Add iterator method .rfold(init, function); the reverse of fold#44682

bluss · 2017-09-18T20:02:50Z

rfold is the reverse version of fold.

Fold allows iterators to implement a different (non-resumable) internal
iteration when it is more efficient than the external iteration implemented
through the next method. (Common examples are VecDeque and .chain()).

Introduce rfold() so that the same customization is available for reverse
iteration. This is achieved by both adding the method, and by having the
Rev<I> adaptor connect Rev::rfold → I::fold and Rev::fold → I::rfold.

On the surface, rfold(..) is just .rev().fold(..), but the special case
implementations allow a data structure specific fold to be used through for
example .iter().rev(); we thus have gains even for users never calling exactly
rfold themselves.

Adaptors are things that take iterators and adapt them into other iterators. With this definition, fold is just a usual method, because it doesn't normally make an iterator.

rfold is the reverse version of fold. Fold allows iterators to implement a different (non-resumable) internal iteration when it is more efficient than the external iteration implemented through the next method. (Common examples are VecDeque and .chain()). Introduce rfold() so that the same customization is available for reverse iteration. This is achieved by both adding the method, and by having the Rev<I> adaptor connect Rev::rfold -> I::fold, Rev::fold -> I::rfold.

With both in place, we can cross them over in rev, and we give rfold behaviour to .rev().fold() and so on.

rust-highfive · 2017-09-18T20:03:01Z

r? @dtolnay

(rust_highfive has picked a reviewer for you, use r? to override)

aturon · 2017-09-18T23:00:04Z

cc @rust-lang/libs

cuviper · 2017-09-18T23:53:56Z

Note that I added FlatMap::fold in #44577 -- it probably deserves rfold too.

dtolnay · 2017-09-19T03:10:00Z

What determines whether this goes on the DoubleEndedIterator trait like rfind or on the Iterator trait like rposition?

fn rfold<B, F>(self, accum: B, f: F) -> B
where
    F: FnMut(B, Self::Item) -> B,
    Self: Sized + DoubleEndedIterator;

dtolnay · 2017-09-19T03:15:35Z

Ah, we already had that discussion in #39399 and #39480.

I would be on board with this addition.

alexcrichton · 2017-09-19T14:06:54Z

Seems like a fine addition to me!

sfackler · 2017-09-19T15:04:00Z

SGTM

dtolnay

@bluss r=me but please file a tracking issue and include the issue number in the #[unstable].

bluss · 2017-09-19T19:30:21Z

great, done.

@bors r=dtolnay

bors · 2017-09-19T19:30:22Z

📌 Commit 41a4226 has been approved by dtolnay

bors · 2017-09-21T18:04:23Z

⌛ Testing commit 41a4226 with merge 23a84ab...

Add iterator method .rfold(init, function); the reverse of fold rfold is the reverse version of fold. Fold allows iterators to implement a different (non-resumable) internal iteration when it is more efficient than the external iteration implemented through the next method. (Common examples are VecDeque and .chain()). Introduce rfold() so that the same customization is available for reverse iteration. This is achieved by both adding the method, and by having the Rev\<I> adaptor connect Rev::rfold → I::fold and Rev::fold → I::rfold. On the surface, rfold(..) is just .rev().fold(..), but the special case implementations allow a data structure specific fold to be used through for example .iter().rev(); we thus have gains even for users never calling exactly rfold themselves.

bors · 2017-09-21T20:12:18Z

💔 Test failed - status-travis

alexcrichton · 2017-09-21T21:04:43Z

@bors: retry

Looks like this was spuriously canceled? not sure why...

bors · 2017-09-21T23:44:17Z

⌛ Testing commit 41a4226 with merge 17600c1...

Add iterator method .rfold(init, function); the reverse of fold rfold is the reverse version of fold. Fold allows iterators to implement a different (non-resumable) internal iteration when it is more efficient than the external iteration implemented through the next method. (Common examples are VecDeque and .chain()). Introduce rfold() so that the same customization is available for reverse iteration. This is achieved by both adding the method, and by having the Rev\<I> adaptor connect Rev::rfold → I::fold and Rev::fold → I::rfold. On the surface, rfold(..) is just .rev().fold(..), but the special case implementations allow a data structure specific fold to be used through for example .iter().rev(); we thus have gains even for users never calling exactly rfold themselves.

bors · 2017-09-22T02:01:09Z

☀️ Test successful - status-appveyor, status-travis
Approved by: dtolnay
Pushing 17600c1 to master...

@bluss

Short-circuiting internal iteration with Iterator::try_fold & try_rfold These are the core methods in terms of which the other methods (`fold`, `all`, `any`, `find`, `position`, `nth`, ...) can be implemented, allowing Iterator implementors to get the full goodness of internal iteration by only overriding one method (per direction). Based off the `Try` trait, so works with both `Result` and `Option` (:tada: #42526). The `try_fold` rustdoc examples use `Option` and the `try_rfold` ones use `Result`. AKA continuing in the vein of PRs #44682 & #44856 for more of `Iterator`. New bench following the pattern from the latter of those: ``` test iter::bench_take_while_chain_ref_sum ... bench: 1,130,843 ns/iter (+/- 25,110) test iter::bench_take_while_chain_sum ... bench: 362,530 ns/iter (+/- 391) ``` I also ran the benches without the `fold` & `rfold` overrides to test their new default impls, with basically no change. I left them there, though, to take advantage of existing overrides and because `AlwaysOk` has some sub-optimality due to #43278 (which 45225 should fix). If you're wondering why there are three type parameters, see issue #45462 Thanks for @bluss for the [original IRLO thread](https://internals.rust-lang.org/t/pre-rfc-fold-ok-is-composable-internal-iteration/4434) and the rfold PR and to @cuviper for adding so many folds, [encouraging me](#45379 (comment)) to make this PR, and finding a catastrophic bug in a pre-review.

bluss added 5 commits September 18, 2017 21:45

core: Small fix in fold docs

ffd171e

Adaptors are things that take iterators and adapt them into other iterators. With this definition, fold is just a usual method, because it doesn't normally make an iterator.

core: Implement fold / rfold for Rev

31cf26a

With both in place, we can cross them over in rev, and we give rfold behaviour to .rev().fold() and so on.

core: Implement rfold for Map, Cloned, Chain

a59a25d

alloc: Implement rfold for VecDeque iterators

84c90f3

rust-highfive assigned dtolnay Sep 18, 2017

bluss changed the title ~~Add iterator method .rfold(accumulator, function); the reverse of fold~~ Add iterator method .rfold(init, function); the reverse of fold Sep 18, 2017

core: Add feature gate to rfold example code

7e81cee

dtolnay approved these changes Sep 19, 2017

View reviewed changes

bluss mentioned this pull request Sep 19, 2017

Tracking issue for DoubleEndedIterator::rfold (feature iter_rfold) #44705

Closed

core: Assign tracking issue for iter_rfold

41a4226

arielb1 added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Sep 19, 2017

bors merged commit 41a4226 into rust-lang:master Sep 22, 2017

scottmcm mentioned this pull request Oct 28, 2017

Short-circuiting internal iteration with Iterator::try_fold & try_rfold #45595

Merged

scottmcm mentioned this pull request Oct 29, 2017

impl FromIterator<()> for () #45379

Merged

bluss deleted the iter-rfold branch November 10, 2017 19:02

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bluss commented Sep 18, 2017 •

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