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

[Maximilian Hünenberger]

It's true, empty sequences aren't inherently unsafe but an oversight of the nil case can lead to unexpected empty sequences which then lead to bugs. Eg.:

let number = Int(string)
for i in sequence(first: number, next: { $0 * 2 - 1 }).prefix(10) {
    print(i)
}

If the string cannot be converted to an Int the for loop doesn't run at all. However the original intention was to provide a default and the loop should run in either case:

let number = Int(string) ?? 2

This could be a scenario where "first" should be of type "T". And checking for nil isn't that hard if you want to avoid to run the loop in the nil case.

> Am 15.08.2016 um 23:33 schrieb Tim Vermeulen <tvermeulen at me.com>:
> 
> 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> 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>:
>>> 
>>> 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> 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>:
>>>>> 
>>>>> 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> 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>:
>>>>>>> 
>>>>>>> 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...
>>>>>>> _______________________________________________
>>>>>>> swift-evolution mailing list
>>>>>>> swift-evolution at swift.org
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
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