[swift-evolution] Why doesn't removeLast() on Collection return an optional?

[Dave Abrahams]

on Mon Oct 17 2016, Louis D'hauwe <swift-evolution at swift.org> wrote:

> Regarding the removeLast() function on Collection:
>
> The current implementation
> <https://github.com/apple/swift/blob/c3b7709a7c4789f1ad7249d357f69509fb8be731/stdlib/public/core/BidirectionalCollection.swift#L228>
> is:
> @discardableResult
> public mutating func removeLast() -> Iterator.Element {
>  	let element = last!
> 	self = self[startIndex..<index(before: endIndex)]
> 	return element
> }
>
> This makes it so that if you call removeLast() on an empty collection you get a fatal error.
> ("fatal error: can't remove last element from an empty collection")
>
> The documentation for removeLast()
> <https://github.com/apple/swift/blob/c3b7709a7c4789f1ad7249d357f69509fb8be731/stdlib/public/core/BidirectionalCollection.swift#L220>
> even has this noted:
> "The collection must not be empty.".
>
> Surely you could just remove the explicit unwrapping of 'last' and add a guard statement? 
>
> As such:
> @discardableResult
> public mutating func removeLast() -> Iterator.Element? {
>  	guard let element = last else {
> 		return nil
> 	}
> 	self = self[startIndex..<index(before: endIndex)]
> 	return element
> }
>
> It sure seems more "Swifty" to alert at compile time that removing the last item of a collection
> might fail, and make it return nil as a result.

see the popLast() method.

-- 
-Dave