Added core::cmp::Reverse for sort_by_key reverse sorting by mitsuhiko · Pull Request #40720 · rust-lang/rust

mitsuhiko · 2017-03-21T23:13:14Z

I'm not sure if this is the best way to go about proposing this feature but it's pretty useful. It allows you to use sort_by_key and return tuples where a single item is then reversed to how it normally sorts.

I quite miss something like this in Rust currently though I'm not sure if this is the best way to implement it.

rust-highfive · 2017-03-21T23:13:26Z

r? @sfackler

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sfackler · 2017-03-22T07:39:03Z

This seems like a nice ergonomic win to me. Thoughts @rust-lang/libs?

ranma42 · 2017-03-22T08:20:23Z

It would be possible to relax the constraints a bit, so that T is allowed to only implement partial orderings, like this.

nagisa · 2017-03-22T11:49:26Z

Stability attributes should be fixed.

malbarbo · 2017-03-22T13:12:51Z

How about a free function cmp::reverse? Reverse seems to generic...

mitsuhiko · 2017-03-22T13:13:46Z

@malbarbo what would the function do? Create a "Reverse" object?

malbarbo · 2017-03-22T13:17:31Z

@mitsuhiko Yes. This is similar to free functions in iter module.

mitsuhiko · 2017-03-22T13:20:39Z

I was going with std::num::Wrapping in mind which is also just a wrapper / newtype over something. Feels wasteful to add a function that does nothing other than making an enum which needs to be public anyways.

mitsuhiko · 2017-03-22T13:45:28Z

Alternative name proposals:

std::cmp::Rev
std::cmp::Invert
std::cmp::Inv

alexcrichton · 2017-03-27T22:15:33Z

Seems like a nice addition to me!

alexcrichton · 2017-03-27T22:15:50Z

I personally like Reverse but would be ok with Invert as well.

scottmcm · 2017-03-27T23:16:05Z

Good to see it handling PartialOrd as well now.

I don't like Invert, since that sounds like 1/x. I'm not a fan of just Reverse either, since v.reverse() and Reverse(v) would both compile but do very different things.

Maybe ReverseOrder? This doesn't feel like something that needs a particularly short name.

Or maybe something involving Descending? v.sort_by_key(|k| Descending(k)) reads decently...

aturon · 2017-03-28T20:28:49Z

+/// assert_eq!(v, vec![3, 2, 1, 6, 5, 4]);
+/// ```
+#[derive(PartialEq, Eq, Debug)]
+#[stable(feature = "rust1", since = "1.18.0")]


This will need to start out unstable.

mitsuhiko · 2017-03-28T20:29:54Z

In my own project I called it RevKey but I found that confusing. I do like the idea of Descending which is what some SQL frameworks do. For instance in Python's SQLAlchemy column sorts upwards and column.desc() sorts backwards.

aturon · 2017-03-28T22:25:58Z

We discussed this PR in the libs triage meeting today, and everyone was in favor of the addition. We're also fine with both the name and API. (It's a slight shift from other APIs in std, in that it exposes a constructor for a tuple struct directly, but that is where idioms have headed in general).

Other than changing the annotations to unstable, I think this is good to land. r=me after that change.

aturon · 2017-03-28T23:21:11Z

@mitsuhiko Ah sorry, to clarify the unstable attribute will need to point to a fresh "tracking issue" like #40494 (make sure to tag B-unstable). You can go ahead and create that.

mitsuhiko · 2017-03-29T07:07:23Z

@aturon i created the issue but lack the rights to flag it appropriately. So it's untagged for now.

BurntSushi · 2017-03-29T10:54:29Z

@bors r+

bors · 2017-03-29T10:54:30Z

📌 Commit 5d36953 has been approved by BurntSushi

BurntSushi · 2017-03-29T10:54:36Z

Thanks @mitsuhiko! :-)

mitsuhiko · 2017-03-29T10:55:25Z

Wohoo \o/ Thanks :)

…ushi Added core::cmp::Reverse for sort_by_key reverse sorting I'm not sure if this is the best way to go about proposing this feature but it's pretty useful. It allows you to use `sort_by_key` and return tuples where a single item is then reversed to how it normally sorts. I quite miss something like this in Rust currently though I'm not sure if this is the best way to implement it.

Rollup of 5 pull requests - Successful merges: #40720, #40786, #40841, #40866, #40897 - Failed merges:

Implement all PartialOrd methods for Reverse When making a forwarding wrapper we must in general forward all methods, so that we use the type's own `lt` for example instead of the default. Example important case: f32's partial_cmp does several operations but its lt is a primitive. Follow up on rust-lang#40720

rkjnsn · 2017-06-02T22:45:37Z

+/// v.sort_by_key(|&num| (num > 3, Reverse(num)));
+/// assert_eq!(v, vec![3, 2, 1, 6, 5, 4]);
+/// ```
+#[derive(PartialEq, Eq, Debug)]


Why do we derive PartialEq and Eq here, when we are specifying custom implementations below?

There is no custom implementation for PartialEq, only for Ord and PartialOrd.

Right, of course. I don't know where my brain was. Sorry about that.

Added core::cmp::Reverse for sort_by_key reverse sorting
d7d4e67

rust-highfive assigned sfackler Mar 21, 2017

sfackler added the T-libs-api [DEPRECATED; DO NOT USE] label Mar 22, 2017

Fix the test for cmp::Reverse
dabff15

Improved bounds for cmp::Reverse
d005d91

aturon reviewed Mar 28, 2017
View reviewed changes

Changed cmp::Reverse to unstable
7efbb69

mitsuhiko mentioned this pull request Mar 29, 2017
Tracking issue for the cmp::Reverse type #40893
Closed

Updated tracking issue for cmp::Reverse
5d36953

frewsxcv mentioned this pull request Mar 29, 2017
Rollup of 5 pull requests #40899
Merged

bors added a commit that referenced this pull request Mar 29, 2017
Auto merge of#40899- frewsxcv:rollup, r=frewsxcv
e1cec5d
Rollup of 5 pull requests - Successful merges: #40720, #40786, #40841, #40866, #40897 - Failed merges:

bors merged commit 5d36953 into rust-lang:masterMar 29, 2017

bluss mentioned this pull request Mar 30, 2017
Implement all PartialOrd methods for Reverse #40929
Merged

mitsuhiko deleted the feature/rev-key branch April 4, 2017 21:17

malbarbo mentioned this pull request May 24, 2017
Add a min heap option to PriorityQueue #15947
Closed

rkjnsn reviewed Jun 2, 2017
View reviewed changes

dtolnay mentioned this pull request Nov 8, 2017
RFC: Add Option::filter to the standard library rust-lang/rfcs#2124
Merged

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