[swift-evolution] [Idea] Use optionals for non-optional parameters

[Justin Jia]

> On Sep 4, 2016, at 1:35 AM, Thorsten Seitz <tseitz42 at icloud.com> wrote:
> 
> 
> 
>> Am 03.09.2016 um 18:45 schrieb Justin Jia <justin.jia.developer at gmail.com>:
>> 
>> 
>>> On Sep 4, 2016, at 12:19 AM, Thorsten Seitz <tseitz42 at icloud.com> wrote:
>>> 
>>> 
>>> 
>>> Am 15.08.2016 um 19:05 schrieb Justin Jia via swift-evolution <swift-evolution at swift.org>:
>>> 
>>>> I agree that being explicit is nice and I also like to use `guard`.
>>>> 
>>>> But according to my observation, usually it is easier to make mistakes if we choose to use `guard`.
>>>> 
>>>> Let me give you a fake real world example.
>>>> 
>>>> With `guard`, you need to be really careful when you want to add new expression (people usually will add to the end of the function).
>>>> 
>>>> ```
>>>> func updateCell(cell: Cell, data: CellData) {
>>>>   cell.label.text = data.title
>>>>   guard let imageName = data.imageName else { return }
>>>>   cell.sublabel.text = cell.humanize(imageName)
>>>>   guard let image = UIImage(named: imageName) else { return }
>>>>   cell.addBackgroundImage(image)
>>>>   // Let's say we changed the design and added a new heading that depends on image name
>>>>   cell.heading = String(imageName.characters.first) // This won't be called if image is nil!
>>>> }
>>>> ```
>>>> 
>>>> With `if let`, it is really hard to read. This will become more complicated if we add more attributes to cell.
>>>> 
>>>> ```
>>>> func updateCell(cell: Cell, data: CellData) {
>>>>   cell.label.text = data.title
>>>>   if let imageName = data.imageName {
>>>>     cell.sublabel.text = cell.humanize(imageName)
>>>>     if let image = UIImage(name: imageName) {
>>>>       cell.addBackgroundImage(image)
>>>>     }
>>>>     cell.heading = String(imageName.characters.first)
>>>>   }
>>>> }
>>>> ```
>>>> 
>>>> With the proposed syntax:
>>>> 
>>>> ```
>>>> func updateCell(cell: Cell, data: CellData) {
>>>>   cell.label.text = data.title
>>>>   let imageName = data.imageName // imageName is optional
>>>>   cell.sublabel.text = cell.humanize(imageName?)
>>>>   let image = UIImage(named: imageName?) // image is optional
>>>>   cell.addBackgroundImage(image?)
>>>>   cell.heading = String(imageName.characters.first?)
>>>> }
>>>> ```
>>>> 
>>>> This is really easy to read. And everything works correctly.
>>> 
>>> It is even easier if you define the methods on Cell to take optional arguments. 
>>> Then you can write the code like in your last example and don't even need the proposed syntax:
>>> 
>>> class Cell {
>>>     let label = UILabel()
>>>     let sublabel = UILabel()
>>>     var heading: String?
>>>     func humanize(_ string: String?) -> String {...}    // optional argument
>>>     func addBackgroundImage(_ image: UIImage?)    // optional argument
>>> }
>>> 
>>> extension UIImage {
>>>     init?(named imageName: String?) {...}
>>> }
>>> 
>>> extension String {
>>>     init?(named imageName: Character?) {...}
>>> }
>>> 
>>> func updateCell(cell: Cell, data: CellData) {
>>>   cell.label.text = data.title
>>>   let imageName = data.imageName
>>>   cell.sublabel.text = cell.humanize(imageName)
>>>   let image = UIImage(named: imageName)
>>>   cell.addBackgroundImage(image)
>>>   cell.heading = String(imageName?.characters?.first)
>>> }
>>> 
>>> -Thorsten 
>> 
>> 
>> Quoting another email:
>> 
>>> Actually there is an easy fix: make all functions accept optionals. I think this is a really bad idea because sometimes functions are designed to accept non-optionals.
>>> 
>>> e.g.
>>> 
>>> ```
>>> func addSubview(_ view: UIView) { }
>>> ```
>>> 
>>> It doesn’t make sense if we want to add nil as the subview, so we choose to write code like this:
>>> 
>>> ```
>>> if let view = view {
>>>     addSubview(view)
>>> }
>>> ```
>> 
> 
> I agree. The proposal effectively does just that, though: turn every function into a function accepting optionals (returning nil if any argument is nil). I would prefer to do this explicitly in cases where it makes sense and use map or let-bindings elsewhere.
> 
> -Thorsten 

I was thinking about a precondition syntax. But I think it's too complicated. I agree. Maybe do this explicitly is better. 

Justin
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