Manual

Chapter 7: Handling Context Dependencies 87
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 prologue of the parser file is needed to make it
accessible to the actions (see Section 3.1.1 [The Prologue], page 41). You must also write
the code in yylex to obey the flag.
7.3 Lexical Tie-ins and Error Recovery
Lexical tie-ins make strict demands on any error recovery rules you have. See Chapter 6
[Error Recovery], page 83.
The reason for this is that the purpose of an error recovery rule is to abort the parsing
of one construct and resume in some larger construct. For example, in C-like languages, a
typical error recovery rule is to skip tokens until the next semicolon, and then start a new
statement, like this:
stmt: expr ’;’
| IF ’(’ expr ’)’ stmt { ... }
...
error ’;’
{ hexflag = 0; }
;
If there is a syntax error in the middle of a ‘hex (expr)’ construct, this error rule will
apply, and then the action for the completed ‘hex (expr)’ will never run. So hexflag
would remain set for the entire rest of the input, or until the next hex keyword, causing
identifiers to be misinterpreted as integers.
To avoid this problem the error recovery rule itself clears hexflag.
There may also be an error recovery rule that works within expressions. For example,
there could be a rule which applies within parentheses and skips to the close-parenthesis:
expr: ...
| ’(’ expr ’)’
{ $$ = $2; }
| ’(’ error ’)’
...
If this rule acts within the hex construct, it is not going to abort that construct (since
it applies to an inner level of parentheses within the construct). Therefore, it should not
clear the flag: the rest of the hex construct should be parsed with the flag still in effect.
What if there is an error recovery rule which might abort out of the hex construct or
might not, depending on circumstances There is no way you can write the action to
determine whether a hex construct is being aborted or not. So if you are using a lexical
tie-in, you had better make sure your error recovery rules are not of this kind. Each rule
must be such that you can be sure that it always will, or always won’t, have to clear the
flag.