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<a name="The-Universal-Function"></a>
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<a name="The-Universal-Function-1"></a>
<h3 class="subsection">3.1.8 The Universal Function</h3>

<a name="index-universal-function"></a>
<p>A function computing the result of the invisible operator used to
specify the virtual code semantics in <a href="Virtual-Code-Semantics.html#Virtual-Code-Semantics">Virtual Code Semantics</a>, is
easily available by way of a declaration in &lsquo;<tt>apply.h</tt>&rsquo;.
</p>
<dl>
<dt><a name="index-avm_005finitialize_005fapply"></a><u>Function:</u> void <b>avm_initialize_apply</b><i> ()</i></dt>
<dd><p>This function should be called by the client program at least once prior
to the first call to <code>avm_apply</code> or
<code>avm_recoverable_apply</code>. It causes certain internal data structures
and error message texts to be initialized.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fcount_005fapply"></a><u>Function:</u> void <b>avm_count_apply</b><i> ()</i></dt>
<dd><p>This function should be used at the end of a run for the purpose of
detecting and reporting any unreclaimed storage associated with
functions in this section. If the function <code>avm_count_lists()</code> is
also being used, it should be called after this one.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fapply"></a><u>Function:</u> list <b>avm_apply</b><i> (list <var>operator</var>, list <var>operand</var>)</i></dt>
<dd><p>This is the function that evaluates the operator used to describe the
virtual code semantics. For example, the value of <code><var>f</var> <var>x</var></code>
can be obtained as the result returned by
<code>avm_apply(<var>f</var>,<var>x</var>)</code>. 
</p>
<p>Both parameters to this function are deallocated unconditionally and
should not be referenced again by the caller. If the parameters are
needed subsequently, then only copies of them should be passed to
<code>avm_apply</code> using <code>avm_copied</code>.
</p>
<p>This function is not guaranteed to terminate, and may cause a memory
overflow error. In the event of an exceptional condition, the error
message is written to standard error and the program is halted. There is
no externally visible distinction between different levels of error
conditions.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005frecoverable_005fapply"></a><u>Function:</u> list <b>avm_recoverable_apply</b><i> (list <var>operator</var>, list <var>operand</var>, int *<var>fault</var>)</i></dt>
<dd><p>This function is similar to <code>avm_apply</code> but leaves the
responsibility of error handling with the caller. If any
overflow or exceptional condition occurs, the result returned is a list
representing the error message, and the <code><var>fault</var></code> flag is set to
a non-zero value. This behavior contrasts with that of <code>avm_apply</code>,
which will display the message to standard error and kill the process.
</p></dd></dl>

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