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<a name="Action-Features"></a>
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<p>
Next: <a href="Internationalization.html#Internationalization" accesskey="n" rel="next">Internationalization</a>, Previous: <a href="Error-Reporting.html#Error-Reporting" accesskey="p" rel="prev">Error Reporting</a>, Up: <a href="Interface.html#Interface" accesskey="u" rel="up">Interface</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>
</div>
<hr>
<a name="Special-Features-for-Use-in-Actions"></a>
<h3 class="section">4.8 Special Features for Use in Actions</h3>
<a name="index-summary_002c-action-features"></a>
<a name="index-action-features-summary"></a>

<p>Here is a table of Bison constructs, variables and macros that
are useful in actions.
</p>
<dl>
<dt><a name="index-_0024_0024"></a>Variable: <strong>$$</strong></dt>
<dd><p>Acts like a variable that contains the semantic value for the
grouping made by the current rule.  See <a href="Actions.html#Actions">Actions</a>.
</p></dd></dl>

<dl>
<dt><a name="index-_0024n"></a>Variable: <strong>$<var>n</var></strong></dt>
<dd><p>Acts like a variable that contains the semantic value for the
<var>n</var>th component of the current rule.  See <a href="Actions.html#Actions">Actions</a>.
</p></dd></dl>

<dl>
<dt><a name="index-_0024_003ctypealt_003e_0024"></a>Variable: <strong>$&lt;<var>typealt</var>&gt;$</strong></dt>
<dd><p>Like <code>$$</code> but specifies alternative <var>typealt</var> in the union
specified by the <code>%union</code> declaration.  See <a href="Action-Types.html#Action-Types">Data
Types of Values in Actions</a>.
</p></dd></dl>

<dl>
<dt><a name="index-_0024_003ctypealt_003en"></a>Variable: <strong>$&lt;<var>typealt</var>&gt;<var>n</var></strong></dt>
<dd><p>Like <code>$<var>n</var></code> but specifies alternative <var>typealt</var> in the
union specified by the <code>%union</code> declaration.
See <a href="Action-Types.html#Action-Types">Data Types of Values in Actions</a>.
</p></dd></dl>

<dl>
<dt><a name="index-YYABORT-2"></a>Macro: <strong>YYABORT</strong> <em><code>;</code></em></dt>
<dd><p>Return immediately from <code>yyparse</code>, indicating failure.
See <a href="Parser-Function.html#Parser-Function">The Parser Function <code>yyparse</code></a>.
</p></dd></dl>

<dl>
<dt><a name="index-YYACCEPT-2"></a>Macro: <strong>YYACCEPT</strong> <em><code>;</code></em></dt>
<dd><p>Return immediately from <code>yyparse</code>, indicating success.
See <a href="Parser-Function.html#Parser-Function">The Parser Function <code>yyparse</code></a>.
</p></dd></dl>

<dl>
<dt><a name="index-YYBACKUP"></a>Macro: <strong>YYBACKUP</strong> <em>(<var>token</var>, <var>value</var>)<code>;</code></em></dt>
<dd><a name="index-YYBACKUP-1"></a>
<p>Unshift a token.  This macro is allowed only for rules that reduce
a single value, and only when there is no lookahead token.
It is also disallowed in GLR parsers.
It installs a lookahead token with token type <var>token</var> and
semantic value <var>value</var>; then it discards the value that was
going to be reduced by this rule.
</p>
<p>If the macro is used when it is not valid, such as when there is
a lookahead token already, then it reports a syntax error with
a message &lsquo;<samp>cannot back up</samp>&rsquo; and performs ordinary error
recovery.
</p>
<p>In either case, the rest of the action is not executed.
</p></dd></dl>

<dl>
<dt><a name="index-YYEMPTY"></a>Macro: <strong>YYEMPTY</strong></dt>
<dd><p>Value stored in <code>yychar</code> when there is no lookahead token.
</p></dd></dl>

<dl>
<dt><a name="index-YYEOF"></a>Macro: <strong>YYEOF</strong></dt>
<dd><p>Value stored in <code>yychar</code> when the lookahead is the end of the input
stream.
</p></dd></dl>

<dl>
<dt><a name="index-YYERROR-1"></a>Macro: <strong>YYERROR</strong> <em><code>;</code></em></dt>
<dd><p>Cause an immediate syntax error.  This statement initiates error
recovery just as if the parser itself had detected an error; however, it
does not call <code>yyerror</code>, and does not print any message.  If you
want to print an error message, call <code>yyerror</code> explicitly before
the &lsquo;<samp>YYERROR;</samp>&rsquo; statement.  See <a href="Error-Recovery.html#Error-Recovery">Error Recovery</a>.
</p></dd></dl>

<dl>
<dt><a name="index-YYRECOVERING"></a>Macro: <strong>YYRECOVERING</strong></dt>
<dd><a name="index-YYRECOVERING-1"></a>
<p>The expression <code>YYRECOVERING ()</code> yields 1 when the parser
is recovering from a syntax error, and 0 otherwise.
See <a href="Error-Recovery.html#Error-Recovery">Error Recovery</a>.
</p></dd></dl>

<dl>
<dt><a name="index-yychar"></a>Variable: <strong>yychar</strong></dt>
<dd><p>Variable containing either the lookahead token, or <code>YYEOF</code> when the
lookahead is the end of the input stream, or <code>YYEMPTY</code> when no lookahead
has been performed so the next token is not yet known.
Do not modify <code>yychar</code> in a deferred semantic action (see <a href="GLR-Semantic-Actions.html#GLR-Semantic-Actions">GLR Semantic Actions</a>).
See <a href="Lookahead.html#Lookahead">Lookahead Tokens</a>.
</p></dd></dl>

<dl>
<dt><a name="index-yyclearin-1"></a>Macro: <strong>yyclearin</strong> <em><code>;</code></em></dt>
<dd><p>Discard the current lookahead token.  This is useful primarily in
error rules.
Do not invoke <code>yyclearin</code> in a deferred semantic action (see <a href="GLR-Semantic-Actions.html#GLR-Semantic-Actions">GLR Semantic Actions</a>).
See <a href="Error-Recovery.html#Error-Recovery">Error Recovery</a>.
</p></dd></dl>

<dl>
<dt><a name="index-yyerrok"></a>Macro: <strong>yyerrok</strong> <em><code>;</code></em></dt>
<dd><p>Resume generating error messages immediately for subsequent syntax
errors.  This is useful primarily in error rules.
See <a href="Error-Recovery.html#Error-Recovery">Error Recovery</a>.
</p></dd></dl>

<dl>
<dt><a name="index-yylloc"></a>Variable: <strong>yylloc</strong></dt>
<dd><p>Variable containing the lookahead token location when <code>yychar</code> is not set
to <code>YYEMPTY</code> or <code>YYEOF</code>.
Do not modify <code>yylloc</code> in a deferred semantic action (see <a href="GLR-Semantic-Actions.html#GLR-Semantic-Actions">GLR Semantic Actions</a>).
See <a href="Actions-and-Locations.html#Actions-and-Locations">Actions and Locations</a>.
</p></dd></dl>

<dl>
<dt><a name="index-yylval"></a>Variable: <strong>yylval</strong></dt>
<dd><p>Variable containing the lookahead token semantic value when <code>yychar</code> is
not set to <code>YYEMPTY</code> or <code>YYEOF</code>.
Do not modify <code>yylval</code> in a deferred semantic action (see <a href="GLR-Semantic-Actions.html#GLR-Semantic-Actions">GLR Semantic Actions</a>).
See <a href="Actions.html#Actions">Actions</a>.
</p></dd></dl>

<dl>
<dt><a name="index-_0040_0024"></a>Value: <strong>@$</strong></dt>
<dd><p>Acts like a structure variable containing information on the textual
location of the grouping made by the current rule.  See <a href="Tracking-Locations.html#Tracking-Locations">Tracking Locations</a>.
</p>




</dd></dl>

<dl>
<dt><a name="index-_0040n"></a>Value: <strong>@<var>n</var></strong></dt>
<dd><a name="index-_0040n-1"></a>
<p>Acts like a structure variable containing information on the textual
location of the <var>n</var>th component of the current rule.  See <a href="Tracking-Locations.html#Tracking-Locations">Tracking Locations</a>.
</p></dd></dl>

<hr>
<div class="header">
<p>
Next: <a href="Internationalization.html#Internationalization" accesskey="n" rel="next">Internationalization</a>, Previous: <a href="Error-Reporting.html#Error-Reporting" accesskey="p" rel="prev">Error Reporting</a>, Up: <a href="Interface.html#Interface" accesskey="u" rel="up">Interface</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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