为什么我应该更喜欢 `Option::ok_or_else` 而不是 `Option::ok_or`? [英] Why should I prefer `Option::ok_or_else` instead of `Option::ok_or`?

问题描述

我刚刚在拉取请求中看到以下更改:

I just saw the following change in a pull request:

- .ok_or(Error::new(ErrorKind::Other, "Decode error"));
+ .ok_or_else(|| Error::new(ErrorKind::Other, "Decode error"));

我知道的唯一区别是:

1. 在 ok_or 中，我们已经通过 Error::new 创建了 Error 并将其传递给适配器.
2. 在ok_or_else中，我们传递了一个闭包，它会产生这样的值，但如果Option中有Some数据，它可能不会被调用>.

1. In ok_or we have already created Error by Error::new and passed it into a adaptor.
2. In ok_or_else we have passed a closure which will produce such a value but it may not be called if there is Some data in the Option.

我错过了什么吗?

推荐答案

使用 ok_or_else 或 any 的 ..._or_else 的主要原因code> methods 是为了避免在不需要时执行函数.对于Option::ok_or_else 或Option::unwrap_or_else，当Option 为Some 时，不需要运行额外的代码.这可以使代码更快，具体取决于错误情况下发生的情况

The primary reason to use ok_or_else or any of the ..._or_else methods is to avoid executing a function when it's not needed. In the case of Option::ok_or_else or Option::unwrap_or_else, there's no need to run extra code when the Option is Some. This can make code faster, depending on what happens in the error case

在这个例子中，Error::new 可能执行分配，但它也可以写入标准输出，发出网络请求，或者任何 Rust 代码可以做的事情；从外面很难分辨.将此类代码放在闭包中通常更安全，这样您就不必担心成功案例发生时的额外副作用.

In this example, Error::new likely performs allocation, but it could also write to standard out, make a network request, or anything any piece of Rust code can do; it's hard to tell from the outside. It's generally safer to place such code in a closure so you don't have to worry about extraneous side effects when the success case happens.

Clippy 也为您整理了这个:

Clippy lints this for you as well:

fn main() {
    let foo = None;
    foo.unwrap_or("hello".to_string());
}

warning: use of `unwrap_or` followed by a function call
 --> src/main.rs:3:9
  |
3 |     foo.unwrap_or("hello".to_string());
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| "hello".to_string())`
  |
  = note: `#[warn(clippy::or_fun_call)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#or_fun_call

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