shift-reduce-conflictsablecc

shift/reduce conflict with SableCC


I'm at my first experience with SableCC and grammar definition. I have the following grammar (a part of it):

query =
           {atop} attroperator |
           {query_par} l_par query r_par |
           {query_and} [q1]:query logic_and [q2]:query  |
           {query_or} [q1]:query logic_or [q2]:query |
           {query_not} logic_not query ;

I have the following errors:

shift/reduce conflict in state [stack: PCommand TLogicNot PQuery *] on
TRPar in {
       [ PQuery = PQuery * TRPar ] (shift),
       [ PQuery = TLogicNot PQuery * ] followed by TRPar (reduce)
}

shift/reduce conflict in state [stack: PCommand TLogicNot PQuery *] on
TLogicAnd in {
       [ PQuery = PQuery * TLogicAnd PQuery ] (shift),
       [ PQuery = TLogicNot PQuery * ] followed by TLogicAnd (reduce)
}

shift/reduce conflict in state [stack: PCommand TLogicNot PQuery *] on
TLogicOr in {
       [ PQuery = PQuery * TLogicOr PQuery ] (shift),
       [ PQuery = TLogicNot PQuery * ] followed by TLogicOr (reduce)
}

I solved them by adding l_par and r_par to all alternatives which, by the way, should increase readability but is there a way to do it in an elegant manner?

Thanks.


Solution

  • So, I've solved the problem. What I've done is basically define three levels of associativity.

    query = 
        {query_or} query logic_or term | 
        {query_term} term ;
    
    term =
        {term_and} term logic_and factor |
        {term_factor} factor ;
    
    factor = 
        {atop} attroperator |
        {query_not} logic_not attroperator |
        {query_par} l_par query r_par ;
    

    It's the classic associativity scheme +,* with an unary operator like - where + = logic_or, * = logic_and, - = logic_not.