Incorrect Scala code for OneOfThree type example in Section 6.3 Sum Types

[jasonrute]

In the section 6.3 Sum Types, there is an example called OneOfThree with the following Scala code:

sealed trait OneOfThree[A, B, C]
final case class Sinistral[A](v: A) extends OneOfThree[A, Nothing, Nothing]
final case class Medial[B](v: B) extends OneOfThree[Nothing, B, Nothing]
final case class Dextral[C](v: C) extends OneOfThree[Nothing, Nothing, C]

While this will compile, it won't work. It is impossible (at least without type coercion) to get a type of, say, OneOfThree[Int, Int, Int]. The solution is to make the type parameters covariant by adding +s to the first line as follows:

sealed trait OneOfThree[+A, +B, +C]

This would agree with the examples of Either[+A,+B] and Option[+A] in the same section of the book, and it is idiomatic Scala.

(There is another possible solution which is to change all the other lines to something like final case class Sinistral[A, B, C](v: A) extends OneOfThree[A, B, C], but that would be unwieldy in practice and be different from the other examples in the book.)

[hmemcpy]

Isn't variance/subtyping fun :) This is a great suggestion, this is how Scala ADTs are typically defined. I would really appreciate a PR for this. Thanks!

[jasonrute]

Sure. I'll make a PR for this file (https://github.com/hmemcpy/milewski-ctfp-pdf/blob/master/src/content/1.6/code/scala/snippet22.scala) later today or tomorrow.

[hmemcpy]

Fixed in #180.