I forgot to specify the type of a parameter and the error message was as follows:
error: expected one of `:` or `@`, found `)`
--> src/main.rs:2:12
|
2 | fn func(arg)
| ^ expected one of `:` or `@` here
Which raises the question: what can you do with an @ symbol? I don't remember reading about using the @ symbol for anything. I also did some Googling and couldn't find anything. What does @ do?
You can use the @ symbol to bind a pattern to a name. As the Rust Reference demonstrates:
let x = 1;
match x {
e @ 1 ... 5 => println!("got a range element {}", e),
_ => println!("anything"),
}
Assignments in Rust allow pattern expressions (provided they are complete) and argument lists are no exception. In the specific case of @, this isn't very useful because you can already name the matched parameter. However, for completeness, here is an example which compiles:
enum MyEnum {
TheOnlyCase(u8),
}
fn my_fn(x @ MyEnum::TheOnlyCase(_): MyEnum) {}
@ is shorthand for guards? E.g., e if 1 <= e && e <= 5? Not saying the shorthand wouldn't be helpful / possibly make the source clearer (assuming the syntax is understood) – I just want to make sure I'm not missing a use case of @ in the spirit of "what can you do with the @ symbol".
@ let's you bind different parts of a structure to various names: a @ Some(p @ Point { x = 5, .. }) now a has type Option<Point> and p has type Point.
The @ symbol allows you to reuse a variable holding a value that's used simultaneously in a match pattern.
For example:
enum Student {
Junior{ id: u32}
}
let me = Student::Junior{ id: 10 };
match me {
Student::Junior{ id: id_val @ 0..=10 } => println!("in range with id: {}", id_val),
Student::Junior{ id: 11..=20 } => println!("in range 11->20"),
Student::Junior{ id } => println!("outside range with id: {}", id)
}
In this example, by binding id to the id_val the match can be used in the println!(), otherwise this won't be possible as seen in the second match expression.
