parsing - bison info doc - precedence in recursive parsing

Question

in info doc of bison, it is mentioned that rule gets its precendence from last terminal symbol. pasted below: https://www.gnu.org/software/bison/manual/html_node/How-Precedence.html#How-Precedence

The first effect of the precedence declarations is to assign precedence levels to the terminal symbols declared. The second effect is to assign precedence levels to certain rules: each rule gets its precedence from the last terminal symbol mentioned in the components.

For below example, how the parsing will happen if above is the case:

nonterm1 : nonterm2
                 | nonterm1 term1 nonterm3 nonterm4
                 | nonterm1 term2 nonterm5 nonterm6

One more example:

nonterm1 : nonterm2 nonterm3
                  | nonterm1 term1 nonterm2 nonterm3
                  | nonterm1 term2 nonterm2 nonterm3

Consider first example. First parser will consume nonterm2 then for term1 rule, nonterm1 will hold nonterm2 value and term2 higher precedence than term1, parser have to pass value to nonterm1 in term2 rule but it is invalid if we provide term1 precedence to term1 rule because as the statement suggests, last terminal token gives rule precedence. Assigning value of nonterm3 nonterm4 to nonterm1 in term2 rule is not possible or is it?

Value is for term or nonterm but not partial set of symbols in rule or is it possible?

Does the info doc says last terminal symbol meant to be "last but one" symbol which if it is terminal?

标签： parsingbisonoperator-precedence

Answer 1

默认情况下，产生式的优先级是产生式右侧最后一个终端的声明优先级，无论后面有多少（或几个）非终端。

如果该终端没有声明的优先级，则生产也没有声明的优先级。

在序言中使用 、 和 声明来%left声明%right优先%precedence级。%nonassoc

优先级仅用于解决歧义。如果语法完全确定解析操作，则不参考优先级。