impl FromParallelIterator for tuple pairs

[cuviper]

Like #604 for ParallelExtend, implementing FromParallelIterator for tuple pairs opens up new possibilities for nesting collect. The possibility of short-circuiting into Result<(T, U), E> for #801 is particularly motivating.

• FromParallelIterator<(A, B)> for (FromA, FromB) works like unzip
• FromParallelIterator<Either<L, R>> for (A, B) works like partition_map

[nikomatsakis]

@cuviper

The possibility of short-circuiting into Result<(T, U), E> for #801 is particularly motivating.

Can you explain this a bit more? Is that potential future work or did I miss something.

[cuviper]

It's a composition. Result<X, E> implements a short-circuiting FromParallelIterator for any X: FromParallelIterator. With this PR, that X can be (T, U), so we now have a short-circuiting unzip/partition_map via collect::<Result<(T, U), E>>().

[nikomatsakis]

I see.

[cuviper]

I finally got around to adding doc-examples for this.

[nikomatsakis]

I think this will not get accepted in the stdlib, right?

[cuviper]

I think this will not get accepted in the stdlib, right?

I expect not, but I haven't even proposed it yet. I think given how IntoIterator rust-lang/rust#78204 went, it's unlikely that FromIterator would be accepted. But I also think it's unlikely that FromIterator for tuples would ever be added with different semantics, so I'm not worried about us being incompatible.

And I do think it's still justified here, because it's otherwise really hard to collect multiple things in parallel. With serial Iterator, it's trivial to just write a manual for loop that unpacks items and stuffs them into however many collections you want. With ParallelIterator, collection is much more complicated to do manually, probably best as fold+reduce, or try_fold+try_reduce if you want short-circuiting. Yet this PR is a relatively small addition that enables a lot of power easily.

[nikomatsakis]

That makes sense.

[nikomatsakis]

I'd be r+ on landing this.

[cuviper]

bors r=nikomatsakis

[bors]

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