Swift Array.indexOf containing nil values

Question

I'm working on an app and I need to know something, because I cannot continue without a solution to my problem. I have an array of the following type:

var samples : [HKSample?]!

Now I want to know if this array contains a specific element and what's the index of that element. When I'm trying to get the index like this

let anotherSample : HKSample = otherSamples.first
let index = samples.indexOf(anotherSample)

I get the following error:

"Cannot convert value of type 'HKSample?' to expected argument type '@noescpae (HKSample?) throws -> Bool'

Please help me!

Rob Napier · Accepted Answer · 2016-03-09 14:25:52Z

4

let anotherSample : HKSample = otherSamples.first

This is incorrect (and shouldn't compile). first will return HKSample? here.

Given that, you're searching for an HKSample? within an [HKSample?]. The problem is that Optional is not Equatable, so you have to use the predicate form of indexOf:

let index = samples.indexOf { $0 == anotherSample }

(where anotherSample is HKSample?, not HKSample.)

It is true that Optional has an == function available when its underlying type is Equatable, but it isn't itself Equatable because you can't currently conform to a protocol with a where clause. In order to make Optional conform, you'd have to be able to write:

extension Optional: Equatable where Wrapped: Equatable {}

But that's not currently supported in Swift. You'll get:

error: extension of type 'Optional' with constraints cannot have an inheritance clause

edited Mar 9, 2016 at 14:25

answered Mar 9, 2016 at 14:21

Rob Napier

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6 Comments

beeef Over a year ago

But when I need the type 'HKSample?' and .first returns 'HKSample?' why isn't my approach working?

Rob Napier Over a year ago

@MartinR Yeah; I was just writing that :D It suddenly occurred to me that it couldn't be ambiguous.

beeef Over a year ago

That sounds a bit complicated. I'm trying to find a workaround for that. :D But thank you very much!

Rob Napier Over a year ago

let index = samples.indexOf { $0 == anotherSample } is complicated? That's very normal Swift.

beeef Over a year ago

I mean the extension for Optional etc.

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