Intel XENIX 286 Programmers Guide (86) - Tenox.tc

yacc: Compiler-Compiler
Consider the following:
% {
int dflag;
% }
... other declarations ...
%%
prog : decls stats
decls :/* empty *I
{ dflag = 1; }
I decls declaration
stats :/* empty *I
{ dflag = 0; }
I stats statement
... other rules ...
XENIX Programming
The flag dflag is now 0 when reading statements, and 1 when reading declarations,
except for the first token in the first statement. This token must be seen by the parser
before it can tell that the declaration section has ended and the statements have begun.
In many cases, this single token exception does not affect the lexical scan.
This kind of approach can be overdone. Nevertheless, it represents a way of doing some
things that are difficult to do otherwise.
Handling Reserved Words
Some programming languages permit the user to use words like if, that are normally
reserved, as label or variable names, provided that such use does not conflict with the
legal use of these names in the programming language. This is extremely hard to do in
the framework of yacc; it is difficult to pass information to the lexical analyzer telling
it "this instance of 'if' is a keyword, and that instance is a variable." It is best that
keywords be reserved, that is, be forbidden for use as variable names.
Simulating Error and Accept in Actions
The parsing actions of error and accept can be simulated in an action by use of the
macros YYACCEPT and YYERROR. YYACCEPT causes yyparse to return the value 0;
YYERROR causes the parser to behave as if the current input symbol had been a syntax
error; yyerror is called, and error recovery takes place. These mechanisms can be used
to simulate parsers with multiple endmarkers or context-sensitive syntax checking.
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