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Often the precedence of an operator depends on the context. This sounds
outlandish at first, but it is really very common. For example, a minus sign
typically has a very high precedence as a unary operator, and a somewhat lower
precedence (lower than multiplication) as a binary operator.
The b() precedence directives, %left, %right and %nonassoc, can only be used
once for a given token; so a token has only one precedence declared in this
way. For context-dependent precedence, you need to use an additional
mechanism: the %prec modifier for rules.
The %prec modifier declares the precedence of a particular (non-empty) rule by
specifying a terminal symbol whose precedence should be used for that
rule. It's not necessary for that symbol to appear otherwise in the rule. The
modifier's syntax is:
verb(%prec terminal-symbol)
and it is written after the components of the rule. Its effect is to assign
the rule the precedence of terminal-symbol, overriding the precedence that
would be deduced for it in the ordinary way. The altered rule precedence then
affects how conflicts involving that rule are resolved (see section Operator
Precedence).
Here is how %prec solves the problem of unary minus. First, declare a
precedence for a fictitious terminal symbol named UMINUS. There are no tokens
of this type, but the symbol serves to stand for its precedence:
verb(
...
%left '+' '-'
%left '*'
%left UMINUS
)
Now the precedence of UMINUS can be used in specific rules:
verb(
exp:
...
| exp '-' exp
...
| '-' exp %prec UMINUS
)
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