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122 lines
5.3 KiB
HTML
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Copyright (C) 2024 Tomas Volf
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<title>Simple Invocation (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Simple-Invocation">
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<p>
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Next: <a href="Creating-a-Procedure.html" accesskey="n" rel="next">Creating and Using a New Procedure</a>, Previous: <a href="Procedures-as-Values.html" accesskey="p" rel="prev">Procedures as Values</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="Simple-Procedure-Invocation"><span>3.2.2 Simple Procedure Invocation<a class="copiable-link" href="#Simple-Procedure-Invocation"> ¶</a></span></h4>
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<p>A procedure invocation in Scheme is written like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(<var class="var">procedure</var> [<var class="var">arg1</var> [<var class="var">arg2</var> ...]])
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</pre></div>
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<p>In this expression, <var class="var">procedure</var> can be any Scheme expression whose
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value is a procedure. Most commonly, however, <var class="var">procedure</var> is simply
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the name of a variable whose value is a procedure.
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</p>
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<p>For example, <code class="code">string-append</code> is a standard Scheme procedure whose
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behavior is to concatenate together all the arguments, which are
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expected to be strings, that it is given. So the expression
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(string-append "/home" "/" "andrew")
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</pre></div>
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<p>is a procedure invocation whose result is the string value
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<code class="code">"/home/andrew"</code>.
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</p>
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<p>Similarly, <code class="code">string-length</code> is a standard Scheme procedure that
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returns the length of a single string argument, so
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(string-length "abc")
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</pre></div>
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<p>is a procedure invocation whose result is the numeric value 3.
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</p>
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<p>Each of the parameters in a procedure invocation can itself be any
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Scheme expression. Since a procedure invocation is itself a type of
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expression, we can put these two examples together to get
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(string-length (string-append "/home" "/" "andrew"))
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</pre></div>
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<p>— a procedure invocation whose result is the numeric value 12.
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</p>
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<p>(You may be wondering what happens if the two examples are combined the
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other way round. If we do this, we can make a procedure invocation
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expression that is <em class="emph">syntactically</em> correct:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(string-append "/home" (string-length "abc"))
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</pre></div>
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<p>but when this expression is executed, it will cause an error, because
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the result of <code class="code">(string-length "abc")</code> is a numeric value, and
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<code class="code">string-append</code> is not designed to accept a numeric value as one of
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its arguments.)
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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="Creating-a-Procedure.html">Creating and Using a New Procedure</a>, Previous: <a href="Procedures-as-Values.html">Procedures as Values</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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