5.3.3 Specifying Precedence Only

Since POSIX Yacc defines only %left, %right, and %nonassoc, which all defines precedence and associativity, little attention is paid to the fact that precedence cannot be defined without defining associativity. Yet, sometimes, when trying to solve a conflict, precedence suffices. In such a case, using %left, %right, or %nonassoc might hide future (associativity related) conflicts that would remain hidden.

The dangling else ambiguity (see Shift/Reduce Conflicts) can be solved explicitly. This shift/reduce conflicts occurs in the following situation, where the period denotes the current parsing state:

if e1 then if  e2 then s1 • else s2

The conflict involves the reduction of the rule ‘IF expr THEN stmt’, which precedence is by default that of its last token (THEN), and the shifting of the token ELSE. The usual disambiguation (attach the else to the closest if), shifting must be preferred, i.e., the precedence of ELSE must be higher than that of THEN. But neither is expected to be involved in an associativity related conflict, which can be specified as follows.

%precedence THEN
%precedence ELSE

The unary-minus is another typical example where associativity is usually over-specified, see Infix Notation Calculator: calc. The %left directive is traditionally used to declare the precedence of NEG, which is more than needed since it also defines its associativity. While this is harmless in the traditional example, who knows how NEG might be used in future evolutions of the grammar…