I’m trying to learn more about generics. I current...

# announcements

[kyleg]

I’m trying to learn more about generics. I currently have

interface Id

inline class BarId(val value: Int): Id
inline class BazId(val value: Int): Id

sealed class Foo<T: Id> {
   abstract val id: T
   data class Bar(...): Foo<BarId>()
   data class Baz(...): Foo<BazId>()
}

fun <T: Foo<U>, U: Id> myFun(entity: T): U = TODO()

Suppose I need to add a function to Foo that will take an Id param and spit back out a child class (the one that is affiliated with the Id implementation in question). So myBar.changeId should take BarId and spit out Bar. For example.

sealed class Foo<T: Id> {
   ...
   abstract fun <T: Foo<U>, U: Id> changeId(id: U): T
}

I’m sitting here with

data class Bar(...): Foo<BarId>() {
   override fun changeId(id: BarId): Bar = TODO()
}

not really knowing what to put for changeId. Obviously the implementation is going to be changeId(id: BarId): Bar = this(id=id) but I have to put some generics in the signature somewhere or else compiler/linter tells me this does not match the signature of Foo.changeId, which it is intended to override.

[StavFX]

Since Bar and Baz are data classes, you can just use the copy method to create a new instance with a different ID.
Maybe you simplified your use-case before posting here, so not sure this will actually work for your real world purposes.

[StavFX]

Also, this is pretty nasty, but it works

sealed class Foo<U : Id, S : Foo<U, S>> {
    abstract val id: U
    abstract fun changeId(id: U): S

    data class Bar(override val id: BarId) : Foo<BarId, Bar>() {
        override fun changeId(id: BarId): Bar = this.copy(id = id)
    }

    data class Baz(override val id: BazId) : Foo<BazId, Baz>() {
        override fun changeId(id: BazId): Baz = this.copy(id = id)
    }
}

[kyleg]

Thanks. copy is what I was using, but I omitted it so my question wouldn’t push potential answers in the direction I’d (possibly suboptimally) chosen. My issue was more around how to keep using the generics. I didn’t realize that I could reuse the class generic param in the function without re-declaring a generic in the function header itself.

Problem solved, thanks!

[StavFX]

Yeah.. that type of “recursive generics” (not sure if that’s a proper name, just what it looks like to me) is a bit odd.. but can help define a “self” type