[swift-evolution] Passing an optional first argument to sequence(first:next:)

[Tim Vermeulen]

> On 16 Aug 2016, at 02:11, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> On Mon, Aug 15, 2016 at 7:03 PM, Tim Vermeulen <tvermeulen at me.com <mailto:tvermeulen at me.com>> wrote:
> If I'm not mistaken, the type of T can be inferred from the `next` closure here. $0 is an optional, so you end up with a sequence of optionals.
> 
> Currently, yes. But if we have both 
> 
> public func sequence<T>(first: T, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T> { ... }
> public func sequence<T>(first: T?, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T> { ... }
> 
> then $0 could be inferred to be of type `T` or `T?`, right?

I was indeed talking about replacing the first function by the second, not having them side by side.

> 
> And if we replace the first function with the second (instead of having both), then the same code that gives you an UnfoldFirstSequence<T?> today would give you an UnfoldFirstSequence<T> tomorrow, which would be the most pernicious way to break code (subtle, hard to catch unless you know what you're looking for, with a potentially massive difference in behavior).

I’m not sure I follow. First of all, I think an unfold sequence with an optional element is pretty far-fetched, but fair enough, they may exist. But why would the type suddenly change? Type inference would still figure out the right type, no? An example would be greatly appreciated, because I don’t see how this could form a problem.

> 
> 
> On 16 Aug 2016, at 00:51, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
> 
>> Given:
>> 
>> let foo: T? = something()
>> let bar = sequence(first: foo, next: { $0?.frobnicate() })
>> 
>> If first could be of type `T` or `T?`, is bar of type `UnfoldSequence<T>` or `UnfoldSequence<T?>`?
>> On Mon, Aug 15, 2016 at 17:15 Tim Vermeulen via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> I doubt that’s it, in no way is an an empty sequence inherently unsafe. The entire standard library is built with empty sequences in mind. I’m more inclined to think it’s an oversight.
>> 
>>> On 15 Aug 2016, at 23:15, Maximilian Hünenberger <m.huenenberger at me.com <mailto:m.huenenberger at me.com>> wrote:
>>> 
>>> Ok, I see. However this could be a potential source of bugs/performance issues where you don't consider the nil case and you do some unnecessary work. By prohibiting to pass nil you have to manually unwrap and you immediately see the "optionality".
>>> 
>>> Am 15.08.2016 um 22:36 schrieb Tim Vermeulen <tvermeulen at me.com <mailto:tvermeulen at me.com>>:
>>> 
>>>> Oh, that’s true, I misunderstood your previous message. It’s not about passing nil, but it’s about passing optionals. The point is to be able to do something like this:
>>>> 
>>>> let number = functionThatReturnsAnOptional()
>>>> sequence(first: number, next: { $0 % 2 == 0 ? $0 / 2 : nil })
>>>> 
>>>>> On 15 Aug 2016, at 22:26, Maximilian Hünenberger <m.huenenberger at me.com <mailto:m.huenenberger at me.com>> wrote:
>>>>> 
>>>>> Probably I didn't understand your proposal. What do you want to change exactly?
>>>>> 
>>>>> I thought:
>>>>> public func sequence<T>(first: T, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T> { ... }
>>>>> 
>>>>> To:
>>>>> public func sequence<T>(first: T?, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T> { ... }
>>>>> 
>>>>>> Am 15.08.2016 um 22:17 schrieb Tim Vermeulen <tvermeulen at me.com <mailto:tvermeulen at me.com>>:
>>>>>> 
>>>>>> You can’t; the `first` parameter has type `T`, not `T?`.
>>>>>> 
>>>>>>> On 15 Aug 2016, at 22:10, Maximilian Hünenberger <m.huenenberger at me.com <mailto:m.huenenberger at me.com>> wrote:
>>>>>>> 
>>>>>>> Hi Tim,
>>>>>>> 
>>>>>>> If you pass "nil" to "first" isn't this an empty sequence? So it would be redundant.
>>>>>>> 
>>>>>>> Best regards
>>>>>>> Maximilian
>>>>>>> 
>>>>>>>> Am 15.08.2016 um 01:27 schrieb Tim Vermeulen via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>>>>>>> 
>>>>>>>> sequence(first:next:) takes a non-optional first argument. Is there a reason for that? sequence(state:next:) allows empty sequences, and I don’t see why sequence(first:next:) shouldn’t. The fix would be to simply add the `?` in the function signature; no other changes are required to make it work.
>>>>>>>> 
>>>>>>>> I considered just filing a bug report, but since this is a change of the public API...
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>>>>>> 
>>>> 
>> 
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