How to use sealed classes with generics in kotlin

Question

I can't find a solution to this problem

data class Data(val s: String)
sealed class Base<T>(val t: T, val f: Base<T>.() -> Unit)
class A(data: Data, f: A.() -> Unit) : Base<Data>(data, f)

Type mismatch.Required:Base<Data>.() → Unit Found: A.() → Unit

Please tell me what should be the correct usage

Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. — Community, Jun 01 '23 at 10:10

k314159 · Answer 1 · 2023-06-01T08:56:19.437

The problem has nothing to do with sealed classes, and nothing to do with generics. It is the same problem as the following:

fun fb(block: Number.() -> Unit) {
    BigDecimal.ONE.block()
}

fun fd(block: Int.() -> Unit) {
    fb(block)  // Error: required: Number.() -> Unit, found: Int.() -> Unit
}

As you can see from the above simplification of your problem, the reason for the error is that you can't pass a Subclass.() -> Unit block to something that expects a Base.() -> Unit block. If it were possible, then you would be able to call fd with a block that acts on an Int but fb passes it a BigDecimal instead. So you need to change your class A to:

class A(data: Data, f: Base<Data>.() -> Unit) : Base<Data>(data, f)

How to use sealed classes with generics in kotlin

1 Answers1