Constructor that enables creation of schema for sum types.

Each constructor of the sum type I is represented as a member of the list of alternatives. Each alternative defines a prism between a single constructor of the sum type, and an underlying type describing the arguments demanded by that constructor.

Consider the following sum type. The first constructor takes no arguments; the second takes two.

sealed trait Role

case object User extends Role
case class Administrator(department: String, subordinateCount: Int) extends Role

A schema value for this type looks like:

val roleSchema = oneOf(
  alt[Unit, Prim, Role, Unit](
    "user",
    Schema.empty,
    (_: Unit) => User,
    {
      case User => Some(Unit)
      case _ => None
    }
  ) ::
  alt[Unit, Prim, Role, Administrator](
    "administrator",
    rec[Prim, Administrator](
      (
        required("department", Prim.str, (_: Administrator).department),
        required("subordinateCount", Prim.int, (_: Administrator).subordinateCount)
      ).mapN(Administrator.apply)
    ),
    identity,
    {
      case a @ Administrator(_, _) => Some(a)
      case _ => None
    }
  ) :: Nil
)