> %token ATOM > %left '+' > %left REP > > block: ATOM > | REP block > | block block %prec '+'
> 5 block: REP block . > 6 | block . block > > ATOM shift, and go to state 3 > > ATOM [reduce using rule 5 (block)] This says that yacc isn't sure how to decide, having read REP block, between shifting ATOM or applying the REP block reduction. > 6 block: block . block > 6 | block block . > > ATOM shift, and go to state 3 > > ATOM [reduce using rule 6 (block)] This says that yacc isn't sure how to decide, having read block block, between shifting ATOM or applying the block block reduction. To know how to decide, yacc needs a precedence for the thing being shifted and the rule. You've given precedences for each rule (REP block has REP's precedence, and block block has +'s thanks to the override) but not to ATOM. Concretely, when yacc sees REP block ATOM it isn't sure whether that's (REP block) ATOM or REP (block ATOM). Instead of > %token ATOM > %left '+' > %left REP you probably want %left '+' %left REP %nonassoc ATOM This wasn't an issue in the first grammar because the shift/reduce decision had to be made when looking at the '+' instead of when looking at ATOM, and the '+' did have a precedence. Russ
