Question

I have the following parser grammar (this is a small sample):

expr:
        ident assignop expr
        {
            $$ = new NAssignment(new NAssignmentIdentifier(*$1), $2, *$3);
        } |
        STAR expr %prec IDEREF
        {
            $$ = new NDereferenceOperator(*$2);
        } |
        STAR expr assignop expr %prec IDEREF
        {
            $$ = new NAssignment(new NAssignmentDereference(*$2), $3, *$4);
        } |
        ... ;

...

assignop:
        ASSIGN_EQUAL |
        ASSIGN_ADD |
        ASSIGN_SUBTRACT |
        ASSIGN_MULTIPLY |
        ASSIGN_DIVIDE ;

Now I’m trying to parse any of the following lines:

*0x8000 = 0x7000;
*mem = 0x7000;

However, Bison keeps seeing “*mem” and reducing on the ‘STAR expr’ rule and not performing look-ahead to see whether ‘STAR expr assignop…’ matches. As far as I understand Bison, it should be doing this look-ahead. My closest guess is that %prec is turning off look-ahead or something strange like that, but I can’t see why it would do so (since the prec values are equivalent).

How do I make it perform look-ahead in this case?

EDIT:

The state that it enters when encountering ‘STAR expr’ is:

state 45

   28 expr: STAR expr .
   29     | STAR expr . assignop expr
   35     | expr . binaryop expr

    $default  reduce using rule 28 (expr)

    assignop  go to state 81
    binaryop  go to state 79

So I don’t understand why it’s picking $default when it could pick assignop (note that the order of the rules in the parser.y file don’t affect which one it picks in this case; I’ve tried reordering the assignop one above the standard ‘STAR expr’).

Answer 1

I ended up solving this problem by creating another rule ‘deref’ like so:

deref:
        STAR ident
        {
            $$ = new NDereferenceOperator(*$<ident>2);
        } |
        STAR numeric
        {
            $$ = new NDereferenceOperator(*$2);
        } |
        STAR CURVED_OPEN expr CURVED_CLOSE
        {
            $$ = new NDereferenceOperator(*$3);
        } |
        deref assignop expr
        {
            if ($1->cType == "expression-dereference") // We can't accept NAssignments as the deref in this case.
                $$ = new NAssignment(new NAssignmentDereference(((NDereferenceOperator*)$1)->expr), $2, *$3);
            else
                throw new CompilerException("Unable to apply dereferencing assignment operation to non-dereference operator based LHS.");
        } ;

replacing both rules in ‘expr’ with a single ‘deref’.