is there some kind of magic type parameter that re...

# getting-started

[Bernhard]

is there some kind of magic type parameter that refers always to the current instance like Self? I'm in front of an api with an inherited updated method that always returns the same class as the current object:

open class Model {
    fun update(): Model {
        // logic in here
        TODO()
    }
}

open class User: Model()

open class SpecialUser: User()

obviously User.update() should return a User and SpecialUser.update() a special user but I don't really see a way of having the type defs work other than overriding it with a more special one in each subtype

PS: it's a JS API, just trying to figure out what the limits of Kotlin's type system are 🙂

[Riccardo Lippolis]

you can do something like this:

open class Model<out T: Model<T>> {
    fun update(): T {
        // logic in here
        TODO()
    }
}

open class User: Model<User>()

open class SpecialUser: Model<SpecialUser>()

[Bernhard]

SpecialUser needs to inherit User though

[Riccardo Lippolis]

ah, missed that requirement 😅

[Bernhard]

yeah, typo first; it's the thing that really messes this all up

[Bernhard]

you basically need to pass up a concrete type through the chain while also not being allowed to have a type parameter

[Rob Elliot]

There’s a YouTrack issue for Self types you could vote on:
https://youtrack.jetbrains.com/issue/KT-6494/Self-types

[Youssef Shoaib [MOD]]

Well you could separate it into 2 methods:

open class Model {
  fun doUpdate() {
    // logic in here
  }
}
fun <M: Model> M.update(): M = this.also { doUpdate() }

[Bernhard]

hm, that's nifty, basically using a generic function parameter instead of defining that on the method

[Bernhard]

however, I'm unsure if this exact solution would work for external js type definitions since I guess that both method signatures must be the same

[Bernhard]

and also only returns the current instance, not the new instance returned from the doUpdate() method

[Bernhard]

fun <T : Model> T.typesafeUpdate(): T = update() as T

[Bernhard]

@Youssef Shoaib [MOD] thanks, I think this is good enough

[Youssef Shoaib [MOD]]

Sure yeah as long as you're sure that update always returns the same type then this is fine.
I wonder how this is implemented on the JS side? Prototype shenanigans?

[Bernhard]

@Youssef Shoaib [MOD]

await this.constructor.update()

[Bernhard]

that one is a static method which uses this

[Bernhard]

there's a lot of weird stuff, like "here's a static method, but we bound the this to an instance"

[Youssef Shoaib [MOD]]

Huh fun! Is the whole hierarchy in JS? If so then I guessed this'll likely be safe.

[Bernhard]

right 🙂

[Bernhard]

everyone told them to not do it since it couldn't be typed properly in TS

[Bernhard]

but they wanted something akin to Django models