> On Jun 23, 2016, at 1:30 PM, Slava Pestov <[email protected]> wrote:
>
>
>> On Jun 23, 2016, at 1:27 PM, Xiaodi Wu <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> When you mention the difficulty of an alternative, is that to say that it's
>> not feasible for the GenericBox in the last example to be resolved as
>> GenericBox<T>? From an end-user point of view, that seems to be the most
>> sensible behavior.
>
> With my proposed change, GenericBox would be resolved as GenericBox<T> in the
> last example. Right now it fails to type check.
This should make it clearer:
struct GenericBox<Contents> {
let contents: Contents
func transform<Result>(f: (Contents) -> Result) -> GenericBox<Result> {
// If you change this to just ‘GenericBox(contents: …)’, it does not type
check
return GenericBox<Result>(contents: f(contents))
}
}
func transform<Contents, Result>(box: GenericBox<Contents>,
f: (Contents) -> Result) -> GenericBox<Result> {
// But this is totally fine!
return GenericBox(contents: f(box.contents))
}
I suspect most people do not expect the first case to fail, and it is not
immediately obvious why it fails when the second example type checks.
>
> Here is an example that works right now, but would not work with my proposed
> change:
>
> struct GenericBox<Contents> {
> // Currently Swift resolves this as ‘GenericBox<Contents>’
> // With the new rule, we cannot infer the parameter, because there’s no
> expression to infer it from
> func combine(other: GenericBox) {
> …
> }
> }
>
> Basically the meaning of ‘GenericBox’ right now depends on whether it appears
> inside its own definition or extension thereof, or not. The behavior when it
> appears elsewhere is more general — we infer the parameters from the
> surrounding expression, instead of assuming they’re equal to the context
> parameters.
>
> This is a subtle change — definitely let me know if I’m not explaining it
> well.
>
> Slava
>
>> On Thu, Jun 23, 2016 at 15:14 Slava Pestov via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>> Simpler interpretation of a reference to a generic type with no arguments
>>
>> Proposal: SE-9999
>> <https://github.com/slavapestov/swift-evolution/blob/silly-proposals/proposals/9999-simplify-unbound-generic-type.md>
>> Author: Slava Pestov <https://github.com/slavapestov>
>> Status: Awaiting review
>> Review manager: TBD
>>
>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#introduction>Introduction
>>
>> This proposal cleans up the semantics of a reference to a generic type when
>> no generic arguments are applied.
>>
>> Swift-evolution thread: Discussion thread topic for that proposal
>> <http://news.gmane.org/gmane.comp.lang.swift.evolution>
>>
>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#motivation>Motivation
>>
>> Right now, we allow a generic type to be referenced with no generic
>> arguments applied in a handful of special cases. The two primary rules here
>> are the following:
>>
>> If the scope from which the reference is made is nested inside the
>> definition of the type or an extension thereof, omitting generic arguments
>> just means to implicitly apply the arguments from context.
>>
>> For example,
>>
>> struct GenericBox<Contents> {
>> let contents: Contents
>>
>> // Equivalent to: func clone() -> GenericBox<Contents>
>> func clone() -> GenericBox {
>> return GenericBox(contents: contents)
>> }
>> }
>>
>> extension GenericBox {
>> func print() {
>> // Equivalent to: let cloned: GenericBox<Contents>
>> let cloned: GenericBox = clone()
>> print(cloned.contents)
>> }
>> }
>> If the type is referenced from an unrelated scope, we attempt to infer the
>> generic parameters.
>>
>> For example,
>>
>> func makeABox() -> GenericBox<Int> {
>> // Equivalent to: GenericBox<Int>(contents: 123)
>> return GenericBox(contents: 123)
>> }
>> The problem appears when the user expects the second behavior, but instead
>> encounters the first. For example, the following does not type check:
>>
>> extension GenericBox {
>>
>> func transform<T>(f: Contents -> T) -> GenericBox<T> {
>> // We resolve 'GenericBox' as 'GenericBox<Contents>', rather than
>> // inferring the type parameter
>> return GenericBox(contents: f(contents))
>> }
>> }
>>
>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#proposed-solution>Proposed
>> solution
>>
>> The proposed solution is to remove the first rule altogether. If the generic
>> parameters cannot be inferred from context, they must be specified
>> explicitly with the usual Type<Args...> syntax.
>>
>>
>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#detailed-design>Detailed
>> design
>>
>> This really just involves removing an existing piece of logic from the type
>> resolver code.
>>
>>
>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#impact-on-existing-code>Impact
>> on existing code
>>
>> This will have a small impact on existing code that uses a pattern similar
>> to the above.
>>
>>
>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#alternatives-considered>Alternatives
>> considered
>>
>>
>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#status-quo>Status
>> quo
>>
>> We could keep the current behavior, but one can argue it is not very useful,
>> and adds a special case where one is not needed.
>>
>>
>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#more-complex-inference-of-generic-parameters>More
>> complex inference of generic parameters
>>
>> We could attempt to unify the two rules for resolving a reference to a
>> generic type with no arguments, however this presents theoretical
>> difficulties with our constraint solver design. Even if it were easy to
>> implement, it would increase type checking type by creating new
>> possibilities to consider, with very little actual benefit.
>> _______________________________________________
>> swift-evolution mailing list
>> [email protected] <mailto:[email protected]>
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>
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