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143 lines
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Immediate Objects (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="Immediate-Objects">
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Next: <a href="Non_002dImmediate-Objects.html" accesskey="n" rel="next">Non-Immediate Objects</a>, Previous: <a href="Relationship-Between-SCM-and-scm_005ft_005fbits.html" accesskey="p" rel="prev">Relationship Between <code class="code">SCM</code> and <code class="code">scm_t_bits</code></a>, Up: <a href="The-SCM-Type-in-Guile.html" accesskey="u" rel="up">The SCM Type in Guile</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="subsubsection" id="Immediate-Objects-1"><span>9.2.5.2 Immediate Objects<a class="copiable-link" href="#Immediate-Objects-1"> ¶</a></span></h4>
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<p>A Scheme object may either be an immediate, i.e. carrying all
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necessary information by itself, or it may contain a reference to a
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<em class="dfn">heap object</em> which is, as the name implies, data on the heap.
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Although in general it should be irrelevant for user code whether an
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object is an immediate or not, within Guile’s own code the distinction
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is sometimes of importance. Thus, the following low level macro is
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provided:
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</p>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-SCM_005fIMP"><span class="category-def">Macro: </span><span><code class="def-type">int</code> <strong class="def-name">SCM_IMP</strong> <code class="def-code-arguments">(SCM <var class="var">x</var>)</code><a class="copiable-link" href="#index-SCM_005fIMP"> ¶</a></span></dt>
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<dd><p>A Scheme object is an immediate if it fulfills the <code class="code">SCM_IMP</code>
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predicate, otherwise it holds an encoded reference to a heap object. The
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result of the predicate is delivered as a C style boolean value. User
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code and code that extends Guile should normally not be required to use
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this macro.
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</p></dd></dl>
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<p>Summary:
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</p><ul class="itemize mark-bullet">
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<li>Given a Scheme object <var class="var">x</var> of unknown type, check first
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with <code class="code">SCM_IMP (<var class="var">x</var>)</code> if it is an immediate object.
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</li><li>If so, all of the type and value information can be determined from the
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<code class="code">scm_t_bits</code> value that is delivered by <code class="code">SCM_UNPACK
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(<var class="var">x</var>)</code>.
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</li></ul>
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<p>There are a number of special values in Scheme, most of them documented
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elsewhere in this manual. It’s not quite the right place to put them,
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but for now, here’s a list of the C names given to some of these values:
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</p>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-SCM_005fEOL"><span class="category-def">Macro: </span><span><code class="def-type">SCM</code> <strong class="def-name">SCM_EOL</strong><a class="copiable-link" href="#index-SCM_005fEOL"> ¶</a></span></dt>
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<dd><p>The Scheme empty list object, or “End Of List” object, usually written
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in Scheme as <code class="code">'()</code>.
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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_005fEOF_005fVAL"><span class="category-def">Macro: </span><span><code class="def-type">SCM</code> <strong class="def-name">SCM_EOF_VAL</strong><a class="copiable-link" href="#index-SCM_005fEOF_005fVAL"> ¶</a></span></dt>
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<dd><p>The Scheme end-of-file value. It has no standard written
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representation, for obvious reasons.
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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_005fUNSPECIFIED"><span class="category-def">Macro: </span><span><code class="def-type">SCM</code> <strong class="def-name">SCM_UNSPECIFIED</strong><a class="copiable-link" href="#index-SCM_005fUNSPECIFIED"> ¶</a></span></dt>
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<dd><p>The value returned by some (but not all) expressions that the Scheme
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standard says return an “unspecified” value.
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</p>
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<p>This is sort of a weirdly literal way to take things, but the standard
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read-eval-print loop prints nothing when the expression returns this
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value, so it’s not a bad idea to return this when you can’t think of
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anything else helpful.
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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_005fUNDEFINED"><span class="category-def">Macro: </span><span><code class="def-type">SCM</code> <strong class="def-name">SCM_UNDEFINED</strong><a class="copiable-link" href="#index-SCM_005fUNDEFINED"> ¶</a></span></dt>
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<dd><p>The “undefined” value. Its most important property is that is not
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equal to any valid Scheme value. This is put to various internal uses
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by C code interacting with Guile.
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</p>
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<p>For example, when you write a C function that is callable from Scheme
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and which takes optional arguments, the interpreter passes
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<code class="code">SCM_UNDEFINED</code> for any arguments you did not receive.
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</p>
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<p>We also use this to mark unbound variables.
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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_005fUNBNDP"><span class="category-def">Macro: </span><span><code class="def-type">int</code> <strong class="def-name">SCM_UNBNDP</strong> <code class="def-code-arguments">(SCM <var class="var">x</var>)</code><a class="copiable-link" href="#index-SCM_005fUNBNDP"> ¶</a></span></dt>
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<dd><p>Return true if <var class="var">x</var> is <code class="code">SCM_UNDEFINED</code>. Note that this is not a
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check to see if <var class="var">x</var> is <code class="code">SCM_UNBOUND</code>. History will not be kind
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to us.
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</p></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="Non_002dImmediate-Objects.html">Non-Immediate Objects</a>, Previous: <a href="Relationship-Between-SCM-and-scm_005ft_005fbits.html">Relationship Between <code class="code">SCM</code> and <code class="code">scm_t_bits</code></a>, Up: <a href="The-SCM-Type-in-Guile.html">The SCM Type in Guile</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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