Section on ADTs

Author: odersky

docs/docs/reference/adts.md

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+# Algebraic Data Types
+
+The `enum` concept is general enough to also support algebraic data
+types (ADTs) and their generalized version (GADTs). Here's an example
+how an `Option` type can be represented as an ADT:
+
+```scala
+enum Option[+T] {
+  case Some[+T](x: T)
+  case None
+}
+```
+
+This example introduces `Option` enum class with a covariant type
+parameter `T`, together with two cases, `Some` and `None`. `Some` is
+parameterized with a type parameter `T` and a value parameter `x`. It
+is a shorthand for writing a case class that extends `Option`. Since
+`None` is not parameterized it is treated as a normal enum value.
+
+The `extends` clauses that were omitted in the example above can also
+be given explicitly:
+
+```scala
+enum Option[+T] {
+  case Some[T](x: T) extends Option[T]
+  case None          extends Option[Nothing]
+}
+```
+
+Note that the parent type of `None` is inferred as
+`List[Nothing]`. Generally, all covariant type parameters of the enum
+class are minimized in a compiler-generated extends clause whereas all
+contravariant type parameters are maximized. If `Option` was non-variant,
+you'd need to give the extends clause of `None` explicitly.
+
+As for normal enum values, the cases of an `enum` are all defined in
+the `enum`s companion object. So it's `Option.Some` and `Option.None`
+unless the definitions are "pulled out" with an import:
+
+```scala
+scala> Option.Some("hello")
+val res1: t2.Option[String] = Some(hello)
+scala> Option.None
+val res2: t2.Option[Nothing] = None
+```
+
+Note that the type of the expressions above is always `Option`. That
+is, the implementation case classes are not visible in the result
+types of their `apply` methods. This is a subtle difference with
+respect to normal case classes. The classes making up the cases do
+exist, and can be unvealed by constructing them directly with a `new`.
+
+```scala
+val res3: t2.Option.Some[Int] = Some(2)
+scala> scala> new Option.Some(2)
+```
+
+
+
+
+

tests/pos/reference/adts.scala

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+object t1 {
+
+enum Option[+T] {
+  case Some[T](x: T)
+  case None
+}
+
+}
+
+object t2 {
+
+enum Option[+T] {
+  case Some[T](x: T) extends Option[T]
+  case None          extends Option[Nothing]
+}
+
+
+}
+
+enum Color(val rgb: Int) {
+  case Red   extends Color(0xFF0000)
+  case Green extends Color(0x00FF00)
+  case Blue  extends Color(0x0000FF)
+  case Mix(mix: Int) extends Color(mix)
+}
+