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

[Joe Groff]

> On Mar 16, 2016, at 9:59 AM, Erica Sadun <erica at ericasadun.com> wrote:
> 
>> 
>> On Mar 16, 2016, at 10:41 AM, Joe Groff <jgroff at apple.com <mailto:jgroff at apple.com>> wrote:
>> 
>>> 
>>> On Mar 16, 2016, at 8:24 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> On Mar 8, 2016, at 7:29 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> One minor change to what I've been proposing: Instead of merely saying that it's implementation-defined, we should expressly say that invoking next() after it has previously returned nil may return nil or it may return an implementation-defined value, but it should not fatalError() (unless some other GeneratorType requirement has been violated). Which is to say, after a GeneratorType has returned nil from next(), it should always be safe to invoke next() again, it's just up to the particular implementation to determine what value I get by doing that.
>>>> 
>>>> -Kevin Ballard
>>> 
>>> I'm torn about sequences that end with nil and should continue always return nil thereafter and 
>>> (pulling a name out of the air) "samples" that may return nil or non-nil values over time. I'd prefer there
>>> to be two distinct contracts between an iterator and another construct that may return an implementation-defined
>>> value after nil.
>> 
>> If your sequence produces optional values, then the result of its generator should be double-optional. If next() returns `.some(nil)`, that would be a nil value in the sequence; if it returns `nil`, that's the end.
>> 
>> -Joe
> 
> The use case I was thinking of was real-world sampling, where there was actually a value available or not. 
> Using double-optionals as a sequence would work for that. Since that approach might be intuitively
> obvious, maybe should be clarified through documentation?

The sequence itself would have a singly-optional element type—it's only next() that adds optionality on top of that. For most use cases, next() is just an implementation detail.

-Joe

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