Anyone got ideas for refactoring this? The nested EitherTs don't seem finished to me, although they're compact.

CallStrategy.scala

trait CallStrategy[C, E] {

  type State[_]

  def call
      (desc: CallDesc[C],
        state: State[C],
        pool: ExecutionContextExecutorService)
      : Task[E \/ Media]

  def getState
      (desc: CallDesc[C], prevState: Option[State[C]])
      : Task[E \/ State[C]]

  def nextState
      (desc: CallDesc[C], error: E, prevState: State[C])
      : Task[E \/ State[C]]

  def callRetryOnce
      (desc: CallDesc[C],
        pool: ExecutionContextExecutorService)
      : EitherT[Task, E, Media] =
    EitherT(getState(desc, none)) >>= { s0 =>
      EitherT(call(desc, s0, pool)).swap >>= { e =>
        EitherT(nextState(desc, e, s0)) >>= { s1 =>
          EitherT(call(desc, s1, pool))
        } swap
      } swap
    }

}