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<title>SRFI-171 Reducers (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="SRFI_002d171-Reducers">
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<h4 class="subsubsection" id="Reducers"><span>7.5.47.3 Reducers<a class="copiable-link" href="#Reducers"> &para;</a></span></h4>
<a class="index-entry-id" id="index-transducers-reducers"></a>
<dl class="first-deffn">
<dt class="deffn" id="index-rcons"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">rcons</strong><a class="copiable-link" href="#index-rcons"> &para;</a></span></dt>
<dd><p>a simple consing reducer. When called without values, it returns its
identity, <code class="code">'()</code>. With one value, which will be a list, it reverses
the list (using <code class="code">reverse!</code>). When called with two values, it conses
the second value to the first.
</p>
<div class="example">
<pre class="example-preformatted">(list-transduce (tmap (lambda (x) (+ x 1)) rcons (list 0 1 2 3))
&rArr; (1 2 3 4)
</pre></div>
</dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-reverse_002drcons"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">reverse-rcons</strong><a class="copiable-link" href="#index-reverse_002drcons"> &para;</a></span></dt>
<dd><p>same as rcons, but leaves the values in their reversed order.
</p><div class="example">
<pre class="example-preformatted">(list-transduce (tmap (lambda (x) (+ x 1))) reverse-rcons (list 0 1 2 3))
&rArr; (4 3 2 1)
</pre></div>
</dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-rany"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">rany</strong> <var class="def-var-arguments">pred?</var><a class="copiable-link" href="#index-rany"> &para;</a></span></dt>
<dd><p>The reducer version of any. Returns <code class="code">(reduced (pred? value))</code> if
any <code class="code">(pred? value)</code> returns non-#f. The identity is #f.
</p>
<div class="example">
<pre class="example-preformatted">(list-transduce (tmap (lambda (x) (+ x 1))) (rany odd?) (list 1 3 5))
&rArr; #f
(list-transduce (tmap (lambda (x) (+ x 1))) (rany odd?) (list 1 3 4 5))
&rArr; #t
</pre></div>
</dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-revery"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">revery</strong> <var class="def-var-arguments">pred?</var><a class="copiable-link" href="#index-revery"> &para;</a></span></dt>
<dd><p>The reducer version of every. Stops the transduction and returns
<code class="code">(reduced #f)</code> if any <code class="code">(pred? value)</code> returns #f. If every
<code class="code">(pred? value)</code> returns true, it returns the result of the last
invocation of <code class="code">(pred? value)</code>. The identity is #t.
</p>
<div class="example">
<pre class="example-preformatted">(list-transduce
(tmap (lambda (x) (+ x 1)))
(revery (lambda (v) (if (odd? v) v #f)))
(list 2 4 6))
&rArr; 7
(list-transduce (tmap (lambda (x) (+ x 1)) (revery odd?) (list 2 4 5 6))
&rArr; #f
</pre></div>
</dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-rcount"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">rcount</strong><a class="copiable-link" href="#index-rcount"> &para;</a></span></dt>
<dd><p>A simple counting reducer. Counts the values that pass through the
transduction.
</p><div class="example">
<pre class="example-preformatted">(list-transduce (tfilter odd?) rcount (list 1 2 3 4)) &rArr; 2.
</pre></div>
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Next: <a href="SRFI_002d171-Transducers.html">Transducers</a>, Previous: <a href="SRFI_002d171-Applying-Transducers.html">Applying Transducers</a>, Up: <a href="SRFI_002d171.html">Transducers</a> &nbsp; [<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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