rust - 具有相同分隔符的并排宏重复

Question

真的不能创建这样的宏还是我做错了:

sample!("hello", "there") => 

println!("{:?}", "hello");
println!("{:?}", "there");

sample!("hello", "there", a "second type", a "another second type") => 

println!("{:?}", "hello");
println!("{:?}", "there");
println!("second {:?}", "second type");
println!("second {:?}", "another second type");

我试过的是这个(playground link):

macro_rules! sample {
  ( 
    $( $first:literal ),*
    $( a $second:literal ),*
  ) => {
      $(
        println!("{:?}", $first);
      )*
      $(
        println!("second {:?}", $second);
      )*
  };
}

失败的原因是:

error: no rules expected the token `a`
  --> main.rs:18:20
   |
1  | macro_rules! sample {
   | ------------------- when calling this macro
...
18 |   sample!("hello", a "testing");
   |                    ^ no rules expected this token in macro call

error: aborting due to previous error

Answer 1

Rust 宏对分隔符非常严格。

macro_rules! sample {
  ( 
    $( $first:literal, )*
    $( a $second:literal ),*
  ) => {
      $(println!("{:?}", $first);)*
      $(println!("second {:?}", $second);)*
  };
}

fn main() {
  sample!("hello", a "testing");
}

此示例有效，您能发现其中的变化吗？我将逗号从第一个 $( ... ) 外部移到内部。区别在于:

• $( $a:literal ),* 只接受 "a", "b", "c" (不允许尾随逗号)
• $( $a:literal, )* 仅接受 "a", "b", "c", (需要尾随逗号)

在您的宏中，中间逗号不匹配作为第一次或第二次重复的一部分。该错误基本上是说它期望另一个 $first 而不是 $second 因为这就是重复所说的。

您可以通过引入可选的逗号来修复它:

macro_rules! sample {
  ( 
    $( $first:literal ),*
    $(,)? // <----------------
    $( a $second:literal ),*
  ) => {
      $(println!("{:?}", $first);)*
      $(println!("second {:?}", $second);)*
  };
}

这更宽松，但会允许这样奇怪的事情，这可能会也可能不会，这取决于你想要什么。

sample!("hello", "there",);
sample!(, a "testing");
sample!("hello" a "testing");

不幸的是，如果不使用不同的 ARM ，我不知道一个完美的解决方案:

macro_rules! sample {
  ($( $first:literal ),*) => { };
  ($( $first:literal, )* $( a $second:literal ),+) => { };
  ($( a $second:literal ),*) => { };
}

fn main() {
  sample!("hello", "there");
  sample!("hello", "there", a "testing");
  sample!(a "second type", a "another second type");
  // sample!("hello", "there",);
  // sample!(, a "testing");
  // sample!("hello" a "testing");
}

另见:

• How do I create a Rust macro with optional parameters using repetitions?
• How to allow optional trailing commas in macros?