Tracking Issue for `option_array_transpose`

[Urgau]

Feature gate: #![feature(option_array_transpose)]

This is a tracking issue for the option_array_transpose.

This feature permits going from an [Option<T>; N] to Option<[T; N]>.

Public API

// core::option

impl<T, const N: usize> [Option<T>; N] {
    fn transpose(self) -> Option<[T; N]>;
}

Steps / History

• Implementation: Add [Option<T>; N]::transpose #130829
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• The operation is irreversible (contrary to the other transpose methods), should it be named differently? Add [Option<T>; N]::transpose #130829 (comment)

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[tgross35]

Should there be something similar for the other way, Option<[T; N]> -> [Option<T>; N]?

[Nemo157]

This doesn't feel like a "transpose" operation to me, afaik all existing transpose methods are information-preserving, they only affect the type system, this one allows [None, Some(1)] -> None destroying the information that some elements were not None.

[Urgau]

@Nemo157 Look at [MaybeUninit<T>; N]::transpose for a precedent.

[Nemo157]

That also doesn't destroy any runtime data, you can array_of_maybe_uninit.transpose().transpose() and get back the same value.

[ada4a]

Regarding the name -- this seems to do a similar thing to Option::collect, but for arrays instead of iterators, so could it be named collect as well, or would that cause some kind of a name clash with the existing method? In that case, maybe collect_array could be another option?

[tgross35]

@ada4a did you mean Iterator::collect? Afaik there is no Option::collect.

That does seem like a good possible direction, this operation is similar to the Iterator method try_collect or maybe try_fold.

[GrigorenkoPV]

Hoogle tells me this thing is called sequence in Haskell.

But I would propose something like try_unwrap_all.

Originally posted by @GrigorenkoPV in #130829 (comment)

[ada4a]

@ada4a did you mean Iterator::collect? Afaik there is no Option::collect.

Sorry, yes, that's what I meant.

That does seem like a good possible direction, this operation is similar to the Iterator method try_collect or maybe try_fold.

Exactly!