> On Dec 31, 2015, at 5:44 AM, Tino Heth <[email protected]> wrote:
>
> This is some sort of a cross-post from another thread ["automatic protocol
> forwarding"] — for anyone who wants to follow, I recommend to read
> https://kotlinlang.org/docs/reference/classes.html
> <https://kotlinlang.org/docs/reference/classes.html>
> The idea of "function-like class declaration" has been introduced here by Joe
> Groff, but apparently, its benefits have been underestimated.
>
>> If you feel Kotlin’s approach is better please respond to the memberwise
>> initialization thread with some examples written in both Kotlin and in Swift
>> using the memberwise initialization feature I am proposing to demonstrate
>> how and why you think it is better.
>
>> Here is Kotlin:
>> class Person constructor(firstName: String, lastName: String) {
>> }
> That is a bad start — do you want to make Kotlin look worse than it is? You
> can just write "class Person(firstName: String, lastName: String)", the other
> syntax merely exists to allow designated constructors with special access
> restrictions.
I was not trying to make Kotlin look worse. I copied from the Kotlin web page.
I do see that constructor can be omitted when no modifiers are necessary.
>
>> Here is Swift under my proposal:
>> class Person {
>> var firstName: String
>> var lastName: String
>> // compiler synthesizes memberwise init
>> }
>>
>> However, my proposal gives you a lot of additional flexibility:
> I deny that, and even if it is true, there is a price to pay — and that is
> more than the lines of code that are required…
Personally, I think it is a lot more readable to put members separate lines,
but if you don’t like doing that:
struct Person { var firstName: String; var lastName: String
// compiler synthesizes memberwise init
}
There are no additional lines of code when you do it this way. You need to
introduce the member declarations with `let` or `var` but that is not
necessarily a bad thing.
>
>> 1. It interacts well with access control
> Better than Kotlin? Please prove this.
Please look at the examples I have in the Access Control section of the
proposal. I spent some time reading the Kotlin docs and it isn’t clear to me
that Kotlin can do this. But maybe it can. I don’t know Kotlin well. It
sounds like you do, so if it can, please show how this is done in Kotlin.
>
>> 2. Partial memberwise initialization is possible
> The same with Kotlin — and imho at least as easy
That isn’t clear from the Kotlin docs. It may well be that they are just
missing examples. Please post some samples showing how this is handled.
>
>> 3. It allows memberwise initializers to accept non-memberwise parameters to
>> initialize private state
> I think this can be achieved with less effort using function-like class
> declaration (afair Joe already gave an example)
I don’t see either of the examples Joe posted doing this. Here is an example
showing what I mean:
struct S {
let s: String
private let i: Int
// user declares:
memberwise init(other: S, ...) {
i = other.i
}
// compiler synthesizes (suppressing memberwise initialization for
properties with lower visibility):
init(other: S, s: String) {
/* synthesized */ self.s = s
// body of the user's initializer remains
i = other.i
}
}
>
>> 4. More than one memberwise initializer is possible
> Kotlin has no need for memberwise initializers at all, and I see this as a
> big advantage
Please explain how it is an advantage. How does Kotlin handle a case where you
have some private state that needs to be initialized internally to protect
invariants, but also some members which users can initialize (such as
appearance attributes on a UI widget)?
>
>> 5. Memberwise initialization of properties with declaration modifiers,
>> behaviors / delegates is possible
> https://kotlinlang.org/docs/reference/delegated-properties.html
> <https://kotlinlang.org/docs/reference/delegated-properties.html>
> (afaik this is not only possible, it's handled by the current compiler for a
> long time)
Yes, I know Kotlin has this feature. It isn’t clear from the docs how
initialization of such properties is handled (for example an Observable
property). Maybe you can provide some examples of how this works.
>
>> And probably more. My approach was to design a solution that fits into the
>> current Swift language and is orthogonal to other language features as much
>> as possible.
> Afaics there not much room left for the promised additional flexibility… and
> the problem with default values for constants just doesn't exist in Kotlin at
> all.
This is currently a problem in Swift. I am confident that it can be solved one
way or another. I don’t think a solution should be tied to the “type
initializer parameter list” syntax.
>
> I neither state your proposal is bad, nor that we should simply copy an
> existing solution, but even as a big fan of Swift, I have to admit that
> Kotlin (which I probably will never use in real projects) clearly performs
> better in this area:
> It's more concise, it doesn't need "required" nor "convenience", it offers an
> intuitive syntax for forwarding and it is easy to grasp.
>
> I know that I'm not just challenging your own, personal ideas here, and have
> no illusions about the outcome of this bold move — but it is my honest
> opinion this proposal might (in total) be better than the status quo, but is
> inferior to Kotlin in every aspect (whereas the current scheme has the edge
> of familiarity on its side).
If you can clearly demonstrate how Kotlin is superior in a specific area I will
give that great consideration. I want this proposal to be the best it can be.
However, you’re going to need to do more than just link to the docs which I
have already looked at.
The good news is that as far as I can tell the things you like about what
Kotlin is doing are not mutually exclusive with this proposal at all. Think of
it this way - this proposal provides a flexible and orthogonal foundation for
memberwise initialization. If desired, a future enhancement could easily be
developed to provide additional syntactic sugar on top of it. The example Joe
posted shows how that might work.
If this proposal is accepted and you want to pursue a proposal for that
additional layer of syntactic sugar to get closer to Kotlin syntax I encourage
you to do that. The new syntax should be evaluated independently as its own
proposal. I would be happy to help show how your desired syntax could be
transformed into existing syntax (including the memberwise initialization
syntax if this proposal is accepted).
Matthew_______________________________________________
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