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181 lines
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Multiple Values (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Multiple-Values">
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Next: <a href="Exceptions.html" accesskey="n" rel="next">Exceptions</a>, Previous: <a href="Continuations.html" accesskey="p" rel="prev">Continuations</a>, Up: <a href="Control-Mechanisms.html" accesskey="u" rel="up">Controlling the Flow of Program Execution</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="Returning-and-Accepting-Multiple-Values"><span>6.11.7 Returning and Accepting Multiple Values<a class="copiable-link" href="#Returning-and-Accepting-Multiple-Values"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-multiple-values"></a>
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<a class="index-entry-id" id="index-receive"></a>
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<p>Scheme allows a procedure to return more than one value to its caller.
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This is quite different to other languages which only allow
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single-value returns. Returning multiple values is different from
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returning a list (or pair or vector) of values to the caller, because
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conceptually not <em class="emph">one</em> compound object is returned, but several
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distinct values.
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</p>
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<p>The primitive procedures for handling multiple values are <code class="code">values</code>
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and <code class="code">call-with-values</code>. <code class="code">values</code> is used for returning
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multiple values from a procedure. This is done by placing a call to
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<code class="code">values</code> with zero or more arguments in tail position in a
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procedure body. <code class="code">call-with-values</code> combines a procedure returning
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multiple values with a procedure which accepts these values as
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parameters.
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</p>
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<a class="index-entry-id" id="index-values-2"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-values"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">values</strong> <var class="def-var-arguments">arg …</var><a class="copiable-link" href="#index-values"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fvalues"><span class="category-def">C Function: </span><span><strong class="def-name">scm_values</strong> <var class="def-var-arguments">(args)</var><a class="copiable-link" href="#index-scm_005fvalues"> ¶</a></span></dt>
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<dd><p>Delivers all of its arguments to its continuation. Except for
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continuations created by the <code class="code">call-with-values</code> procedure,
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all continuations take exactly one value. The effect of
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passing no value or more than one value to continuations that
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were not created by <code class="code">call-with-values</code> is unspecified.
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</p>
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<p>For <code class="code">scm_values</code>, <var class="var">args</var> is a list of arguments and the
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return is a multiple-values object which the caller can return. In
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the current implementation that object shares structure with
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<var class="var">args</var>, so <var class="var">args</var> should not be modified subsequently.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fc_005fvalues"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_c_values</strong> <code class="def-code-arguments">(SCM *base, size_t n)</code><a class="copiable-link" href="#index-scm_005fc_005fvalues"> ¶</a></span></dt>
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<dd><p><code class="code">scm_c_values</code> is an alternative to <code class="code">scm_values</code>. It creates
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a new values object, and copies into it the <var class="var">n</var> values starting from
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<var class="var">base</var>.
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</p>
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<p>Currently this creates a list and passes it to <code class="code">scm_values</code>, but we
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expect that in the future we will be able to use a more efficient
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representation.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fc_005fnvalues"><span class="category-def">C Function: </span><span><code class="def-type">size_t</code> <strong class="def-name">scm_c_nvalues</strong> <code class="def-code-arguments">(SCM obj)</code><a class="copiable-link" href="#index-scm_005fc_005fnvalues"> ¶</a></span></dt>
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<dd><p>If <var class="var">obj</var> is a multiple-values object, returns the number of values
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it contains. Otherwise returns 1.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fc_005fvalue_005fref"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_c_value_ref</strong> <code class="def-code-arguments">(SCM obj, size_t idx)</code><a class="copiable-link" href="#index-scm_005fc_005fvalue_005fref"> ¶</a></span></dt>
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<dd><p>Returns the value at the position specified by <var class="var">idx</var> in
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<var class="var">obj</var>. Note that <var class="var">obj</var> will ordinarily be a
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multiple-values object, but it need not be. Any other object
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represents a single value (itself), and is handled appropriately.
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</p></dd></dl>
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<a class="index-entry-id" id="index-call_002dwith_002dvalues-2"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-call_002dwith_002dvalues"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">call-with-values</strong> <var class="def-var-arguments">producer consumer</var><a class="copiable-link" href="#index-call_002dwith_002dvalues"> ¶</a></span></dt>
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<dd><p>Calls its <var class="var">producer</var> argument with no values and a
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continuation that, when passed some values, calls the
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<var class="var">consumer</var> procedure with those values as arguments. The
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continuation for the call to <var class="var">consumer</var> is the continuation
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of the call to <code class="code">call-with-values</code>.
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</p>
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<div class="example">
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<pre class="example-preformatted">(call-with-values (lambda () (values 4 5))
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(lambda (a b) b))
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⇒ 5
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</pre></div>
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<div class="example">
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<pre class="example-preformatted">(call-with-values * -)
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⇒ -1
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</pre></div>
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</dd></dl>
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<p>In addition to the fundamental procedures described above, Guile has a
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module which exports a syntax called <code class="code">receive</code>, which is much
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more convenient. This is in the <code class="code">(ice-9 receive)</code> and is the
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same as specified by SRFI-8 (see <a class="pxref" href="SRFI_002d8.html">SRFI-8 - receive</a>).
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(use-modules (ice-9 receive))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-receive-1"><span class="category-def">library syntax: </span><span><strong class="def-name">receive</strong> <var class="def-var-arguments">formals expr body</var><a class="copiable-link" href="#index-receive-1"> ¶</a></span></dt>
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<dd><p>Evaluate the expression <var class="var">expr</var>, and bind the result values (zero
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or more) to the formal arguments in <var class="var">formals</var>. <var class="var">formals</var> is a
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list of symbols, like the argument list in a <code class="code">lambda</code>
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(see <a class="pxref" href="Lambda.html">Lambda: Basic Procedure Creation</a>). After binding the variables, the <var class="var">body</var> is
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evaluated to produce the result of the <code class="code">receive</code> expression.
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</p>
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<p>For example getting results from <code class="code">partition</code> in SRFI-1
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(see <a class="pxref" href="SRFI_002d1.html">SRFI-1 - List library</a>),
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</p>
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<div class="example">
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<pre class="example-preformatted">(receive (odds evens)
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(partition odd? '(7 4 2 8 3))
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(display odds)
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(display " and ")
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(display evens))
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-| (7 3) and (4 2 8)
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</pre></div>
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</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="Exceptions.html">Exceptions</a>, Previous: <a href="Continuations.html">Continuations</a>, Up: <a href="Control-Mechanisms.html">Controlling the Flow of Program Execution</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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