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scala-fx

Getting started

Scala-fx is an effects library for Scala 3 that introduces structured concurrency and an abilities system to describe pure functions and programs.

The example below is a pure program that returns Int and requires the context capability Bind. Bind enables the bind syntax over values of Either and other types.

import fx.*

val program: Int =
    Right(1).bind + Right(2).bind
// program: Int = 3

Using Scala3 features such as context functions we can encode pure programs in terms of capabilities with minimal overhead. Capabilities can be introduced a la carte and will be carried as given contextual evidences through call sites until you proof you can get rid of them.

import fx.*

def runProgram: Int | String =
    val program: Errors[String] ?=> Int =
      Right(1).bind + Right(2).bind + "oops".raise[Int]

    run(program)

println(runProgram)
// oops

Users and library authors may define their own Capabilities. Here is how Bind for Either[E, A] is declared

/** Brings the capability to perform Monad bind in place. Types may
  * access [[Control]] to short-circuit as necessary
  */
extension [R, A](fa: Either[R, A])
  def bind: Errors[R] ?=> A = fa.fold(_.shift, identity)

Scala Fx supports a structured concurrency model backed by the non-blocking StructuredExecutorTask where you can fork and join cancellable fibers and scopes.

Popular functions like parallel support arbitrary typed arity in arguments and return types.

import fx.*

def runProgram: (String, Int, Double) =
  val results: Structured ?=> (String, Int, Double) =
    parallel(
      () => "1",
      () => 0,
      () => 47.03
    )

  structured(results)

println(runProgram)
// (1,0,47.03)

Continuations based on Control Throwable or a non-blocking model like Loom are useful because they allow us to intermix async and sync programs in the same syntax without the need for boxing as is frequently the case in most scala effect libraries.

Build and run in your local environment:

Pre-requisites:

  1. Download the latest Project Loom Early-Access build for your system architecture.

    This is easy to do if you are using SDKMAN. First list java versions with sdk list java, and sdk install java 19-ea-<XX>-open, where XX is the latest Java.net version 19 release number. Make it the default with sdk default java 19-ea-<XX>-open.

  2. Set your JAVA_HOME to the path you extracted above.

Unnecessary if you have set the ea build to be your default in sdkman.

You can now compile and run the tests:

env JAVA_OPTS='--enable-preview --add-modules jdk.incubator.concurrent' sbt "clean; compile; test"

NOTE: The Loom project is defined as an incubator module that is a means to distribute APIs which are not final or completed to get feedback from the developers. You should include the -add-module Java option to add the module to the class path of the project.