169 lines
7.4 KiB
HTML
169 lines
7.4 KiB
HTML
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>R6RS Incompatibilities (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="R6RS-Support.html" rel="up" title="R6RS Support">
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<body lang="en">
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<div class="subsection-level-extent" id="R6RS-Incompatibilities">
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<div class="nav-panel">
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<p>
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Next: <a href="R6RS-Standard-Libraries.html" accesskey="n" rel="next">R6RS Standard Libraries</a>, Up: <a href="R6RS-Support.html" accesskey="u" rel="up">R6RS 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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</div>
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<hr>
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<h4 class="subsection" id="Incompatibilities-with-the-R6RS"><span>7.6.1 Incompatibilities with the R6RS<a class="copiable-link" href="#Incompatibilities-with-the-R6RS"> ¶</a></span></h4>
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<p>There are some incompatibilities between Guile and the R6RS. Some of
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them are intentional, some of them are bugs, and some are simply
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unimplemented features. Please let the Guile developers know if you
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find one that is not on this list.
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</p>
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<ul class="itemize mark-bullet">
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<li>The R6RS specifies many situations in which a conforming implementation
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must signal a specific error. Guile doesn’t really care about that too
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much—if a correct R6RS program would not hit that error, we don’t
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bother checking for it.
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</li><li>Multiple <code class="code">library</code> forms in one file are not yet supported. This
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is because the expansion of <code class="code">library</code> sets the current module, but
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does not restore it. This is a bug.
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</li><li>R6RS unicode escapes within strings are disabled by default, because
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they conflict with Guile’s already-existing escapes. The same is the
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case for R6RS treatment of escaped newlines in strings.
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<p>R6RS behavior can be turned on via a reader option. See <a class="xref" href="String-Syntax.html">String Read Syntax</a>, for more information.
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</p>
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</li><li>Guile does not yet support Unicode escapes in symbols, such as
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<code class="code">H\x65;llo</code> (the same as <code class="code">Hello</code>), or <code class="code">\x3BB;</code> (the same
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as <code class="code">λ</code>).
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</li><li>A <code class="code">set!</code> to a variable transformer may only expand to an
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expression, not a definition—even if the original <code class="code">set!</code>
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expression was in definition context.
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</li><li>Instead of using the algorithm detailed in chapter 10 of the R6RS,
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expansion of toplevel forms happens sequentially.
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<p>For example, while the expansion of the following set of toplevel
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definitions does the correct thing:
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</p>
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<div class="example">
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<pre class="example-preformatted">(begin
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(define even?
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(lambda (x)
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(or (= x 0) (odd? (- x 1)))))
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(define-syntax odd?
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(syntax-rules ()
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((odd? x) (not (even? x)))))
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(even? 10))
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⇒ #t
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</pre></div>
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<p>The same definitions outside of the <code class="code">begin</code> wrapper do not:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define even?
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(lambda (x)
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(or (= x 0) (odd? (- x 1)))))
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(define-syntax odd?
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(syntax-rules ()
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((odd? x) (not (even? x)))))
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(even? 10)
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<unnamed port>:4:18: In procedure even?:
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<unnamed port>:4:18: Wrong type to apply: #<syntax-transformer odd?>
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</pre></div>
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<p>This is because when expanding the right-hand-side of <code class="code">even?</code>, the
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reference to <code class="code">odd?</code> is not yet marked as a syntax transformer, so
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it is assumed to be a function.
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</p>
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<p>This bug will only affect top-level programs, not code in <code class="code">library</code>
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forms. Fixing it for toplevel forms seems doable, but tricky to
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implement in a backward-compatible way. Suggestions and/or patches would
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be appreciated.
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</p>
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</li><li>The <code class="code">(rnrs io ports)</code> module is incomplete. Work is
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ongoing to fix this.
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</li><li>Guile does not prevent use of textual I/O procedures on binary ports, or
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vice versa. All ports in Guile support both binary and textual I/O.
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See <a class="xref" href="Encoding.html">Encoding</a>, for full details.
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</li><li>Guile’s implementation of <code class="code">equal?</code> may fail to terminate when
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applied to arguments containing cycles.
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</li></ul>
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<p>Guile exposes a procedure in the root module to choose R6RS 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_002dr6rs_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">install-r6rs!</strong><a class="copiable-link" href="#index-install_002dr6rs_0021"> ¶</a></span></dt>
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<dd><p>Alter Guile’s default settings to better conform to the R6RS.
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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 R6RS
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conventions. See <a class="xref" href="Load-Paths.html">Load Paths</a>. Also, enable R6RS unicode escapes in
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strings; see the discussion above.
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</p></dd></dl>
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<p>Finally, note that the <code class="code">--r6rs</code> command-line argument will call
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<code class="code">install-r6rs!</code> before calling user code. R6RS users probably want
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to pass this argument to their Guile.
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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="R6RS-Standard-Libraries.html">R6RS Standard Libraries</a>, Up: <a href="R6RS-Support.html">R6RS 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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</div>
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</body>
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</html>
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