doc/html/yap.html

   1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
   2 <html>
   3 <head>
   4 <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
   5 <title>Chapter&#160;47.&#160;Boost.YAP</title>
   6 <link rel="stylesheet" href="../../doc/src/boostbook.css" type="text/css">
   7 <meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
   8 <link rel="home" href="index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
   9 <link rel="up" href="libraries.html" title="Part&#160;I.&#160;The Boost C++ Libraries (BoostBook Subset)">
  10 <link rel="prev" href="xpressive/appendices.html" title="Appendices">
  11 <link rel="next" href="boost_yap/manual.html" title="Manual">
  12 </head>
  13 <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
  14 <table cellpadding="2" width="100%"><tr>
  15 <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../boost.png"></td>
  16 <td align="center"><a href="../../index.html">Home</a></td>
  17 <td align="center"><a href="../../libs/libraries.htm">Libraries</a></td>
  18 <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
  19 <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
  20 <td align="center"><a href="../../more/index.htm">More</a></td>
  21 </tr></table>
  22 <hr>
  23 <div class="spirit-nav">
  24 <a accesskey="p" href="xpressive/appendices.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="boost_yap/manual.html"><img src="../../doc/src/images/next.png" alt="Next"></a>
  25 </div>
  26 <div class="chapter">
  27 <div class="titlepage"><div>
  28 <div><h2 class="title">
  29 <a name="yap"></a>Chapter&#160;47.&#160;Boost.YAP</h2></div>
  30 <div><div class="author"><h3 class="author">
  31 <span class="firstname">Zach</span> <span class="surname">Laine</span>
  32 </h3></div></div>
  33 <div><p class="copyright">Copyright &#169; 2018 T. Zachary Laine</p></div>
  34 <div><div class="legalnotice">
  35 <a name="yap.legal"></a><p>
  36         Distributed under the Boost Software License, Version 1.0. (See accompanying
  37         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>)
  38       </p>
  39 </div></div>
  40 </div></div>
  41 <div class="toc">
  42 <p><b>Table of Contents</b></p>
  43 <dl class="toc">
  44 <dt><span class="section"><a href="yap.html#boost_yap.introduction">Introduction</a></span></dt>
  45 <dt><span class="section"><a href="boost_yap/manual.html">Manual</a></span></dt>
  46 <dd><dl>
  47 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.an_expression_template_primer">An Expression
  48       Template Primer</a></span></dt>
  49 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.the_yap_way">The YAP Way</a></span></dt>
  50 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.expressions">Expressions</a></span></dt>
  51 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.mix_and_match_expression_templates">Mix-and-Match
  52       Expression Templates</a></span></dt>
  53 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.kinds_of_expressions">Kinds of Expressions</a></span></dt>
  54 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.operators">Operators</a></span></dt>
  55 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.transforming_expressions">Transforming
  56       Expressions</a></span></dt>
  57 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.evaluating_expressions">Evaluating Expressions</a></span></dt>
  58 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.operator_macros">Operator Macros</a></span></dt>
  59 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.how_expression_operands_are_treated">How
  60       Expression Operands Are Treated</a></span></dt>
  61 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.printing">Printing</a></span></dt>
  62 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.examples">Examples</a></span></dt>
  63 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.header_organization">Header Organization</a></span></dt>
  64 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.configuration">Configuration</a></span></dt>
  65 <dt><span class="section"><a href="boost_yap/manual.html#boost_yap.manual.object_code">Object Code</a></span></dt>
  66 </dl></dd>
  67 <dt><span class="section"><a href="boost_yap/concepts.html">Concepts</a></span></dt>
  68 <dt><span class="section"><a href="boost_yap/compiler_support.html">Compiler Support</a></span></dt>
  69 <dt><span class="section"><a href="boost_yap/dependencies.html">Dependencies</a></span></dt>
  70 <dt><span class="section"><a href="yap/reference.html">Reference</a></span></dt>
  71 <dd><dl><dt><span class="section"><a href="yap/reference.html#headers">Headers</a></span></dt></dl></dd>
  72 <dt><span class="section"><a href="boost_yap/rationale.html">Rationale</a></span></dt>
  73 </dl>
  74 </div>
  75 <div class="section">
  76 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  77 <a name="boost_yap.introduction"></a><a class="link" href="yap.html#boost_yap.introduction" title="Introduction">Introduction</a>
  78 </h2></div></div></div>
  79 <p>
  80       "I like to start documentation with a quote. A nice, pithy one."
  81     </p>
  82 <p>
  83       <span class="emphasis"><em><span class="bold"><strong>&#8212; Eric Niebler (paraphrased)</strong></span></em></span>
  84     </p>
  85 <h4>
  86 <a name="boost_yap.introduction.h0"></a>
  87       <span class="phrase"><a name="boost_yap.introduction.motivation"></a></span><a class="link" href="yap.html#boost_yap.introduction.motivation">Motivation</a>
  88     </h4>
  89 <p>
  90       Expression templates are rad. They are used in lots of libraries; here are
  91       just three of the most impressive:
  92     </p>
  93 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  94 <li class="listitem">
  95           <a href="http://boost-spirit.com" target="_top">Boost.Spirit</a> allows you to
  96           write an EBNF-style grammar that gets transformed into a PEG parser.
  97         </li>
  98 <li class="listitem">
  99           <a href="http://eigen.tuxfamily.org" target="_top">Eigen</a> allows you to do
 100           linear algebra using a very natural and mathematical expression syntax
 101           that <a href="http://eigen.tuxfamily.org" target="_top">Eigen</a> uses to heavily
 102           optimize your expressions.
 103         </li>
 104 <li class="listitem">
 105           <a href="https://www.lri.fr/~falcou/nt2" target="_top">NT2</a> takes slightly
 106           modified MatLab code and allows it to be parsed and run as highly optimized
 107           C++ code.
 108         </li>
 109 </ul></div>
 110 <p>
 111       However, this can come at a high cost. Expression templates are costly to implement
 112       and maintain. Each of <a href="http://eigen.tuxfamily.org" target="_top">Eigen</a>
 113       and Boost.Ublas has a large volume of complex expression template code that
 114       cannot be reused elsewhere.
 115     </p>
 116 <p>
 117       With the language facilities available in the C++14 and C++17 standards, an
 118       expression template library is now straightforward to write and use, and has
 119       very reasonable compile times.
 120     </p>
 121 <p>
 122       As a quick example, let's say we are doing a bit of matrix math, and we write
 123       this statement:
 124     </p>
 125 <pre class="programlisting"><span class="identifier">D</span> <span class="special">=</span> <span class="identifier">A</span> <span class="special">*</span> <span class="identifier">B</span> <span class="special">+</span> <span class="identifier">C</span><span class="special">;</span>
 126 </pre>
 127 <p>
 128       in which all the variables are matrices. It turns out that making a temporary
 129       for <code class="computeroutput"><span class="identifier">A</span> <span class="special">*</span>
 130       <span class="identifier">B</span></code> and then another temporary for
 131       the resulting product plus <code class="computeroutput"><span class="identifier">C</span></code>
 132       is very inefficient. Most matrix math libraries will have a single function
 133       that does it in one go:
 134     </p>
 135 <pre class="programlisting"><span class="identifier">mul_add_assign</span><span class="special">(</span><span class="identifier">D</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">);</span>
 136 </pre>
 137 <p>
 138       If you use a matrix library that offers both kinds of syntax, you have to notice
 139       when some bit of operator-using code should be replaced with some more efficient
 140       function; this is tedious and error-prone. If the library does not provide
 141       the operator syntax at all, only providing the more-efficient function calls,
 142       code using the library is a lot less writable and readable.
 143     </p>
 144 <p>
 145       Using Boost.YAP, you can write some library code that enables expressions like
 146       <code class="computeroutput"><span class="identifier">D</span> <span class="special">=</span>
 147       <span class="identifier">A</span> <span class="special">*</span> <span class="identifier">B</span> <span class="special">+</span> <span class="identifier">C</span></code>
 148       to be automatically transformed into expressions like <code class="computeroutput"><span class="identifier">mul_add_assign</span><span class="special">(</span><span class="identifier">D</span><span class="special">,</span>
 149       <span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">)</span></code>.
 150     </p>
 151 <p>
 152       Consider another example. Many of us have used Unix command line tools to remove
 153       duplicate lines in a file:
 154     </p>
 155 <pre class="programlisting"><span class="identifier">sort</span> <span class="identifier">file_with_duplicates</span> <span class="special">|</span> <span class="identifier">uniq</span> <span class="special">&gt;</span> <span class="identifier">file_without_duplicates</span>
 156 </pre>
 157 <p>
 158       We can do something very similar with the standard algorithms, of course:
 159     </p>
 160 <p>
 161 </p>
 162 <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">v1</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">7</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">8</span><span class="special">};</span>
 163 <span class="identifier">std</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">v1</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v1</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
 164 <span class="keyword">auto</span> <span class="identifier">it</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span><span class="identifier">v1</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">v1</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
 165 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">v2</span><span class="special">(</span><span class="identifier">v1</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">it</span><span class="special">);</span>
 166 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">v2</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;({</span><span class="number">0</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">7</span><span class="special">,</span> <span class="number">8</span><span class="special">}));</span>
 167 </pre>
 168 <p>
 169     </p>
 170 <p>
 171       However, it would be much better if our code did exactly that, but with a more
 172       concise syntax:
 173     </p>
 174 <p>
 175 </p>
 176 <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">v1</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">7</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">8</span><span class="special">};</span>
 177 <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">v2</span> <span class="special">=</span> <span class="identifier">sort</span><span class="special">(</span><span class="identifier">v1</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">unique</span><span class="special">;</span>
 178 <span class="identifier">assert</span><span class="special">(</span><span class="identifier">v2</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;({</span><span class="number">0</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="number">7</span><span class="special">,</span> <span class="number">8</span><span class="special">}));</span>
 179 </pre>
 180 <p>
 181     </p>
 182 <p>
 183       This looks much more similar to the Unix command line above. (Let's pretend
 184       that <a href="https://github.com/ericniebler/range-v3" target="_top">Range-v3</a>
 185       doesn't already do almost exactly this.)
 186     </p>
 187 <p>
 188       Boost.YAP can be used to do both of these things, in a pretty small amount
 189       of code. In fact, you can jump right into the <a class="link" href="boost_yap/manual.html#boost_yap.manual.examples.pipable_algorithms" title="Pipable Algorithms">Pipable
 190       Algorithms</a> example if you want to see how the second one can be implemented.
 191     </p>
 192 <h4>
 193 <a name="boost_yap.introduction.h1"></a>
 194       <span class="phrase"><a name="boost_yap.introduction.features"></a></span><a class="link" href="yap.html#boost_yap.introduction.features">Features</a>
 195     </h4>
 196 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 197 <li class="listitem">
 198           Simple <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplate</a>
 199           and <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expression">Expression</a> concepts
 200           easily modeled by user code. Member and non-member functions on <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplates</a>
 201           and <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expression">Expressions</a> can
 202           be added with compact macros, and a reference template that models <a class="link" href="boost_yap/concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplate</a>
 203           exists for prototyping or experimentation.
 204         </li>
 205 <li class="listitem">
 206           Evaluation of Boost.YAP expressions matches the semantics of builtin C++
 207           expressions as closely as possible. This leads to clearer understanding
 208           of the semantics of expression evaluation, because the definitions are
 209           local to the types involved.
 210         </li>
 211 <li class="listitem">
 212           Expressions may be transformed explicitly in a user-defined way. This is
 213           accomplished with overloaded call operators in a transform class, which
 214           are matched against subexpressions in the overall expression. While these
 215           member functions may transform a subexpression into anything, a common
 216           pattern is to transform only some subexpressions into either new subexpressions
 217           or appropriate values and to leave other subexpressions unchanged. This
 218           <code class="computeroutput"><span class="identifier">evaluate</span><span class="special">(</span><span class="identifier">transform</span><span class="special">(</span><span class="identifier">expr</span><span class="special">))</span></code>
 219           idiom is expected to be one of the most common ways of using Yap to manipulate
 220           and evaluate expressions.
 221         </li>
 222 <li class="listitem">
 223           Functions that operate on or create expressions. Functions are provided
 224           (and used within Boost.YAP) that manipulate expressions or their subexpressions.
 225           These simplify the process of writing user-defined transforms, for example.
 226         </li>
 227 </ul></div>
 228 </div>
 229 </div>
 230 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 231 <td align="left"><p><small>Last revised: December 10, 2019 at 00:26:26 GMT</small></p></td>
 232 <td align="right"><div class="copyright-footer"></div></td>
 233 </tr></table>
 234 <hr>
 235 <div class="spirit-nav">
 236 <a accesskey="p" href="xpressive/appendices.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="boost_yap/manual.html"><img src="../../doc/src/images/next.png" alt="Next"></a>
 237 </div>
 238 </body>
 239 </html>