(bison.info) Tie-in Recovery

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(bison.info) Lexical Tie-ins

(bison.info) Context Dependency

 
 Lexical Tie-ins and Error Recovery
 ==================================
 
    Lexical tie-ins make strict demands on any error recovery rules you
 have.   Error Recovery.
 
    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.
 

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(bison.info) Lexical Tie-ins

(bison.info) Context Dependency