure. But that’s exactly what a lambda expression is! So we
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can use a lambda expression directly in a procedure invocation, like
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this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">((lambda (name address)
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(string-append "Name=" name ":Address=" address))
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"FSF"
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"Cambridge")
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</pre></div>
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<p>This is a valid procedure invocation expression, and its result is the
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string:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">"Name=FSF:Address=Cambridge"
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</pre></div>
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<p>It is more common, though, to store the procedure value in a variable —
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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
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(lambda (name address)
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(string-append "Name=" name ":Address=" address)))
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</pre></div>
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<p>— and then to use the variable name in the procedure invocation:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(make-combined-string "FSF" "Cambridge")
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</pre></div>
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<p>Which has exactly the same result.
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</p>
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<p>It’s important to note that procedures created using <code class="code">lambda</code> have
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exactly the same status as the standard built in Scheme procedures, and
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can be invoked, passed around, and stored in variables in exactly the
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same ways.
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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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Next: <a href="Lambda-Alternatives.html">Lambda Alternatives</a>, Previous: <a href="Simple-Invocation.html">Simple Procedure Invocation</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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