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

[Xiaodi Wu]

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> 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>
> 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...
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