Atomic Cell

A synchronized, concurrent, mutable reference.

Provides safe concurrent access and modification of its contents, by ensuring only one fiber can operate on them at the time. Thus, all operations may semantically block the calling fiber.

abstract class AtomicCell[F[_], A] {
  def get: F[A]
  def set(a: A): F[Unit]
  def modify[B](f: A => (A, B)): F[B]
  def evalModify[B](f: A => F[(A, B)]): F[B]
  def evalUpdate(f: A => F[A]): F[Unit]
  // ... and more
}

Using AtomicCell

The AtomicCell can be treated as a combination of Semaphore and Ref:

import cats.effect.{IO, Ref}
import cats.effect.std.Semaphore

trait State
class Service(sem: Semaphore[IO], ref: Ref[IO, State]) {
  def modify(f: State => IO[State]): IO[Unit] = 
    sem.permit.surround {
      for {
        current <- ref.get
        next <- f(current)
        _ <- ref.set(next) 
      } yield ()
    }
}

The following is the equivalent of the example above:

import cats.effect.IO
import cats.effect.std.AtomicCell

trait State
class Service(cell: AtomicCell[IO, State]) {
  def modify(f: State => IO[State]): IO[Unit] = 
    cell.evalUpdate(current => f(current))
}