ZIO approach with TF

playground.scala

import cats.~>

object playground {

  // typeclasses

  trait MIO[F[_], R, +E, +A] {

    def map[B](f: A => B): MIO[F, R, E, B]
    def flatMap[R1 <: R, E1 >: E, B](k: A => MIO[F, R1, E1, B]): MIO[F, R1, E1, B]

    def transform[G[_]](nt: F ~> G): MIO[G, R, E, A]

  }

  trait EnvAccess[F[_]] {

    def accessM[R, E, A](f: R => MIO[F, Any, E, A]): MIO[F, R, E, A]

  }

  object EnvAccess {

    def apply[F[_]](implicit F: EnvAccess[F]): EnvAccess[F] = F

    def accessM[F[_]: EnvAccess, R, E, A](f: R => MIO[F, Any, E, A]): MIO[F, R, E, A] =
      EnvAccess[F].accessM(f)

  }

  // db

  trait EvalDB[F[_], DB[_]] {

    def evalDb: DB ~> F

  }

  // model

  trait Currency

  // repository

  trait CurrencyRepository[F[_]] {

    def currencyRepository: CurrencyRepository.Api[F]

  }

  object CurrencyRepository {

    trait Api[F[_]] {

      def findAllCurrencies: MIO[F, Any, Nothing, Seq[Currency]]

    }

  }

  // service

  trait CurrencyService[F[_], DB[_]] {

    def currencyService: CurrencyService.Api[F, DB]

  }

  object CurrencyService {

    trait Api[F[_], DB[_]] {

      def findAllCurrencies: MIO[F, CurrencyRepository[DB] with EvalDB[F, DB], Nothing, Seq[Currency]]

    }

    class Impl[F[_]: EnvAccess, DB[_]] extends Api[F, DB] {

      override def findAllCurrencies: MIO[F, CurrencyRepository[DB] with EvalDB[F, DB], Nothing, Seq[Currency]] =
        EnvAccess.accessM { env =>
          env.currencyRepository.findAllCurrencies.transform[F](env.evalDb)
        }

    }

  }

}