[swift-evolution] [Pitch] Simpler interpretation of a reference to a generic type with no arguments

[Xiaodi Wu]

Sorry, it's I who is saying things all wrong. I meant to ask, is it
feasible to keep both behaviors but have #2 "win" over #1, instead of
getting rid of behavior #1 entirely?

On Thu, Jun 23, 2016 at 15:30 Slava Pestov <spestov at apple.com> wrote:

> On Jun 23, 2016, at 1:27 PM, Xiaodi Wu <xiaodi.wu at gmail.com> 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.
>
> 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 <
> swift-evolution at swift.org> 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.
>> _______________________________________________
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>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>
>
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