[swift-evolution] [Pitch] Make try? + optional chain flattening work together

[John McCall]

> On Jan 12, 2018, at 12:53 PM, BJ Homer via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I agree that this behavior is annoying. However, wouldn’t it be source-breaking to change this now?

Source compatibility means that we can't change the behavior of Swift 3 / Swift 4 source.  This would be a semantic change when building Swift 5 source (or later).  There is no technical reason we couldn't make this change.  It does need to meet a very high bar, because we are trying to avoid making significant semantic breaks.  My personal sense is that it meets that bar because double-optionals can be very confusing for novices and very annoying for everyone else.

I think most use sites probably do want the optional-collapsing behavior.  'try?' is already "sugar" syntax and should aim to be as convenient as possible for the majority of use cases.  Much like the collapsing done by optional chaining, I think you can come up with examples where somebody would want the non-collapsing behavior, but it doesn't seem unreasonable to say that they just shouldn't use the sugar.

John.

> 
> -BJ
> 
>> On Jan 12, 2018, at 10:25 AM, Russ Bishop via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Greetings swift-evolution!
>> 
>> There is currently a disconnect between optional chaining and try? when it comes to optional flattening:
>> 
>> 
>> struct SomeType {
>>     func nonThrow() -> SomeType? { return self }
>>     func doThrow() throws -> SomeType? { return self }
>>     func nonOptional() throws -> SomeType { return self }
>> }
>> 
>> let w = SomeType().nonThrow()?.nonThrow()?.nonThrow()?.nonThrow()
>> // w has type SomeType?
>> 
>> let x = try? SomeType().nonOptional().nonOptional().nonOptional().nonOptional()
>> // x has type SomeType?
>> 
>> let y = try! SomeType().doThrow()?.doThrow()?.doThrow()?.doThrow()
>> // y has type SomeType?
>> 
>> let z = try? SomeType().doThrow()?.doThrow()?.doThrow()?.doThrow()
>> // z has type SomeType??
>> 
>> 
>> We get a double-optional only when combining try? and optional-chaining. That is inconvenient and it would be natural to have the compiler do the flattening here.
>> 
>> 
>> If anyone is interested in working on the proposal or implementation please let me know. It would make a nice self-contained task if you're looking to start contributing.
>> 
>> 
>> Russ Bishop
>>  Simulator
>> 
>> 
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