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

[Tino Heth]

> The same-file mentality comes from Swift 3, which introduced fileprivate (which, since Swift 4, got merged into private within a single file scope). With that feature, implementors don't have to choose between access protection and code locality and can get both.
You weren't here when this was discussed, were you?
It was nearly exactly the other way round:
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.

>> 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.

>> 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.

>> [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?

>>  and if you have a huge entity, it doesn't get better just because you split it and hide its complexity.
> 
> Splitting and hiding complexity is by far the only reasonable way of dealing with huge entities. If the entity gains too much responsibility, it's probably a good idea to split it into several smaller entities. If the entity contains a large amount of accidental complexity that solely serves the purpose of enabling a select set of intended features, then it's probably a good idea to hide the accidental complexity away from users of the entity.
> 
> In fact, that's exactly why I always wished that protocols could get private requirements that are there solely for use in protocol extensions and are otherwise hidden from existence. I haven't talked about this in detail because I don't see a reasonable way of implementing it yet.

Like protected (you can override/implement, but never call)?
Guess this will haunt us forever ;-) — but that door is imho closed now.


- Tino

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