Added Option::reduce

[zesterer]

This PR adds a function that I and others have needed on a regular occurrence (more frequently than at least half of those on Option already) that does not have a concise inline alternative, nor can it seemingly be composed from existing functions on Option.

I've found this function to be particularly useful in the context of compiler development when one has to resolve two potentially non-existent values together, although it crops up in many other contexts. For example, two error values can be resolved like so:

err0.reduce(err1, |err0, err1| err0.merge_with(err1))

This expands to:

match (err0, err1) {
    (Some(err0), Some(err1)) => Some(err0.merge_with(err1)),
    (Some(err0), None) => Some(err0),
    (None, Some(err1)) => Some(err1),
    (None, None) => None,
}

(I've expanded this example for the sake of clarity)

Additionally, although not implemented in this PR, there is another, similar function that crops up almost as frequently:

impl<T> Option<T> {
    pub fn reduce<F>(self, other: T, f: F)
        where F: FnOnce(T, T) -> T
    {
        match self {
            Some(a) => f(a, other),
            None => other,
        }
    }
}

However, this is trivially emulated with a.into_iter().reduce(b, f).

[rust-highfive]

r? @yaahc

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[the8472]

Bikeshed, this looks a lot like Iterator::reduce, so it could be named that way.

[zesterer]

Bikeshed, this looks a lot like Iterator::reduce, so it could be named that way.

I quite like that idea, very much open to changing it to that if others agree.

[fee1-dead]

These do not look like real use cases to me. At a first glance, you are trying to sum these up if any of them has a value, and get None when both of them do not. What does this achieve? You can either resolve this by responding to me or changing the example.

[zesterer]

I've altered the example to hopefully make the use-case clearly. Regardless, it is only an example. It's difficult to demonstrate a real use-case in such a limited, controlled environment.

[zesterer]

On the recommendation by @the8472 I've renamed this method to reduce. Additionally, I've cleaned up and improved the documentation & example.

[yaahc]

Looks like this is a duplicate of #87036 which we rejected last month.

From the other thread:

Thanks for your PR.

I'm not convinced we should add this, though. I can't think of a lot of use cases that aren't already covered by Iterator::reduce, and I don't think it'd be clear to a reader what this function does, which I think is especially important for methods on commonly used types like Option.

Pinging @rust-lang/libs in case anyone feels like this would be an important addition. Otherwise I'll close this PR.

I still feel the same way as I did when the last issue came up, but since this has come up twice in as many months perhaps we should re-evaluate.

[zesterer]

Looks like this is a duplicate of #87036 which we rejected last month.

From the other thread:

Thanks for your PR.
I'm not convinced we should add this, though. I can't think of a lot of use cases that aren't already covered by Iterator::reduce, and I don't think it'd be clear to a reader what this function does, which I think is especially important for methods on commonly used types like Option.
Pinging @rust-lang/libs in case anyone feels like this would be an important addition. Otherwise I'll close this PR.

I still feel the same way as I did when the last issue came up, but since this has come up twice in as many months perhaps we should re-evaluate.

Thanks for the response.

The function I'm proposing here does not do the same thing as Iterator::reduce (I discuss the similar but distinct function that does do a similar thing in the issue body). Instead, it performs an operation that I believe is not possible to compose out of existing Option/Iterator methods.

I'm not sure which past issue you're referring to (I believe you might have accidentally linked to this one).

[yaahc]

Oops, I linked this PR instead of the other one: #84695

The function I'm proposing here does not do the same thing as Iterator::reduce (I discuss the similar but distinct function that does do a similar thing in the issue body). Instead, it performs an operation that I believe is not possible to compose out of existing Option/Iterator methods.

I'm not following how this can't be done with Iterator. This example seems to reproduce what you're describing: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=92c6ea1718d4992b2a09a5f4327013a4

[zesterer]

Oops, I linked this PR instead of the other one: #84695

The function I'm proposing here does not do the same thing as Iterator::reduce (I discuss the similar but distinct function that does do a similar thing in the issue body). Instead, it performs an operation that I believe is not possible to compose out of existing Option/Iterator methods.

I'm not following how this can't be done with Iterator. This example seems to reproduce what you're describing: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=92c6ea1718d4992b2a09a5f4327013a4

Ah, I forget that Iterator::chain takes an impl IntoIterator! That's more concise than past alternatives I've used.

I still believe that it's a common enough occurrence to warrant a dedicated function, but I won't resurrect a debate that's been so recently discussed (when I opened the issue, I didn't search for "reduce" due to the aforementioned name change).

Thanks for the clarification!

[WaffleLapkin]

I've made a micro crate that provides the function, in case anyone needs it too: opt_reduce.