Agent

Agent: Reference with effectful mutators

Agent is like cats-effect Ref but it allows effectful updates of referenced value. It also allows enqueuing of mutations without waiting for their completion and mutation of values by filter(partial function).

trait Agent[F[_], A] {

  def get: F[A]

  def updateM(f: A => F[A]): F[A]

  def fireUpdateM(f: A => F[A]): F[Unit]

  def modifyM[B](f: A => F[(B, A)]): F[B]

  def updateSomeM(f: PartialFunction[A, F[A]]): F[A]

  def modifySomeM[B](default: B)(f: PartialFunction[A, F[(B, A)]]): F[B]
}

Agent’s companion object contains default implementation that can be built from cats-effect’s Ref and Semaphore:

object Agent {

  final case class SemRef[F[_]: Monad: Fire, A](ref: Ref[F, A], sem: Semaphore[F]) extends Agent[F, A] {
  // impl code...
  }
}

Creation

One can create Agent of some value with helper MakeAgent.

It allows different effects for creating and running Agent:

trait MakeAgent[I[_], F[_]] {

  def agentOf[A](a: A): I[Agent[F, A]]
}

MakeAgent has a companion object that offers easier creation of Agent instances. Agent could also be constructed from tofu.concurrent’s MakeRef and MakeSemaphore implicit instances.

object MakeAgent {
def apply[I[_], F[_]](implicit mkAgent: MakeAgent[I, F]): Applier[I, F] = // impl

  implicit def byRefAndSemaphore[I[_]: FlatMap, F[_]: Monad: Fire](
      implicit
      refs: MakeRef[I, F],
      sems: MakeSemaphore[I, F]
  ) = // impl
}

If you are using the same effect for creation and running you can use Agents type alias defined in tofu.concurrent package object.

package tofu

package object concurrent {
   // other aliases...
   type Agents[F[_]] = MakeAgent[F, F]
   // other aliases...
}

Examples

Using Agents:

import cats.effect.Sync
import cats.implicits._
import cats.Monad
import cats.syntax.flatMap._
import tofu.common.Console
import tofu.concurrent.Agents

  def example[F[_]: Agents: Sync: Monad: Console]: F[Unit] =
       for {
         _ <- Monad[F].unit
         agent <- Agents[F].of(42)
         newValue <- agent.updateM(a => Console[F].putStrLn(s"current value is $a") *> Monad[F].pure(a + 27))
         _ <- Console[F].putStrLn(s"new value is $newValue") // new value is 69
       } yield ()

Using MakeAgent:

import cats.effect.Sync
import cats.implicits._
import cats.Monad
import cats.syntax.flatMap._
import tofu.common.Console
import tofu.concurrent.{Agents, MakeAgent, MakeRef, MakeSemaphore, Refs, Semaphores}
import tofu.Fire

 def example[F[_]: Agents: Fire: Monad: Console: Sync: Refs: Semaphores](
     implicit
     refs: MakeRef[Option, F],
     sems: MakeSemaphore[Option, F]
 ): F[Unit] =
   for {
     _        <- Monad[F].unit
     agent    <- MakeAgent[Option, F].of(42).map(Monad[F].pure(_)).getOrElse(Agents[F].of(42))
     newValue <- agent.updateM(a => Console[F].putStrLn(s"current value is $a") *> Monad[F].pure(a + 27))
     _        <- Console[F].putStrLn(s"new value is $newValue") // new value is 69
  } yield ()