[swift-evolution] [Pre-pitch] Conditional default arguments
Dave Abrahams
dabrahams at apple.com
Sat Nov 25 17:04:28 CST 2017
This sort of “it compiles if it’s syntactically valid, regardless of declared constraints” thing is deliberately avoided in Swift’s generics design with good reason; it’s possible that in this instance there are no problems, but I’m skeptical.
Sent from my iPhone
> On Nov 25, 2017, at 1:16 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
>
> On Sat, Nov 25, 2017 at 15:06 Matthew Johnson <matthew at anandabits.com> wrote:
>>> On Nov 25, 2017, at 1:28 PM, Tony Allevato via swift-evolution <swift-evolution at swift.org> wrote:
>>>
>>> On Fri, Nov 24, 2017 at 7:18 PM Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
>>>
>>>
>>>
>>>> It's kludgy, but we could have something like:
>>>>
>>>> ```
>>>> @defaultArgument(configuration = (), where R.Configuration == Void)
>>>> @defaultArgument(actionHandler = { _ in }, where R.Action == Never)
>>>> func makeResource(with configuration: R.Configuration, actionHandler: @escaping (R.Action) -> Void) -> R { ... }
>>>> ```
>>>>
>>>> I don't like that we'd be setting a default argument on something lexically before even encountering it in the declaration, but it's serviceable.
>>>
>>>
>>> What if we could take advantage of the fact that you can have non-constant expressions in default arguments? Overload resolution could already do most of the job—what we need on top of that is a way for the author to say that “if no overload matches, then it’s not an error—just don’t have a default argument in that case”. Something like SFINAE in C++, but more explicit.
>>>
>>> I’m imagining something like this:
>>>
>>> func defaultConfiguration() -> Void {
>>> return ()
>>> }
>>>
>>> func defaultActionHandler() -> (Never) -> Void {
>>> return { _ in }
>>> }
>>>
>>> struct ResourceDescription<R: Resource> {
>>> func makeResource(
>>> with configuration: R.Configuration =? defaultConfiguration(),
>>> actionHandler: @escaping (R.Action) -> Void =? defaultActionHandler()
>>> ) -> R {
>>> // create a resource using the provided configuration
>>> // connect the action handler
>>> // return the resource
>>> }
>>> }
>>> The main difference here is the strawman =? syntax, which would indicate that “the default argument exists if there is a way the RHS can be satisfied for some instances of the generic arguments; otherwise, there is no default”, instead of today’s behavior where it would be an error. There could be multiple overloads of defaultConfiguration and defaultActionHandler (even ones that are themselves generic) and it would do the right thing when there are matches and when there aren’t.
>>>
>>> I like this approach because it mostly takes advantage of existing language features and is fairly lightweight in terms of how it’s expressed in code compared to regular default arguments—we’d just need to design the new operator and type-checker logic around it.
>>>
>>
>> This is an interesting approach. One advantage to something in this direction is that it could support defining different defaults for the same argument under different constraints by overloading the default argument factories on their return type.
>>
>> One concern I have is that it doesn’t allows us to clearly define under which constraints a default argument is available. I suspect this might be problematic especially for public interfaces where source compatibility is a concern.
>
> It's certainly an interesting idea but it would suggest that the constraints under which a default argument is available can change at runtime. I'm concerned, like you, that this is difficult to reason about. It is still unclear to me how widespread the underlying issue is that requires conditional default arguments, but the conversation thus far has been about compile-time constraints and Tony's design seems to envision much more than that.
>
>> I think I prefer Xiaodi’s suggestion for that reason. His approach could also support multiple defaults for the same parameter as long as the constraints are not allowed to overlap (overlapping constraints would result in ambiguity similar to ambiguous overload resolution) or an explicit argument is required if they do.
>>
>>>
>>>
>>>
>>>>
>>>>
>>>>> On Fri, Nov 24, 2017 at 8:36 PM, T.J. Usiyan via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> I am all for this. are many types where there is an obvious 'zero' or 'default' value and the ability to express "use that when possible" without an overload is welcome.
>>>>>
>>>>>
>>>>> The best thing that I can think of right now, in terms of syntax, is actually using @overload
>>>>>
>>>>> ```
>>>>> struct ResourceDescription<R: Resource> {
>>>>>
>>>>> func makeResource(with configuration: R.Configuration, actionHandler: @escaping (R.Action) -> Void) -> R
>>>>> @overload(R.Configuration == Void) func makeResource(actionHandler: @escaping (R.Action) -> Void) -> R
>>>>> @overload(R.Action == Never) func makeResource(with configuration: R.Configuration) -> R
>>>>> {
>>>>> // create a resource using the provided configuration
>>>>> // connect the action handler
>>>>> // return the resource
>>>>> }
>>>>> }
>>>>> ```
>>>>>
>>>>>
>>>>> This isn't great though…
>>>>>
>>>>>> On Fri, Nov 24, 2017 at 6:11 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>> As mentioned in my prior message, I currently have a PR open to update the generics manifesto (https://github.com/apple/swift/pull/13012). I removed one topic from that update at Doug Gregor’s request that it be discussed on the list first.
>>>>>>
>>>>>> The idea is to add the ability to make default arguments conditional (i.e. depend on generic constraints). It is currently possible to emulate conditional default arguments using an overload set. This is verbose, especially when several arguments are involved. Here is an example use case using the overload method to emulate this feature:
>>>>>>
>>>>>> ```swift
>>>>>> protocol Resource {
>>>>>> associatedtype Configuration
>>>>>> associatedtype Action
>>>>>> }
>>>>>> struct ResourceDescription<R: Resource> {
>>>>>> func makeResource(with configuration: R.Configuration, actionHandler: @escaping (R.Action) -> Void) -> R {
>>>>>> // create a resource using the provided configuration
>>>>>> // connect the action handler
>>>>>> // return the resource
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> extension ResourceDescription where R.Configuration == Void {
>>>>>> func makeResource(actionHandler: @escaping (R.Action) -> Void) -> R {
>>>>>> return makeResource(with: (), actionHandler: actionHandler)
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> extension ResourceDescription where R.Action == Never {
>>>>>> func makeResource(with configuration: R.Configuration) -> R {
>>>>>> return makeResource(with: configuration, actionHandler: { _ in })
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> extension ResourceDescription where R.Configuration == Void, R.Action == Never {
>>>>>> func makeResource() -> R {
>>>>>> return makeResource(with: (), actionHandler: { _ in })
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> ```
>>>>>>
>>>>>> Adding language support for defining these more directly would eliminate a lot of boilerplate and reduce the need for overloads. Doug mentioned that it may also help simplify associated type inference (https://github.com/apple/swift/pull/13012#discussion_r152124535).
>>>>>>
>>>>>> The reason that I call this a pre-pitch and one reason Doug requested it be discussed on list is that I haven’t thought of a good way to express this syntactically. I am interested in hearing general feedback on the idea. I am also looking for syntax suggestions.
>>>>>>
>>>>>> Matthew
>>>>>>
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>>>>>
>>>>>
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