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<title>avram - a virtual machine code interpreter: 2.7.4.2 Semantics</title>

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<a name="Semantics"></a>
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<tr><td valign="middle" align="left">[<a href="Syntax.html#Syntax" title="Previous section in reading order"> &lt; </a>]</td>
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<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left">[<a href="Virtual-Machine-Specification.html#Virtual-Machine-Specification" title="Beginning of this chapter or previous chapter"> &lt;&lt; </a>]</td>
<td valign="middle" align="left">[<a href="A-Simple-Lisp-Like-Language.html#A-Simple-Lisp-Like-Language" title="Up section"> Up </a>]</td>
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<hr size="1">
<a name="Semantics-1"></a>
<h4 class="subsubsection">2.7.4.2 Semantics</h4>

<p>Declarations in <code>silly</code> should be understood in the obvious way as
preprocessor directives to perform parenthetic textual substitutions (similar to
<code>#define id (exp)</code> in C). All identifiers in expressions are thereby
eliminated during the preprocessing phase.
</p>
<a name="index-semantic-function"></a>
<p>The overall meaning of the program is the meaning of the expression in
the last declaration.  A denotational semantics for expressions is given
by the following equations, where [[<code><var>x</var></code>]] should be read as
&ldquo;the meaning of <code><var>x</var></code>&rdquo;, and <code><var>x</var></code>, <code><var>y</var></code> and <code><var>z</var></code>
are metavariables. (That is, they stand for any source code fragment
that could fit there subject to the constraint, informally speaking,
that it has to correspond to a connected subtree of the parse tree as enforced
by the unambiguous grammar in the context of the rest of the program.)
</p>
<table><tr><td>&nbsp;</td><td><pre class="display">
[[<code>()</code>]] = <code>nil</code>

[[<code>(<var>x</var>)</code>]] = [[<code><var>x</var></code>]]

[[<code>(<var>x</var>,<var>y</var>)</code>]] = <code>cons(</code>[[<code><var>x</var></code>]]<code>,</code>[[<code><var>y</var></code>]]<code>)</code>

[[<code><var>x</var> <var>y</var></code>]] = [[<code><var>x</var>(<var>y</var>)</code>]] = [[<code><var>x</var></code>]] [[<code><var>y</var></code>]]

[[<code><var>x</var> (<var>y</var>,<var>z</var>)</code>]] = [[<code><var>x</var>(<var>y</var>,<var>z</var>)</code>]] = [[<code><var>x</var></code>]] [[<code>(<var>y</var>,<var>z</var>)</code>]]

</pre></td></tr></table>

<p>Toy languages like this are among the few situations a
<a name="index-denotational-semantics"></a>
denotational semantics stands a chance of clarifying more than it
obfuscates, so the reader is invited to take a moment to savor it.
</p>
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<td valign="middle" align="left">[<a href="Library-Reference.html#Library-Reference" title="Next chapter"> &gt;&gt; </a>]</td>
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<td valign="middle" align="left"> &nbsp; </td>
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