Question

I am using YACC to do my compiler homework project. I found that my program could not get the syntax tree. So I printed it all out to see what is happening. According to my result, it seems that ClassDecl does not reduce to ClassDeclList here. But I can’t understand why… can anyone help me out?

The sample input is:

   program ex11;
   class ab {
   }

It printed out as:

    programXXXX ex11ID
    semicon abID
    RBRACEnum 
    ClassBody ClassDecl ClassDecl1 Error!

The first three lines are messages I printed from my LEX file, to ensure that the characters are recognized correctly.

According to the information, the parser successfully reduces {} to ClassBody and class ab {} to ClassDecl. And then it does not reduce to ClassDeclList, is it because I am writing a left recursive grammar here?

This is the part of my YACC rule base for the inference:

    Program: PROGRAMnum IDnum SEMInum ClassDeclList
{printf("program"); $$ = MakeTree(ProgramOp,$4, MakeLeaf(IDNode,$2)); printtree($$,0);};

   ClassDeclList: ClassDecl 
{printf("ClassDeclList1");$$ = MakeTree(ClassOp,NullExp(),$1); printf("ClassDeclListend");};
                  |ClassDecl ClassDeclList 
{printf("ClassDeclList2");$$ = MakeTree(ClassOp,$2,$1); printf("ClassDeclList");};

    ClassDecl: CLASSnum IDnum ClassBody
{printf("ClassDecl");$$=MakeTree(ClassDefOp,$3,MakeLeaf(IDNode,$2)); printf("ClassDecl1");};

Answer 1

Have you tried

| ClassDeclList ClassDecl

instead of

| ClassDecl ClassDeclList

?

I remember this fixing many problems when I used to use CUP.