Re: Help with Yacc reduce/reduce conflict

I found this in your yacc grammar:


          assignment
          : left_hand_side assignment_operator assignment_expression
          | left_hand_side EQEQ assignment_expression
              /* XXX: The above rule causes 4 reduce/reduce conflicts */
              /* Code here to warn about == in assignments */
          | left_hand_side assignment_operator error
          ;


I would remove the line with the EQEQ in it. While the C language
allows the statement:


          a == b;


It is really a conditional-statement and not an assignment statement.
Outside of an if-statement it does nothing, unless you have something
like this:


          a == b++;


I would not allow EQEQ to be used as an assignment operator (it's not
one). Also I would not allow conditional-expressions like this one to
exist outside of if-statements.


If you do as I suggest, then the assignment statement:


          a == b;


would cause a syntax error at the '=='


To handle the other problem:


        if (a = b) ...


I would not allow an expression inside of an if-statement unless it
were a conditional-expression (which would have to contain a
conditional- operator).


This would cause a syntax error at the ')' and force the student to
code:


          if ((a=b) == c) ...


or


          if (a == b) ...


Whatever the student was attempting to express. I think it can be
done in the grammar, as long as you make a distinction between
expressions and conditional-expressions, and where they can be used.


Paul B Mann

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