递归下降解析器问题 [英] Recursive descent parser questions

问题描述

我有两个关于如何编写递归下降解析器的问题：

I have two questions about how to write a recursive descent parser:

第一个是什么时候有一个非终结符可以匹配几个不同的终结符之一？如何检查哪种方式是正确的？

The first is what when you have a nonterminal that can match one of a few different nonterminals? How do you check which way is correct?

其次，如何构建AST？使用YACC，我可以只写一段代码来为非终结符的每个实例执行，并且它有特殊的变量，引用规则的值。如何在递归下降解析器中做类似的事情？

Second, how do you build an AST? Using YACC, I can just write a piece of code to execute for every instance of a nonterminal and it has special variables which refer to the "values" of the rules. How do you do a similar thing in a recursive descent parser?

推荐答案

1. ，然后在故障时回溯。或者您证明自己的语言位于 LL（ k ）


2. 对于规则的每一次成功解析，都要从子规则的结果中构造一个对象。

例如

class ASTNode {
  public:
    virtual int eval() = 0;
    virtual ~ASTNode() = 0;
};

// construct this when parsing an integer literal
class Value : ASTNode {
    int v;
  public:
    Value(int v_) : v(v_) {}
    virtual int eval() { return v; }
    virtual ~Value() {}
};

// construct this when parsing "x+y"
class Addition : ASTNode {
    ASTNode *left, *right;
  public:
    Addition(ASTNode *l, ASTNode *r) : left(l), right(r) {}
    virtual int eval() { return l->eval() + r->eval(); }
    virtual ~Addition() { delete left; delete right; }
};

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