133 lines
6.2 KiB
HTML
133 lines
6.2 KiB
HTML
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Value History (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<div class="subsection-level-extent" id="Value-History">
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<div class="nav-panel">
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<p>
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Next: <a href="REPL-Commands.html" accesskey="n" rel="next">REPL Commands</a>, Previous: <a href="Readline.html" accesskey="p" rel="prev">Readline</a>, Up: <a href="Using-Guile-Interactively.html" accesskey="u" rel="up">Using Guile Interactively</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="Value-History-1"><span>4.4.3 Value History<a class="copiable-link" href="#Value-History-1"> ¶</a></span></h4>
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<p>Just as Readline helps you to reuse a previous input line, <em class="dfn">value
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history</em> allows you to use the <em class="emph">result</em> of a previous evaluation in
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a new expression. When value history is enabled, each evaluation result
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is automatically assigned to the next in the sequence of variables
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<code class="code">$1</code>, <code class="code">$2</code>, …. You can then use these variables in
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subsequent expressions.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">scheme@(guile-user)> (iota 10)
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$1 = (0 1 2 3 4 5 6 7 8 9)
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scheme@(guile-user)> (apply * (cdr $1))
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$2 = 362880
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scheme@(guile-user)> (sqrt $2)
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$3 = 602.3952191045344
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scheme@(guile-user)> (cons $2 $1)
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$4 = (362880 0 1 2 3 4 5 6 7 8 9)
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</pre></div>
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<p>Value history is enabled by default, because Guile’s REPL imports the
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<code class="code">(ice-9 history)</code> module. Value history may be turned off or on within the
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repl, using the options interface:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">scheme@(guile-user)> ,option value-history #f
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scheme@(guile-user)> 'foo
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foo
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scheme@(guile-user)> ,option value-history #t
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scheme@(guile-user)> 'bar
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$5 = bar
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</pre></div>
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<p>Note that previously recorded values are still accessible, even if value history
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is off. In rare cases, these references to past computations can cause Guile to
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use too much memory. One may clear these values, possibly enabling garbage
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collection, via the <code class="code">clear-value-history!</code> procedure, described below.
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</p>
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<p>The programmatic interface to value history is in a module:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(use-modules (ice-9 history))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-value_002dhistory_002denabled_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">value-history-enabled?</strong><a class="copiable-link" href="#index-value_002dhistory_002denabled_003f"> ¶</a></span></dt>
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<dd><p>Return true if value history is enabled, or false otherwise.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-enable_002dvalue_002dhistory_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">enable-value-history!</strong><a class="copiable-link" href="#index-enable_002dvalue_002dhistory_0021"> ¶</a></span></dt>
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<dd><p>Turn on value history, if it was off.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-disable_002dvalue_002dhistory_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">disable-value-history!</strong><a class="copiable-link" href="#index-disable_002dvalue_002dhistory_0021"> ¶</a></span></dt>
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<dd><p>Turn off value history, if it was on.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-clear_002dvalue_002dhistory_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">clear-value-history!</strong><a class="copiable-link" href="#index-clear_002dvalue_002dhistory_0021"> ¶</a></span></dt>
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<dd><p>Clear the value history. If the stored values are not captured by some other
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data structure or closure, they may then be reclaimed by the garbage collector.
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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="REPL-Commands.html">REPL Commands</a>, Previous: <a href="Readline.html">Readline</a>, Up: <a href="Using-Guile-Interactively.html">Using Guile Interactively</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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