ex7 - https://medium.com/@lu4nm3/monad-transformers-vs-mtl-d3a1167e2649

ex7_mvm.scala

final class IndexedReaderWriterStateT[F[_], E, L, SA, SB, A](val runF: F[(E, SA) => F[(L, SB, A)]]) {

  def map[B](f: A => B)(implicit F: Functor[F]): IndexedReaderWriterStateT[F, E, L, SA, SB, B] = {
    transform { (l, s, a) => (l, s, f(a)) }
  }
  
  def transform[LL, SC, B](f: (L, SB, A) => (LL, SC, B))
                          (implicit F: Functor[F]): IndexedReaderWriterStateT[F, E, LL, SA, SC, B] = {
    IndexedReaderWriterStateT.applyF {
      F.map(runF) { rwsfa =>
        (e: E, s: SA) => F.map(rwsfa(e, s)) { case (l, sb, a) =>
          val (ll, sc, b) = f(l, sb, a)
          (ll, sc, b)
        }
      }
    }
  }
  
  def flatMap[SC, B](f: A => IndexedReaderWriterStateT[F, E, L, SB, SC, B])
                    (implicit F: FlatMap[F], L: Semigroup[L]): IndexedReaderWriterStateT[F, E, L, SA, SC, B] = {
    IndexedReaderWriterStateT.applyF {
      F.map(runF) { rwsfa =>
        (e: E, sa: SA) =>
          F.flatMap(rwsfa(e, sa)) { case (la, sb, a) =>
            F.flatMap(f(a).runF) { rwsfb =>
              F.map(rwsfb(e, sb)) { case (lb, sc, b) =>
                (L.combine(la, lb), sc, b)
              }
            }
          }
      }
    }
  }

  ...
}