MTL.scala

/** Enable MTL-style programming by avoiding ambiguous implicits. */
class MTL[M](val M: M) extends AnyVal
object MTL {
  type MonadMTL[F[_]] <: Monad[F]
  implicit def mtl[M](implicit M: M): MTL[M] = new MTL[M](M)

  private def MonadMTL[M[f[_]] <: Monad[f], F[_]](M: M[F]): MonadMTL[F] =
    M.asInstanceOf[MonadMTL[F]]

  /** An artificial hierarchy for MTL type classes. */
  object Hierarchy extends Hierarchy1
  sealed abstract class Hierarchy0 extends Hierarchy1 {
    implicit def monadP[F[_]](implicit mtl: MTL[MonadPlus[F]]): MonadMTL[F] = MonadMTL(mtl.M)
  }
  sealed abstract class Hierarchy1 extends Hierarchy2 {
    implicit def monadE[F[_], E](implicit mtl: MTL[MonadError[F, E]]): MonadMTL[F] = MonadMTL(mtl.M)
  }
  sealed abstract class Hierarchy2 extends Hierarchy3 {
    implicit def monadS[F[_], S](implicit mtl: MTL[MonadState[F, S]]): MonadMTL[F] = MonadMTL(mtl.M)
  }
  sealed abstract class Hierarchy3 extends Hierarchy4 {
    implicit def monadR[F[_], R](implicit mtl: MTL[MonadReader[F, R]]): MonadMTL[F] = MonadMTL(mtl.M)
  }
  sealed abstract class Hierarchy4 extends Hierarchy5 {
    implicit def monadT[F[_], W](implicit mtl: MTL[MonadTell[F, W]]): MonadMTL[F] = MonadMTL(mtl.M)
  }
  sealed abstract class Hierarchy5 {
    implicit def monadPlus[F[_]](implicit mtl: MTL[MonadPlus[F]]): MonadPlus[F] = mtl.M
    implicit def monadError[F[_], E](implicit mtl: MTL[MonadError[F, E]]): MonadError[F, E] = mtl.M
    implicit def monadState[F[_], S](implicit mtl: MTL[MonadState[F, S]]): MonadState[F, S] = mtl.M
    implicit def monadReader[F[_], R](implicit mtl: MTL[MonadReader[F, R]]): MonadReader[F, R] = mtl.M
    implicit def monadTell[F[_], W](implicit mtl: MTL[MonadTell[F, W]]): MonadTell[F, W] = mtl.M
  }
}