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160 lines
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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>Shift and Reset (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="Prompts.html" rel="up" title="Prompts">
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<link href="Prompt-Primitives.html" rel="prev" title="Prompt Primitives">
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<div class="subsubsection-level-extent" id="Shift-and-Reset">
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<div class="nav-panel">
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<p>
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Previous: <a href="Prompt-Primitives.html" accesskey="p" rel="prev">Prompt Primitives</a>, Up: <a href="Prompts.html" accesskey="u" rel="up">Prompts</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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<hr>
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<h4 class="subsubsection" id="Shift_002c-Reset_002c-and-All-That"><span>6.11.5.2 Shift, Reset, and All That<a class="copiable-link" href="#Shift_002c-Reset_002c-and-All-That"> ¶</a></span></h4>
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<p>There is a whole zoo of delimited control operators, and as it does not
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seem to be a bounded set, Guile implements support for them in a
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separate module:
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (ice-9 control))
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</pre></div>
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<p>Firstly, we have a helpful abbreviation for the <code class="code">call-with-prompt</code>
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operator.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-_0025"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">%</strong> <var class="def-var-arguments">expr</var><a class="copiable-link" href="#index-_0025"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-_0025-1"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">%</strong> <var class="def-var-arguments">expr handler</var><a class="copiable-link" href="#index-_0025-1"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-_0025-2"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">%</strong> <var class="def-var-arguments">tag expr handler</var><a class="copiable-link" href="#index-_0025-2"> ¶</a></span></dt>
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<dd><p>Evaluate <var class="var">expr</var> in a prompt, optionally specifying a tag and a
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handler. If no tag is given, the default prompt tag is used.
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</p>
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<p>If no handler is given, a default handler is installed. The default
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handler accepts a procedure of one argument, which will be called on
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the captured continuation, within a prompt.
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</p>
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<p>Sometimes it’s easier just to show code, as in this case:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (default-prompt-handler k proc)
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(% (default-prompt-tag)
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(proc k)
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default-prompt-handler))
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</pre></div>
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<p>The <code class="code">%</code> symbol is chosen because it looks like a prompt.
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</p></dd></dl>
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<p>Likewise there is an abbreviation for <code class="code">abort-to-prompt</code>, which
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assumes the default prompt tag:
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-abort"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">abort</strong> <var class="def-var-arguments">val1 val2 …</var><a class="copiable-link" href="#index-abort"> ¶</a></span></dt>
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<dd><p>Abort to the default prompt tag, passing <var class="var">val1</var> <var class="var">val2</var> …
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to the handler.
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</p></dd></dl>
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<p>As mentioned before, <code class="code">(ice-9 control)</code> also provides other
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delimited control operators. This section is a bit technical, and
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first-time users of delimited continuations should probably come back to
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it after some practice with <code class="code">%</code>.
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</p>
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<p>Still here? So, when one implements a delimited control operator like
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<code class="code">call-with-prompt</code>, one needs to make two decisions. Firstly, does
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the handler run within or outside the prompt? Having the handler run
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within the prompt allows an abort inside the handler to return to the
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same prompt handler, which is often useful. However it prevents tail
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calls from the handler, so it is less general.
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</p>
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<p>Similarly, does invoking a captured continuation reinstate a prompt?
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Again we have the tradeoff of convenience versus proper tail calls.
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</p>
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<p>These decisions are captured in the Felleisen <em class="dfn">F</em> operator. If
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neither the continuations nor the handlers implicitly add a prompt, the
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operator is known as <em class="dfn">–F–</em>. This is the case for Guile’s
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<code class="code">call-with-prompt</code> and <code class="code">abort-to-prompt</code>.
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</p>
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<p>If both continuation and handler implicitly add prompts, then the
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operator is <em class="dfn">+F+</em>. <code class="code">shift</code> and <code class="code">reset</code> are such
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operators.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-reset"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">reset</strong> <var class="def-var-arguments">body1 body2 …</var><a class="copiable-link" href="#index-reset"> ¶</a></span></dt>
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<dd><p>Establish a prompt, and evaluate <var class="var">body1</var> <var class="var">body2</var> … within
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that prompt.
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</p>
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<p>The prompt handler is designed to work with <code class="code">shift</code>, described
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below.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-shift"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">shift</strong> <var class="def-var-arguments">cont body1 body2 …</var><a class="copiable-link" href="#index-shift"> ¶</a></span></dt>
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<dd><p>Abort to the nearest <code class="code">reset</code>, and evaluate <var class="var">body1</var> <var class="var">body2</var>
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… in a context in which the captured continuation is bound to
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<var class="var">cont</var>.
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</p>
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<p>As mentioned above, taken together, the <var class="var">body1</var> <var class="var">body2</var> …
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expressions and the invocations of <var class="var">cont</var> implicitly establish a
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prompt.
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</p></dd></dl>
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<p>Interested readers are invited to explore Oleg Kiselyov’s wonderful web
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site at <a class="uref" href="http://okmij.org/ftp/">http://okmij.org/ftp/</a>, for more information on these
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operators.
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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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Previous: <a href="Prompt-Primitives.html">Prompt Primitives</a>, Up: <a href="Prompts.html">Prompts</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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