What is the conflict in this yacc parser?

Question

I used the -v option in yacc to produce a y.output file. At the top of the file it says

State 98 conflicts: 1 shift/reduce

Further down in the file is the following:

state 98

   30 selection_stmt: IF '(' expression ')' statement .
   31               | IF '(' expression ')' statement . ELSE statement

    ELSE  shift, and go to state 101

    ELSE      [reduce using rule 30 (selection_stmt)]
    $default  reduce using rule 30 (selection_stmt)

What is the conflict, and how can it be fixed?

Answer 1

Try something like this:

election_stmt: IF '(' expression ')' statement . selection_stmt_else_part;
selection_stmt_else_part: ELSE statement 
                        | 
                        ;

Answer 2

Just about every shift/reduce error with if/then/else statement is the infamous dangling else problem.

With this code segment:

if (f1):
    if (f2):
        c1
    else:
        c2

you (and Python due to it's bizarre indentation rules) know which if the else belongs to, but a parser is not so clever.

It can't tell whether the else belongs to the first or second if.

This link shows how to convert the LR(n) to an LR(1) equivalent which should solve the problem.

Another alternative is to change your base language definition (if possible) so that the ambiguity disappears:

: IF '(' cond ')' THEN statement ENDIF
| IF '(' cond ')' THEN statement ELSE statement ENDIF