[swift-evolution] [Pre-pitch] Conditional default arguments
Tony Allevato
tony.allevato at gmail.com
Mon Nov 27 16:55:17 CST 2017
On Mon, Nov 27, 2017 at 2:39 PM Matthew Johnson <matthew at anandabits.com>
wrote:
> On Nov 27, 2017, at 4:25 PM, Tony Allevato <tony.allevato at gmail.com>
> wrote:
>
>
>
> On Mon, Nov 27, 2017 at 2:19 PM Matthew Johnson <matthew at anandabits.com>
> wrote:
>
>> On Nov 27, 2017, at 3:56 PM, Howard Lovatt via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> Really like Tony’s suggestion, much cleaner than yet another annotation
>> rammed into the signature. Also the idea of a static factory that could
>> accept previously initialized arguments would be very powerful.
>>
>>
>> It is syntactically cleaner at the site of the function declaration for
>> sure. Wouldn’t you agree that it is semantically less clear though? Both
>> are important, but which is more important? If not semantics, why not?
>>
>> It’s also worth noting that it would be more verbose in at least some use
>> cases (when a declaration such as `defaultConfiguration()` is used in the
>> conditional default expression and is not otherwise necessary).
>>
>
> (Apologies for the earlier blank reply—why is Cmd+Enter a keyboard
> shortcut for Send something someone would think is a good idea?)
>
> That verbosity is kind of a feature of my design, in the sense that it
> describes exactly what's going on at the location in code where someone
> expects to see default values. If anything, the default factory name is an
> opportunity to add context where normally there might be none.
>
> It's also worth noting that this design works well if you have multiple
> methods that need to share the same defaults. Instead of repeating the same
> annotations for each method that needs them, you just define the functions
> once and refer to them everywhere. (Could you do the same with the
> annotation based method? Probably, if you allow arbitrary expressions
> within them. But that seems less obvious compared to this approach.)
>
>
> You make a really good point here. The annotations would need to be
> applied to every declaration that uses them whereas your proposed syntax
> would just rely on overload resolution succeeding or failing in a given
> type context. The use case I have would benefit in this respect as the
> conditional defaults would be shared by several declarations.
>
> Howard’s idea of restricting conditional defaults to only use declarations
> in the same file seems somewhat arbitrary but it would go a long way
> towards helping an author understand clearly what the provided defaults are
> as the rest of the module and imported symbols would not need to be
> considered. I wonder if this approach could be refined a bit so it feels
> less arbitrary. Users would probably need to rely on tooling to discover
> defaults but I think I’m ok with that. I am mostly concerned with the
> ability of an author to reason locally about the API contract they are
> publishing. Do you have any ideas on how to refine Howard's idea?
>
I mentioned earlier in the thread (with a few messed up details) that there
are already some restrictions on default arguments that I think already
work well here. Declarations referenced in the default value expression of
a function today must be accessible within the scope of the declaration
being defined, so I'm not sure if we need to go further than that.
Here's an example that I'll admit is completely contrived, but should I be
prevented from doing this? Let's say I define an "Identities" module with
this type:
```
enum Identities {
func identity() -> Int { return 0 } // let's ignore additive vs.
multiplicative for a moment
func identity() -> Double { return 0.0 }
func identity() -> () { return () }
func identity() -> String { return "" }
func identity<T>() -> (T) -> Void { return { _ in } }
// and so on
}
```
Then somewhere I want to define a conditional default, using those
identities:
```
import Identities
func someWeirdThing<T>( ...contrived args..., defaultValue: T =?
Identities.identity()) { ... }
```
Swift today already allows this with regular default value expressions, so
the problem of tooling being needed to discover defaults already exists and
this hypothetical construct doesn't change that. We *could* restrict such
defaults to same file or same module and it seems reasonable to do so, but
should we? If the defaults I want happen to live elsewhere, why not let me
use them? Or, if it's a serious concern, why not lock down all default
expressions the same way?
I think the bigger concern is the other one Xiaodi mentioned—we probably
don't want people to be able to retroactively add overloads that would
introduce a default where previously there was none. (This wouldn't be
possible for global functions in different modules, but could be for
non-private type members.) This might be something that Just Works Out™
depending on how and when the compiler resolves such expressions—but I
don't know the compiler deeply enough to say for sure without investigating
more.
>
>
>
>
>>
>> -- Howard.
>>
>> On 26 Nov 2017, at 9:25 am, Tony Allevato via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> On Sat, Nov 25, 2017 at 1:16 PM Xiaodi Wu <xiaodi.wu at gmail.com> 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.
>>>
>>
>> This runtime/reasoning problem *already exists* today with default
>> arguments, because you can write something like this:
>>
>> struct Foo {
>> static var defaultExponent = 2.0
>>
>> func raise(_ x: Double, to exponent: Double = defaultExponent) {
>> print(pow(x, exponent))
>> }
>> }
>> Foo().raise(4) // "16.0"Foo.defaultExponent = 3.0Foo().raise(4) // "64.0"
>>
>> Swift lets you write a default value expression that references static
>> (but not instance) vars of the enclosing type, as well as anything else
>> that’s visible from that expression’s scope. Should people do this?
>> Probably not, for the reasons that you described.
>>
>> But the point is that my example is no more harmful or difficult to
>> reason about than default arguments in the language today. My proposed
>> solution *in no way* changes the runtime behavior of default argument
>> expressions. I’m not envisioning anything more than what default arguments
>> can already do except for adding a way to choose different default
>> factories (or choose none without error) based on the *static* types of
>> the generic arguments that are bound at a particular call site.
>>
>>
>>
>>> 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
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> 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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>>>>
>>>>
>>>>
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