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

[Justin Jia]

> On Aug 17, 2016, at 11:59 PM, Tino Heth <2th at gmx.de> wrote:
> 
> 
>> Let’s discuss something else. Actually there is an easy fix: make all functions accept optionals. I think this is a really bad idea
> Indeed.
> I don't think there is a good motivation to change the status quo:
> You can't "sugar away" everything; sometimes, you just have to write a line of code that does what you want.
> As you can "if let" convert as many optionals as you like in a single statement, it really doesn't hurt that much.
> 
> The precondition stuff looks exactly like guard inside the function to me… what's the benefit of that?

It won't change the original function. It's like adding a new function. It won't affect the original function. For example: if view is non optional then the function won't throw. 

func addSubview(_ view: UIView?) throw
func addSubview(_ view: UIView)

It's only a very rough idea. But this is not limited to checking nil. *Maybe* in the future we can achieve something like this:

----
func randomInt() -> Int { }

precondition count > 0 else { return nil }
func getCellSize(forItemsInRow count: Int) -> CGSize { }

let numberOfCells = randomInt()
let cellSize = getCellSize(forItemsInRow: numberOfCells) // cellSize is optional, since numberOfCells can be smaller or equal to zero

let cellSize = getCellSize(forItemInRow: 4) // cellSize is non optional, since 4 > 0
----

Even this:

----
func randomPositiveInt() -> Int { }
postcondition > 0 else { fatalError() }

precondition count > 0 else { return nil }
func getCellSize(forItemsInRow count: Int) -> CGSize { }

let numberOfCells = randomPositiveInt()
let cellSize = collectionView.getCellSize(forItemsInRow: numberOfCells) // cellSize is non-optional, since numberOfCells is always positive 
----
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