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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_yap.concepts"></a><a class="link" href="concepts.html" title="Concepts">Concepts</a>
</h2></div></div></div>
<h4>
<a name="boost_yap.concepts.h0"></a>
<span class="phrase"><a name="boost_yap.concepts.expression"></a></span><a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>
</h4>
<p>
<a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a> is the central
concept in Boost.YAP. It must contain at least an <code class="computeroutput"><a class="link" href="../boost/yap/expr_kind.html" title="Type expr_kind">expr_kind</a></code> and a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span></code>
of values.
</p>
<p>
Here is a summary of the requirements on <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
In the tables below, <code class="computeroutput"><span class="identifier">E</span></code> is a
type that models <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>;
<code class="computeroutput"><span class="identifier">e</span></code> is an object of type <code class="computeroutput"><span class="identifier">E</span></code>; <code class="computeroutput"><span class="identifier">Tuple</span></code>
is an instantiation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span></code>;
and <code class="computeroutput"><span class="identifier">t</span></code> is an object of type
<code class="computeroutput"><span class="identifier">Tuple</span></code>.
</p>
<div class="table">
<a name="boost_yap.concepts.expression_requirements"></a><p class="title"><b>Table 48.6. Expression Requirements</b></p>
<div class="table-contents"><table class="table" summary="Expression Requirements">
<colgroup>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Expression
</p>
</th>
<th>
<p>
Type
</p>
</th>
<th>
<p>
Description
</p>
</th>
<th>
<p>
Notes
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">E</span><span class="special">::</span><span class="identifier">kind</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><a class="link" href="../boost/yap/expr_kind.html" title="Type expr_kind">expr_kind</a></code>
</p>
</td>
<td>
<p>
The kind of expression <code class="computeroutput"><span class="identifier">E</span></code>
is.
</p>
</td>
<td>
<p>
Must be a compile-time constant.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">e</span><span class="special">.</span><span class="identifier">elements</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">Tuple</span></code>
</p>
</td>
<td>
<p>
The child expressions of <code class="computeroutput"><span class="identifier">e</span></code>.
</p>
</td>
<td>
<p>
The types of the elements must be appropriate to the kind of the
expression.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">E</span> <span class="identifier">e</span><span class="special">{</span><span class="identifier">t</span><span class="special">}</span></code>
</p>
</td>
<td>
</td>
<td>
<p>
Construction/initialization of <code class="computeroutput"><span class="identifier">e</span></code>.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">t</span></code> must be stored
in <code class="computeroutput"><span class="identifier">e</span><span class="special">.</span><span class="identifier">elements</span></code>.
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
As stated above, the <code class="computeroutput"><span class="identifier">elements</span></code>
data member must match the kind of the expression:
</p>
<div class="table">
<a name="boost_yap.concepts.expression_requirements0"></a><p class="title"><b>Table 48.7. Expression Requirements</b></p>
<div class="table-contents"><table class="table" summary="Expression Requirements">
<colgroup>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
<code class="computeroutput"><span class="identifier">E</span><span class="special">::</span><span class="identifier">kind</span></code>
</p>
</th>
<th>
<p>
<code class="computeroutput"><span class="identifier">hana</span><span class="special">::</span><span class="identifier">size</span><span class="special">(</span><span class="identifier">e</span><span class="special">.</span><span class="identifier">elements</span><span class="special">)</span></code>
</p>
</th>
<th>
<p>
Possible Tuple Element Types
</p>
</th>
<th>
<p>
Notes
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">expr_ref</span></code>
</p>
</td>
<td>
<p>
1
</p>
</td>
<td>
<p>
Any non-<a class="link" href="../boost/yap/expr_kind.html#boost.yap.expr_kind.expr_ref"><code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">expr_ref</span></code></a> <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">terminal</span></code>
</p>
</td>
<td>
<p>
1
</p>
</td>
<td>
<p>
Any non-<a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
<p>
A terminal with a <code class="computeroutput"><a class="link" href="../boost/yap/placeholder.html" title="Struct template placeholder">placeholder&lt;&gt;</a></code>
value will be treated as a placeholder.
</p>
</td>
</tr>
<tr>
<td>
<p>
Any unary operator
</p>
</td>
<td>
<p>
1
</p>
</td>
<td>
<p>
Any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
Any binary operator
</p>
</td>
<td>
<p>
2
</p>
</td>
<td>
<p>
Any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">if_else</span></code>
</p>
</td>
<td>
<p>
3
</p>
</td>
<td>
<p>
Any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">call</span></code>
</p>
</td>
<td>
<p>
Any number &gt;= 1.
</p>
</td>
<td>
<p>
Any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h4>
<a name="boost_yap.concepts.h1"></a>
<span class="phrase"><a name="boost_yap.concepts.expressiontemplate"></a></span><a class="link" href="concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplate</a>
</h4>
<p>
<a class="link" href="concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplate</a>
is any template with two parameters that, when instantiated with an <code class="computeroutput"><a class="link" href="../boost/yap/expr_kind.html" title="Type expr_kind">expr_kind</a></code>
and a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span></code>, results in an <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
<h4>
<a name="boost_yap.concepts.h2"></a>
<span class="phrase"><a name="boost_yap.concepts.transform"></a></span><a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a>
</h4>
<p>
<code class="computeroutput"><a class="link" href="../boost/yap/transform.html" title="Function template transform">transform()</a></code> takes a <a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a>
as its second parameter. A <a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a>
is a <a href="http://en.cppreference.com/w/cpp/concept/Callable" target="_top">Callable</a>
that takes expressions and returns values of unconstrained type. There are
two sorts of overloads <a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a>
may use: <a class="link" href="concepts.html#boost_yap.concepts.expressiontransform">ExpressionTransform</a>
and <a class="link" href="concepts.html#boost_yap.concepts.tagtransform">TagTransform</a>.
</p>
<p>
A <a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a> may have any
number of overloads, including none.
</p>
<h4>
<a name="boost_yap.concepts.h3"></a>
<span class="phrase"><a name="boost_yap.concepts.expressiontransform"></a></span><a class="link" href="concepts.html#boost_yap.concepts.expressiontransform">ExpressionTransform</a>
</h4>
<p>
<a class="link" href="concepts.html#boost_yap.concepts.expressiontransform">ExpressionTransform</a>
takes an <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a> as
its only parameter. Here are some examples.
</p>
<p>
This one takes any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">xform</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">auto</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
This one takes any type of <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>
that satisfies the constraints imposed by its template parameters:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr1</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr2</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr3</span><span class="special">&gt;</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="identifier">xform</span> <span class="special">(</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">expression</span><span class="special">&lt;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">plus</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">expression</span><span class="special">&lt;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span>
<span class="identifier">Expr1</span><span class="special">,</span>
<span class="identifier">Expr2</span>
<span class="special">&gt;</span>
<span class="special">&gt;,</span>
<span class="identifier">Expr3</span>
<span class="special">&gt;</span>
<span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span>
<span class="special">)</span> <span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
</pre>
<p>
This one takes only a specific type:
</p>
<pre class="programlisting"><span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="identifier">xform</span> <span class="special">(</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="identifier">term</span><span class="special">&lt;</span><span class="identifier">number</span><span class="special">&gt;{{</span><span class="number">0.0</span><span class="special">}}</span> <span class="special">*</span> <span class="identifier">number</span><span class="special">{}</span> <span class="special">+</span> <span class="identifier">number</span><span class="special">{})</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span>
<span class="special">)</span> <span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
</pre>
<h4>
<a name="boost_yap.concepts.h4"></a>
<span class="phrase"><a name="boost_yap.concepts.tagtransform"></a></span><a class="link" href="concepts.html#boost_yap.concepts.tagtransform">TagTransform</a>
</h4>
<p>
<a class="link" href="concepts.html#boost_yap.concepts.tagtransform">TagTransform</a> takes a
tag-type as its first parameter, and the individual elements of an expression
as the remaining parameters.
</p>
<p>
Tags are named such that the tag for an expression with <code class="computeroutput"><a class="link" href="../boost/yap/expr_kind.html" title="Type expr_kind">expr_kind</a></code> <code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">foo</span></code> is
named <code class="computeroutput"><span class="identifier">foo_tag</span></code>. Here are some
examples.
</p>
<p>
This one takes any terminal that contains a <code class="computeroutput"><span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span></code>
(or reference to such a terminal):
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">xform</span>
<span class="special">{</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">terminal_tag</span><span class="special">,</span> <span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">n</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
This one takes any plus expression that contains a pair of <code class="computeroutput"><span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span></code>
terminals (or references to terminals):
</p>
<pre class="programlisting"><span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="identifier">xform</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">plus_tag</span><span class="special">,</span> <span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span> <span class="identifier">rhs</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
</pre>
<p>
This one takes any negation expression:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">xform</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">negate_tag</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
This one takes any call expression with two terminals (or references to terminals)
containing values convertible to <code class="computeroutput"><span class="keyword">double</span></code>:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">xform</span>
<span class="special">{</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">call_tag</span><span class="special">,</span> <span class="identifier">tag_type</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">b</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2018 T. Zachary Laine<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>)
</p>
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