In the meantime, let's explore Scala's third option for building parsers: parser combinators, built entirely from the functional side of Scala.
同时，我们来探索Scala构建解析器的第3个选项：解析器组合子(parsercombinators)，它完全是从Scala的函数性方面构建的。
To change it to produce instances of our case class AST hierarchy (Expr objects), the return type of the combinators has to be changed to be Parser[Expr].
要将它更改为生成case类AST层次结构的实例（Expr对象），组合子的返回类型必须更改为Parser[Expr]。
If you make that change on its own, it will fail to compile; the three combinators know how to take in Strings but not how to produce Expr objects from what's parsed.
如果让它自行更改，编译将会失败；这三个组合子知道如何获取String，但不知道如何根据解析的内容生成Expr对象。
Parsers "combine" to provide higher and higher levels of abstraction over the concept of parsing (hence the name "parser combinators"; elements which combine together to provide parsing behavior).
“组合”解析器可以提供解析概念的越来越高级的抽象(因此称为“解析器组合子”；组合在一起的元素提供解析行为)。
Parsers "combine" to provide higher and higher levels of abstraction over the concept of parsing (hence the name "parser combinators"; elements which combine together to provide parsing behavior).
“组合”解析器可以提供解析概念的越来越高级的抽象(因此称为“解析器组合子”；组合在一起的元素提供解析行为)。
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