Programming

Kotlin now has sealed classes. I think they primarily resemble the concept of "tagged union"s. Rust has a similar elegant construct in form of enums.

sealed class Type () {
    class Named(val name: String): Type()
    class Nested {
        class Function(val param: Type, 
                       val result: Type): Type()
    }
    object Top: Type()
}

when (type) {
    is Named -> println(name)
    is Nested.Function -> println("$param -> $result")
    // Alternatively, we can omit is here
    is Top -> println("TOP")
}

• Sealed Class Hierarchies and when-expressions, https://github.com/JetBrains/kotlin/blob/2dea17a3a937940bd1f668307db7110edc6b9da3/spec-docs/sealed-hierarchies-and-when.md
• Support exhaustive when for sealed class hierarchies, https://youtrack.jetbrains.com/issue/KT-7606
• https://doc.rust-lang.org/book/enums.html