Question

I have the following bison grammar (as part of a more complex grammar):

classDeclaration : CLASS ID EXTENDS ID LBRACE variableDeclarationList methodDeclarationList RBRACE
                 ;

variableDeclarationList : variableDeclarationList variableDeclaration 
                        | /* empty */
                        ;

variableDeclaration : type ID SEMICOLON
                    ;

type : NATTYPE | ID
     ;

methodDeclarationList : methodDeclarationList methodDeclaration
                      | /* empty */
                      ;

methodDeclaration : type ID LPAREN parameterDeclarationList RPAREN variableExpressionBlock
          ;

which is supposed to describe class declarations which look like this:

class foo extends object
{
    nat number;

    nat divide(nat aNumber)
    {
        0;
    }
}

or this:

class foo extends object
{
    nat divide(nat aNumber)
    {
        0;
    }
}

or this:

class foo extends object
{
}

Problem is that there is ambiguity where variable declarations end and method declarations begin (2 shift/reduce conflicts). For example, the method declaration looks like a variable declaration until it sees the parenthesis.

How can I rewrite this grammar to eliminate this ambiguity?

To clarify: the class body can be empty, the only constraint is that variable declarations come before method declarations if there are any.

Answer 1

Another way to do it is to not even have empty rules, and instead use multiple options, one with the nonterminal and one without.

classDeclaration : CLASS ID EXTENDS ID LBRACE RBRACE
                 | CLASS ID EXTENDS ID LBRACE methodDeclarationList RBRACE
                 | CLASS ID EXTENDS ID LBRACE variableDeclarationList RBRACE
                 | CLASS ID EXTENDS ID LBRACE variableDeclarationList methodDeclarationList RBRACE           
                 ;

variableDeclarationList : variableDeclaration 
                        | variableDeclarationList variableDeclaration
                        ;

variableDeclaration : type ID SEMICOLON
                    ;

type : NATTYPE 
     | ID
     ;

methodDeclarationList : methodDeclaration 
                      | methodDeclarationList methodDeclaration
                      ;

methodDeclaration :  type ID LPAREN RPAREN variableExpressionBlock
                  |  type ID LPAREN parameterList RPAREN variableExpressionBlock
                  ;