[swift-evolution] [Proposal] Change guarantee for GeneratorType.next() to always return nil past end

[Dmitri Gribenko]

On Sat, Mar 5, 2016 at 5:40 PM, Brent Royal-Gordon via swift-evolution
<swift-evolution at swift.org> wrote:
>> In fact, I don't think the post-nil behavior of generators is even the biggest potential gotcha. A much bigger issue is how generators behave when you make copies of them. If you ever copy a generator you're supposed to never touch the original again, but it's easy to violate that accidentally, and most generators actually behave sanely when copied anyway.
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> That's true, but it's just not addressable until we have some kind of borrowing system in place.
>
>>> (Another reason to lean towards always returning `nil`: it's testable. In fact, I was thinking it might be nice to have something in XCTest where you could hand it a Sequence, or perhaps a closure that creates one, and an array containing the values you expect it to have, and it would assert the basic properties we expect from a Sequence, like that the elements come out in the right order, it returns nil at the end, `underestimateCount()` is not incorrect, etc. The `nil` semantic, unlike the other possible ones, is something you could include in that test.)
>>
>> Defining the post-nil behavior as implementation-defined means that, by definition, you don't need to test how arbitrary generators behave there.
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> True, but then you're imposing a constraint on the clients of generators: "Don't call next() again after it turns nil". I think it will be far easier to write generic tests of any generator than it will be to write generic tests of any function that *uses* a generic generator.
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> (Although I suppose you could assist in that testing by writing a wrapper around any sequence/collection/generator which enforces all rules as strictly as possible.)

See stdlib/private/StdlibCollectionUnittest/MinimalCollections.swift.gyb.

Dmitri

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