Lexical Tie-ins

One way to handle context-dependency is the lexical tie-in: a flag which is set by Bison actions, whose purpose is to alter the way tokens are parsed.

For example, suppose we have a language vaguely like C, but with a special construct `hex (hex-expr)'. After the keyword hex comes an expression in parentheses in which all integers are hexadecimal. In particular, the token `a1b' must be treated as an integer rather than as an identifier if it appears in that context. Here is how you can do it:

%{
int hexflag;
%}
%%
...
expr:   IDENTIFIER
        | constant
        | HEX '('
                { hexflag = 1; }
          expr ')'
                { hexflag = 0;
                   $$ = $4; }
        | expr '+' expr
                { $$ = make_sum ($1, $3); }
        ...
        ;

constant:
          INTEGER
        | STRING
        ;

Here we assume that yylex looks at the value of hexflag; when it is nonzero, all integers are parsed in hexadecimal, and tokens starting with letters are parsed as integers if possible.

The declaration of hexflag shown in the C declarations section of the parser file is needed to make it accessible to the actions (see section The C Declarations Section). You must also write the code in yylex to obey the flag.

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