Reported by Аскар Сафин. http://lists.gnu.org/archive/html/bug-bison/2012-11/msg00024.html
* doc/bison.texi (Reduce/Reduce): Fix the resulting ambiguity using precedence/associativity directives. --- NEWS | 4 ++++ THANKS | 1 + doc/bison.texi | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 57 insertions(+) diff --git a/NEWS b/NEWS index 59549b5..b7b0a22 100644 --- a/NEWS +++ b/NEWS @@ -2,6 +2,10 @@ GNU Bison NEWS * Noteworthy changes in release ?.? (????-??-??) [?] +** Documentation + + The sections about Shift/Reduce and Reduce/Reduce conflict resolution have + been fixed and extended. * Noteworthy changes in release 2.6.5 (2012-11-07) [stable] diff --git a/THANKS b/THANKS index e815f8e..b355b57 100644 --- a/THANKS +++ b/THANKS @@ -1,6 +1,7 @@ Bison was originally written by Robert Corbett. It would not be what it is today without the invaluable help of these people: +Аскар Сафин [email protected] Airy Andre [email protected] Akim Demaille [email protected] Albert Chin-A-Young [email protected] diff --git a/doc/bison.texi b/doc/bison.texi index bc3237a..e2e6a81 100644 --- a/doc/bison.texi +++ b/doc/bison.texi @@ -7117,6 +7117,58 @@ redirects: @end group @end example +Yet this proposal introduces another kind of ambuguity! The input +@samp{word word} can be parsed as a single @code{words} composed of two +@samp{word}s, or as two one-@code{word} @code{words} (and likewise for +@code{redirect}/@code{redirects}). However this ambiguity is now a +shift/reduce conflict, and therefore it can now be addressed with precedence +directives. + +To simplify the matter, we will procede with @code{word} and @code{redirect} +being tokens: @code{"word"} and @code{"redirect"}. + +To prefer the longest @code{words}, the conflict between the token +@code{"word"} and the rule @samp{sequence: sequence words} must be resolved +as a shift. To this end, we use the same techniques as exposed above, see +@ref{Non Operators,, Using Precedence For Non Operators}. One solution +relies on precedences: use @code{%prec} to give a lower precedence to the +rule: + +@example +%nonassoc "word" +%nonassoc "sequence" +%% +@group +sequence: + /* empty */ +| sequence word %prec "sequence" +| sequence redirect %prec "sequence" +; +@end group + +@group +words: + word +| words "word" +; +@end group +@end example + +Another solution relies on associativity: provide both the token and the +rule with the same precedence, but make them right-associative: + +@example +%right "word" "redirect" +%% +@group +sequence: + /* empty */ +| sequence word %prec "word" +| sequence redirect %prec "redirect" +; +@end group +@end example + @node Mysterious Conflicts @section Mysterious Conflicts @cindex Mysterious Conflicts -- 1.8.0
