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boost_fusion/doc/html/fusion/algorithm.html
Joel de Guzman ef1e821173 added info on how to map to and from MPL 
[SVN r47647]
2008-07-21 07:25:52 +00:00

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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="fusion.algorithm"></a><a href="algorithm.html" title="Algorithm">Algorithm</a></h2></div></div></div>
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<dt><span class="section"><a href="algorithm/iteration.html">Iteration</a></span></dt>
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<dt><span class="section"><a href="algorithm/iteration/functions.html">Functions</a></span></dt>
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<a name="fusion.algorithm.lazy_evaluation"></a><h3>
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<a href="algorithm.html#fusion.algorithm.lazy_evaluation">Lazy Evaluation</a>
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<p>
Unlike <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, Fusion
algorithms are lazy and non sequence-type preserving. What does that mean?
It means that when you operate on a sequence through a Fusion algorithm that
returns a sequence, the sequence returned may not be of the same class as the
original. This is by design. Runtime efficiency is given a high priority. Like
<a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, and unlike
<a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>,
fusion algorithms are functional in nature such that algorithms are non mutating
(no side effects). However, due to the high cost of returning full sequences
such as vectors and lists, <span class="emphasis"><em>Views</em></span> are returned from Fusion
algorithms instead. For example, the <a href="algorithm/transformation/functions/transform.html" title="transform"><tt class="computeroutput"><span class="identifier">transform</span></tt></a> algorithm does not actually
return a transformed version of the original sequence. <a href="algorithm/transformation/functions/transform.html" title="transform"><tt class="computeroutput"><span class="identifier">transform</span></tt></a> returns a <a href="view/transform_view.html" title="transform_view"><tt class="computeroutput"><span class="identifier">transform_view</span></tt></a>. This view holds a
reference to the original sequence plus the transform function. Iteration over
the <a href="view/transform_view.html" title="transform_view"><tt class="computeroutput"><span class="identifier">transform_view</span></tt></a>
will apply the transform function over the sequence elements on demand. This
<span class="emphasis"><em>lazy</em></span> evaluation scheme allows us to chain as many algorithms
as we want without incurring a high runtime penalty.
</p>
<a name="fusion.algorithm.sequence_extension"></a><h3>
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<a href="algorithm.html#fusion.algorithm.sequence_extension">Sequence Extension</a>
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The <span class="emphasis"><em>lazy</em></span> evaluation scheme where <a href="algorithm.html" title="Algorithm">Algorithms</a>
return <a href="view.html" title="View">Views</a> also allows operations such
as <a href="algorithm/transformation/functions/push_back.html" title="push_back"><tt class="computeroutput"><span class="identifier">push_back</span></tt></a> to be totally generic. In
Fusion, <a href="algorithm/transformation/functions/push_back.html" title="push_back"><tt class="computeroutput"><span class="identifier">push_back</span></tt></a> is actually a generic algorithm
that works on all sequences. Given an input sequence <tt class="computeroutput"><span class="identifier">s</span></tt>
and a value <tt class="computeroutput"><span class="identifier">x</span></tt>, Fusion's <a href="algorithm/transformation/functions/push_back.html" title="push_back"><tt class="computeroutput"><span class="identifier">push_back</span></tt></a> algorithm simply returns
a <a href="view/joint_view.html" title="joint_view"><tt class="computeroutput"><span class="identifier">joint_view</span></tt></a>:
a view that holds a reference to the original sequence <tt class="computeroutput"><span class="identifier">s</span></tt>
and the value <tt class="computeroutput"><span class="identifier">x</span></tt>. Functions
that were once sequence specific and need to be implemented N times over N
different sequences are now implemented only once. That is to say that Fusion
sequences are cheaply extensible. However, an important caveat is that the
result of a sequence extending operation like <a href="algorithm/transformation/functions/push_back.html" title="push_back"><tt class="computeroutput"><span class="identifier">push_back</span></tt></a> does not retain the properties
of the original sequence such as associativity of <a href="container/set.html" title="set"><tt class="computeroutput"><span class="identifier">set</span></tt></a>(s). To regain the original sequence,
<a href="container/conversion/functions.html" title="Functions">Conversion</a> functions
are provided. You may use one of the <a href="container/conversion/functions.html" title="Functions">Conversion</a>
functions to convert back to the original sequence type.
</p>
<a name="fusion.algorithm.header"></a><h3>
<a name="id579083"></a>
<a href="algorithm.html#fusion.algorithm.header">Header</a>
</h3>
<pre class="programlisting">
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2007 Joel de Guzman, Dan Marsden, Tobias
Schwinger<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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