Previous: Compiling a grammar, Up: Wisent Grammar [Contents][Index]

### 2.4 Conflicts

Normally, a grammar should produce an automaton where at each state the parser has only one action to do (see Wisent Parsing).

In certain cases, a grammar can produce an automaton where, at some states, there are more than one action possible. Such a grammar is ambiguous, and generates conflicts.

The parser can’t be driven by an automaton which isn’t completely deterministic, that is which contains conflicts. It is necessary to resolve the conflicts to eliminate them. Wisent resolves conflicts like Bison does.

There are two sorts of conflicts:

shift/reduce conflicts

  When either a shift or a reduction would be valid at the same state.

  Such conflicts are resolved by choosing to shift, unless otherwise directed by operator precedence declarations. See (bison)Shift/Reduce, in the Bison manual for more information.

reduce/reduce conflicts

  That 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.

  Such conflicts are resolved 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. See (bison)Reduce/Reduce, in the Bison manual for more information.