gistfile1.txt

[info] Compiling 9 Scala sources to /home/robert/Git/mobile-apps-api/common/target/scala-2.10/classes...
[error] /home/robert/Git/mobile-apps-api/common/app/lib/PlayJson.scala:17: type mismatch;
[error]  found   : (play.api.libs.json.Reads[Nothing], play.api.libs.json.Reads[Nothing]) => play.api.libs.json.Reads[Nothing]
[error]  required: (play.api.libs.json.Reads[_ <: A], play.api.libs.json.Reads[_ <: A]) => play.api.libs.json.Reads[_ <: A]
[error]     def oneOf[A](reads: Seq[Reads[_ <: A]]): Reads[A] = reads.reduce(either)
[error]                                                                      ^
[error] /home/robert/Git/mobile-apps-api/common/app/lib/PlayJson.scala:17: type mismatch;
[error]  found   : play.api.libs.json.Reads[_$1] where type _$1 <: A
[error]  required: play.api.libs.json.Reads[A]
[error] Note: _$1 <: A, but trait Reads is invariant in type A.
[error] You may wish to define A as +A instead. (SLS 4.5)
[error]     def oneOf[A](reads: Seq[Reads[_ <: A]]): Reads[A] = reads.reduce(either)

PLAY SUCKS DAMMIT

  // Reads is covariant, dunno why it isn't.
  def readsUpCast[A](r: Reads[_ <: A]): Reads[A] = r.map(x => x)

  def mapx[A](reads: Seq[Reads[_ <: A]]): Seq[Reads[A]] = reads.map(readsUpCast[A] _)

  def oneOf[A](reads: Seq[Reads[_ <: A]]): Reads[A] = mapx[A](reads).reduce(_ orElse _)

Let's see if someone wiser could answer: http://stackoverflow.com/questions/25567572/why-reads-is-not-declared-covariant