Option counter method to ok_or

libs
1

Would there be any desire to have a complementing err_or and err_or_else methods on Option<T>. There already are ok_or and ok_or_else so it seems only natural to have the opposite.

Signatures:

impl<E> Option<E> {
    fn err_or<O>(self, ok: O) -> Result<O, E>;

    fn err_or_else<O, F>(self, ok: F) -> Result<O, E> where F : FnOnce() -> O;
}

The alternative (or in addition) would be a method on Result<U, V> called flip which has the following signature:

impl<U, V> Result<U, V> {
    fn flip(self) -> Result<V, U>;
}

These methods would be useful for when you want to return an optional error in conjunction with some other information.

2

That would be useful only if you had to work with existing Option<Error>, but that situation is more idiomatically expressed by Result<(), Error>.

So I think lack of these methods is a valuable reminder that you should be using Result for errors.

Result has very clear Ok/Err sides that you should never need to flip around. For cases where the two types are interchangeble, there is the Either type.

3 Likes
3

I agree that result shouldn't have something like flip. I just suggested it as an option.

Yes, it would be useful in situations of Option<Error>

4

One way to write this today is: opt.map_or(Ok(foo), Err)

3 Likes
5

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