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148 lines
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Eval Procedure (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="Evaluating.html" rel="up" title="Evaluating">
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<div class="subsubsection-level-extent" id="Eval-Procedure">
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<p>
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Next: <a href="Eval-Special.html" accesskey="n" rel="next">Evaluating Special Syntactic Expressions</a>, Previous: <a href="Eval-Variable.html" accesskey="p" rel="prev">Evaluating a Variable Reference</a>, Up: <a href="Evaluating.html" accesskey="u" rel="up">Evaluating Expressions and Executing Programs</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="subsubsection" id="Evaluating-a-Procedure-Invocation-Expression"><span>3.3.1.3 Evaluating a Procedure Invocation Expression<a class="copiable-link" href="#Evaluating-a-Procedure-Invocation-Expression"> ¶</a></span></h4>
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<p>This is where evaluation starts getting interesting! As already noted,
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a procedure invocation expression has the form
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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>where <var class="var">procedure</var> must be an expression whose value, when evaluated,
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is a procedure.
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</p>
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<p>The evaluation of a procedure invocation expression like this proceeds
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by
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</p>
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<ul class="itemize mark-bullet">
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<li>evaluating individually the expressions <var class="var">procedure</var>, <var class="var">arg1</var>,
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<var class="var">arg2</var>, and so on
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</li><li>calling the procedure that is the value of the <var class="var">procedure</var>
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expression with the list of values obtained from the evaluations of
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<var class="var">arg1</var>, <var class="var">arg2</var> etc. as its parameters.
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</li></ul>
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<p>For a procedure defined in Scheme, “calling the procedure with the list
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of values as its parameters” means binding the values to the
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procedure’s formal parameters and then evaluating the sequence of
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expressions that make up the body of the procedure definition. The
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value of the procedure invocation expression is the value of the last
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evaluated expression in the procedure body. The side effects of calling
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the procedure are the combination of the side effects of the sequence of
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evaluations of expressions in the procedure body.
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</p>
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<p>For a built-in procedure, the value and side-effects of calling the
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procedure are best described by that procedure’s documentation.
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</p>
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<p>Note that the complete side effects of evaluating a procedure invocation
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expression consist not only of the side effects of the procedure call,
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but also of any side effects of the preceding evaluation of the
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expressions <var class="var">procedure</var>, <var class="var">arg1</var>, <var class="var">arg2</var>, and so on.
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</p>
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<p>To illustrate this, let’s look again at the procedure invocation
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expression:
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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>In the outermost expression, <var class="var">procedure</var> is <code class="code">string-length</code> and
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<var class="var">arg1</var> is <code class="code">(string-append "/home" "/" "andrew")</code>.
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</p>
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<ul class="itemize mark-bullet">
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<li>Evaluation of <code class="code">string-length</code>, which is a variable, gives a
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procedure value that implements the expected behavior for
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“string-length”.
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</li><li>Evaluation of <code class="code">(string-append "/home" "/" "andrew")</code>, which is
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another procedure invocation expression, means evaluating each of
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<ul class="itemize mark-bullet">
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<li><code class="code">string-append</code>, which gives a procedure value that implements the
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expected behavior for “string-append”
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</li><li><code class="code">"/home"</code>, which gives the string value <code class="code">"/home"</code>
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</li><li><code class="code">"/"</code>, which gives the string value <code class="code">"/"</code>
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</li><li><code class="code">"andrew"</code>, which gives the string value <code class="code">"andrew"</code>
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</li></ul>
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<p>and then invoking the procedure value with this list of string values as
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its arguments. The resulting value is a single string value that is the
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concatenation of all the arguments, namely <code class="code">"/home/andrew"</code>.
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</p></li></ul>
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<p>In the evaluation of the outermost expression, the interpreter can now
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invoke the procedure value obtained from <var class="var">procedure</var> with the value
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obtained from <var class="var">arg1</var> as its arguments. The resulting value is a
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numeric value that is the length of the argument string, which is 12.
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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="Eval-Special.html">Evaluating Special Syntactic Expressions</a>, Previous: <a href="Eval-Variable.html">Evaluating a Variable Reference</a>, Up: <a href="Evaluating.html">Evaluating Expressions and Executing Programs</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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