[swift-evolution] [Pitch] Remove type inference for associated types

[Austin Zheng]

I'm not sure this is a problem. Once you bind the associated types explicitly, the requirements using those associated types need to match the bound types otherwise the type checker will emit an error. If you have default associated types and default implementations, but then bind the associated types differently and do not update your type-specific implementations to use those other types, you will get an error message. This is how explicitly specifying associated types works today.

Austin

> On Jun 28, 2016, at 10:17 PM, Paulo Faria via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Jun 29, 2016, at 1:51 AM, Douglas Gregor <dgregor at apple.com <mailto:dgregor at apple.com>> wrote:
>> 
>> which might reduce the common case for conforming to the protocol to be, e.g.,
> 
> Just discovered another horrible implication of type inference removal for the same use case. :OOO
> 
> This is the full code for the protocol
> 
> public protocol ResourceController {
>     associatedtype DetailID: PathParameterInitializable = UpdateID
>     associatedtype UpdateID: PathParameterInitializable
>     associatedtype DestroyID: PathParameterInitializable = UpdateID
> 
>     associatedtype CreateInput: StructuredDataInitializable = UpdateInput
>     associatedtype UpdateInput: StructuredDataInitializable
> 
>     associatedtype ListOutput: StructuredDataFallibleRepresentable = UpdateOutput
>     associatedtype CreateOutput: StructuredDataFallibleRepresentable = UpdateOutput
>     associatedtype DetailOutput: StructuredDataFallibleRepresentable = UpdateOutput
>     associatedtype UpdateOutput: StructuredDataFallibleRepresentable
> 
>     func list() throws -> [ListOutput]
>     func create(element: CreateInput) throws -> CreateOutput
>     func detail(id: DetailID) throws -> DetailOutput
>     func update(id: UpdateID, element: UpdateInput) throws -> UpdateOutput
>     func destroy(id: DestroyID) throws
> }
> 
> extension ResourceController {
>     public func list() throws -> [ListOutput] {
>         throw ClientError.notFound
>     }
> 
>     public func create(element: CreateInput) throws -> CreateOutput {
>         throw ClientError.notFound
>     }
> 
>     public func detail(id: DetailID) throws -> DetailOutput {
>         throw ClientError.notFound
>     }
> 
>     public func update(id: UpdateID, element: UpdateInput) throws -> UpdateOutput {
>         throw ClientError.notFound
>     }
> 
>     public func destroy(id: DestroyID) throws {
>         throw ClientError.notFound
>     }
> }
> 
> Suppose we have an implementation like this:
> 
> public struct TodoController : ResourceController {
>     public typealias CreateInput = NotTodo
>     public typealias UpdateInput = NotTodo
> 
>     public typealias ListOutput = NotTodo
>     public typealias CreateOutput = NotTodo
>     public typealias DetailOutput = NotTodo
>     public typealias UpdateOutput = NotTodo
> 
>     public typealias DetailID = NotString
>     public typealias UpdateID = NotString
>     public typealias DestroyID = NotString
> 
>     public func list() throws -> [Todo] {
>         ...
>     }
> 
>     public func create(element todo: Todo) throws -> Todo {
>         ...
>     }
> 
>     public func detail(id: String) throws -> Todo {
>         ...
>     }
> 
>     public func update(id: String, element todo: Todo) throws -> Todo {
>         ...
>     }
> 
>     public func destroy(id: String) throws {
>         ...
>     }
> }
> 
> Notice that the typealiases and the function declarations don’t match. But the compiler doesn’t complain because the protocol has default implementations in the protocol extension. This means that if the person implementing the protocol doesn’t make sure the types match exactly, there’s gonna be unexpected behaviour. Which I’m sure he/she will take quite some time to figure out. :(
> 
> 
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