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0771-std-iter-once.md

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Summary

Add a once function to std::iter to construct an iterator yielding a given value one time, and an empty function to construct an iterator yielding no values.

Motivation

This is a common task when working with iterators. Currently, this can be done in many ways, most of which are unergonomic, do not work for all types (e.g. requiring Copy/Clone), or both. once and empty are simple to implement, simple to use, and simple to understand.

Detailed design

once will return a new struct, std::iter::Once<T>, implementing Iterator. Internally, Once<T> is simply a newtype wrapper around std::option::IntoIter<T>. The actual body of once is thus trivial:

pub struct Once<T>(std::option::IntoIter<T>);

pub fn once<T>(x: T) -> Once<T> {
	Once(
		Some(x).into_iter()
	)
}

empty is similar:

pub struct Empty<T>(std::option::IntoIter<T>);

pub fn empty<T>(x: T) -> Empty<T> {
	Empty(
		None.into_iter()
	)
}

These wrapper structs exist to allow future backwards-compatible changes, and hide the implementation.

Drawbacks

Although a tiny amount of code, it still does come with a testing, maintainance, etc. cost.

It's already possible to do this via Some(x).into_iter(), std::iter::repeat(x).take(1) (for x: Clone), vec![x].into_iter(), various contraptions involving iterate...

The existence of the Once struct is not technically necessary.

Alternatives

There are already many, many alternatives to this- Option::into_iter(), iterate...

The Once struct could be not used, with std::option::IntoIter used instead.

Unresolved questions

Naturally, once is fairly bikesheddable. one_time? repeat_once?

Are versions of once that return &T/&mut T desirable?