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Import AnaGram (near-)release tree into Mercurial.
author David A. Holland
date Sat, 22 Dec 2007 17:52:45 -0500
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1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
2 <HTML>
3 <HEAD>
4 <TITLE>Expression Evaluator - Macro preprocessor and C Parser </TITLE>
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11
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24 <P>
25 <H1> Expression Evaluator - Macro preprocessor and C Parser </H1>
26
27 <IMG ALIGN="bottom" SRC="../../images/rbline6j.gif" ALT="----------------------"
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29 <P>
30 <BR>
31
32 <H2>Introduction</H2>
33 <P> The Expression Evaluator module, <tt>ex.syn</tt>, exists only to
34 evaluate the conditional expressions in <tt>#if</tt> and
35 <tt>#elif</tt>
36 statements. The syntax for these expressions has been
37 extracted from a full C syntax and reduced to that which is
38 necessary for this purpose.
39 <P>
40 The rules for the evaluation of constant expressions are
41 given in Kernighan and Ritchie, sections A7.19 and A12.5
42 <P>
43 <BR>
44
45 <H2>Theory of Operation</H2>
46 The only unusual feature of this grammar is the
47 interposition of a token called "nonzero" to avoid division
48 by zero. Otherwise the grammar is a conventional expression
49 grammar.
50 <P>
51 The parser is implemented as an event driven parser. The
52 initializer is <tt>init_ex()</tt>. The parser itself is
53 <tt>ex()</tt>. The
54 parser has been interfaced to the rest of the program by a
55 class definition, <tt>expression_evaluator</tt>.
56 <P>
57 The class definition for <tt>expression_evaluator</tt> is given in
58 <tt>mpp.h</tt>. Each instance of the class has its own parser control
59 block. Therefore the parser makes all of its references
60 indirectly through a pointer, <tt>ex_pcb</tt>. Whenever it is
61 necessary to invoke <tt>init_ex()</tt> or <tt>ex()</tt>,
62 therefore, it is
63 necessary first to make sure <tt>ex_pcb</tt> is correctly set.
64 <P>
65 Input to the parser consists of tokens as defined in
66 <tt>token.h</tt>. Since the input does not consist of simple
67 characters, the <b>input values</b> switch is set. The input
68 procedures store the full token into the input_value field
69 of the pcb and the id field into input_code.
70
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90 AnaGram parser generator - examples<BR>
91 Expression Evaluator - Macro preprocessor and C Parser <BR>
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