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Suppressing Conflict Warnings

Bison normally warns if there are any conflicts in the grammar (see 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 no warning if there are n shift/reduce conflicts and no reduce/reduce conflicts. An error, instead of the usual warning, is given if there are either more or fewer conflicts, or if there are any reduce/reduce conflicts.

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.

Now Bison will stop annoying you about the conflicts you have checked, but it will warn you again if changes in the grammar result in additional conflicts.