How do I create a match macro in rust?

Question

I'm trying to complete Quiz #4 in the Rustlings exercises:

// Write a macro that passes the quiz! No hints this time, you can do it!

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_my_macro_world() {
        assert_eq!(my_macro!("world!"), "Hello world!");
    }

    #[test]
    fn test_my_macro_goodbye() {
        assert_eq!(my_macro!("goodbye!"), "Hello goodbye!");
    }
}

my macro looks like this:

#[macro_export]
macro_rules! my_macro {
    (($val:expr), "world!") => {
        println!("Hello world!");
    };
    (($val:expr), "goodbye!") => {
        println!("Hello goodbye!");
    };
}

This was patterned after the Declarative Macro section of the Rust Language Documentation. I get the following errors:

assert_eq!(my_macro!("world!"), "Hello world!");
   |                              ^^^^^^^^^^ no rules expected this token in macro call

assert_eq!(my_macro!("goodbye!"), "Hello goodbye!");
   |                              ^^^^^^^^^^ no rules expected this token in macro call

I cannot find a solution to this to make it compile. Help!

Try writing the expansion of your macro by hand and see if it makes sense to you. — rodrigo, Jul 04 '20 at 16:33
What's the `($val:expr), ` in your macro? It's not used, what do you expect it to match? — mcarton, Jul 04 '20 at 16:40

score 5 · Answer 1 · answered Aug 14 '20 at 23:12

I think the idea of the exercise is to find a way that by using the macro we get the string "Hello whatever". I'm a newbie at Rust too, but I've been able to do the exercise this way:

#[macro_use]
macro_rules! my_macro {
    ($val:expr) => {
        format!("Hello {}",$val);
    }
}

I'm waiting for feedback!

Deadbeef · Answer 2 · 2021-02-06T03:12:34.400

1

You shouldn't use format!() in a macro like in @Max's answer, you should use concat! instead, because it produces a string slice at compile time instead of formatting at runtime:

macro_rules! my_macro {
    ($val:tt) => {
        concat!("Hello ", $val);
    }
}

edited Feb 06 '21 at 03:12

answered Jan 07 '21 at 13:05

Deadbeef

1,499
8
20

score 0 · Answer 3 · answered Jul 04 '20 at 18:50

After clearing my head and thoroughly examining the error messages I figured it out. Here's the solution:

#[macro_export]
macro_rules! my_macro {
    ($("world!"),*) => {
        "Hello world!"
    };
    ($("goodbye!"),*) => {
        "Hello goodbye!"
    };
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_my_macro_world() {
        assert_eq!(my_macro!("world!"), "Hello world!");
    }

    #[test]
    fn test_my_macro_goodbye() {
        assert_eq!(my_macro!("goodbye!"), "Hello goodbye!");
    }
}

I was making it too complicated.

This way it works, but is a little too complicated, too. Hints: 1) What does the `$(something),*` construction do in your case? 2) What is the difference between branches? 3) Do you know about https://doc.rust-lang.org/stable/std/macro.concat.html? — Cerberus, Jul 05 '20 at 05:09

Ian Githungo · Answer 4 · 2021-12-17T10:02:46.697

0

The question in Quiz4 also requires the use of modules. This works perfectly for me. I also think format! works too but I chose to use conact!.

#[macro_use]
mod macros {
   macro_rules! my_macro {
     ($val: expr) => {
         concat!("Hello ", $val);
      };
   }
}

#[cfg(test)]
mod tests {
  use super::*;

  #[test]
  fn test_my_macro_world() {
     assert_eq!(my_macro!("world!"), "Hello world!");
  }

  #[test]
  fn test_my_macro_goodbye() {
      assert_eq!(my_macro!("goodbye!"), "Hello goodbye!");
  }
}

edited Dec 17 '21 at 10:02

answered Dec 16 '21 at 12:13

Ian Githungo

1
1
1

As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Dec 16 '21 at 15:02

How do I create a match macro in rust?

4 Answers4