General, extensible effect monad for Scala with message-passing semantics
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Tk. A message-passing effect monad

Tk aims to be an efficient, well-behaved "impure effect wrapper monad" in the same vein as Scalaz's IO and Task types. You can use it to transform side-effecting, imperative APIs into ones which can be combined purely and then executed all at once later.

Construct libraries of side-effects happening in the Tk monad and compose them into larger effect chains deferring all side-effect execution until the very end. Tk provides opportunity for some amount of equational reason around side-effecting code.


Alpha. Tk appears functional but needs more testing. Furthermore, Tk is nowhere near as battle-tested as Scalaz's IO which additionally offers more features and integration into a framework of other effects. Finally, while there is promise of good performance with Tk's Church-encoding tricks this has not been tested.


class Main extends App {

  // Create new safe effect-presentations using Safe
  def write(string: String): Tk.Safe[Unit] =
    Tk.Safe { println(string) }

  def read: Tk.Safe[String] =
    Tk.Safe { }

  // Effects which may respond exceptionally can be wrapped by
  // Tk and Tk.fn directly, capturing their exceptions
  // as values.
  // If blowUp were defined using Unsafe.effThunk it would produce a
  // runtime exception.
  def blowUp: Tk[Throwable, Nothing] =
    Tk { throw new Exception("Oh no!") }

  // Compose Tk[A] values using for-comprehensions
  val tk: Tk[Throwable, Unit] =
    for {
      _ <- write("Echo echo echo...")
      line <- read
      _ <- blowUp
      _ <- write(line)
    } yield ()

  // Then, finally, when it's time to perform your effects do so
  // using the Run function.

Connected to the target VM, address: '', transport: 'socket'
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0).
Type in expressions for evaluation. Or try :help.

scala> (new
Echo echo echo...
Hello, Tk

Hello, Tk


The Tk ("task" or "to come" in newspaper editor speak) is a certain kind of free monad over the indexed store comonad. This provides message-passing semantics to "interacting with the real world" which are amenable to good models for concurrent threaded computation (unlike RealWorld token passing models if you're familiar with them).

In particular, Tk is the flattened form of a "codensity transformed" Free monad structure applied to the Store comonad. Even more particularly, it's not actually the Codensity of Free but instead a smaller structure with all of the desired properties arising from applying Yoneda instead.

To understand Tk best, we can look at a simplified model of the free monad over the indexed store comonad

sealed trait Tkish[+A]
case class Pure[+A](a: A) extends Tkish[A]
case class Effect[Req, Resp, +A](
  ffi: FFIOperation[Request, Response],
  request: Request,
  continue: Response => Tkish[A]
) extends Tkish[A]

This has the shape of a free monad embedding either a pure value with Pure or a wrapper of effects with Effect. In the case of Effect we see that we have all of the required components to perform some FFI (impure, basic) operation which converts requests of type Request to responses of type Response after which we use that response to continue the computation.

Assuming we have a method to send and receive messages to the runtime using the FFIOperation type the interpretation behavior of Tkish is now direct. Then, given that Scala is natively an effectful language we immediately have

type FFIOperation[-Req, +Resp] = Req => Resp

In practice, the actual Tk monad is identical to this except (a) it uses a Yoneda transformed version of the Free monad for efficiency and (b) it also contains an exception handler to properly account for the exceptions that may be thrown by FFIOperations.

References and sources

Most of these ideas were elaborated by Ed Kmett in a series of posts on called "Free Monads for Less". Within these posts Ed references many relevant papers describing these techniques. The translation to Scala is novel (to me).