Bison/Yacc Recursion Problem - "Rule never reduced because of conflicts"

[LuckY]

I'm suffering a problem with a yacc specification. I had a similar problem but was able to resolve it. I'm sure the problem is in the recursion madness I'm dealing with, but am unable to find a clear way around it. Any ideas?

Here're the related bits:

atomic_patt : CONSTANT_TK
            | UNDERSCORE_TK
            | LPAREN_TK  pattern_star  RPAREN_TK
            | LBRACE_TK  pattern_star  RBRACE_TK
            | IDENT_TK
            ;

pattern_star : /* epsilon */
             | pattern
             | pattern  COMMA_TK  pattern_star
             ;

pattern : pattern_qual_id  atomic_patt  pattern_type
        | atomic_patt  CONS_TK  atomic_patt
        ;

pattern_qual_id : /* epsilon */  <============= line 152
                | qual_id
                ;

pattern_type : /* epsilon */
             | COLON_TK  type
             ;

qual_id : IDENT_TK
        | IDENT_TK  DOT_TK  qual_id
        ;

And the error (warning really, but hey):

mwb.y:152.16: warning: rule never reduced because of conflicts: pattern_qual_id: /* empty */

[Sebastiani]

not sure - what is the meaning of the /* epsilon */ in place of a rule?
and shouldn't the longer rules be defined first:

| pattern
| pattern COMMA_TK pattern_star

seems to me the second would never get evaluated (or does bison account for that)?

[LuckY]

The epsilon represents nothing. In other words a regular expression like ab?c could be accpeted by a grammar like:

S -> aTc
T -> b | epsilon

In any case, I have found the problem, and it was in "pattern." The problem was that Bison could not determine if it should choose the first or second production because pattern_qual_id could evaluate to epsilon which would then make both productions begin with atomic_patt. So instead of using the epsilon, I just split the two options into two productions.

The correction of pattern looks like this (and pattern_qual_id is now gone):

pattern : atomic_patt  pattern_type
        | qual_id atomic_patt  pattern_type
        | atomic_patt  CONS_TK  atomic_patt
        ;