3.7.8 Suppressing Conflict Warnings

Bison normally warns if there are any conflicts in the grammar (see section Shift/Reduce Conflicts), but most real grammars have harmless shift/reduce conflicts which are resolved in a predictable way and would be difficult to eliminate. It is desirable to suppress the warning about these conflicts unless the number of conflicts changes. You can do this with the %expect declaration.

The declaration looks like this:

%expect n

Here n is a decimal integer. The declaration says there should be n shift/reduce conflicts and no reduce/reduce conflicts. Bison reports an error if the number of shift/reduce conflicts differs from n, or if there are any reduce/reduce conflicts.

For normal LALR(1) parsers, reduce/reduce conflicts are more serious, and should be eliminated entirely. Bison will always report reduce/reduce conflicts for these parsers. With GLR parsers, however, both kinds of conflicts are routine; otherwise, there would be no need to use GLR parsing. Therefore, it is also possible to specify an expected number of reduce/reduce conflicts in GLR parsers, using the declaration:

%expect-rr n

In general, using %expect involves these steps:

• Compile your grammar without %expect. Use the ‘-v’ option to get a verbose list of where the conflicts occur. Bison will also print the number of conflicts.
• Check each of the conflicts to make sure that Bison's default resolution is what you really want. If not, rewrite the grammar and go back to the beginning.
• Add an %expect declaration, copying the number n from the number which Bison printed. With GLR parsers, add an %expect-rr declaration as well.

Now Bison will warn you if you introduce an unexpected conflict, but will keep silent otherwise.