Choreo

A library for choregraphic programming in Scala.

The implementation is based on the paper HasChor: Functional Choreographic Programming for All by Gan Shen, Shun Kashiwa and Lindsey Kuper, and its Haskell implementation.

Example

package choreo
package examples
package kv

import cats.effect.IO
import cats.effect.IO.asyncForIO
import cats.effect.kernel.Ref
import cats.syntax.all.*

type State = Map[String, String]

enum Request:
  case Get(key: String)
  case Put(key: String, value: String)

type Response = Option[String]

val client: "client" = "client"
val server: "server" = "server"

def main: IO[Unit] =
  for
    backend <- Backend.local(List(client, server))
    clientTask = choreo.run(backend, client)
    serverTask = choreo.run(backend, server)
    _ <- (clientTask, serverTask).parTupled
  yield ()

def choreo: Choreo[IO, Unit] =
  for
    stateS <- server.locally(Ref.of[IO, State](Map.empty))
    _ <- step(stateS).foreverM
  yield ()

def step(stateS: Ref[IO, State] @@ "server"): Choreo[IO, Unit] =
  for
    reqC <- client.locally(readRequest)
    resC <- kvs(reqC, stateS)
    _ <- client.locally:
      resC.!.fold(IO.println("Key not found")):
        IO.print("> ") *> IO.println(_)
  yield ()

def kvs(
    reqC: Request @@ "client",
    stateS: Ref[IO, State] @@ "server"
): Choreo[IO, Response @@ "client"] =
  for
    reqS <- client.send(reqC).to(server)
    resS <- server.locally(handleRequest(reqS.!, stateS.!))
    resC <- server.send(resS).to(client)
  yield resC

def handleRequest(
    req: Request,
    state: Ref[IO, State]
): IO[Response] =
  req match
    case Request.Get(key) =>
      state.get.map(_.get(key))

    case Request.Put(key, value) =>
      state.update(_.updated(key, value)).as(Some(value))

def readRequest: IO[Request] =
  for
    _ <- IO.print("> ")
    line <- IO.readLine
    req <- line.split(" ") match
      case Array("GET", key) =>
        IO.pure(Request.Get(key))

      case Array("PUT", key, value) =>
        IO.pure(Request.Put(key, value))

      case _ =>
        IO.raiseError(new Exception("Invalid request"))
  yield req

License

This library is released under the same license as HasChor, namely the BSD-3 license.

Please see the LICENSE within this repository for the full text of the license.