Question

我正在使用Jflex，byacc，java来解析类似于

的语句

where expr,expr having expr

，语法看起来像

%token WHERE HAVING COMMA
%%

stmt : whereclause havingclause

whereclause : WHERE conditionlist { move tmpList to whereClause};
havingclause : HAVING conditionlist { move tmpList to havingClause};

conditionlist : condition | condition COMMA conditionlist;

condition : expr { tmpList.add($1); 
                  /How do I know this is being executed for whereclause or havingclause /};

expr : ....

如果条件是whereclause或havingclause的一部分，我无法区分，所以我将条件存储在临时列表中，然后转到右边的子句。这样做的正确方法是什么？

Answer 1

最常见的是，您在规则操作中构建数据结构，将指针分配给$$，然后在更高级别的规则中读取它们。类似的东西：

%token WHERE HAVING COMMA
%union {
    class List *listptr;
    class Expr *exprptr;
}
%type<listptr> whereclasue havingclause conditionlist
%type<exprptr> condition expr
%destructor { delete $$; } <listptr>
%destructor { delete $$; } <exprptr>
%%

stmt : whereclause havingclause { do something with $1 and $2... }

whereclause : WHERE conditionlist { $$ = $2; };
havingclause : HAVING conditionlist { $$ = $2 };

conditionlist : condition { $$ = new List($1); }
              | condition COMMA conditionlist { $$ = $2->prepend($1); };

condition : expr { $$ = $1; }

expr : ....

请注意%destructor操作是一个bison扩展，并且在语法错误时需要避免内存泄漏。

yacc / bison识别结果

1 个答案: