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

[Mike Sanderson]

It seems that this discussion got sidetracked from the original proposal,
that aimed to provide more visibility about default implementation for
protocols, to the merits of writing functionality for classes and structs
in same-file extensions.

1. `default` for Protocols

First, the issue the original proposal wants to solve is real-- knowing
what methods have default implementations and therefore what you actually
need to implement to conform to a complex protocol can get complex. (Chris
Eidhof has a great extended discussion about conforming to `Collection` in
Advanced Swift that covers this in detail. It's advanced.)

My understanding of the proposal is that marking a method `default` would
force the protocol author to write a default method, and provide a label
for anyone reading the protocol that this method has a default
implementation.

However, if non-default methods can still have protocol extensions (the
alternative of not allowing this would be a huge restriction on protocol
extensions) then after these extensions are written, the methods would have
default implementations but not be marked default-- we're back to
the original problem. This would be whenever the person writing the
extension can't modify the protocol to add default, which is probably
often.

So while I agree with the issue, I'm not sure adding a `default` label
solves it. Maybe a better way to link to any protocol extensions from the
protocol declaration, or better way to see where a conforming type gets its
conforming methods, but these might be Xcode features.

2. Same-file extensions

The issue of using same-file extensions to organize code probably should be
discussed as it's own topic. My understanding is it first emerged as a
suggestion from 2014 WWDC labs specifically because Swift did not have
pragma marks (https://www.natashatherobot.com/using-swift-extensions/,
https://stackoverflow.com/a/24069206/4474705). For self-contained, purely
additive protocol conformance I think it makes sense, such as for adding
`CustomDebugStringConvertable`. But it always seemed weird to me for things
like UITableViewDataSource on a view controller that exists to support a
table view-- even if all the methods for the delegate are there, they will
rely on access to stored properties for the model, rely on helper methods,
rely on lifecycle functions to have called `registerClass:forReuseIdentifer`
etc. The extension is just for grouping methods, which is better than
nothing, but which it does under the guise of a language feature.

Still, this style of extensions is widely used. I'm not sure if same-file
extensions needs language-level endorsement and codification, or if it's a
programming style/usage issue and people should argue about it on their own
time. But I think this thread shows regardless it's an open question.

Mike Sanderson




On Wed, Aug 2, 2017 at 11:44 AM, Gor Gyolchanyan via swift-evolution <
swift-evolution at swift.org> wrote:

>
> On Aug 2, 2017, at 6:18 PM, Tino Heth <2th at gmx.de> wrote:
>
> 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".
>
>
> I'll reiterate: the problem with the inline solution is that it forces all
> code to be bunched up in a single place and sacrifices readability.
>
> - 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?
>
>
> The extra guarantee that all non-same-file extensions don't have: the
> guarantee that the extension can be deterministically merged with the type
> definition before the type definition is emitted.
>
> 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
>
> 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
> <https://en.wikipedia.org/wiki/Don't_repeat_yourself>
>
>
> The Don't Repeat Yourself principle states that providing generic
> solutions to domains of problems is superior to providing largely similar
> solutions to specific problems. If anything, separating related bunches of
> code helps reduce extra work by localizing the code in a way that makes
> refactoring and debugging easier.
> The separation of concerns is a well-known software design and
> implementation principle whereby code quality (which encompasses
> readability, maintainability, extensibility, portability) are improved
> through strict separation of logically unrelated (or loosely related) parts
> of the code.
>
>
> 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
> 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.
>
>
> Your example demonstrated a hypothetical lack of a compiler limitation
> that would make extensions more convenient and I demonstrated that such a
> limitation (namely, forcing protocol conformance and implementation of
> protocol requirements to be in a single extension) would not be a good idea.
> Extensions are not decoration. They're at the very least access control
> tool. Extensions can have private members that are only visible to the
> extension and all other extensions in the same file. You can put many
> extensions into separate files and have them all operate on private members
> without exposing those private members and without having to bunch them all
> up into one place.
>
> [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 ;-)
>
>
> The answer is no. Extensions are specifically designed to group related
> methods and they have minimal overhead. Protocols have overhead (even if
> that overhead is not nearly as much as a class overhead) and serve the
> purpose of providing polymorphism. Protocols also pollute the global
> namespace and introduce complexity to the interface, so they also have an
> overhead of complexity. Extensions do none of those things.
> Using protocols simply for bunching up related methods is similar to using
> a bulldozer for digging a hole in a sandbox.
>
>
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>
>
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