Is there a way to model this? ```interface Foo { ...

# getting-started

[Mark]

Is there a way to model this?

interface Foo {
    // withBar always returns same type as `this`
    fun withBar(bar: Bar): Foo
}

class SubFoo(val bar: Bar? = null): Foo {  
    override fun withBar(bar: Bar): SubFoo = SubFoo(bar)
}

Full code in thread

[Mark]

class Bar

interface Foo {
    // withBar always returns same type as `this`
    fun withBar(bar: Bar): Foo
}

class SubFoo(val bar: Bar? = null): Foo {  
    override fun withBar(bar: Bar): SubFoo = SubFoo(bar)
}

// will not compile: Type mismatch
// fun <F: Foo> List<F>.mapFoos(bar: Bar): List<F> = this.map { foo ->
//     foo.withBar(bar)
// }

// alternative approach which seems to work
fun <F: Foo> List<F>.mapFoos(bar: Bar, withBar: (F, Bar) -> F): List<F> = this.map { foo ->
    withBar(foo, bar)
}

// convenience for when don't care about sub type. In this case
// wouldn't need to use mapFoos because of mapFoos simple
// implementation, but imagine mapFoos is much more complex.
fun List<Foo>.mapFoos(bar: Bar): List<Foo> = mapFoos(bar, Foo::withBar)

fun main() {
    val subFoo = SubFoo()
    val bar = Bar()
    val mappedSubFoo = listOf(subFoo).mapFoos(bar, SubFoo::withBar) // List<SubFoo>
}

[Mark]

https://pl.kotl.in/nD8bEGzcb

[Ruckus]

https://pl.kotl.in/cLeiYOXkE

[Mark]

Ah, yes I’ve used this technique in the past. But I really don’t like it. Making Foo generic just clutters the code IMO.

[Wout Werkman]

Some other languages do have a language feature for this, such as Swift, who calls this Self. But the only way to model this in Kotlin is with Generics.
One such example is Comparable, which practically contains a function: compare(other: Self): Int. But since Java too did not have Self, Comparable takes a generic argument as a workaround.