porting http://www.craftinginterpreters.com/scanning.html

语法分析器从词法分析器获得一个由词法单元组成的串，并验证这个串可以由源语言的文法生成

递归下降解析器

• Recursive descent is considered a top-down parser because it starts from the top or outermost grammar rule (here expression) and works its way down into the nested subexpressions before finally reaching the leaves of the syntax tree.

• A recursive descent parser is a literal translation of the grammar’s rules straight into imperative code.

• Each rule becomes a function.

• Each rule here only matches expressions at its precedence level or higher.

1. 解释器模式
2. 访问者模式

  interface PastryVisitor {
    void visitBeignet(Beignet beignet);
    void visitCruller(Cruller cruller);
  }

  abstract class Pastry {
    abstract void accept(PastryVisitor visitor);
  }

  class Beignet extends Pastry {
    @Override
    void accept(PastryVisitor visitor) {
      visitor.visitBeignet(this);
    }
  }

  class Cruller extends Pastry {
    @Override
    void accept(PastryVisitor visitor) {
      visitor.visitCruller(this);
    }
  }