[swift-evolution] [Pre-pitch] Conditional default arguments

[Tony Allevato]

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.



>
> 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
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>>
>>
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>>
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