Add RFC float-next-up-down. #3173

orlp · 2021-09-06T15:38:45Z

This RFC adds two argumentless methods to f32/f64, next_up and next_down. These functions are specified in the IEEE 754 standard, and provide the capability to enumerate floating point values in order.

Rendered.

There's also already an associated implementation pull request (rust-lang/rust#88728), should this RFC pass or be deemed unnecessary.

The quoted API was a re-export from Java, which was already accounted for.

kennytm · 2021-09-06T17:41:29Z

i think you could file a PR directly? https://github.com/rust-lang/rfcs/blob/master/libs_changes.md#is-an-rfc-required

orlp · 2021-09-06T18:04:32Z

@kennytm I believe it falls under the 'new API' section, which suggests making an RFC.

clarfonthey · 2021-09-06T23:45:25Z

Whether new APIs require RFCs generally depends on surface area. Regardless, you've certainly put in the time to justify the change.

leonardo-m · 2021-09-07T11:26:43Z

I'm in favour of these two functions, for me their usage is rare but I've used them once or twice in other languages. The point of this RFC is to link to it from those function docs, to give complete documentation.

(Regarding the names: as long as the documentation contains the common C function names, I think that next_float and prev_float are simpler to understand for common humans. Rust stdlib generally prefers more self-explanatory function names instead of the common and often more cryptic names. And I agree with this because remembering tens of strange function names isn't nice).

nagisa

Much like the others, I feel like API addition of this scope does not necessarily warrant a full RFC process. Since we do have a nicely written RFC, perhaps consider making a PR with an implementation in parallel to this RFC as well? I suspect that it would be merged and made available to nightly users significantly before the RFC process concludes.

nagisa · 2021-09-07T11:45:26Z

text/0000-float-next-up-down.md

+Two more functions get added to `f32` and `f64`, which may be considered
+already cluttered by some.
+
+Additionally, there is a minor pitfall regarding signed zero. Repeatedly calling


This behaviour follows from the definition of the function, doesn't it? The definition says:

Returns the largest/smallest number more/less than self.

Since 0.0 and -0.0 compare equal, 0.0.next_down() == -0.0 would violate the contract.

Yes, if we didn't want this we would need to change the definition of the function, and depart from IEEE 754 compliance. But I still felt I needed to mention this potential pitfall.

nagisa · 2021-09-07T11:49:16Z

text/0000-float-next-up-down.md

+then `x` is supposed to be returned. Besides error signaling and NaNs, that is
+the complete specification.
+
+We did not choose this function for three reasons:


next_up and next_down are also easier to implement more efficiently as they don't need to dispatch algorithm based on 2 inputs.

nagisa · 2021-09-07T11:51:09Z

text/0000-float-next-up-down.md

+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+ - Which is the better pair of names, `next_up` and `next_down` or `next_float`


_float is already implied by the type in Rust.

orlp · 2021-09-07T12:04:36Z

Much like the others, I feel like API addition of this scope does not necessarily warrant a full RFC process.

Part of the reason I chose to come directly in full force is because next_after was part of Rust, but got deprecated. I felt I really had to make my case in a thought-out manner.

Since we do have a nicely written RFC, perhaps consider making a PR with an implementation in parallel to this RFC as well?

I could do that, I also have written tests, not part of the RFC.

yaahc · 2021-10-14T23:51:57Z

i think you could file a PR directly? https://github.com/rust-lang/rfcs/blob/master/libs_changes.md#is-an-rfc-required

Wow, I have somehow never seen this doc and the last change to it was 6 years ago... I'm gonna go ahead and create a zulip thread about deduplicating that info...

But as everyone else said, this change certainly does not require an RFC. The most up to date documentation on when an RFC is necessary is documented here: https://github.com/rust-lang/governance/blob/master/teams/libs/subteam-api.md#making-changes-to-the-standard-libraries. Either way though the RFC is appreciated and certainly doesn't hurt.

yaahc · 2021-10-22T17:41:08Z

@rfcbot merge

Also, I want to note that I've already approved the PR adding the methods to std since they're unstable. I figure if we decide to not merge the RFC we can always remove them.

rfcbot · 2021-10-22T17:41:09Z

Team member @yaahc has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

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.

scottmcm · 2021-10-22T17:53:09Z

I like following IEEE for the names and semantics. I don't think we're likely to find a name that's better enough to be worth diverging from that.

I've done the hacky incorrect version of this multiple times (f32::from_bits(x.to_bits() + 1)), so would be happy to have the proper one.

Added next_up and next_down for f32/f64. This is a pull request implementing the features described at rust-lang/rfcs#3173.

rfcbot · 2021-11-10T23:13:06Z

🔔 This is now entering its final comment period, as per the review above. 🔔

rfcbot · 2021-11-20T23:14:16Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

yaahc · 2021-11-30T19:08:43Z

Thank you!

yaahc · 2021-11-30T19:32:27Z

Huzzah! The @rust-lang/libs-api team has decided to accept this RFC.

To track further discussion, subscribe to the tracking issue here: rust-lang/rust#91399

…ottmcm Added next_up and next_down for f32/f64. This is a pull request implementing the features described at rust-lang/rfcs#3173.

Add next_up and next_down for f32/f64 - take 2 This is a revival of rust-lang#88728 which staled due to inactivity of the original author. I've address the last review comment. --- This is a pull request implementing the features described at rust-lang/rfcs#3173. `@rustbot` label +T-libs-api -T-libs r? `@scottmcm` cc `@orlp`

Add next_up and next_down for f32/f64 - take 2 This is a revival of rust-lang/rust#88728 which staled due to inactivity of the original author. I've address the last review comment. --- This is a pull request implementing the features described at rust-lang/rfcs#3173. `@rustbot` label +T-libs-api -T-libs r? `@scottmcm` cc `@orlp`

orlp added 4 commits September 6, 2021 17:36

Add RFC float-next-up-down.

55850e0

Updated float-next-up-down RFC pull request number.

f787513

Minor typos/wording in float-next-up-down.

f3accee

Removed C# from the prior art in float-next-up-down.

1f4541b

The quoted API was a re-export from Java, which was already accounted for.

nagisa reviewed Sep 7, 2021

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This was referenced Sep 7, 2021

Added next_up and next_down for f32/f64. rust-lang/rust#88728

Closed

f32::MAX_EXP and f64::MAX_EXP are documented incorrectly, and other associated constant woes rust-lang/rust#88734

Open

ehuss added the T-libs-api Relevant to the library API team, which will review and decide on the RFC. label Oct 5, 2021

rfcbot added proposed-final-comment-period Currently awaiting signoff of all team members in order to enter the final comment period. disposition-merge This RFC is in PFCP or FCP with a disposition to merge it. labels Oct 22, 2021

rfcbot added final-comment-period Will be merged/postponed/closed in ~10 calendar days unless new substational objections are raised. and removed proposed-final-comment-period Currently awaiting signoff of all team members in order to enter the final comment period. labels Nov 10, 2021

rfcbot added finished-final-comment-period The final comment period is finished for this RFC. to-announce and removed final-comment-period Will be merged/postponed/closed in ~10 calendar days unless new substational objections are raised. labels Nov 20, 2021

Merge 3173-float-next-up-down RFC

8793a4d

yaahc merged commit faf7fb6 into rust-lang:master Nov 30, 2021

Urgau mentioned this pull request Aug 15, 2022

Add next_up and next_down for f32/f64 - take 2 rust-lang/rust#100578

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Add RFC float-next-up-down. #3173

Add RFC float-next-up-down. #3173

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Add RFC float-next-up-down. #3173

Add RFC float-next-up-down. #3173

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