如何使用可变参数宏来调用嵌套构造函数? [英] How to use variadic macros to call nested constructors?

问题描述

我正在尝试在Rust中创建一个宏，以便我编写

I'm trying to create a macro in Rust that lets me write

make_list!(1, 2, 3)

代替

Node::new(1, Node::new(2, Node::new(3, None)))

应该适用于任意数量的参数"，包括零.这是我到目前为止的内容:

which should work for an arbitrary number of "parameters" including zero. This is what I have so far:

macro_rules! make_list(
    () => (
        None
    );
        ( $x:expr, $( $more:expr ),* ) => (
        Node::new($x, make_list!( $( $more ),* ))
    )
);

但出现以下错误:

error: unexpected end of macro invocation
  --> src/main.rs:19:42
   |
19 |             Node::new($x, make_list!( $( $more ),* ))
   |                                          ^^^^^

我对此不太了解.据我所知，它应该有效.我做错了什么?

I can't make much sense of this. From what I can tell, it should work. What did I do wrong?

完整代码:

type List<T> = Option<Box<Node<T>>>;

struct Node<T> {
    value: T,
    tail: List<T>,
}

impl<T> Node<T> {
    fn new(val: T, tai: List<T>) -> List<T> {
        Some(Box::new(Node::<T> {
            value: val,
            tail: tai,
        }))
    }
}

macro_rules! make_list(
    () => (
        None
    );
    ( $x:expr, $( $more:expr ),* ) => (
        Node::new($x, make_list!( $( $more ),* ))
    )
);

fn main() {
    let _list: List<i32> = make_list!(1, 2, 3, 4, 5, 6, 7, 8, 9);
}

推荐答案

扩展错误:您只剩下一个值的情况，因此它写为make_list!(1).但是，没有规则可以匹配该规则，对于第二个规则，在使用表达式x之后，需要一个逗号(未提供).

Expanding on the error: you get down to the case where there is only one value, and so it writes make_list!(1). However, there is no rule that will match that, for the second rule, after consuming the expression x, wants a comma, which is not provided.

因此，您需要制作它，使其适用于make_list!(1)，而不仅仅适用于make_list!(1,).为此，请在重复部分内添加逗号，如下所示:

So you need to make it so that it will work for make_list!(1) and not just (in fact, just not) make_list!(1,). To achieve this, get the comma inside the repeating part, like this:

macro_rules! make_list(
    () => (
        None
    );
    ( $x:expr $( , $more:expr )* ) => (
        Node::new($x, make_list!( $( $more ),* ))
    )
);

奖金:如果需要，您可以写make_list![1, 2, 3]而不是make_list!(1, 2, 3).

Bonus: you can write make_list![1, 2, 3] instead of make_list!(1, 2, 3) if you want.

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