我将它分成几部分，并将案例分析推送到|中.这是组合器和真正的LL(*)解析的优点之一:

I would split it up a bit and push the case analysis into the |. This is one of the advantages of combinators and really LL(*) parsing in general:

def factor: Parser[ExprTree] = ( wholeNumber ^^ { Number(_.toInt) }
                               | "(" ~> expr <~ ")" 
                               | ident ^^ { Variable(_) } )

如果您对下划线语法不熟悉，我深表歉意.基本上，这仅表示将第 n 个参数替换为封闭函数的值".因此{ Variable(_) }等效于{ x => Variable(x) }.

I apologize if you're not familiar with the underscore syntax. Basically it just means "substitute the nth parameter to the enclosing function value". Thus { Variable(_) } is equivalent to { x => Variable(x) }.

另一个语法魔术是~>和<~运算符代替了~.这些运算符意味着该术语的解析应包括两个parens的语法，但是结果应仅由expr的结果确定.因此，"(" ~> expr <~ ")"与"(" ~ expr ~ ")"完全相同，但是不需要额外的案例分析就可以从expr检索内部结果值.

Another bit of syntax magic here is the ~> and <~ operators in place of ~. These operators mean that the parsing of that term should include the syntax of both the parens, but the result should be solely determined by the result of expr. Thus, the "(" ~> expr <~ ")" matches exactly the same thing as "(" ~ expr ~ ")", but it doesn't require the extra case analysis to retrieve the inner result value from expr.