lforite

case class Kleisli[F[_], A, B](run: A => F[B])

lforite

type Traceable[T] = CorrelationId ?=> T

//then
given cid: CorrelationId = ...
def f(x: Int): Traceable[Int] = ...

f(2) using cid   // explicit argument
f(2)             // argument is inferred

lforite

trait ServiceA {
  def createA(a: EntityA): RIO[_]
}

trait ClientB {
  def getB(bId: EntityBID): RIO[_]
}

case class ServiceA(clientB: ClientB) extends ServiceA {
  override def createA(a: EntityA): RIO[_] = {

lforite

type RIO[A] = Kleisli[IO, CorrelationId, A]

lforite

case class HttpClientB(httpClient: HttpClient) extends ClientB {
  def getB(bId: EntityBID): Klesli[IO, CorrelationId, EntityB] = Kleisli { cid =>
     httpClient
       .url(s"https://my-b-service-url.com/api/b-entity/bId")
       .headers("X-Correlation-Id" -> cid)
       .get()
  }
}

lforite

case class ApiA(serviceA: ServiceA) {
  def postA() = Route { request =>
     val correlationId = request.headers("X-Correlation-Id")
         .map(CorrelationId)
         .getOrElse(CorrelationId.new)
         
     serviceA.createA(request.body.decodeJson[EntityA])
       .run(correlationId)
  }
}

lforite

trait ServiceA {
  def createA(a: EntityA): Klesli[IO, CorrelationId, _]
}

trait ClientB {
  def getB(bId: EntityBID): Klesli[IO, CorrelationId, _]
}

case class ServiceA(clientB: ClientB) extends ServiceA {
  override def createA(a: EntityA): Klesli[IO, CorrelationId, _] = {

lforite

trait Logger {
  def info(msg: String): Kleisli[IO, CorrelationId, Unit] = Kleisli { cid =>
    IO(underlyingLogger.info(s"[$cid] $msg"))  
  }  
}

lforite

val context: String = "Kleisli, is that you ?"
val kleisli: Kleisli[IO, String, String] = Kleisli.ask()

val io: IO[String] = kleisli.run(context)
io.unsafeRunSync()
// returns string "Kleisli, is that you?

lforite

val context: String = "Kleisli, is that you ?"
val kleisli: Kleisli[IO, String, Unit] = Kleisli { (context: String) =>  
    IO(println(s"printing context: $context")) 
}
val io: IO[Unit] = kleisli.run(context)
io.unsafeRunSync()
//"printing context: Kleisli, is that you?