Manual

Chapter 5: The Bison Parser Algorithm 77
{ printf ("empty sequence\n"); }
| sequence word
{ printf ("added word %s\n", $2); }
;
Here is another common error that yields a reduce/reduce conflict:
sequence: /* empty */
| sequence words
| sequence redirects
;
words: /* empty */
| words word
;
redirects:/* empty */
| redirects redirect
;
The intention here is to define a sequence which can contain either word or redirect
groupings. The individual definitions of sequence, words and redirects are error-free,
but the three together make a subtle ambiguity: even an empty input can be parsed in
infinitely many ways!
Consider: nothing-at-all could be a words. Or it could be two words in a row, or three,
or any number. It could equally well be a redirects, or two, or any number. Or it could
be a words followed by three redirects and another words. And so on.
Here are two ways to correct these rules. First, to make it a single level of sequence:
sequence: /* empty */
| sequence word
| sequence redirect
;
Second, to prevent either a words or a redirects from being empty:
sequence: /* empty */
| sequence words
| sequence redirects
;
words:
word
| words word
;
redirects:redirect
| redirects redirect
;
5.7 Mysterious Reduce/Reduce Conflicts
Sometimes reduce/reduce conflicts can occur that don’t look warranted. Here is an example: