1 Revision History

1.1 R0

• Initial revision.

2 Abstract

This paper proposes a fix that makes the views::zip() an infinite range views::repeat(tuple()) instead of the current views::empty<tuple<>>

3 Design Considerations

3.1 Rationale for the Current zip() Design

3.1.1 P2321R2

According to [P2321R2], the reason for the current behaviour of zip() was:

As in range-v3, zipping nothing produces an empty_view of the appropriate type.

Searching through the historical commits in range-v3, the behaviour of zip() was added in this commit [range-v3].

In this commit, both nullary cartesian_product() and zip() were added, both yield an empty range, and both are incorrect. And the rationale for this commit was

This allows us to remove the nasty MSVC special case for empty cartesian_products.

In conclusion, this behaviour did not seem to be a result of careful design, but just chosen to work around a “nasty MSVC special case”.

3.1.2 P2540R1

The discussion on the [reflector] indicated that cartesian_product() should be single(tuple()) and zip() should be repeat(tuple()).

Luckily, [P2540R1] fixed cartesian_product. So it no longer follows the “range-v3” behaviour but the correct one views::single(tuple()).

However, the paper intentionally left zip’s behaviour unchanged, after .

The main reason behind this decision was

In particular, zip has the property that it is the inner join of the indexed sets, and is the main diagonal of the Cartesian product. However, the identity element for zip is repeat(tuple<>), the infinite range of repeated empty tuples. If we allowed zip of an empty range of ranges to be its identity element, we would be introducing an inconsistency into the system, where two different formulations of notionally the same thing produces different answers.

However, the authors believe that the above conclusion is flawed.

TODO: explain the main diagonal and identity yield same results

3.2 TODO

• explain any_of/all_of’s identity
• explain cartesian_product
• explain the identify component of “minimum” operation should be infinity
• we prefer the result to be infinite range. if this cannot be agreed on, at the minimum, zip() should be ill-formed

3.3 Other Languages

3.3.1 Julia

Julia’s zip() produces an infinite range of empty tuples, which is in line with the authors’ view.

3.3.2 Haskell

Haskell’s ZipList’s pure implementation is [haskell_source]

pure x = ZipList (repeat x)

This is also what authors expected.

Haskell documentation [haskell_doc] clearly stated that

The only way to ensure zipWith ($) fs never removes elements is making fs infinite.

3.3.3 Rust

Rust’s compiler actively rejects multizip or izip! calls with no argument. This is much better than giving the wrong results.

3.3.4 Java, C

These languages either only supports a zip with two ranges, or provide zip3, zip4, … functions for more ranges, possibly due to lack of support for variadic arguments. However, they don’t provide a zip0, for a good reason.

3.3.5 Python

Python got it wrong too. zip() is an empty iterable. It is unclear what drives the design decision on this. The author has asked this question on StackOverflow’s Python community long time ago [stackoverflow], but there is no satisfying answer.

3.4 An Infinite Range is not a ranges::range

Whether an infinite range is a ranges::range is out of the scope of this paper. After all, this paper only delegates this special case of views::zip to views::repeat without inventing anything new. Whatever fix that is needed to fix ranges::range concept to work nicely with infinite ranges will fix all the existing views, i.e. views::iota(0), views::repeat(x) and so on in one go.

3.5 Conclusion

The authors propose to change the current incorrect results of zip() to repeat(tuple()). If the resistance of this change is too significant, the authors believe a compromise is to make zip() ill-formed.

4 Implementation Experience

5 Wording

Modify [range.zip.overview] section (2.1) as

• (2.1) auto(views​::​empty<tuple<>>) views::repeat(tuple()) if Es is an empty pack,

5.1 Feature Test Macro

Bump the FTM __cpp_lib_ranges_zip

6 References