Are all iterators lazy?

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1

From the documentation of BTreeSet:

Difference
A lazy iterator producing elements in the difference of BTreeSets.
ExtractIf
An iterator produced by calling extract_if on BTreeSet.
Intersection
A lazy iterator producing elements in the intersection of BTreeSets.
IntoIter
An owning iterator over the items of a BTreeSet in ascending order.
Iter
An iterator over the items of a BTreeSet.
Range
An iterator over a sub-range of items in a BTreeSet.
SymmetricDifference
A lazy iterator producing elements in the symmetric difference of BTreeSets.
Union
A lazy iterator producing elements in the union of BTreeSets.

Why “lazy” ? Isn’t it the case that all Rust iterators are “lazy”? Is there some subtle distinction I am missing here?

2

Yes. The doc comments you are referring to were originally created pre-1.0 Rust, maybe back then eager iterators were a thing, I don't know.

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3

I think it might just be there to clarify that the set operation itself is lazy.

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4

Yes, that does seem to be the pattern, maybe because it could be surprising I suppose.

5

Sometimes when an iterator is described as being lazy, it means that nothing is accessed in the underlying collection until you call for a value. I remember this distinction from Scala; IIRC their iterator for a Stream tried to preload the first value, which could in theory cause side effects or even an exception. I have no idea whether that distinction is relevant to BTreeSets.

6

Note that things that aren't iterators, but can kind of be treated like them sometimes are not covered by the blanket "yes".

This has tripped me a few times:

let option = Some(whatever);
option.map(f); // Not lazy
option.iter().map(f); // Lazy
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7

BTreeSet::difference does do some processing on construction of the iterator, including calling element comparison functions (which could panic), so I don’t think this what it means.

8

"Owning iterator" in that listing, on the other hand, is important - it means that on Drop, the iterator will be exhausted and all values destroyed.