Abstract out DisjunctionT?

[adelbertc]

I'll just leave this here..

/** Laws? */
trait SumElim[F[_, _], G[_], A] {
  def elim[B, C](f: F[A, B])(left: A => C, right: B => C): G[C]
}

object SumElim {
  implicit def either[A]: SumElim[Either, Id, A] =
    new SumElim[Either, Id, A] {
      def elim[B, C](f: Either[A, B])(left: A => C, right: B => C): C = f.fold(left, right)
    }

  implicit def disjunction[A]: SumElim[Disjunction, Id, A] =
    new SumElim[Disjunction, Id, A] {
      def elim[B, C](f: Disjunction[A, B])(left: A => C, right: B => C): C = f.fold(left, right)
    }

  implicit def disjunctionT[G[_] : Functor, A]: SumElim[DisjunctionT[G, ?, ?], G, A] =
    new SumElim[DisjunctionT[G, ?, ?], G, A] {
      def elim[B, C](f: DisjunctionT[G, A, B])(left: A => C, right: B => C): G[C] = f.fold(left, right)
    }

  implicit val future: SumElim[({type l[e, a] = Future[a]})#l, Future, Throwable] =
    new SumElim[({type l[e, a] = Future[a]})#l, Future, Throwable] {
      def elim[B, C](f: Future[B])(left: Throwable => C, right: B => C): Future[C] =
        f.map(right).recover(left)
    }
}