You can use ? on an Option if your type returns an Option. If it returns a Resul...

vmarsy · on Jan 10, 2019

I see, not very familiar with both those idioms `?` and `ok_or()?`

My current understanding is that those would return an Error only? I was more describing cases where you do want to return, but not necessarily return an `Error`.

For instance in a simplified example function that returns a boolean, you could decide to return `false`. is it possible there?

    // Function that returns a boolean value
    fn is_equal_to_ten(n: Option<u32>) -> bool {
        // some one liner  that checks for None, if it's not none, gives you `x` when `n` matches content of `Some(x)` (not real code): 
        if let Some(x) = n else { return false; /* what to do in case it's a None*/ } 
        
        // `x` is available here:
        return (x == 10);        
    }

Would this be considered bad practice in Rust?

steveklabnik · on Jan 10, 2019

The question mark operator works via a trait, Try. Both Option and Result implement Try. If you use ? on a None value, it will return None, just like using ? on an Err returns an Err.

ok_or is a method on Option that would let you manually convert it to a Result. You could then combine it with ?, turning a None into a specific Err.

It won’t help for stuff that returns bool, it’s true.

vmarsy · on Jan 10, 2019

Thanks for the detailed answer :)

Thiez · on Jan 10, 2019

Often such functions can be rewritten to perform some operations "inside" the option. For example:

    fn is_equal_to_ten(n: Option<u32>) -> bool {
        n.map(|n|n == 10).unwrap_or_default()
    }