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(bison.info)Reduce/Reduce

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Reduce/Reduce Conflicts
=======================

   A reduce/reduce conflict occurs if there are two or more rules that
apply to the same sequence of input.  This usually indicates a serious
error in the grammar.

   For example, here is an erroneous attempt to define a sequence of
zero or more `word' groupings.

     sequence: /* empty */
                     { printf ("empty sequence\n"); }
             | maybeword
             | sequence word
                     { printf ("added word %s\n", $2); }
             ;
     
     maybeword: /* empty */
                     { printf ("empty maybeword\n"); }
             | word
                     { printf ("single word %s\n", $1); }
             ;

The error is an ambiguity: there is more than one way to parse a single
`word' into a `sequence'.  It could be reduced to a `maybeword' and
then into a `sequence' via the second rule.  Alternatively,
nothing-at-all could be reduced into a `sequence' via the first rule,
and this could be combined with the `word' using the third rule for
`sequence'.

   There is also more than one way to reduce nothing-at-all into a
`sequence'.  This can be done directly via the first rule, or
indirectly via `maybeword' and then the second rule.

   You might think that this is a distinction without a difference,
because it does not change whether any particular input is valid or
not.  But it does affect which actions are run.  One parsing order runs
the second rule's action; the other runs the first rule's action and
the third rule's action.  In this example, the output of the program
changes.

   Bison resolves a reduce/reduce conflict by choosing to use the rule
that appears first in the grammar, but it is very risky to rely on
this.  Every reduce/reduce conflict must be studied and usually
eliminated.  Here is the proper way to define `sequence':

     sequence: /* empty */
                     { 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
             ;