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

[Joe Groff]

> On Mar 16, 2016, at 8:24 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
> 
> On Mar 8, 2016, at 7:29 PM, Kevin Ballard via swift-evolution <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