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<h3 class="section">1.6 Locations</h3>

<p><a name="index-location-57"></a><a name="index-textual-location-58"></a><a name="index-location_002c-textual-59"></a>
Many applications, like interpreters or compilers, have to produce verbose
and useful error messages.  To achieve this, one must be able to keep track of
the <dfn>textual location</dfn>, or <dfn>location</dfn>, of each syntactic construct. 
Bison provides a mechanism for handling these locations.

   <p>Each token has a semantic value.  In a similar fashion, each token has an
associated location, but the type of locations is the same for all tokens and
groupings.  Moreover, the output parser is equipped with a default data
structure for storing locations (see <a href="Locations.html#Locations">Locations</a>, for more details).

   <p>Like semantic values, locations can be reached in actions using a dedicated
set of constructs.  In the example above, the location of the whole grouping
is <code>@$</code>, while the locations of the subexpressions are <code>@1</code> and
<code>@3</code>.

   <p>When a rule is matched, a default action is used to compute the semantic value
of its left hand side (see <a href="Actions.html#Actions">Actions</a>).  In the same way, another default
action is used for locations.  However, the action for locations is general
enough for most cases, meaning there is usually no need to describe for each
rule how <code>@$</code> should be formed.  When building a new location for a given
grouping, the default behavior of the output parser is to take the beginning
of the first symbol, and the end of the last symbol.

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bison-doc 1:2.5-1 / usr / share / doc / bison-doc / html / Locations-Overview.html
