3.7.9 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 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 deterministic 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

You may wish to be more specific in your specification of expected conflicts. To this end, you can also attach %expect and %expect-rr modifiers to individual rules. The interpretation of these modifiers differs from their use as declarations. When attached to rules, they indicate the number of states in which the rule is involved in a conflict. You will need to consult the output resulting from -v to determine appropriate numbers to use. For example, for the following grammar fragment, the first rule for empty_dims appears in two states in which the ‘[’ token is a lookahead. Having determined that, you can document this fact with an %expect modifier as follows:

dims:
  empty_dims
| '[' expr ']' dims
;

empty_dims:
  %empty   %expect 2
| empty_dims '[' ']'
;

Mid-rule actions generate implicit rules that are also subject to conflicts (see Conflicts due to Midrule Actions). To attach an %expect or %expect-rr annotation to an implicit mid-rule action’s rule, put it before the action. For example,

%glr-parser
%expect-rr 1

%%

clause:
  "condition" %expect-rr 1 { value_mode(); } '(' exprs ')'
| "condition" %expect-rr 1 { class_mode(); } '(' types ')'
;

Here, the appropriate mid-rule action will not be determined until after the ‘(’ token is shifted. Thus, the two actions will clash with each other, and we should expect one reduce/reduce conflict for each.

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 that Bison printed. With GLR parsers, add an %expect-rr declaration as well.
• Optionally, count up the number of states in which one or more conflicted reductions for particular rules appear and add these numbers to the affected rules as %expect-rr or %expect modifiers as appropriate. Rules that are in conflict appear in the output listing surrounded by square brackets or, in the case of reduce/reduce conflicts, as reductions having the same lookahead symbol as a square-bracketed reduction in the same state.

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