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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="phoenix.modules.scope.let"></a><a class="link" href="let.html" title="let">let</a>
  28 </h4></div></div></div>
  29 <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">phoenix</span><span class="special">/</span><span class="identifier">scope</span><span class="special">/</span><span class="identifier">let</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  30 </pre>
  31 <p>
  32           You declare local variables using the syntax:
  33         </p>
  34 <pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">local</span><span class="special">-</span><span class="identifier">declarations</span><span class="special">)</span>
  35 <span class="special">[</span>
  36     <span class="identifier">let</span><span class="special">-</span><span class="identifier">body</span>
  37 <span class="special">]</span>
  38 </pre>
  39 <p>
  40           <code class="computeroutput"><span class="identifier">let</span></code> allows 1..N local variable
  41           declarations (where N == <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>).
  42           Each declaration follows the form:
  43         </p>
  44 <pre class="programlisting"><span class="identifier">local</span><span class="special">-</span><span class="identifier">id</span> <span class="special">=</span> <span class="identifier">lambda</span><span class="special">-</span><span class="identifier">expression</span>
  45 </pre>
  46 <div class="note"><table border="0" summary="Note">
  47 <tr>
  48 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td>
  49 <th align="left">Note</th>
  50 </tr>
  51 <tr><td align="left" valign="top"><p>
  52             You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>,
  53             the predefined maximum local variable declarations in a let expression.
  54             By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>
  55             is set to <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>.
  56           </p></td></tr>
  57 </table></div>
  58 <p>
  59           Example:
  60         </p>
  61 <pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="number">123</span><span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="number">456</span><span class="special">)</span>
  62 <span class="special">[</span>
  63     <span class="identifier">_a</span> <span class="special">+</span> <span class="identifier">_b</span>
  64 <span class="special">]</span>
  65 </pre>
  66 <p>
  67           <span class="bold"><strong>Reference Preservation</strong></span>
  68         </p>
  69 <p>
  70           The type of the local variable assumes the type of the lambda- expression.
  71           Type deduction is reference preserving. For example:
  72         </p>
  73 <pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="number">456</span><span class="special">)</span>
  74 </pre>
  75 <p>
  76           <code class="computeroutput"><span class="identifier">_a</span></code> assumes the type of
  77           <code class="computeroutput"><span class="identifier">arg1</span></code>: a reference to an
  78           argument, while <code class="computeroutput"><span class="identifier">_b</span></code> has
  79           type <code class="computeroutput"><span class="keyword">int</span></code>.
  80         </p>
  81 <p>
  82           Consider this:
  83         </p>
  84 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
  85
  86 <span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">arg1</span><span class="special">)</span>
  87 <span class="special">[</span>
  88     <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">--</span><span class="identifier">_a</span> <span class="special">&lt;&lt;</span> <span class="char">' '</span>
  89 <span class="special">]</span>
  90 <span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
  91
  92 <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
  93 </pre>
  94 <p>
  95           the output of above is : 0 0
  96         </p>
  97 <p>
  98           While with this:
  99         </p>
 100 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
 101
 102 <span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">val</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">))</span>
 103 <span class="special">[</span>
 104     <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">--</span><span class="identifier">_a</span> <span class="special">&lt;&lt;</span> <span class="char">' '</span>
 105 <span class="special">]</span>
 106 <span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
 107
 108 <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">i</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
 109 </pre>
 110 <p>
 111           the output is : 0 1
 112         </p>
 113 <p>
 114           Reference preservation is necessary because we need to have L-value access
 115           to outer lambda-scopes (especially the arguments). <code class="computeroutput"><span class="identifier">arg</span></code>s
 116           and <code class="computeroutput"><span class="identifier">ref</span></code>s are L-values.
 117           <code class="computeroutput"><span class="identifier">val</span></code>s are R-values.
 118         </p>
 119 <p>
 120           <span class="bold"><strong>Visibility</strong></span> <a name="phoenix.modules.scope.let.visibility"></a>
 121         </p>
 122 <p>
 123           The scope and lifetimes of the local variables is limited within the let-body.
 124           <code class="computeroutput"><span class="identifier">let</span></code> blocks can be nested.
 125           A local variable may hide an outer local variable. For example:
 126         </p>
 127 <pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_x</span> <span class="special">=</span> <span class="identifier">_1</span><span class="special">,</span> <span class="identifier">_y</span> <span class="special">=</span> <span class="identifier">_2</span><span class="special">)</span>
 128 <span class="special">[</span>
 129     <span class="comment">// _x here is an int: 1</span>
 130
 131    <span class="identifier">let</span><span class="special">(</span><span class="identifier">_x</span> <span class="special">=</span> <span class="identifier">_3</span><span class="special">)</span> <span class="comment">// hides the outer _x</span>
 132    <span class="special">[</span>
 133        <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">_x</span> <span class="special">&lt;&lt;</span> <span class="identifier">_y</span> <span class="comment">// prints "Hello, World"</span>
 134    <span class="special">]</span>
 135 <span class="special">](</span><span class="number">1</span><span class="special">,</span><span class="string">" World"</span><span class="special">,</span><span class="string">"Hello,"</span><span class="special">);</span>
 136 </pre>
 137 <p>
 138           The actual values of the parameters _1, _2 and _3 are supplied from the
 139           bracketed list at the end of the <code class="computeroutput"><span class="identifier">let</span></code>.
 140         </p>
 141 <p>
 142           There is currently a limitation that the inner <code class="computeroutput"><span class="identifier">let</span></code>
 143           cannot be supplied with a constant e.g. <code class="computeroutput"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_x</span> <span class="special">=</span> <span class="number">1</span><span class="special">)</span></code>.
 144         </p>
 145 <p>
 146           The RHS (right hand side lambda-expression) of each local-declaration cannot
 147           refer to any LHS local-id. At this point, the local-ids are not in scope
 148           yet; they will only be in scope in the let-body. The code below is in error:
 149         </p>
 150 <pre class="programlisting"><span class="identifier">let</span><span class="special">(</span>
 151     <span class="identifier">_a</span> <span class="special">=</span> <span class="number">1</span>
 152   <span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="identifier">_a</span> <span class="comment">// Error: _a is not in scope yet</span>
 153 <span class="special">)</span>
 154 <span class="special">[</span>
 155     <span class="comment">// _a and _b's scope starts here</span>
 156     <span class="comment">/*. body .*/</span>
 157 <span class="special">]</span>
 158 </pre>
 159 <p>
 160           However, if an outer let scope is available, this will be searched. Since
 161           the scope of the RHS of a local-declaration is the outer scope enclosing
 162           the let, the RHS of a local-declaration can refer to a local variable of
 163           an outer scope:
 164         </p>
 165 <pre class="programlisting"><span class="identifier">let</span><span class="special">(</span><span class="identifier">_a</span> <span class="special">=</span> <span class="number">1</span><span class="special">)</span>
 166 <span class="special">[</span>
 167     <span class="identifier">let</span><span class="special">(</span>
 168         <span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">_1</span>
 169       <span class="special">,</span> <span class="identifier">_b</span> <span class="special">=</span> <span class="identifier">_a</span> <span class="comment">// Ok. _a refers to the outer _a</span>
 170     <span class="special">)</span>
 171     <span class="special">[</span>
 172         <span class="comment">/*. body .*/</span>
 173     <span class="special">]</span>
 174 <span class="special">](</span><span class="number">1</span><span class="special">)</span>
 175 </pre>
 176 </div>
 177 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 178 <td align="left"></td>
 179 <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2010, 2014 Joel de Guzman, Dan Marsden, Thomas Heller, John
 180       Fletcher<p>
 181         Distributed under the Boost Software License, Version 1.0. (See accompanying
 182         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>)
 183       </p>
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