rust 中函数接受闭包作为参数或返回闭包的惯用方式是什么？

Question

What is the idiomatic way in rust for a function accepts a closure as argument or return a closure?

I see it can be done in at least the below 3 ways:

// 1
pub fn run_with_envs_guard1(envs: &HashMap<&str, &str>, f: &dyn FnOnce()) {}

// 2
pub fn run_with_envs_guard2(envs: &HashMap<&str, &str>, f: Box<dyn FnOnce()>) {}

// 3
pub fn run_with_envs_guard3<F: FnOnce()>(envs: &HashMap<&str, &str>, f: F) {}

Are there really some differences among these 3 ways? If yes, pls help to clarify, and which way is more idiomatic i should choose?

I am learning rust still, sorry if all the above ways are some bad/strange things.

Answer 1

I would prefer the third one. Because the rust documentation suggest to Use FnOnce as a bound when you want to accept a parameter of function-like type and only need to call it once .

pub fn run_with_envs_guard3<F: FnOnce()>(envs: &HashMap<&str, &str>, f: F) {}

This means that the F to be bound by FnOnce (ie, F must implement FnOnce )

Answer 2

Abdul answers the first half of your question (and I agree completely with what he said), so I'll take a stab at the second half.

If you want to return a closure from a function, you can't return a type parameter, because that implies that you're returning an instance of any FnOnce , at the caller's choice. You can't return a &FnOnce , because you (usually) need to pass ownership to the caller. You could make it work with Box<FnOnce> , but that tends to just be clunky to work with. When returning closures from functions, I'm partial to the impl trait syntax .

pub fn test() -> impl FnOnce() {
  || { println!("It worked!") }
}

In argument position, writing impl FnOnce() as the type of something is equivalent to defining a type argument, as Abdul did in his answer. However, in return position, it's an entirely new feature that returns an opaque value. It says "I'm returning an FnOnce , and I'm not telling you which one it is". It's the same concept as a trait object, but without the overhead of throwing it in a box.