[swift-evolution] Allow strengthening argument types in functions declared for protocol conformance

[Ian Ynda-Hummel]

Sorry, that should be

    extension FooDelegate where Self: BarController {
        func didPerformSomeAction(object: Bar) {}
    }


On Wed, Dec 16, 2015 at 3:32 PM Ian Ynda-Hummel <ianynda at gmail.com> wrote:

> -1
>
> I think the basic problem is saying the runtime should fail. I feel like a
> large part of the power of Swift is letting the compiler and the programmer
> make strong assumptions about types specifically to prevent the runtime
> from dealing with it.
>
> In fact, the type system can save us here! I wrote some code you can play
> with here: http://swiftstub.com/318278733
>
> The basic idea is that if you do something like
>
>     extension FooDelegate where Self: BarController {
>         func didPerformSomeAction(object: Foo) {}
>     }
>
> calls to didPerformSomeAction with an argument of type Bar will call the
> right one for the type! There might be better ways to do that, but that is
> what immediately came to mind.
>
> On Wed, Dec 16, 2015 at 2:43 PM Terrence Katzenbaer via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> Because APIs are designed to be generic, protocols that must be conformed
>> generally use types like Object or other base classes for a given
>> framework. This introduces type casting verbosity when implementing the
>> protocol for a specific use. I would like to see the ability to strengthen
>> argument types in functions declared for protocol conformance.
>>
>> An example:
>>
>> class Foo { }
>>
>>
>> class Bar: Foo { }
>>
>>
>> protocol FooDelegate {
>>
>>     func didPerformSomeAction(object: Foo)
>>
>> }
>>
>>
>> class FooController: FooDelegate {
>>
>>     func didPerformSomeAction(var object: Foo) {
>>
>>         // I know that object must be a Bar instance
>>
>>         object = object as! Bar
>>
>>         // do something with object
>>
>>     }
>>
>> }
>>
>>
>> class ProposedFooController: FooDelegate { // Type
>> 'ProposedFooController' does not conform to protocol 'FooDelegate'
>>
>>     func didPerformSomeAction(object: Bar) {
>>
>>         // do something with Bar instance
>>
>>     }
>>
>> }
>>
>> The glaring issue I see outright is how the runtime should fail when
>> `didPerformSomeAction` in `ProposedFooController` is called with a non-Bar
>> instance... But perhaps it /should/ fail outright because the programmer
>> has explicitly stated that the type should be Bar.
>>
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>
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