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Previous: <a rel="previous" accesskey="p" href="Actions-and-Locations.html#Actions-and-Locations">Actions and Locations</a>,
Up: <a rel="up" accesskey="u" href="Locations.html#Locations">Locations</a>
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<h4 class="subsection">3.6.3 Default Action for Locations</h4>
<p><a name="index-YYLLOC_005fDEFAULT-162"></a><a name="index-GLR-parsers-and-_0040code_007bYYLLOC_005fDEFAULT_007d-163"></a>
Actually, actions are not the best place to compute locations. Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each
rule. The <code>YYLLOC_DEFAULT</code> macro is invoked each time a rule is
matched, before the associated action is run. It is also invoked
while processing a syntax error, to compute the error's location.
Before reporting an unresolvable syntactic ambiguity, a GLR
parser invokes <code>YYLLOC_DEFAULT</code> recursively to compute the location
of that ambiguity.
<p>Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.
<p>The <code>YYLLOC_DEFAULT</code> macro takes three parameters. The first one is
the location of the grouping (the result of the computation). When a
rule is matched, the second parameter identifies locations of
all right hand side elements of the rule being matched, and the third
parameter is the size of the rule's right hand side.
When a GLR parser reports an ambiguity, which of multiple candidate
right hand sides it passes to <code>YYLLOC_DEFAULT</code> is undefined.
When processing a syntax error, the second parameter identifies locations
of the symbols that were discarded during error processing, and the third
parameter is the number of discarded symbols.
<p>By default, <code>YYLLOC_DEFAULT</code> is defined this way:
<pre class="smallexample"> # define YYLLOC_DEFAULT(Current, Rhs, N) \
do \
if (N) \
{ \
(Current).first_line = YYRHSLOC(Rhs, 1).first_line; \
(Current).first_column = YYRHSLOC(Rhs, 1).first_column; \
(Current).last_line = YYRHSLOC(Rhs, N).last_line; \
(Current).last_column = YYRHSLOC(Rhs, N).last_column; \
} \
else \
{ \
(Current).first_line = (Current).last_line = \
YYRHSLOC(Rhs, 0).last_line; \
(Current).first_column = (Current).last_column = \
YYRHSLOC(Rhs, 0).last_column; \
} \
while (0)
</pre>
<p>where <code>YYRHSLOC (rhs, k)</code> is the location of the <var>k</var>th symbol
in <var>rhs</var> when <var>k</var> is positive, and the location of the symbol
just before the reduction when <var>k</var> and <var>n</var> are both zero.
<p>When defining <code>YYLLOC_DEFAULT</code>, you should consider that:
<ul>
<li>All arguments are free of side-effects. However, only the first one (the
result) should be modified by <code>YYLLOC_DEFAULT</code>.
<li>For consistency with semantic actions, valid indexes within the
right hand side range from 1 to <var>n</var>. When <var>n</var> is zero, only 0 is a
valid index, and it refers to the symbol just before the reduction.
During error processing <var>n</var> is always positive.
<li>Your macro should parenthesize its arguments, if need be, since the
actual arguments may not be surrounded by parentheses. Also, your
macro should expand to something that can be used as a single
statement when it is followed by a semicolon.
</ul>
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