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<a name="Precedence-Only"></a>
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<p>
Next: <a href="Precedence-Examples.html#Precedence-Examples" accesskey="n" rel="next">Precedence Examples</a>, Previous: <a href="Using-Precedence.html#Using-Precedence" accesskey="p" rel="prev">Using 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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<a name="Specifying-Precedence-Only"></a>
<h4 class="subsection">5.3.3 Specifying Precedence Only</h4>
<a name="index-_0025precedence-1"></a>

<p>Since POSIX Yacc defines only <code>%left</code>, <code>%right</code>, and
<code>%nonassoc</code>, which all defines precedence and associativity, little
attention is paid to the fact that precedence cannot be defined without
defining associativity.  Yet, sometimes, when trying to solve a
conflict, precedence suffices.  In such a case, using <code>%left</code>,
<code>%right</code>, or <code>%nonassoc</code> might hide future (associativity
related) conflicts that would remain hidden.
</p>
<p>The dangling <code>else</code> ambiguity (see <a href="Shift_002fReduce.html#Shift_002fReduce">Shift/Reduce
Conflicts</a>) can be solved explicitly.  This shift/reduce conflicts occurs
in the following situation, where the period denotes the current parsing
state:
</p>
<div class="example">
<pre class="example">if <var>e1</var> then if  <var>e2</var> then <var>s1</var> . else <var>s2</var>
</pre></div>

<p>The conflict involves the reduction of the rule &lsquo;<samp>IF expr THEN
stmt</samp>&rsquo;, which precedence is by default that of its last token
(<code>THEN</code>), and the shifting of the token <code>ELSE</code>.  The usual
disambiguation (attach the <code>else</code> to the closest <code>if</code>),
shifting must be preferred, i.e., the precedence of <code>ELSE</code> must be
higher than that of <code>THEN</code>.  But neither is expected to be involved
in an associativity related conflict, which can be specified as follows.
</p>
<div class="example">
<pre class="example">%precedence THEN
%precedence ELSE
</pre></div>

<p>The unary-minus is another typical example where associativity is
usually over-specified, see <a href="Infix-Calc.html#Infix-Calc">Infix Notation
Calculator - <code>calc</code></a>.  The <code>%left</code> directive is traditionally
used to declare the precedence of <code>NEG</code>, which is more than needed
since it also defines its associativity.  While this is harmless in the
traditional example, who knows how <code>NEG</code> might be used in future
evolutions of the grammar&hellip;
</p>



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bison-doc 1:3.0.4-1 / usr / share / doc / bison-doc / html / Precedence-Only.html
