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

[Kevin Ballard]

On Wed, Mar 16, 2016, at 09:59 AM, Erica Sadun wrote:
>
>> On Mar 16, 2016, at 10:41 AM, Joe Groff <jgroff at apple.com> wrote:
>>
>>>
>>> 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
>
> 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?
 
Double-optionals makes this pattern useless. The whole point of using
the Generator pattern here is so you can easily process all of the currently-
available "samples" and then hit nil and stop, and then later when you
try again you may or may not get more values. Using a generator that
returns a double-optional value, the generator would never actually
return nil directly, it would always return a .Some (either .Some(nil)
or .Some(value)), and at this point there's no reason to be using a
Generator at all over just having a method that samples it.
 
-Kevin Ballard
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