Why should I prefer `Option::ok_or_else` instead of `Option::ok_or`?

Question

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"));

The only differences I know are:

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.

Did I miss anything?

Answer 1

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

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 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

Answer 2

In addition to performance implications, more complex arguments in ok_or might yield unexpected results if one is not careful enough; consider the following case:

fn main() {
    let value: Option<usize> = Some(1);

    let result = value.ok_or({ println!("value is not Some!"); 0 }); // actually, it is

    assert_eq!(result, Ok(1)); // this holds, but "value is not Some!" was printed
}

This would have been avoided with ok_or_else (and the same goes for other *_or_else functions), because the closure is not evaluated if the variant is Some.