TofuStreamAdapter.scala

trait MyService[F[_], S[_]] {
  def getItem(id: Long): F[Item]
  def getItemsByGroup(groupId): F[S[Item]]
}

class MyServiceImpl[F[_], S[_]](xa: Transactor[F]) extends MyService {
  
  def getItemsByGroup(groupId): S[Item] = {
    val stream = sql"select * from items where group = $groupId"
      .query[Item]
      .stream
      .transact(xa)
    // Convers fs2.Stream[F, Item] to S[_] in both of them have `tofu.streams` instances.
    StreamAdapter(stream) 
  }
}