[swift-evolution] Proposal: Extend Optional Chaining to Function, Initializer, and Subscript Parameters

[Jordan Rose]

Hi, Liam. The particular issue we’ve seen when exploring this feature is that it’s unclear whether or not other arguments get evaluated:

print(a.foo()?, b.bar()?, c.baz()?)

If b.bar() turns out to be nil, does c.baz() still get evaluated?

(I’m pretty sure the right answer is “yes”, but it’s still something that should be addressed explicitly.)

The added complexity and the existence of Optional.map led us not to pursue this direction in the past.

Jordan


> On Jul 12, 2016, at 07:16, Liam Stevenson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Optional chaining is one of the great, useful features of Swift. It can be used “for querying and calling properties, methods, and subscripts on an optional that might currently be nil,” to quote Apple's "The Swift Programming Language.” However, often it is necessary to call a function, subscript, or initializer conditionally based on if one or more parameters are nil. The proposed solution is to allow a question mark (?) to be placed after an optional value wished to be used as a parameter. Then, the function, initializer, or subscript will be called if and only if the parameter's value is not nil. If it has a return type, it will return an optional, which will be nil if the parameter is nil.
> 
> Old way (with seemingly unnecessary if statement considering the flexibility provided by optional chaining):
> 	var arr = ["apples", "oranges", "pears", "bananas"]
> 	let index: Int? = 2
> 
> 	var removedElement: String?
> 	if let index = index {
>   		removedElement = arr.removeAtIndex(index) //sets removedElement to "pears"
> 	}
> Using this proposal:
> 	var arr = ["apples", "oranges", "pears", "bananas"]
> 	let index: Int? = 2
> 
> 	var removedElement: String?
> 	removedElement = arr.removeAtIndex(index?) //sets removedElement to “pears"
> Another similar example:
> Old way:
> 	var arr = ["apples", "oranges", "pears", "bananas"]
> 	let index: Int? = nil
> 
> 	var removedElement: String?
> 	if let index = index {
>   		removedElement = arr.removeAtIndex(index) //never called
> 	}
> Using this proposal:
> 	var arr = ["apples", "oranges", "pears", "bananas"]
> 	let index: Int? = nil
> 
> 	var removedElement: String?
> 	removedElement = arr.removeAtIndex(index?) //removeAtIndex is never called, and removedElement is set to nil
> 
> What does everyone think of this proposal? It is additive so it will not break any existing code, and in the future it will provide conciseness and clarity since the syntax is similar to the existing optional chaining syntax.
> 
> View the full proposal on GitHub here: https://github.com/liam923/swift-evolution/blob/master/proposals/NNNN-extend-optional-chaining-to-function-initializer-and-subscript-parameters.md <https://github.com/liam923/swift-evolution/blob/master/proposals/NNNN-extend-optional-chaining-to-function-initializer-and-subscript-parameters.md>
> 
> Liam
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160712/ea7139b8/attachment.html>