Add then and then_with for ordering.

[rednum]

Fixes#37053 (see discussion in rust-lang/rfcs#1677).

[rust-highfive]

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[apasel422]

s/Chain/Chains/

[apasel422]

Chains the ordering with the given function.

[rednum]

Fixed doctests, can you please take another look? Should I rename this as discussed in the rust-lang/rfcs#1677 ?

[alexcrichton]

@rfcbot fcp merge

Seems like a nifty API to me!

[rfcbot]

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

• @BurntSushi
• @Kimundi
• @alexcrichton
• @aturon
• @brson
• @sfackler

Concerns:

• Naming resolved by Add then and then_with for ordering. #37054 (comment)

Once these reviewers reach consensus, 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.

[aturon]

@rfcbot concern Naming

This looks like handy functionality, but I think we should consider the naming a bit. I imagine the proposed name is based on analogy with methods on e.g. Option? What this is implementing is something like "lexicographic ordering" (aka "dictionary ordering"), but I'm not sure whether those terms would be helpful/ergonomic to use.

My basic fear is that, when reading code, or does not immediately suggest the actual semantics here.

Unfortunately, nothing better is leaping to mind. Anybody else share this concern, or have alternative ideas?

[alexcrichton]

Hm yeah that is true. In retrospect if I initially saw something like Ordering::Less.or(Ordering::Greater) I wouldn't actually know what that would do!

Other thoughts brought up in rust-lang/rfcs#1677 were and/and_then, chain/chain_then.

I sort of like the name chain but agree that the closure-taking variant would sound odd.

[rednum]

I've used the name or as analogy to Option, but after giving it a second thought I agree it's not the obvious naming. In Java 8 similar feature is called thenComparing. So I think another possibilty would be to call it then (as also suggested in original RFC) and the functional counterpart then_fn (I think and_then would be a bit confusing, because it already does something else for Option).

[aturon]

@rednumthen indeed sounds better! I'd suggest then_with for the closure variant.

[aturon]

@rfcbot resolved Naming

[bors]

☔ The latest upstream changes (presumably #37299) made this pull request unmergeable. Please resolve the merge conflicts.

[brson]

I think the names without precedent are unfortunate, but can't think of anything better.

[rfcbot]

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

psst @alexcrichton, I wasn't able to add the final-comment-period label, please do so.

[alexcrichton]

@bors: r+

[bors]

📌 Commit 655effe has been approved by alexcrichton

[bors]

⌛ Testing commit 655effe with merge acfe959...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: alexcrichton
Pushing acfe959 to master...