udalov · March 28, 2016 11:31 · Mar 28, 2016
diff --git a/spec-sealed-data.md b/spec-sealed-data.md
@@ -0,0 +1,89 @@
+# Summary
+
+Allow data classes as sealed class subclasses.
+```
+sealed class Base {
+    data class Derived(...) : Base()
+}
+```
+
+# Motivation / use cases
+
+- Algebraic data types
+  https://discuss.kotlinlang.org/t/sealed-classes-design/82
+  http://stackoverflow.com/questions/35921234/kotlin-sealed-class-cannot-contain-data-classes-why
+
+# Description
+
+`data` modifier should now be allowed on subclasses of sealed classes.
+
+`componentN`, `equals`, `hashCode`, `toString`, `copy` are generated exactly as for usual data classes.
+Only primary constructor parameters of the data class itself should participate in all of those functions.
+
+```
+sealed class Base(val sealedParam: Any?) {
+    data class Derived(val dataParam: Any?) : Base(null)
+}
+
+fun test(d: Base.Derived) {
+    val (x) = d       // ok, x = d.dataParam
+    val (x, y) = d    // error, no component2 in Derived
+}
+```
+
+<...>
+
+# Effort
+
+- Front-end: little
+    - relax the diagnostic
+    - improve some existing diagnostics: "data class cannot inherit from class" is no longer true
+- Back-end (JVM):
+    - always generate equals/hashCode/toString for data classes (KT-6206)
+- Standard library: none
+- IDE: ???
+- Tools: none
+
+# Drawbacks 
+
+None (?)
+
+# Open questions
+
+- Sealed class can have super-classes itself, will there be any problems with that?
+  Consider sealed subclasses of sealed classes
+
+```
+sealed class Base {
+    data class Derived(val s: String) : Base()
+    override fun toString() = "Base"
+}
+
+Base.Derived("ok").toString() // should return "Derived(s='ok')", not "Base"
+```
+
+- For uniformity consider generating toString() to object declarations, similar to data classes (KT-4107)
+
+# Related issues
+
+Some issues were closed around 1.0 because data class inheritance had been forbidden. They should be located and investigated again wrt this proposal.
+
+- KT-11306: won't be able to e.g. specify abstract toString() in sealed class
+
+<...>
+
+# Alternatives
+
+We could support sealed interfaces. Some of the use cases will be covered because data classes can inherit from sealed interfaces.
+However, it's not possible to store data in an interface. Also it's possible to inherit such interface from Java, which would break exhaustive `when`s.
+
+# Possible future advancements
+
+- Allow data classes extend other classes?
+- Allow non-val/var (incl. vararg) trailing constructor parameters for data classes?
+
+# (???) Comparison to other languages?
+
+
+
+