ARROW-6847: [C++] Add range_expression adapter to Iterator

[bkietz]

Allows Iterator<T> to be used in a range-for loop

[github-actions]

https://issues.apache.org/jira/browse/ARROW-6847

[pitrou]

Getting the Status on the side is a bit weird. Should we return a Result<> when iterating from the range instead?
@JohanMabille @wolfv What would be your thoughts on this?

[pitrou]

(also needs rebasing @bkietz )

[bkietz]

@pitrou returning a Result<> as the range value requires placing a status check inside the loop body:

for (Result<int> i_result : ints.AsRange()) {
  ARROW_ASSIGN_OR_RAISE(int i, i_result);
  // loop body
}

IMHO, this seems less natural to me than placing the check outside:

Status status;
for (int i : ints.AsRange(&status)) {
  // loop body
}
RETURN_NOT_OK(status);

But neither is a problem. @fsaintjacques ?

[bkietz]

@fsaintjacques @pitrou I've refactored so the range's elements are Result<>

[pitrou]

Perhaps you should add some tests for Status and Result (in)equality?

[codecov-io]

Codecov Report

Merging #5621 into master will increase coverage by <.01%.
The diff coverage is 94.91%.

@@            Coverage Diff             @@
##           master    #5621      +/-   ##
==========================================
+ Coverage   89.49%   89.49%   +<.01%     
==========================================
  Files         898      796     -102     
  Lines      125350   118804    -6546     
  Branches     1501        0    -1501     
==========================================
- Hits       112182   106325    -5857     
+ Misses      13157    12479     -678     
+ Partials       11        0      -11

Impacted Files	Coverage Δ
cpp/src/arrow/util/iterator.h	99.37% <100%> (+0.08%)	⬆️
cpp/src/arrow/result.h	90.38% <100%> (+0.18%)	⬆️
cpp/src/arrow/util/iterator_test.cc	98.66% <100%> (+0.18%)	⬆️
cpp/src/arrow/status.h	91.26% <70%> (-2.29%)	⬇️
python/pyarrow/plasma.py	58.9% <0%> (-1.37%)	⬇️
cpp/src/arrow/compute/kernels/hash_test.cc	100% <0%> (ø)	⬆️
js/src/util/fn.ts
js/src/builder/index.ts
js/src/enum.ts
js/src/Arrow.node.ts
... and 102 more

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[bkietz]

@pitrou I've added inequality and tests

@fsaintjacques The EqualityComparable helper ought to help with https://issues.apache.org/jira/browse/ARROW-6772

[JohanMabille]

Getting the Status on the side is a bit weird. Should we return a Result<> when iterating from the range instead?
@JohanMabille @wolfv What would be your thoughts on this?

Not sure to get why you need the iterator to fill a Status object. Anyway, the second loop written by @bkietz

for (int i : ints.AsRange(&status)) {
  // loop body
}

is indeed more natural in C++ than having to check anything in the loop body.

[bkietz]

@JohanMabille the Iterator may return an error status when attempting to retrieve the next sequence element. That error needs to be accessible by the consumer of the Iterator, hence the result/side status loops.

[fsaintjacques]

Very handy work, this will definitively improve the ergonomics. I've restarted the tests, failed due to external errors (github or dockerhub network).

[fsaintjacques]

Is this safe to override the destructor just for a static_assert?

[bkietz]

The default generated destructor is just an empty block like this. I think it's fine, but if you prefer I can replace with void CheckEqualsImpemented() or so