[swift-evolution] [Proposal] Explicit Non-Default-Implemented Protocol Requirements

[Tino Heth]

Hi Gor,

[I'll continue to answer below, but that wont help you with your proposal…]

The question is wether this
protocol Equatable {

	static func == (_ some: Self, _ other: Self) -> Bool

	default static func != (_ some: Self, _ other: Self) -> Bool
}

extension Equatable {

	static func != (_ some: Self, _ other: Self) -> Bool {
		[Your Code Here]
	}

}

is better than

protocol Equatable {

	static func == (_ some: Self, _ other: Self) -> Bool

	static func != (_ some: Self, _ other: Self) -> Bool {
		[Your Code Here]
	}

}

I don't think so, and therefore, I don't want to add a new keyword (or, more precise, a new meaning for an existing keyword).
Both solutions could coexist, but I doubt that implementation inside protocol body leaves enough room for "default func".

- Tino

>> Fileprivate has been there for years, it just was called private — and because the "church of extensions" ;-) has been so powerful, we finally ended up with what we have now.
> 
> Yes, I was here. The fact that we started off with private behaving the way it does now is history now. The point stands: extensions in the same file as the type definition have an extra guarantee that enable the compiler to provide extra features based on that: the guarantee that the extension and the type definition will always be visible simultaneously, thus, implicitly merging the extension into the type definition is possible. It's not possible to do in any other way, because a file is the only unit of compilation that's more-or-less guaranteed to be atomically parsed. A module is separated into files, which can be compiled into objects separately, making it impossible for the compiler to know if any extension in any of the other files will change the layout of the type.
The compiler won't care where if a method is defined in the type declaration, or in an extension in the same file.
What extra guarantee could those extensions offer?

>>>> They are recommended in style guides, influencers blog about them, and they motivated a ridiculous complex change in the access rights system. Yet I haven't seen any evidence that they offer real benefit.
>>> 
>>> Extensions are a tool for decentralizing code. There are some critical limits on extensions that make main type definition subject to unavoidable bloating (required and designated initializers, stored properties, the deinitializer, and open methods), but everything else is a prime candidate for decentralizing. Putting as little code as possible into the type definition and semantically grouping the implementation into extensions improves readability and maintainability dramatically.
>> The thing is: This is just a claim that gets repeated over and over. There is no proof, and I even don't know a single study on that topic.
> 
> Take a look at this article: https://en.wikipedia.org/wiki/Separation_of_concerns <https://en.wikipedia.org/wiki/Separation_of_concerns>There's nothing about Swift, neither about extensions, in this article.
I don't want to start throwing around buzzwords, but there's also https://en.wikipedia.org/wiki/Don%27t_repeat_yourself

> 
>>>> Extensions are great for adding useful helpers to existing types, and still allow you to selectively expose details of your own classes — but most people seem to ignore those options and focus on something can be done better with plain old comments.
>>> 
>>> Relying on comments for invariants and preconditions is a poor design decision, because there's no way of enforcing them and the whole integrity of the code is thrown at the mercy of a human's carefulness (which is a horrible fate to befall upon any code). By writing the code in such a way that makes it impossible to be misused (by way of compiler enforcement), the code becomes resilient and no amount of clumsy usage can lead to incredibly obscure bugs that would take a week of debugging to catch.
>> But extensions are no tool to do so: They have no features that offer any protection, exactly like comments — they are just more typing and don't show up properly in Xcode.
>> 
>> import UIKit
>> 
>> class MyViewController: UIViewController {
>> }
>> 
>> extension MyViewController: UITableViewDataSource {
>> 
>>     func tableView(_ tableView: UITableView, numberOfRowslnSection: Int) -> Int {
>>         return 1
>>     }
>> 
>>     func tableView(_ tableView: UITableView, didSelectRowAt: IndexPath) {
>>         print("Hu, isn't this a delegate method?")
>>     }
>> }
>> 
>> extension MyViewController: UITableViewDelegate {
>>     func tableView(_ tableView: UITableView, numberOfRowsInSection: Int) -> Int {
>>         return 99
>>     }
>> 
>>     func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
>>         let cell = UITableViewCell(style: .default, reuseIdentifier: "")
>>         cell.textLabel?.text = "I should get my data from a datasource"
>>         return cell
>>     }
>> }
>> 
>> This is perfectly valid Swift, and it is build on extensions — but does it increase the quality of the code?
>> Same file extensions are nothing but a different pair of parenthesis to surround your code, and since Swift 4, you can shuffle around those delimiters however you like, and it has still the same meaning for the compiler.
>> They enforce nothing, and so far, I haven't seen any ideas to increase their power.
> 
> The compiler also doesn't stop you from making a wide variety of design choices, a comprehensive list of which can be found here: https://en.wikipedia.org/wiki/Anti-pattern#Programming <https://en.wikipedia.org/wiki/Anti-pattern#Programming>
> For instance, have you tried reading the implementation of sin function in glibc? It's a very thick spaghetti code, sprinkled with a bucketful of magic numbers. It's absolutely impossible to comprehend.
> 
> Any language limitation should only be implemented if it passes the threshold where the gain of safety and convenience outweighs the loss of flexibility and the cost of implementing the limitation.
> In your example, the gain of convenience is much lower than the cost of implementing it and loss of flexibility.
What?
I didn't talk about adding limitations, and my example just shows that that "extension-oriented programming" doesn't add safety, just like comments — simply because the compiler strips both kinds of decoration.

>>>> [sorry for the rant — but I think a critical look at extensions is long overdue: I rarely see someone questioning their role, so basically, we are making important decisions based on pure superstition]
>>>> 
>>>> A protocol itself is already a vehicle to group related methods,
>>> 
>>> A protocol is a vehicle for generic programming and separation of abstractions.
>> True — but does that stop a protocol from being a way to group related methods?
> 
> Protocols are not free. They come at a cost of wrapping the object in an existential container and make indirect calls via the witness table. Extensions on value types are free.
All true — but not an answer to the question ;-)

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