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99 lines
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>R7RS Incompatibilities (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="R7RS-Incompatibilities">
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Next: <a href="R7RS-Standard-Libraries.html" accesskey="n" rel="next">R7RS Standard Libraries</a>, Up: <a href="R7RS-Support.html" accesskey="u" rel="up">R7RS Support</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="Incompatibilities-with-the-R7RS"><span>7.7.1 Incompatibilities with the R7RS<a class="copiable-link" href="#Incompatibilities-with-the-R7RS"> ¶</a></span></h4>
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<p>As the R7RS is a much less ambitious standard than the R6RS
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(see <a class="pxref" href="Guile-and-Scheme.html">Guile and Scheme</a>), it is very easy for Guile to support. As
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such, Guile is a fully conforming implementation of R7RS, with the
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exception of the occasional bug and a couple of unimplemented features:
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</p>
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<ul class="itemize mark-bullet">
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<li><a class="index-entry-id" id="index-datum-labels"></a>
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The R7RS specifies a syntax for reading circular data structures using
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<em class="dfn">datum labels</em>, such as <code class="code">#0=(1 2 3 . #0#)</code>. Guile’s reader
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does not support this syntax currently;
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<a class="url" href="https://bugs.gnu.org/38236">https://bugs.gnu.org/38236</a>.
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</li><li>As with R6RS, a number of lexical features of R7RS conflict with Guile’s
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historical syntax. In addition to <code class="code">r6rs-hex-escapes</code> and
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<code class="code">hungry-eol-escapes</code> (see <a class="pxref" href="R6RS-Incompatibilities.html">Incompatibilities with the R6RS</a>), the
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<code class="code">r7rs-symbols</code> reader feature needs to be explicitly enabled.
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</li></ul>
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<p>Guile exposes a procedure in the root module to choose R7RS defaults
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over Guile’s historical defaults.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-install_002dr7rs_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">install-r7rs!</strong><a class="copiable-link" href="#index-install_002dr7rs_0021"> ¶</a></span></dt>
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<dd><p>Alter Guile’s default settings to better conform to the R7RS.
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</p>
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<p>While Guile’s defaults may evolve over time, the current changes that
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this procedure imposes are to add <code class="code">.sls</code> and <code class="code">.guile.sls</code> to
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the set of supported <code class="code">%load-extensions</code>, to better support R7RS
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conventions. See <a class="xref" href="Load-Paths.html">Load Paths</a>. <code class="code">install-r7rs!</code> will also enable
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the reader options mentioned above.
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</p></dd></dl>
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<p>Finally, note that the <code class="code">--r7rs</code> command-line argument will call
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<code class="code">install-r7rs!</code> before calling user code. R7RS users probably want
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to pass this argument to their Guile.
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</p>
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