Hi Zach, please correct me if I am wrong, but I think that a parser generated by bison does not use the look-ahead symbol to decide when to reduce. I suppose, that when the right-hand side of a production has been pushed onto the stack, the parser recognizes this and reduces. (If there would be a look-ahead in this situation that could be shifted instead of reduced, there is a shift/reduce conflict present.)
I am also unsure what you mean by that the parser checks the lookahead and than may shift a different token. I guess the parser calls the scanner only once for each token and in this moment the scanner may look at a variable set by the parser and decide which token to return. In case of [1]: I suppose that bison pushes for example HEX ( onto the stack and recognizes that it is time to reduce. Now the semantic action sets the variable and the new context for the next scanner-call is well defined. Cheers, Alex Zachary Carter wrote: > I'm curious on how Bison's algorithm works wrt lexical tie-ins[1]. The > situation appears to me as a bit of a catch-22. The parser checks the > lookahead token to decide when to reduce (right?) but the semantic action > may alter the scanner so that the next token shifted is actually different > than what the lookahead token had been. It seems counter intuitive, but is > that really how it works? > > [1]: http://www.gnu.org/software/bison/manual/bison.html#Lexical-Tie_002dins _______________________________________________ help-bison@gnu.org http://lists.gnu.org/mailman/listinfo/help-bison
