108 lines
4.9 KiB
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108 lines
4.9 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>SRFI-171 (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="SRFI_002d171">
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<p>
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Previous: <a href="SRFI_002d119.html" accesskey="p" rel="prev">SRFI-119 Wisp: simpler indentation-sensitive Scheme.</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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<h4 class="subsection" id="Transducers"><span>7.5.47 Transducers<a class="copiable-link" href="#Transducers"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-SRFI_002d171"></a>
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<a class="index-entry-id" id="index-transducers"></a>
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<p>Some of the most common operations used in the Scheme language are those
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transforming lists: map, filter, take and so on. They work well, are well
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understood, and are used daily by most Scheme programmers. They are however not
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general because they only work on lists, and they do not compose very well
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since combining N of them builds <code class="code">(- N 1)</code> intermediate lists.
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</p>
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<p>Transducers are oblivious to what kind of process they are used in, and
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are composable without building intermediate collections. This means we
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can create a transducer that squares all odd numbers:
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</p>
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<div class="example">
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<pre class="example-preformatted">(compose (tfilter odd?) (tmap (lambda (x) (* x x))))
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</pre></div>
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<p>and reuse it with lists, vectors, or in just about any context where
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data flows in one direction. We could use it as a processing step for
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asynchronous channels, with an event framework as a pre-processing step,
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or even in lazy contexts where you pass a lazy collection and a
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transducer to a function and get a new lazy collection back.
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</p>
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<p>The traditional Scheme approach of having collection-specific procedures
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is not changed. We instead specify a general form of transformations
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that complement these procedures. The benefits are obvious: a clear,
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well-understood way of describing common transformations in a way that
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is faster than just chaining the collection-specific counterparts. For
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guile in particular this means a lot better GC performance.
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</p>
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<p>Notice however that <code class="code">(compose …)</code> composes transducers
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left-to-right, due to how transducers are initiated.
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</p>
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<ul class="mini-toc">
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<li><a href="SRFI_002d171-General-Discussion.html" accesskey="1">SRFI-171 General Discussion</a></li>
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<li><a href="SRFI_002d171-Applying-Transducers.html" accesskey="2">Applying Transducers</a></li>
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<li><a href="SRFI_002d171-Reducers.html" accesskey="3">Reducers</a></li>
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<li><a href="SRFI_002d171-Transducers.html" accesskey="4">Transducers</a></li>
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<li><a href="SRFI_002d171-Helpers.html" accesskey="5">Helper functions for writing transducers</a></li>
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</ul>
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</div>
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<hr>
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<p>
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Previous: <a href="SRFI_002d119.html">SRFI-119 Wisp: simpler indentation-sensitive Scheme.</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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