Quick Start

Freestyle is a library that enables the building of large-scale modular Scala applications and libraries on top of Free monads/applicatives.

Getting Started

Freestyle is compatible with both Scala JVM and Scala.js.

This project supports Scala 2.11 and 2.12. The project is based on scalameta.

To use the project, add the following to your build.sbt:

addCompilerPlugin("org.scalameta" % "paradise" % "3.0.0-M9" cross CrossVersion.full)

For Scala.jvm:

libraryDependencies += "io.frees" %% "freestyle" % "0.3.1"

For Scala.js:

libraryDependencies += "io.frees" %%% "freestyle" % "0.3.1"


Freestyle’s core feature is the definition of Free boilerplate-free algebras that support both sequential and parallel style computations and all the implicit machinery required to turn them into modular programs.

In the example below, we will define two algebras with intermixed sequential and parallel computations.

import freestyle._
// import freestyle._

import freestyle.implicits._
// import freestyle.implicits._

@free trait Validation {
  def minSize(s: String, n: Int): FS[Boolean]
  def hasNumber(s: String): FS[Boolean]
// defined trait Validation
// defined object Validation

@free trait Interaction {
  def tell(msg: String): FS[Unit]
  def ask(prompt: String): FS[String]
// defined trait Interaction
// defined object Interaction

Learn more about algebras in the extended documentation.


Freestyle algebras can be combined into @module definitions which provide aggregation and unification over the parameterization of Free programs.

@module trait Application {
  val validation: Validation
  val interaction: Interaction
// defined trait Application
// defined object Application

Freestyle automatically wires all dependencies through implicit evidences that are generated, so you don’t have to worry about the boilerplate required to build Free-based programs.

Once you have these abstract definitions, you can combine them in whichever way you want. Freestyle supports nested modules enabling onion-style architectures of any arbitrary depth.

Learn more about modules in the extended documentation.

Building programs

Abstract definitions are all it takes to start building programs that support sequential and parallel operations that are entirely decoupled from their runtime interpretation.

The example below combines both algebras to produce a more complex program:

def program[F[_]](implicit A: Application[F]) = {
  import A._
  import cats.implicits._

  for {
    userInput <- interaction.ask("Give me something with at least 3 chars and a number on it")
    valid <- (validation.minSize(userInput, 3) |@| validation.hasNumber(userInput)).map(_ && _).freeS
    _ <- if (valid)
            interaction.tell(s"$userInput is not valid")
  } yield ()
// program: [F[_]](implicit A: Application[F])cats.free.Free[[β$0$]cats.free.FreeApplicative[F,β$0$],Unit]

Running programs

In order to run programs, we need interpreters. We define interpreters providing implementations for the operations defined in our algebras:

import scala.concurrent.ExecutionContext.Implicits.global
// import scala.concurrent.ExecutionContext.Implicits.global

import scala.concurrent.Future
// import scala.concurrent.Future

implicit val validationHandler = new Validation.Handler[Future] {
  override def minSize(s: String, n: Int): Future[Boolean] = Future(s.size >= n)
  override def hasNumber(s: String): Future[Boolean] = Future(s.exists(c => "0123456789".contains(c)))
// validationHandler: Validation.Handler[scala.concurrent.Future] = $anon$1@3011acde

implicit val interactionHandler = new Interaction.Handler[Future] {
  override def tell(s: String): Future[Unit] = Future.successful(println(s))
  override def ask(s: String): Future[String] = Future.successful { println(s); "This could have been user input 1" }
// interactionHandler: Interaction.Handler[scala.concurrent.Future] = $anon$1@73f189bd

The mere fact that you provide implicit evidences for the individual steps enables Freestyle to automatically discover and unify all interpreters involved in a program definition.

At this point, we can run our pure programs at the edge of the world:

import cats.implicits._
// import cats.implicits._

import scala.concurrent.duration.Duration
// import scala.concurrent.duration.Duration

import scala.concurrent.Await
// import scala.concurrent.Await

val futureValue = program[Application.Op].interpret[Future]
// Give me something with at least 3 chars and a number on it
// futureValue: scala.concurrent.Future[Unit] = Future(<not completed>)

Await.result(futureValue, Duration.Inf) //blocking only for demo purposes. Don't do this at home.

There is more

You may want to consider using Freestyle if among your concerns are:

  • Decoupling program declaration from runtime interpretation.
  • Automatic composition of dispair monadic/applicative style actions originating from independent ADTs.
  • Automatic onion-style architectures through composable modules without the complexity of manually aligning Coproducts and interpreters.
  • Boilerplate-free application and libraries.

Freestyle includes ready to go Algebras and Integrations for the most common application concerns:

Learn more about how Freestyle works behind the scenes in the extended documentation and check out a the reference application with examples of multiple algebras in use.