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]。

youdao

A command indicates a type of pattern to match, and a modifier can change the behavior that occurs in case of such a match.

命令显示欲匹配的此类模式，修饰符可以改变此类匹配中发生的行为。

youdao

A command indicates a type of pattern to match, and a modifier can change the behavior that occurs in case of such a match.

命令显示欲匹配的此类模式，修饰符可以改变此类匹配中发生的行为。

youdao