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<title>Bison 3.0.4: Non Operators</title>

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<a name="Non-Operators"></a>
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<p>
Previous: <a href="How-Precedence.html#How-Precedence" accesskey="p" rel="prev">How Precedence</a>, Up: <a href="Precedence.html#Precedence" accesskey="u" rel="up">Precedence</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Terms.html#Index-of-Terms" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Using-Precedence-For-Non-Operators"></a>
<h4 class="subsection">5.3.6 Using Precedence For Non Operators</h4>

<p>Using properly precedence and associativity directives can help fixing
shift/reduce conflicts that do not involve arithmetics-like operators.  For
instance, the &ldquo;dangling <code>else</code>&rdquo; problem (see <a href="Shift_002fReduce.html#Shift_002fReduce">Shift/Reduce Conflicts</a>) can be solved elegantly in two different ways.
</p>
<p>In the present case, the conflict is between the token <code>&quot;else&quot;</code> willing
to be shifted, and the rule &lsquo;<samp>if_stmt: &quot;if&quot; expr &quot;then&quot; stmt</samp>&rsquo;, asking
for reduction.  By default, the precedence of a rule is that of its last
token, here <code>&quot;then&quot;</code>, so the conflict will be solved appropriately
by giving <code>&quot;else&quot;</code> a precedence higher than that of <code>&quot;then&quot;</code>, for
instance as follows:
</p>
<div class="example">
<pre class="example">%precedence &quot;then&quot;
%precedence &quot;else&quot;
</pre></div>

<p>Alternatively, you may give both tokens the same precedence, in which case
associativity is used to solve the conflict.  To preserve the shift action,
use right associativity:
</p>
<div class="example">
<pre class="example">%right &quot;then&quot; &quot;else&quot;
</pre></div>

<p>Neither solution is perfect however.  Since Bison does not provide, so far,
&ldquo;scoped&rdquo; precedence, both force you to declare the precedence
of these keywords with respect to the other operators your grammar.
Therefore, instead of being warned about new conflicts you would be unaware
of (e.g., a shift/reduce conflict due to &lsquo;<samp>if test then 1 else 2 + 3</samp>&rsquo;
being ambiguous: &lsquo;<samp>if test then 1 else (2 + 3)</samp>&rsquo; or &lsquo;<samp>(if test then 1
else 2) + 3</samp>&rsquo;?), the conflict will be already &ldquo;fixed&rdquo;.
</p>



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