Manual

Chapter 5: The Bison Parser Algorithm 75
can also specify explicitly the precedence of a rule. See Section 5.4 [Context-Dependent
Precedence], page 75.)
Finally, the resolution of conflicts works by comparing the precedence of the rule being
considered with that of the look-ahead token. If the token’s precedence is higher, the choice
is to shift. If the rule’s precedence is higher, the choice is to reduce. If they have equal
precedence, the choice is made based on the associativity of that precedence level. The
verbose output file made by ‘-v’ (see Chapter 9 [Invoking Bison], page 97) says how each
conflict was resolved.
Not all rules and not all tokens have precedence. If either the rule or the look-ahead
token has no precedence, then the default is to shift.
5.4 Context-Dependent Precedence
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 Bison precedence declarations, %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 contextdependent
precedence, you need to use an additional mechanism: the %prec modifier for
rules.
The %prec modifier declares the precedence of a particular 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:
%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 5.3 [Operator Precedence], page 73).
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:
...
%left ’+’ ’-’
%left ’*’
%left UMINUS
Now the precedence of UMINUS can be used in specific rules:
exp: ...
| exp ’-’ exp
...
| ’-’ exp %prec UMINUS
5.5 Parser States
The function yyparse is implemented using a finite-state machine. The values pushed
on the parser stack are not simply token type codes; they represent the entire sequence of