# \[Pitch\] Adding safety to arrays

**URL:** <https://forums.swift.org/t/pitch-adding-safety-to-arrays/5876>\
**Category:** Pitches\
**Created:** [May 7, 2017, 3:41pm UTC](https://forums.swift.org/t/pitch-adding-safety-to-arrays/5876 "2017-05-07T15:41:30Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![MutatingFunk](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/mutatingfunk/32/984_2.png) [@MutatingFunk](https://forums.swift.org/u/MutatingFunk)\
**Post date:** [May 7, 2017, 3:41pm UTC](https://forums.swift.org/t/pitch-adding-safety-to-arrays/5876/1 "2017-05-07T15:41:30Z")

</div>

Array's (Collection's?) `.first` property already returns element 0 as an optional, so your first use case is already covered.

For the second point, I'll back this up with a less hacky example: accessing the detail viewcontroller, if present, from a SplitViewController loaded from a storyboard.

Your string parsing example, and perhaps parsing in general, seem like the strongest use-case for this feature.

I would also like to see an optional subscript or method added to Array, but since this is additive I'm not expecting it any time soon.

------------ Begin Message ------------  
Group: gmane.comp.lang.swift.evolution  
MsgID: \<CAHO0oD9Odg4WPe2jOTqJz\_kW=w3V0+H=9Nb0UJQT2NkOurZGXQ@mail.gmail.com\>

To respond to one of the main points Ive seen, which is that this isnt  
necessary for arrays because the index youre using is often tied to data  
related to the array, such as in the example I gave originally, where in  
`cellForRowAtIndexPath`, the index path is probably informed by the length  
of the array.

Thats true for data-backed code, but on iOS arrays are used in other  
scenarios. I wanted to provide 3 additional examples where optional arrays  
would be helpful that Ive run across recently:

1. When determining the video dimensions, frame rate or other property of  
an AVAsset which you know is a video, you need to first use  
assetTracks(withMediaType: String), which returns an array. While Im  
fairly sure that these video assets will all contain a video track, its  
not a certainty, so to be safe I need to check that one exists. Currently  
my code looks like this:

let videoTracks = asset.tracks(withMediaType: AVMediaTypeVideo)  
if videoTracks.count \> 0 {  
&nbsp;&nbsp;&nbsp;&nbsp;let videoTrack = videoTracks[0]  
&nbsp;&nbsp;&nbsp;&nbsp;print("dimensions: \(videoTrack.naturalSize)")  
}

With optional arrays, this could be shortened to this:

if let videoTrack = asset.tracks(withMediaType: AVMediaTypeVideo)[0] {  
&nbsp;&nbsp;&nbsp;&nbsp;print("dimensions: \(videoTrack.naturalSize)")  
}

2. UIVisualEffect can display a blurred view on top of underlying views. If  
you wish to add a tint color, it's simply added on top of the blur view's  
own background color (by default, 0.97 white with 0.8 alpha) which is  
provided by a subview. Your tint color being placed on top of the blur  
views background color therefore degrades how that color affects the  
content underneath, and also the overall opacity of the content underneath.  
You might also not want the overlay to be as strong as 0.8. To get around  
this, I might customise the UIVisualEffectView to display my own base color  
displayed over the blur. I know that the background color is set in the 2nd  
subview of visualEffectView, so Ill modify it like so:

if visualEffectView.subviews.count \>= 2 {  
&nbsp;&nbsp;&nbsp;&nbsp;let tintedView = self.visualEffectView.subviews[1]  
&nbsp;&nbsp;&nbsp;&nbsp;tintedView.backgroundColor = UIColor.red.withAlphaComponent(0.5)  
}

The checking for the subviews count is important here - while its possible  
that iOS may change the implementation of UIVisualEffectViews in future,  
and break my UI, I would much prefer that over the app crashing.

With optional arrays, this UI code could be shortened to this:

visualEffectView.subviews[1]?.backgroundColor UIColor.red.withAlphaComponent(0.5)

Obviously this kind of manipulation is hacky and isnt encouraged, but its  
an example of some of the more finicky UI work that in reality is often  
required on iOS in order to achieve certain goals.

3. I also think optional arrays can be used for clarity of purpose, in the  
same way that dictionary keys are. In a different scenario, such as when  
attempting to extract data from a string, this is how code might currently  
look:

let components = coordinatesString.components(separatedBy: ",")

if components.count \< 2 {  
&nbsp;&nbsp;&nbsp;&nbsp;return nil  
}

let latitudeString = components[0]  
let longitudeString = components[1]

Notice that in advance youre saying ok, so I know therell be 2  
components I require later on, so Ill check to make sure I have at least 2  
components, and then after that Ill explain what those are for. Instead,  
it could look like this:

let components = coordinatesString.components(separatedBy: ",)  
let latitudeString = components[0]  
let longitudeString = components[1]

if latitudeString == nil || longitudeString == nil {  
&nbsp;&nbsp;&nbsp;&nbsp;return nil  
}

The intent is much clearer here. You want to extract latitude and  
longitude, but if either of those dont exist then return nil.

> **···**
>
> On 17 April 2017 at 20:22:10, Riley Testut (rileytestut-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org) wrote:
> 
> Dynamic programming comes to mind.
> 
> Wouldnt you then be able to use Array(repeating:count:) and repeat 0 (or  
> something else) to achieve this then?
> 
> Yes, less performant than allocing array (since we need to fill in default  
> values), but doing otherwise would go against Swifts safety model. If you  
> truly wanted that behavior, you can use the UnsafePointer methods anyway  
> (AFAIK).
> 
> On Apr 16, 2017, at 9:18 PM, Saagar Jha \<saagar-tskU2QEWZ+aB+jHODAdFcQ@public.gmane.org\> wrote:
> 
> Dynamic programming comes to mind.
> 
> Saagar Jha
> 
> On Apr 16, 2017, at 19:33, Riley Testut \<rileytestut-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org\> wrote:
> 
> My bad, should have phrased my response better :^)
> 
> Under what circumstances would you need to be able to assign elements in an  
> array out of order, while also requiring Array size/performance? (Genuinely  
> curious, not trying to attack).
> 
> IMO, if the differences between Array and Dictionary would cause that much  
> of an issue for your implementation, my guess is you have more important  
> priorities than the need to assign elements out-of-order = I don't think  
> we'd need to add another type to the standard library for this use case.
> 
> On Apr 16, 2017, at 11:22 AM, Saagar Jha \<saagar-tskU2QEWZ+aB+jHODAdFcQ@public.gmane.org\> wrote:
> 
> A Dictionary uses a lot more space than an Array, though, and allow for  
> bogus keys like -1, etc.
> 
> Saagar Jha
> 
> On Apr 16, 2017, at 10:34, Riley Testut via swift-evolution \< swift-evolution-m3FHrko0VLzYtjvyW6yDsg@public.gmane.org\> wrote:
> 
> Personally, the only valid use-case I can think of is when you want to  
> initialise an Arrays elements out-of-order - i.e., you want to set a value  
> for myArray[2] despite myArray[0] and [1] not being populated. In that  
> case, it would be better to have some kind of SparseArray type, and for us  
> to have a proper API for unsafe initialisation of stdlib types.
> 
> Wouldn't the same functionality be accomplished by a Dictionary with Int as  
> the key type?
> 
> On Apr 14, 2017, at 10:00 AM, Karl Wagner via swift-evolution \< swift-evolution-m3FHrko0VLzYtjvyW6yDsg@public.gmane.org\> wrote:
> 
> I'd actually say the #1 reason not to add this feature is that a lot of  
> developers don't seem to understand this, and they're likely to use the  
> feature to make their code try to continue in the face of programmer error  
> instead of trapping like it properly should. A program in an inconsistent  
> state is dangerous; best to stop it quickly before it does some damage.)
> 
> Right, so I think the reason is actually that a lot of developers dont  
> understand what an Array is. There are two use-cases for an Array:
> 
> 1) As a string of items, dont care about the length. The maximum prior  
> knowledge you can have is that the order may or may not be significant.  
> This includes operations like iteration, mapping, reducing and filtering.  
> 2) As a string of items of specific length. You have prior knowledge about  
> what you expect to find at each location. This includes operations like  
> random-access subscripting, which is what were talking about.
> 
> Basically, the interesting part of a statement such as let someValue myArray[2] is: why index 2? Whats so special about that element; why  
> couldn't someValue be the item at any index N instead? Its because we know  
> to expect something of special significance at index 2.
> 
> In that case, the only time myArray[2] will fail is when your prior  
> knowledge breaks down. The type-system has no way to encode and check for  
> the length of an Array, and that has allowed somebody to pass in a bad  
> value. So what to do?
> 
> A) If you absolutely require a value for myArray[2]: Insert a precondition  
> check.  
> B) If you can still continue without myArray[2]: Check the length of the  
> Array. Your logic will be branching anyway in this case, to account for the  
> value (and subsequent values) being/not being present.
> 
> Personally, the only valid use-case I can think of is when you want to  
> initialise an Arrays elements out-of-order - i.e., you want to set a value  
> for myArray[2] despite myArray[0] and [1] not being populated. In that  
> case, it would be better to have some kind of SparseArray type, and for us  
> to have a proper API for unsafe initialisation of stdlib types.
> 
> - Karl  
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