[swift-evolution] Optional Argument Chaining

[Jared Khan]

Even this small example I think this is still a little fiddly. If we add another required parameter to the Markdown initializer:
	let readme = String(contentsOfFile: “README.md”).flatMap { Markdown(string: $0, flavor: .github) }
this starts to feel a little inside-out and crufty to me. 

> On 12 Dec 2017, at 05:54, Félix Cloutier <felixcloutier at icloud.com> wrote:
> 
> You talk about flatMap without giving an example. The readme isn't that bad with it, IMO:
> 
> if let readme = String(contentsOfFile: "README.md").flatMap(Markdown) {
> 	// use contents here
> }
> 
> That doesn't work when you need multiple optional parameters. In my own experience, that hasn't been a huge problem, though.
> 
> Félix
> 
>> Le 11 déc. 2017 à 08:30, Jared Khan via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>> 
>> Hi all,
>> 
>> I'd like to propose a syntax addition that acts to ease some things that I believe should fall under the umbrella of 'optional chaining'. Optional chaining allows us to access the properties of an optional value and return nil if any link in that chain breaks. I propose we introduce syntax to allow similar chaining when passing optional valued parameters to functions that expect that parameter to be non-optional.
>> 
>> The example below is taken from a project I'm working on at the moment:
>> 
>> 
>> // Current
>> let readme: Markdown?
>> if let rawMarkdown = String(contentsOfFile: "README.md") {
>> 	readme = Markdown(string: rawMarkdown)
>> } else {
>> 	readme = nil
>> }
>> In this example we want to perform an operation, the initialisation of a 'Markdown' type, with our raw text if it exists and get nil otherwise. This is rather verbose
>> 
>> I propose the following syntax for an alternative:
>> 
>> 
>> // Proposed alternative
>> let readme = Markdown(string: String(contentsOfFile: "README.md")?)
>> 
>> The ? is familiar in its use for optional chaining.
>> 
>> This would act like syntactic sugar for the flatMap method on Optional. For example:
>> 
>> (where `john` is of a `Person` type with a property `address: Address?`)
>> // func getZipCode(fromAddress address: Address) -> ZipCode
>> getZipCode(fromAddress: john.address?)
>> 
>> // Would be equivalent to…
>> john.address.flatMap {
>> 	getZipCode($0)
>> }
>> An example with multiple parameters:
>> 
>> // func getPostageEstimate(source: Address, destination: Address, weight: Double) -> Int
>> getPostageEstimate(source: john.address?, destination: alice.address?, weight: 2.0)
>> 
>> // Equivalent to
>> john.address.flatMap { freshVar1 in
>> 	alice.address.flatMap { freshVar2 in
>> 		getPostageEstimate(source: freshVar1, destination: freshVar2, weight: 2.0)
>> 	}
>> }
>> 
>> // Or equally:
>> {
>> 	guard let freshVar1 = john.address,
>> 		let freshVar2 = alice.address else {
>> 			return nil
>> 	}
>> 
>> 	return getPostageEstimate(source: freshVar1, destination: freshVar2, weight: 2.0)
>> }()
>> This would only be allowed when the parameter doesn’t already accept Optionals and when the chained value is in fact an optional. We’d want to consider emitting at least the following errors/warnings in the given scenarios:
>> 
>> 
>> let result = myFunc(3?)
>> // error: cannot use optional chaining on non-optional value of type 'Int'
>> 
>> // func myFunc2(x: String?) -> String
>> let result = myFunc2(x: john.address?)
>> // error: cannot use optional argument chaining on argument of optional type
>> let result = myFunc(nil?)
>> // warning: optional argument chaining with nil literal always results in nil
>> 
>> Seeking your thoughts on this idea, the specific syntax, and more use case examples.
>> 
>> Best,
>> 
>> Jared
>> 
>> _______________________________________________
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>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
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> 

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