176 lines
9.7 KiB
HTML
176 lines
9.7 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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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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under the terms of the GNU Free Documentation License, Version 1.3 or
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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>SRFI-38 (Guile Reference Manual)</title>
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<meta name="description" content="SRFI-38 (Guile Reference Manual)">
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<meta name="keywords" content="SRFI-38 (Guile Reference Manual)">
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<link href="index.html" rel="start" title="Top">
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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="SRFI-Support.html" rel="up" title="SRFI Support">
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<link href="SRFI_002d39.html" rel="next" title="SRFI-39">
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<link href="SRFI_002d37.html" rel="prev" title="SRFI-37">
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<style type="text/css">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="SRFI_002d38">
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<div class="nav-panel">
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<p>
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Next: <a href="SRFI_002d39.html" accesskey="n" rel="next">SRFI-39 - Parameters</a>, Previous: <a href="SRFI_002d37.html" accesskey="p" rel="prev">SRFI-37 - args-fold</a>, Up: <a href="SRFI-Support.html" accesskey="u" rel="up">SRFI Support Modules</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="SRFI_002d38-_002d-External-Representation-for-Data-With-Shared-Structure"><span>7.5.26 SRFI-38 - External Representation for Data With Shared Structure<a class="copiable-link" href="#SRFI_002d38-_002d-External-Representation-for-Data-With-Shared-Structure"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-SRFI_002d38"></a>
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<p>This subsection is based on
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<a class="uref" href="http://srfi.schemers.org/srfi-38/srfi-38.html">the specification
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of SRFI-38</a> written by Ray Dillinger.
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</p>
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<p>This SRFI creates an alternative external representation for data
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written and read using <code class="code">write-with-shared-structure</code> and
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<code class="code">read-with-shared-structure</code>. It is identical to the grammar for
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external representation for data written and read with <code class="code">write</code> and
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<code class="code">read</code> given in section 7 of R5RS, except that the single
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production
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</p>
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<div class="example">
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<pre class="example-preformatted"><datum> --> <simple datum> | <compound datum>
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</pre></div>
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<p>is replaced by the following five productions:
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</p>
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<div class="example">
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<pre class="example-preformatted"><datum> --> <defining datum> | <nondefining datum> | <defined datum>
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<defining datum> --> #<indexnum>=<nondefining datum>
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<defined datum> --> #<indexnum>#
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<nondefining datum> --> <simple datum> | <compound datum>
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<indexnum> --> <digit 10>+
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-write_002dwith_002dshared_002dstructure"><span class="category-def">Scheme procedure: </span><span><strong class="def-name">write-with-shared-structure</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-write_002dwith_002dshared_002dstructure"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-write_002dwith_002dshared_002dstructure-1"><span class="category-def">Scheme procedure: </span><span><strong class="def-name">write-with-shared-structure</strong> <var class="def-var-arguments">obj port</var><a class="copiable-link" href="#index-write_002dwith_002dshared_002dstructure-1"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-write_002dwith_002dshared_002dstructure-2"><span class="category-def">Scheme procedure: </span><span><strong class="def-name">write-with-shared-structure</strong> <var class="def-var-arguments">obj port optarg</var><a class="copiable-link" href="#index-write_002dwith_002dshared_002dstructure-2"> ¶</a></span></dt>
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<dd>
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<p>Writes an external representation of <var class="var">obj</var> to the given port.
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Strings that appear in the written representation are enclosed in
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doublequotes, and within those strings backslash and doublequote
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characters are escaped by backslashes. Character objects are written
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using the <code class="code">#\</code> notation.
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</p>
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<p>Objects which denote locations rather than values (cons cells, vectors,
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and non-zero-length strings in R5RS scheme; also Guile’s structs,
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bytevectors and ports and hash-tables), if they appear at more than one
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point in the data being written, are preceded by ‘<samp class="samp">#<var class="var">N</var>=</samp>’ the
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first time they are written and replaced by ‘<samp class="samp">#<var class="var">N</var>#</samp>’ all
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subsequent times they are written, where <var class="var">N</var> is a natural number
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used to identify that particular object. If objects which denote
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locations occur only once in the structure, then
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<code class="code">write-with-shared-structure</code> must produce the same external
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representation for those objects as <code class="code">write</code>.
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</p>
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<p><code class="code">write-with-shared-structure</code> terminates in finite time and
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produces a finite representation when writing finite data.
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</p>
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<p><code class="code">write-with-shared-structure</code> returns an unspecified value. The
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<var class="var">port</var> argument may be omitted, in which case it defaults to the
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value returned by <code class="code">(current-output-port)</code>. The <var class="var">optarg</var>
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argument may also be omitted. If present, its effects on the output and
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return value are unspecified but <code class="code">write-with-shared-structure</code> must
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still write a representation that can be read by
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<code class="code">read-with-shared-structure</code>. Some implementations may wish to use
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<var class="var">optarg</var> to specify formatting conventions, numeric radixes, or
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return values. Guile’s implementation ignores <var class="var">optarg</var>.
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</p>
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<p>For example, the code
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(begin (define a (cons 'val1 'val2))
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(set-cdr! a a)
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(write-with-shared-structure a))
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</pre></div>
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<p>should produce the output <code class="code">#1=(val1 . #1#)</code>. This shows a cons
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cell whose <code class="code">cdr</code> contains itself.
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</p>
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</dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-read_002dwith_002dshared_002dstructure"><span class="category-def">Scheme procedure: </span><span><strong class="def-name">read-with-shared-structure</strong><a class="copiable-link" href="#index-read_002dwith_002dshared_002dstructure"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-read_002dwith_002dshared_002dstructure-1"><span class="category-def">Scheme procedure: </span><span><strong class="def-name">read-with-shared-structure</strong> <var class="def-var-arguments">port</var><a class="copiable-link" href="#index-read_002dwith_002dshared_002dstructure-1"> ¶</a></span></dt>
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<dd>
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<p><code class="code">read-with-shared-structure</code> converts the external representations
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of Scheme objects produced by <code class="code">write-with-shared-structure</code> into
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Scheme objects. That is, it is a parser for the nonterminal
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‘<samp class="samp"><datum></samp>’ in the augmented external representation grammar defined
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above. <code class="code">read-with-shared-structure</code> returns the next object
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parsable from the given input port, updating <var class="var">port</var> to point to the
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first character past the end of the external representation of the
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object.
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</p>
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<p>If an end-of-file is encountered in the input before any characters are
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found that can begin an object, then an end-of-file object is returned.
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The port remains open, and further attempts to read it (by
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<code class="code">read-with-shared-structure</code> or <code class="code">read</code> will also return an
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end-of-file object. If an end of file is encountered after the
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beginning of an object’s external representation, but the external
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representation is incomplete and therefore not parsable, an error is
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signaled.
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</p>
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<p>The <var class="var">port</var> argument may be omitted, in which case it defaults to the
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value returned by <code class="code">(current-input-port)</code>. It is an error to read
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from a closed port.
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</p>
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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="SRFI_002d39.html">SRFI-39 - Parameters</a>, Previous: <a href="SRFI_002d37.html">SRFI-37 - args-fold</a>, Up: <a href="SRFI-Support.html">SRFI Support Modules</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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