[swift-evolution] Allow strengthening argument types in functions declared for protocol conformance
Ian Ynda-Hummel
ianynda at gmail.com
Wed Dec 16 16:10:22 CST 2015
Hm. Taking the NSOutlineViewDelegate example, ideally it would be generic
and you would supply it a type, but that not being the case I would be
hesitant to do the implicit type casting. What happens if you get it wrong
in one place and not the other? Something like
func outlineView(outlineView: NSOutlineView, viewForTableColumn
tableColumn: NSTableColumn, item: String) -> NSView? {
...
}
func outlineView(outlineView: NSOutlineView, shouldSelectItem item:
NSURL) -> Bool {
...
}
I'm not sure how NSOutlineView should function in that case. Does one of
them silently fail? Cause a runtime exception? With AnyObject, imperfect as
it is, you have the opportunity to do the casting yourself and handle
errors gracefully. You could even wrap it in your own generic protocol that
is just a proxy that does the type casting. Or extend the protocol and
overload the methods with more strongly typed arguments.
I actually think making the programmer be explicit about types is an
advantage of things like generic protocols. It helps avoid problems like
the inconsistent types thing. Obviously, you can still get the types wrong,
but that feels like a product of a bad API and not deficiency in the
language.
On Wed, Dec 16, 2015 at 4:40 PM Terrence Katzenbaer <tkatzenbaer at me.com>
wrote:
> Sorry, that last reply was impolitely succinct.
>
> In practice, doing that workaround is strange and difficult to remember
> (especially for beginners). Additionally, it requires that the user bind
> the protocol to a specific type to even be used. Adding generic protocols
> would also require the user to bind the protocol.
>
> Additionally, unless I'm mistaken, Objective-C allows you to conform to
> protocols in the way I'm proposing. Of course, it wouldn't be wise to take
> every feature Objective-C has and implement it into Swift.
>
> On Dec 16, 2015, at 01:32 PM, Terrence Katzenbaer <tkatzenbaer at me.com>
> wrote:
>
> The limitations are that the Cocoa and Cocoa Touch frameworks don't
> implement this behavior.
>
> On Dec 16, 2015, at 1:25 PM, Ian Ynda-Hummel <ianynda at gmail.com> wrote:
>
> I actually think that generic protocols as they exist in Swift 2 are
> expressive enough to do what you want to do.
>
> Let me check my understanding, though. You want a definition of a method
> of the same name with a signature whose arguments are at least as specific
> as those defined in the protocol to implicitly do the appropriate casting,
> correct? So to supply a Swift 2 example, your code would be equivalent to
> this:
>
> class Foo { }
>
> class Bar: Foo { }
>
> protocol FooDelegate {
> typealias FooType: Foo
> func didPerformSomeAction(object: FooType)
> }
>
> class FooController: FooDelegate {
> typealias FooType = Foo
> func didPerformSomeAction(object: Foo) {}
> }
>
> class ProposedFooController: FooDelegate {
> typealias FooType = Bar
> func didPerformSomeAction(object: Bar) {}
> }
>
> What are the limitations there?
>
> On Wed, Dec 16, 2015 at 4:08 PM Terrence Katzenbaer <tkatzenbaer at me.com>
> wrote:
>
>> This seems to be better addressed with a generic protocol.
>>
>> I did consider a generic protocols, and it would be good to have some
>> discussion on this vs. my proposed solution.
>>
>> While there are currently some limitations surrounding generics, and so I
>> won't give specific examples, this has been stated as an area of focus for
>> Swift 3.
>>
>> Isn't this mailing list's purpose to discuss and propose changes to Swift
>> 3? It seems like you're dismissing this as a "wontfix" with hopes that
>> improving generics would resolve this issue in its entirety.
>>
>> On Dec 16, 2015, at 12:58 PM, ilya <ilya.nikokoshev at gmail.com> wrote:
>>
>> > It seems more expressive and more functionally correct to be able
>> to implement a method strengthening AnyObject to my model type while still
>> conforming to the protocol (since my model would pass "is AnyObject").
>>
>> This seems to be better addressed with a generic protocol. While there
>> are currently some limitations surrounding generics, and so I won't give
>> specific examples, this has been stated as an area of focus for Swift 3.
>>
>> > If my NSOutlineView contained objects of a single type, it makes little
>> sense to be forced to implement a method solely for handling AnyObjects or
>> being forced to typecast to my model type.
>>
>> In this example NSOutlineView should be specialized to some type T then.
>>
>> On Wed, Dec 16, 2015 at 23:45 Terrence Katzenbaer via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> Maybe I shouldn't have mentioned anything about the runtime since that's
>>> more about the implementation instead of the general idea... Something more
>>> "swifty" could be a compile check on the newly strengthened arguments.
>>>
>>> If I understand your example code, it's calling
>>> didPerformSomeAction(object: Bar) instead of the Foo version because of
>>> function overloading? It seems this functionally satisfies not having to
>>> typecast, but it doesn't solve the verbosity problem since you're still
>>> forced to implement the extraneous didPerformSomeAction(object: Foo).
>>>
>>> Using the Cocoa framework as an example, NSOutlineViewDelegate has the
>>> method:
>>> optional func outlineView(_ *outlineView*: NSOutlineView
>>> <https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSOutlineView_Class/index.html#//apple_ref/swift/cl/c:objc(cs)NSOutlineView>
>>> ,
>>> shouldSelectItem *item*: AnyObject
>>> <https://developer.apple.com/library/mac/documentation/Swift/Reference/Swift_AnyObject_Protocol/index.html#//apple_ref/swift/intf/s:PSs9AnyObject>)
>>> -> Bool
>>> <https://developer.apple.com/library/mac/documentation/Swift/Reference/Swift_Bool_Structure/index.html#//apple_ref/swift/struct/s:Sb>
>>>
>>> If my NSOutlineView contained objects of a single type, it makes little
>>> sense to be forced to implement a method solely for handling AnyObjects or
>>> being forced to typecast to my model type.
>>>
>>> It seems more expressive and more functionally correct to be able
>>> to implement a method strengthening AnyObject to my model type while still
>>> conforming to the protocol (since my model would pass "is AnyObject").
>>>
>>> On Dec 16, 2015, at 12:35 PM, Ian Ynda-Hummel <ianynda at gmail.com> wrote:
>>>
>>> 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
>>>>>
>>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>
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