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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="phoenix.modules.scope.let"></a><a class="link" href="let.html" title="let">let</a>
</h4></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</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">></span>
</pre>
<p>
You declare local variables using the syntax:
</p>
<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>
<span class="special">[</span>
<span class="identifier">let</span><span class="special">-</span><span class="identifier">body</span>
<span class="special">]</span>
</pre>
<p>
<code class="computeroutput"><span class="identifier">let</span></code> allows 1..N local variable
declarations (where N == <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>).
Each declaration follows the form:
</p>
<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>
</pre>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>,
the predefined maximum local variable declarations in a let expression.
By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LOCAL_LIMIT</span></code>
is set to <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>.
</p></td></tr>
</table></div>
<p>
Example:
</p>
<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>
<span class="special">[</span>
<span class="identifier">_a</span> <span class="special">+</span> <span class="identifier">_b</span>
<span class="special">]</span>
</pre>
<p>
<span class="bold"><strong>Reference Preservation</strong></span>
</p>
<p>
The type of the local variable assumes the type of the lambda- expression.
Type deduction is reference preserving. For example:
</p>
<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>
</pre>
<p>
<code class="computeroutput"><span class="identifier">_a</span></code> assumes the type of
<code class="computeroutput"><span class="identifier">arg1</span></code>: a reference to an
argument, while <code class="computeroutput"><span class="identifier">_b</span></code> has
type <code class="computeroutput"><span class="keyword">int</span></code>.
</p>
<p>
Consider this:
</p>
<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>
<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="special">[</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="special">--</span><span class="identifier">_a</span> <span class="special"><<</span> <span class="char">' '</span>
<span class="special">]</span>
<span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
the output of above is : 0 0
</p>
<p>
While with this:
</p>
<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>
<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>
<span class="special">[</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="special">--</span><span class="identifier">_a</span> <span class="special"><<</span> <span class="char">' '</span>
<span class="special">]</span>
<span class="special">(</span><span class="identifier">i</span><span class="special">);</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">i</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
the output is : 0 1
</p>
<p>
Reference preservation is necessary because we need to have L-value access
to outer lambda-scopes (especially the arguments). <code class="computeroutput"><span class="identifier">arg</span></code>s
and <code class="computeroutput"><span class="identifier">ref</span></code>s are L-values.
<code class="computeroutput"><span class="identifier">val</span></code>s are R-values.
</p>
<p>
<span class="bold"><strong>Visibility</strong></span> <a name="phoenix.modules.scope.let.visibility"></a>
</p>
<p>
The scope and lifetimes of the local variables is limited within the let-body.
<code class="computeroutput"><span class="identifier">let</span></code> blocks can be nested.
A local variable may hide an outer local variable. For example:
</p>
<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>
<span class="special">[</span>
<span class="comment">// _x here is an int: 1</span>
<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>
<span class="special">[</span>
<span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">_x</span> <span class="special"><<</span> <span class="identifier">_y</span> <span class="comment">// prints "Hello, World"</span>
<span class="special">]</span>
<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>
</pre>
<p>
The actual values of the parameters _1, _2 and _3 are supplied from the
bracketed list at the end of the <code class="computeroutput"><span class="identifier">let</span></code>.
</p>
<p>
There is currently a limitation that the inner <code class="computeroutput"><span class="identifier">let</span></code>
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>.
</p>
<p>
The RHS (right hand side lambda-expression) of each local-declaration cannot
refer to any LHS local-id. At this point, the local-ids are not in scope
yet; they will only be in scope in the let-body. The code below is in error:
</p>
<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> <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>
<span class="special">)</span>
<span class="special">[</span>
<span class="comment">// _a and _b's scope starts here</span>
<span class="comment">/*. body .*/</span>
<span class="special">]</span>
</pre>
<p>
However, if an outer let scope is available, this will be searched. Since
the scope of the RHS of a local-declaration is the outer scope enclosing
the let, the RHS of a local-declaration can refer to a local variable of
an outer scope:
</p>
<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>
<span class="special">[</span>
<span class="identifier">let</span><span class="special">(</span>
<span class="identifier">_a</span> <span class="special">=</span> <span class="identifier">_1</span>
<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>
<span class="special">)</span>
<span class="special">[</span>
<span class="comment">/*. body .*/</span>
<span class="special">]</span>
<span class="special">](</span><span class="number">1</span><span class="special">)</span>
</pre>
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