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 and is based on scalameta.

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

addCompilerPlugin("org.scalameta" % "paradise" % "3.0.0-M10" cross CrossVersion.full)

For Scala.jvm:

libraryDependencies += "io.frees" %% "frees-core" % "0.8.2"

For Scala.js:

libraryDependencies += "io.frees" %%% "frees-core" % "0.8.2"


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.free._
// import freestyle.free._

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

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:

@module trait Application {
  val validation: Validation
  val interaction: Interaction
  import cats.implicits._
  def program: FS.Seq[Unit] = 
    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)).mapN(_ && _).freeS
      _         <- if (valid) 
                      interaction.tell(s"$userInput is not valid")
    } yield ()
// 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.

Running programs

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

import cats.effect.IO
// import cats.effect.IO

implicit val validationHandler = new Validation.Handler[IO] {
  override def minSize(s: String, n: Int): IO[Boolean] = IO{s.size >= n}
  override def hasNumber(s: String): IO[Boolean] = IO{s.exists(c => "0123456789".contains(c))}
// validationHandler: Validation.Handler[cats.effect.IO] = $anon$1@2093126c

implicit val interactionHandler = new Interaction.Handler[IO] {
  override def tell(s: String): IO[Unit] = IO{println(s)}
  override def ask(s: String): IO[String] = IO { println(s); "This could have been user input 1" }
// interactionHandler: Interaction.Handler[cats.effect.IO] = $anon$1@454cba20

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 cats.effect.IO
// import cats.effect.IO

// Give me something with at least 3 chars and a number on it
// awesomesauce!

There is more

You may want to consider using Freestyle you have any of the following concerns:

  • 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 the reference application with examples of multiple algebras in use.