Auditing Ideas

auditing.scala

type Timestamp = Long
type Nanos = Long

sealed trait Clock[A]
final case class CurrentMillis extends Clock[Timestamp]
final case class CurrentNanos  extends Clock[Nanos]

final case class EntryId(val asString: String) extends AnyVal

sealed trait Audit[A]
final case class StartEntry(startTs: Timestamp, startNanos: Long, log: Json)
  extends Audit[EntryId]
final case class EndEntry(id: EntryId, endNanos: Long)
  extends Audit[Unit]

// Another possibility
final case class AuditLogEntry(user: User, ts: Timestamp, operation: Json)

sealed trait AuditLog[A]
final case class AddEntry(operation: Json) extends AuditLog[AuditLogEntry]

type ClockF[A] = Coyoneda[Clock, A]
type AuditF[A] = Coyoneda[Audit, A]

def currentMillis[S[_]: Functor](implicit S: ClockF :<: S): Free[S, Timestamp]
def currentNanos[S[_]: Functor](implicit S: ClockF :<: S): Free[S, Nanos]
  
def startEntry[S[_]: Functor](startTs: Timestamp, startNanos: Long, log: Json)(implicit S: AuditF :<: S): Free[S, EntryId]
def endEntry[S[_]: Functor](entryId: EntryId, endNanos: Long)(implicit S: AuditF :<: S): Free[S, Unit]

def log[S[_]: Functor](log: Json, op: Free[S, A])(implicit S0: AuditF :<: S, S1: ClockF :<: S): Free[S, A] =
  for {
    ts  <- currentMillis
    ns  <- currentNanos
    id  <- startEntry(ts, ns, log)
    a   <- op
    ne  <- currentNanos
    _   <- endEntry(id, ne)
  } yield a
  
// or, if you want to log a particular api

sealed trait Api[A]
final case class StrOp extends Api[String]
final case class IntOp extends Api[Int]

type ApiF[A] = Coyoneda[Api, A]

type Eff0[A] = Coproduct[AuditF, ClockF, A]
type Eff[A]  = Coproduct[ApiF, Eff0, A]

val logApi: Api ~> Free[Eff, ?] =
  new (Api ~> Free[Eff, ?]) {
    def apply[A](api: Api[A]) = api match {
      case StrOp =>
        log(JString("StrOp!"), <inject StrOp into Eff, lift into Free>)
        
      case IntOp =>
        log(JString("IntOp!"), <inject IntOp into Eff, lift into Free)
    }
  }
  
val someApiProgram: Free[ApiF, A] = ???

val loggedApiProgram = someApiProgram flatMapSuspension logApi