Assigning a precedence to grammar rules

In general, unary operators must be given a precedence. Sometimes a unary operator and a binary operator have the same symbolic representation but different precedences. An example is the unary and binary minus, (). Unary minus often has the same strength as multiplication, or even higher, while binary minus will have a lower strength than multiplication.

Since the representations of the two operators are the same, the keywords %left and %right cannot be used to set the precedences for both. Rather, the precedence for the unary minus is associated with the ``grammar rule'' in which it appears. The keyword %prec is used to change the precedence level associated with a particular grammar rule. Here is a grammar that involves unary and binary minuses:

   %left  ´+´  ´-´
   %left  ´´  ´/´
   

   %%
   

   expr   :   expr  ´+´  expr
          |   expr  ´-´  expr
          |   expr  ´´  expr
          |   expr  ´/´  expr
          |   ´-´  expr      %prec  ´´
          |   NAME
          ;

If a token appears after one of the keywords %left, %right, and %nonassoc, it does not need to be declared by %token as well, although it is not incorrect to do so.

Precedences and associativities used by yacc to resolve parsing conflicts give rise to the following disambiguating rules:

• The precedence and associativity associated with a grammar rule is that of the last token or literal in the body of the rule. If the %prec construction is used, it overrides this default. Some grammar rules may have no precedence and associativity associated with them.

• When there is a reduce-reduce conflict, or when there is a shift-reduce conflict and either the input symbol or the grammar rule has no precedence and associativity, then the two default disambiguating rules given at the beginning of the section are used and the conflicts are reported.

• If there is a shift-reduce conflict, and both the grammar rule and the input character have precedence and associativity associated with them, then the conflict is resolved in favor of the action (shift or reduce) associated with the higher precedence. If precedences are equal, then associativity is used. Left-associativity implies reduce, right-associativity implies shift, and nonassociativity implies error.

Conflicts resolved by precedence are not counted in the number of shift-reduce and reduce-reduce conflicts that yacc reports. This means that mistakes in the precedence specification may disguise errors in the input grammar. It is a good idea to be sparing with precedence and use them in a cookbook fashion until you are experienced in using it. The y.output file is very useful in deciding whether the parser is actually doing what was intended.