On Fri, 23 Apr 2010, 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?)
Not always. If the parser doesn't need a lookahead because there's only one possible parser action, then it doesn't fetch a lookahead from the scanner. > 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? No, the semantic action would have to modify the stored lookahead (yychar) not the scanner behavior (using hexflag, for example) to make that happen. > [1]: http://www.gnu.org/software/bison/manual/bison.html#Lexical-Tie_002dins Here's the relevant production from that example: expr: HEX '(' { hexflag = 1; } expr ')' { hexflag = 0; $$ = $4; } The assumption is that, after HEX '(', the mid-rule is the only possible action, so hexflag = 1 is set before the next token after '(' is fetched. _______________________________________________ help-bison@gnu.org http://lists.gnu.org/mailman/listinfo/help-bison
