yacc shift/reduce conflict

Question

I faced conflict problem during yacc compilation.

Error message below:

24: shift/reduce conflict (shift 66, reduce 99) on '/'
state 24
arithmetic_leaf : absolute_path .  (99)
absolute_path : absolute_path . '/' relative_path  (102)

Code below:

arithmetic_leaf:  '(' arithmetic_expression ')'
    {
    }
| integer_value
    {
    }
| real_value
    {
    }
| absolute_path
    {
    }
;

absolute_path: '/'
    {
    }
| '/' relative_path
    {
    }
| absolute_path '/' relative_path
    {
    }
;

relative_path: path_segment
    {
    }
| relative_path '/' path_segment
    {
    }
; 

path_segment: V_ATTRIBUTE_IDENTIFIER V_LOCAL_TERM_CODE_REF
    {
    }
| V_ATTRIBUTE_IDENTIFIER '[' V_ARCHETYPE_ID ']'
    {
    }
| V_ATTRIBUTE_IDENTIFIER
    {
    }
;

At this point, 'shift/reduce' conflict will occur.

I don't know what is the problem. How to solve this conflict?

Thanks.

Jerry Coffin Jerry Coffin · Accepted Answer · 2012-07-10T07:18:22

The conflict (appears to me) to be between the alternatives for absolute_path.

It appears that a string like '/a/b will match either the absolute_path '/' relative_path rule, or the '/' relative_path rule.

At least to me, it looks like you just want to eliminate one of the two. I'd probably write it as:

absolute_path: '/'
             | '/' relative_path
             ;

Alternatively, it might make more sense to allow a relative_path to just be an empty string, in which case, you could end up with something like:

absolute_path: '/' relative_path
             ;

relative_path:
             | path_segment
             | relative_path '/' path_segment
             ;

yacc shift/reduce conflict

1 Answers