Add Iterator::map_while
#66577
Add Iterator::map_while
#66577
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Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @sfackler (or someone else) soon. If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes. Please see the contribution instructions for more information. |
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The job Click to expand the log.I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact |
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That method name is a lot, but it does seem potentially useful? @rfcbot fcp merge |
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Team member @sfackler has proposed to merge this. The next step is review by the rest of the tagged team members: Concerns: Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up! See this document for info about what commands tagged team members can give me. |
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For the record, this was previously proposed in #65864. It was not merged because I (randomly assigned reviewer) and the two T-libs members who voiced an opinion after I pinged the team were lukewarm on adding it to std instead of itertools. Take from that what you will. |
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If we want to add this I think |
src/libcore/iter/adapters/mod.rs
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| #[unstable(feature = "iter_take_while_map", reason = "recently added", issue = "0")] |
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Stability attributes currently don't work with trait impls
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Why then there is a #[stable(feature = "rust1", since = "1.0.0")] on Iterator impl for TakeWhile?
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It's unstability attributes that don't work. ;) There is no such thing as an unstable trait impl.
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I don't understand :( You said that there is no unstable trait impl, but
- without
#[unsatble]this code doesn't compile:
- there are other
#[unstable]impls, e.g.:impl TrustedLen for Rev - there are
#[stable]impls too, e.g.:impl Iterator for Rev
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Ping from triage: |
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@JohnCSimon this is still pending FCP waiting on @Kimundi, @KodrAus, @SimonSapin, @dtolnay, and @withoutboats. |
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@rfcbot concern name My view: I don't have strong feelings either way about adding this API, if someone else on the libs team wants to drive it (seems like sfackler does) that's fine with me, but if we do add it this API I want it to be named |
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+1 for calling this map_while. @rfcbot reviewed |
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☔ The latest upstream changes (presumably #67485) made this pull request unmergeable. Please resolve the merge conflicts. |
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☔ The latest upstream changes (presumably #67540) made this pull request unmergeable. Please resolve the merge conflicts. |
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Heh, I'm not sure why my PR didn't gain the same traction this one does, but I'm glad it seems to be going somewhere. I actually almost submitted it as |
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☔ The latest upstream changes (presumably #67596) made this pull request unmergeable. Please resolve the merge conflicts. |
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| #[derive(Clone)] | ||
| pub struct MapWhile<I, P> { | ||
| iter: I, | ||
| finished: bool, |
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Is the finished field needed? It looks like it's only used to make MapWhile fused but that seems unnecessary when you can instead just call .fuse() if needed.
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The whole point of the MapWhile is that it stops on first None just like TakeWhile stops on first false
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It stops by having .next() return None. The finished field isn't needed to do that. After then it doesn't matter what .next() returns unless the iterator implements FusedIterator which MapWhile doesn't.
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I first thought it was to improve the upper bound of size_hint, but it also doesn't matter what that method returns once the iterator has been exhausted. So I suppose you're right. We probably can't remove it from TakeWhile though, since it already implements FusedIterator.
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It can be a bit confusing, so if we do remove finished flag, then I think we need to mention it in the docs
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☔ The latest upstream changes (presumably #68234) made this pull request unmergeable. Please resolve the merge conflicts. |
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Rebased to resolve conflicts with master |
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@Mark-Simulacrum Can you merge this pr? I've added the tracking issue to the |
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@bors r=mark-simulcrum |
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📌 Commit db1a107 has been approved by |
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…=mark-simulcrum
Add `Iterator::map_while`
In `Iterator` trait there is `*_map` version of [`filter`] — [`filter_map`], however, there is no `*_map` version of [`take_while`], that can also be useful.
### Use cases
In my code, I've found that I need to iterate through iterator of `Option`s, stopping on the first `None`. So I've written code like this:
```rust
let arr = [Some(4), Some(10), None, Some(3)];
let mut iter = arr.iter()
.take_while(|x| x.is_some())
.map(|x| x.unwrap());
assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), Some(10));
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), None);
```
Thit code
1) isn't clean
2) In theory, can generate bad bytecode (I'm actually **not** sure, but I think that `unwrap` would generate additional branches with `panic!`)
The same code, but with `map_while` (in the original PR message it was named "take_while_map"):
```rust
let arr = [Some(4), Some(10), None, Some(3)];
let mut iter = arr.iter().map_while(std::convert::identity);
```
Also, `map_while` can be useful when converting something (as in [examples]).
[`filter`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter
[`filter_map`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter_map
[`take_while`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.take_while
[examples]: https://github.com/rust-lang/rust/compare/master...WaffleLapkin:iter_take_while_map?expand=1#diff-7e57917f962fe6ffdfba51e4955ad6acR1042
…crum
Add `Iterator::map_while`
In `Iterator` trait there is `*_map` version of [`filter`] — [`filter_map`], however, there is no `*_map` version of [`take_while`], that can also be useful.
### Use cases
In my code, I've found that I need to iterate through iterator of `Option`s, stopping on the first `None`. So I've written code like this:
```rust
let arr = [Some(4), Some(10), None, Some(3)];
let mut iter = arr.iter()
.take_while(|x| x.is_some())
.map(|x| x.unwrap());
assert_eq!(iter.next(), Some(4));
assert_eq!(iter.next(), Some(10));
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), None);
```
Thit code
1) isn't clean
2) In theory, can generate bad bytecode (I'm actually **not** sure, but I think that `unwrap` would generate additional branches with `panic!`)
The same code, but with `map_while` (in the original PR message it was named "take_while_map"):
```rust
let arr = [Some(4), Some(10), None, Some(3)];
let mut iter = arr.iter().map_while(std::convert::identity);
```
Also, `map_while` can be useful when converting something (as in [examples]).
[`filter`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter
[`filter_map`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter_map
[`take_while`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.take_while
[examples]: https://github.com/rust-lang/rust/compare/master...WaffleLapkin:iter_take_while_map?expand=1#diff-7e57917f962fe6ffdfba51e4955ad6acR1042
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☀️ Test successful - checks-azure |
rfcbot
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…_while, r=LukasKalbertodt Remove `finished` flag from `MapWhile` This PR removes `finished` flag from `MapWhile` as been proposed in rust-lang#66577 (comment). This also resolves open questions of the tracking issue (rust-lang#68537): - `MapWhile` can't implement both + `DoubleEndedIterator` (discussed in rust-lang#66577 (comment) and following comments) + `FusedIterator` (this pr removes `finished` flag, so `MapWhile` isn't fused anymore) - Debug output (this pr removes `finished` flag, so there is no question in including it in debug output) r? @Mark-Simulacrum
In
Iteratortrait there is*_mapversion offilter—filter_map, however, there is no*_mapversion oftake_while, that can also be useful.Use cases
In my code, I've found that I need to iterate through iterator of
Options, stopping on the firstNone. So I've written code like this:Thit code
unwrapwould generate additional branches withpanic!)The same code, but with
map_while(in the original PR message it was named "take_while_map"):Also,
map_whilecan be useful when converting something (as in examples).