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168 lines
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<title>Source Properties (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Source-Properties">
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Next: <a href="Programmatic-Error-Handling.html" accesskey="n" rel="next">Programmatic Error Handling</a>, Previous: <a href="Evaluation-Model.html" accesskey="p" rel="prev">Evaluation and the Scheme Stack</a>, Up: <a href="Debugging.html" accesskey="u" rel="up">Debugging Infrastructure</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="subsection" id="Source-Properties-1"><span>6.26.2 Source Properties<a class="copiable-link" href="#Source-Properties-1"> ¶</a></span></h4>
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<p>How best to associate source locations with datums parsed from a port?
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The right way to do this is to annotate all components of each parsed
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datum. See <a class="xref" href="Annotated-Scheme-Read.html">Reading Scheme Code, For the Compiler</a>, for more on <code class="code">read-syntax</code>.
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</p>
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<a class="index-entry-id" id="index-source-properties"></a>
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<p>Guile only switched to use <code class="code">read-syntax</code> in 2021, however. For the
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previous thirty years, it used a mechanism known as <em class="dfn">source
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properties</em>.
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</p>
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<p>As Guile reads in Scheme code from file or from standard input, it can
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record the file name, line number and column number where each
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expression begins in a side table.
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</p>
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<p>The way that this side table associates datums with source properties
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has a limitation, however: Guile can only associate source properties
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with freshly allocated objects. This notably excludes individual
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symbols, keywords, characters, booleans, or small integers. This
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limitation finally motivated the switch to <code class="code">read-syntax</code>.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-supports_002dsource_002dproperties_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">supports-source-properties?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-supports_002dsource_002dproperties_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fsupports_005fsource_005fproperties_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_supports_source_properties_p</strong> <var class="def-var-arguments">(obj)</var><a class="copiable-link" href="#index-scm_005fsupports_005fsource_005fproperties_005fp"> ¶</a></span></dt>
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<dd><p>Return #t if source properties can be associated with <var class="var">obj</var>,
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otherwise return #f.
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</p></dd></dl>
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<p>The recording of source properties is controlled by the read option
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named “positions” (see <a class="pxref" href="Scheme-Read.html">Reading Scheme Code</a>). This option is switched
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<em class="emph">on</em> by default. Now that <code class="code">read-syntax</code> is available,
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however, Guile may change the default for this flag to off in the
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future.
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</p>
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<p>The following procedures can be used to access and set the source
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properties of read expressions.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-set_002dsource_002dproperties_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">set-source-properties!</strong> <var class="def-var-arguments">obj alist</var><a class="copiable-link" href="#index-set_002dsource_002dproperties_0021"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fset_005fsource_005fproperties_005fx"><span class="category-def">C Function: </span><span><strong class="def-name">scm_set_source_properties_x</strong> <var class="def-var-arguments">(obj, alist)</var><a class="copiable-link" href="#index-scm_005fset_005fsource_005fproperties_005fx"> ¶</a></span></dt>
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<dd><p>Install the association list <var class="var">alist</var> as the source property
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list for <var class="var">obj</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-set_002dsource_002dproperty_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">set-source-property!</strong> <var class="def-var-arguments">obj key datum</var><a class="copiable-link" href="#index-set_002dsource_002dproperty_0021"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fset_005fsource_005fproperty_005fx"><span class="category-def">C Function: </span><span><strong class="def-name">scm_set_source_property_x</strong> <var class="def-var-arguments">(obj, key, datum)</var><a class="copiable-link" href="#index-scm_005fset_005fsource_005fproperty_005fx"> ¶</a></span></dt>
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<dd><p>Set the source property of object <var class="var">obj</var>, which is specified by
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<var class="var">key</var> to <var class="var">datum</var>. Normally, the key will be a symbol.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-source_002dproperties"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">source-properties</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-source_002dproperties"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fsource_005fproperties"><span class="category-def">C Function: </span><span><strong class="def-name">scm_source_properties</strong> <var class="def-var-arguments">(obj)</var><a class="copiable-link" href="#index-scm_005fsource_005fproperties"> ¶</a></span></dt>
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<dd><p>Return the source property association list of <var class="var">obj</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-source_002dproperty"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">source-property</strong> <var class="def-var-arguments">obj key</var><a class="copiable-link" href="#index-source_002dproperty"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fsource_005fproperty"><span class="category-def">C Function: </span><span><strong class="def-name">scm_source_property</strong> <var class="def-var-arguments">(obj, key)</var><a class="copiable-link" href="#index-scm_005fsource_005fproperty"> ¶</a></span></dt>
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<dd><p>Return the property specified by <var class="var">key</var> from <var class="var">obj</var>’s source
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properties.
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</p></dd></dl>
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<p>If the <code class="code">positions</code> reader option is enabled, supported expressions
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will have values set for the <code class="code">filename</code>, <code class="code">line</code> and
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<code class="code">column</code> properties.
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</p>
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<p>Source properties are also associated with syntax objects. Procedural
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macros can get at the source location of their input using the
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<code class="code">syntax-source</code> accessor. See <a class="xref" href="Syntax-Transformer-Helpers.html">Syntax Transformer Helpers</a>, for
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more.
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</p>
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<p>Guile also defines a couple of convenience macros built on
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<code class="code">syntax-source</code>:
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-current_002dsource_002dlocation"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">current-source-location</strong><a class="copiable-link" href="#index-current_002dsource_002dlocation"> ¶</a></span></dt>
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<dd><p>Expands to the source properties corresponding to the location of the
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<code class="code">(current-source-location)</code> form.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-current_002dfilename"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">current-filename</strong><a class="copiable-link" href="#index-current_002dfilename"> ¶</a></span></dt>
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<dd><p>Expands to the current filename: the filename that the
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<code class="code">(current-filename)</code> form appears in. Expands to <code class="code">#f</code> if this
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information is unavailable.
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</p></dd></dl>
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<p>If you’re stuck with defmacros (see <a class="pxref" href="Defmacros.html">Lisp-style Macro Definitions</a>), and want to preserve
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source information, the following helper function might be useful to
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you:
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-cons_002dsource"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">cons-source</strong> <var class="def-var-arguments">xorig x y</var><a class="copiable-link" href="#index-cons_002dsource"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fcons_005fsource"><span class="category-def">C Function: </span><span><strong class="def-name">scm_cons_source</strong> <var class="def-var-arguments">(xorig, x, y)</var><a class="copiable-link" href="#index-scm_005fcons_005fsource"> ¶</a></span></dt>
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<dd><p>Create and return a new pair whose car and cdr are <var class="var">x</var> and <var class="var">y</var>.
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Any source properties associated with <var class="var">xorig</var> are also associated
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with the new pair.
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</p></dd></dl>
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<p>
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Next: <a href="Programmatic-Error-Handling.html">Programmatic Error Handling</a>, Previous: <a href="Evaluation-Model.html">Evaluation and the Scheme Stack</a>, Up: <a href="Debugging.html">Debugging Infrastructure</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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