I just came across the fact that a Show[Throwable] exists.

1. It is not wired up into import Scalaz._, one needs to import scalaz.std.java.throwable._
2. It discards the stack trace entirely.

What's going on with this? :D
Is it OK if I fix both of these (for 7.2 and 7.3)?

In scalaz/scalaz#1958 @fommil uses this optimization:

fa match {
  case Right(b) => f(b)
  case Left(a) => Left(a)
}

fa match {
  case Right(b) => f(b)
  case a => a.asInstanceOf[Either[L, B]]
}

perhaps we could detect this particular case and optimize it automatically.