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Previous: <a href="Keywords.html#Keywords" accesskey="p" rel="prev">Keywords</a>, Up: <a href="Grammar-and-Parser.html#Grammar-and-Parser" accesskey="u" rel="up">Grammar and Parser</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Parser-1"></a>
<h3 class="section">I.2 Parser</h3>
<a name="index-parser"></a>
<p>The parser has a number of variables that affect its internal operation.
These variables are generally documented in the manual alongside the code that
they affect. For example, <code>allow_noninteger_range_as_index</code> is discussed
in the section on index expressions.
</p>
<p>In addition, there are three non-specific parser customization functions.
<code>add_input_event_hook</code> can be used to schedule a user function for
periodic evaluation. <code>remove_input_event_hook</code> will stop a user function
from being evaluated periodically.
</p>
<a name="XREFadd_005finput_005fevent_005fhook"></a><dl>
<dt><a name="index-add_005finput_005fevent_005fhook"></a>Built-in Function: <em><var>id</var> =</em> <strong>add_input_event_hook</strong> <em>(<var>fcn</var>)</em></dt>
<dt><a name="index-add_005finput_005fevent_005fhook-1"></a>Built-in Function: <em><var>id</var> =</em> <strong>add_input_event_hook</strong> <em>(<var>fcn</var>, <var>data</var>)</em></dt>
<dd><p>Add the named function or function handle <var>fcn</var> to the list of functions
to call periodically when Octave is waiting for input. The function should
have the form
</p>
<div class="example">
<pre class="example"><var>fcn</var> (<var>data</var>)
</pre></div>
<p>If <var>data</var> is omitted, Octave calls the function without any
arguments.
</p>
<p>The returned identifier may be used to remove the function handle from
the list of input hook functions.
</p>
<p><strong>See also:</strong> <a href="#XREFremove_005finput_005fevent_005fhook">remove_input_event_hook</a>.
</p></dd></dl>
<a name="XREFremove_005finput_005fevent_005fhook"></a><dl>
<dt><a name="index-remove_005finput_005fevent_005fhook"></a>Built-in Function: <em></em> <strong>remove_input_event_hook</strong> <em>(<var>name</var>)</em></dt>
<dt><a name="index-remove_005finput_005fevent_005fhook-1"></a>Built-in Function: <em></em> <strong>remove_input_event_hook</strong> <em>(<var>fcn_id</var>)</em></dt>
<dd><p>Remove the named function or function handle with the given identifier
from the list of functions to call periodically when Octave is waiting
for input.
</p>
<p><strong>See also:</strong> <a href="#XREFadd_005finput_005fevent_005fhook">add_input_event_hook</a>.
</p></dd></dl>
<p>Finally, when the parser cannot identify an input token it calls a particular
function to handle this. By default, this is the internal function
<code>"__unimplemented__"</code> which makes suggestions about possible Octave
substitutes for <small>MATLAB</small> functions.
</p>
<a name="XREFmissing_005ffunction_005fhook"></a><dl>
<dt><a name="index-missing_005ffunction_005fhook"></a>Built-in Function: <em><var>val</var> =</em> <strong>missing_function_hook</strong> <em>()</em></dt>
<dt><a name="index-missing_005ffunction_005fhook-1"></a>Built-in Function: <em><var>old_val</var> =</em> <strong>missing_function_hook</strong> <em>(<var>new_val</var>)</em></dt>
<dt><a name="index-missing_005ffunction_005fhook-2"></a>Built-in Function: <em></em> <strong>missing_function_hook</strong> <em>(<var>new_val</var>, "local")</em></dt>
<dd><p>Query or set the internal variable that specifies the function to call when
an unknown identifier is requested.
</p>
<p>When called from inside a function with the <code>"local"</code> option, the
variable is changed locally for the function and any subroutines it calls.
The original variable value is restored when exiting the function.
</p>
<p><strong>See also:</strong> <a href="Missing-Components.html#XREFmissing_005fcomponent_005fhook">missing_component_hook</a>.
</p></dd></dl>
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