好吧，现在Kotlin 正式出局了，我又开始玩它了，我很困惑，我需要在sealed和data的优势之间做出 Select ，但不知何故，这两者都不能兼得.
This, for example, seems to make sense to me, but does not compile:
sealed class Expr {
data class Const(val number: Double) : Expr()
data class Sum(val expr1 : Expr, val expr2 : Expr) : Expr()
}
因为数据类不能扩展其他类.
Is there something I am missing?
Shortly before having entered Beta state, Kotlin team had decided to add certain limitations on data classes usage (see this post) because of the problems they caused in class hierarchies.
One of the limitations is that data class should not subtype another class, only interfaces are allowed. Consequently, data classes cannot derive from a sealed class.
This was a necessary measure to avoid further postponing the 1.0 release. Some of the limitations were said to be lifted in future releases, once the problematic cases are thoroughly reviewed and a good design solution is found.