141 lines
6.6 KiB
HTML
141 lines
6.6 KiB
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<!DOCTYPE html>
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<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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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-171 General Discussion (Guile Reference Manual)</title>
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<meta name="description" content="SRFI-171 General Discussion (Guile Reference Manual)">
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<meta name="keywords" content="SRFI-171 General Discussion (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_002d171.html" rel="up" title="SRFI-171">
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<link href="SRFI_002d171-Applying-Transducers.html" rel="next" title="SRFI-171 Applying Transducers">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="SRFI_002d171-General-Discussion">
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<div class="nav-panel">
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<p>
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Next: <a href="SRFI_002d171-Applying-Transducers.html" accesskey="n" rel="next">Applying Transducers</a>, Up: <a href="SRFI_002d171.html" accesskey="u" rel="up">Transducers</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="subsubsection" id="SRFI_002d171-General-Discussion-1"><span>7.5.47.1 SRFI-171 General Discussion<a class="copiable-link" href="#SRFI_002d171-General-Discussion-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-transducers-discussion"></a>
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<h4 class="subheading" id="The-concept-of-reducers"><span>The concept of reducers<a class="copiable-link" href="#The-concept-of-reducers"> ¶</a></span></h4>
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<p>The central part of transducers are 3-arity reducing procedures.
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</p>
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<ul class="itemize mark-bullet">
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<li>no arguments: Produces the identity of the reducer.
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</li><li>(result-so-far): completion. Returns <code class="code">result-so-far</code> either with or
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without transforming it first.
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</li><li>(result-so-far input) combines <code class="code">result-so-far</code> and <code class="code">input</code> to produce
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a new <code class="code">result-so-far</code>.
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</li></ul>
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<p>In the case of a summing <code class="code">+</code> reducer, the reducer would produce, in
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arity order: <code class="code">0</code>, <code class="code">result-so-far</code>, <code class="code">(+ result-so-far
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input)</code>. This happens to be exactly what the regular <code class="code">+</code> does.
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</p>
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<h4 class="subheading" id="The-concept-of-transducers"><span>The concept of transducers<a class="copiable-link" href="#The-concept-of-transducers"> ¶</a></span></h4>
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<p>A transducer is a one-arity procedure that takes a reducer and produces a
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reducing function that behaves as follows:
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</p>
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<ul class="itemize mark-bullet">
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<li>no arguments: calls reducer with no arguments (producing its identity)
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</li><li>(result-so-far): Maybe transform the result-so-far and call reducer with it.
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</li><li>(result-so-far input) Maybe do something to input and maybe call the
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reducer with result-so-far and the maybe-transformed input.
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</li></ul>
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<p>A simple example is as following:
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</p>
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<div class="example">
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<pre class="example-preformatted">(list-transduce (tfilter odd?) + '(1 2 3 4 5))
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</pre></div>
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<p>This first returns a transducer filtering all odd
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elements, then it runs <code class="code">+</code> without arguments to retrieve its
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identity. It then starts the transduction by passing <code class="code">+</code> to the
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transducer returned by <code class="code">(tfilter odd?)</code> which returns a reducing
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function. It works not unlike reduce from SRFI 1, but also checks
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whether one of the intermediate transducers returns a "reduced" value
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(implemented as a SRFI 9 record), which means the reduction finished
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early.
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</p>
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<p>Because transducers compose and the final reduction is only executed in
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the last step, composed transducers will not build any intermediate
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result or collections. Although the normal way of thinking about
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application of composed functions is right to left, due to how the
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transduction is built it is applied left to right. <code class="code">(compose
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(tfilter odd?) (tmap sqrt))</code> will create a transducer that first filters
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out any odd values and then computes the square root of the rest.
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</p>
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<h4 class="subheading" id="State"><span>State<a class="copiable-link" href="#State"> ¶</a></span></h4>
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<p>Even though transducers appear to be somewhat of a generalisation of
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<code class="code">map</code> and friends, this is not really true. Since transducers don’t
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know in which context they are being used, some transducers must keep
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state where their collection-specific counterparts do not. The
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transducers that keep state do so using hidden mutable state, and as
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such all the caveats of mutation, parallelism, and multi-shot
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continuations apply. Each transducer keeping state is clearly described
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as doing so in the documentation.
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</p>
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<h4 class="subheading" id="Naming"><span>Naming<a class="copiable-link" href="#Naming"> ¶</a></span></h4>
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<p>Reducers exported from the transducers module are named as in their
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SRFI-1 counterpart, but prepended with an r. Transducers also follow
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that naming, but are prepended with a t.
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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="SRFI_002d171-Applying-Transducers.html">Applying Transducers</a>, Up: <a href="SRFI_002d171.html">Transducers</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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