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

[Xiaodi Wu]

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.
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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>
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