Product and MultiProduct should implement ExactSizeIterator if all the constituent iterators implement it

[sunshowers]

Thanks for itertools! It's super useful :)

I do have one feature request: I love the Cartesian product iterators because they were just what I wanted for a use case, but I noticed that they're missing ExactSizeIterator implementations. I think the trait can be implemented if all the constituent iterators implement the trait.

I'd expect the result to be the product of the lengths of all the constituent iterators.

I'd be happy to submit a PR for this some time in the next couple of weeks if no one else gets to it. Thanks!

[scottmcm]

Because of the potential for overflow, it's not actually correct to implement ExactSizeIterator for any adapter that makes an iterator longer -- you'll notice that Chain doesn't implement it either.

[sunshowers]

Ah, good point. Could the TrustedLen contract be followed (or is it already followed)? https://doc.rust-lang.org/stable/std/iter/trait.TrustedLen.html

[SkiFire13]

That trait is unstable, so even if the contract could be followed, the trait itself can't be implemented

[sunshowers]

(yeah that's what I meant, just following the contract and documenting that it is followed)

[jswrenn]

Product already overrides size_hint, and so does MultiProduct. Is this sufficient?

[sunshowers]

It does, yes! So it looks like the only thing to do is to verify that the impla follow the TrustedLen contract, update them if they don't, and document that they do.

Rain (they/she)

On Sun, Dec 27, 2020, 14:24 Jack Wrenn [email protected] wrote:

Product already overrides size_hint https://docs.rs/itertools/0.10.0/src/itertools/adaptors/mod.rs.html#330-339, and so does MultiProduct https://docs.rs/itertools/0.10.0/src/itertools/adaptors/multi_product.rs.html#186-206 .

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