Algebraic Data Types are the foundation used to define Free based applications and libraries that express their operations as algebras. At the core of Freestyle algebras is the @free macro annotation. @free expands abstract traits and classes automatically deriving Algebraic Data types and all the machinery needed to compose them from abstract method definitions.

When you build an algebra with Freestyle, you only need to concentrate on the API that you want to be exposed as abstract smart constructors, without worrying how they will be implemented.

In Freestyle, an algebra is a trait or abstract class annotated with @free or @tagless:

// import

case class User(id: Long, name: String)
// defined class User

@free trait Users {
  def get(id: Long): FS[User]
  def save(user: User): FS[User]
  def list: FS[List[User]]
// defined trait Users
// defined object Users

The Users trait declares three smart constructors, named get, save, and list, which generate the basic operations in the algebra. A smart constructor is an abstract method declaration with a return type of the form FS[Ret], where Ret is the type of the data computed by the operation, and FS[_] marks the method as an operation of the algebra. Intuitively, FS means that the method gives a computation within some (generic) context or effect FS. For example, the save smart constructor has a return type FS[User]. Intuitively, this declares save as a computation in a context (or effect) FS, whose result will be a User object.

The @free is a Scala macro annotation, that replaces the annotated trait Users by a modified trait, and generates a companion object, as in the code below:


case class User(id: Long, name: String)

trait Users[F[_]] extends EffectLike[F] {
  def get(id: Long): FS[User]
  def save(user: User): FS[User]
  def getAll(filter: String): FS[List[User]]

object Users {
  import _root_.cats.arrow.FunctionK

  sealed trait Op[A] extends Product with Serializable
  final case class GetOp(id: Long) extends Op[User]
  final case class SaveOp(user: User) extends Op[User]
  final case class GetAllOp(filter: String) extends Op[List[User]]

  class To[L[_]](implicit ii: InjK[Op, L]) extends Users[L] {
    private[this] val inj = FreeS.inject[Op, L](ii)

    def get(id: Long): FS[User] = inj( GetOp(id) )
    def save(user: User): FS[User] = inj( SaveOp(user) )
    def getAll(filter: String): FS[List[User]] = inj( GetAllOp(filter) )

  implicit def to[L[_]](implicit I: InjK[Op, L]): Users[L] =
    new To[L]

  def apply[L[_]](implicit c: Users[L]): Users[L] = c

  trait Handler[M[_]] extends FSHandler[Op, M] {
    protected[this] def get(id: Long): M[User]
    protected[this] def save(user: User): M[User]
    protected[this] def getAll(filter: String): M[List[User]]

    override def apply[A](fa: Op[A]): M[A] = fa match {
      case l @ GetOp(_) => get(
      case l @ SaveOp(_) => save(l.user)
      case l @ GetAllOp(_) => getAll(l.filter)


Let us examine how the @free-annotated trait relates to the code generated from it.

Generalise Trait

The @free macro makes two changes to the trait Users.

  1. It adds a type parameter F[_], of kind * -> *, which is a general type in which operations are constructed. This makes Users[F[_]] resemble a Generalised Abstract Data Type (GADT), an interface to construct the algebra’s operations into a type F[_].

  2. It makes Users to extend EffectLike[F[_]]. This trait defines the type FS[_], used in the smart constructors, as FS[_] = FreeS.Par[F], which in turn is an alias for Free[FreeApplicative[F, ?], ?].

The @free annotation works as a syntactic sugar for writing a GADT in Scala. Without the EffectLike trait, the algebra GADT would be written as follows:

trait Users[F[_]] {
  def get(id: Long): FreeS.Par[F, User]
  def save(user: User): FreeS.Par[F, User]
  def getAll(filter: String): FreeS.Par[F, List[User]]

Algebraic Data Type

From the abstract smart constructors, @free generates an algebraic data type (ADT) of operations inside the companion object. This Algebraic data type contains the shape needed to implement the abstract methods.

  sealed trait Op[A] extends Product with Serializable

  final case class GetOp(id: Long) extends Op[User]
  final case class SaveOp(user: User) extends Op[User]
  final case class GetAllOp(filter: String) extends Op[List[User]]

Some important features of this ADT are the following ones:

  • The root of the ADT is a sealed trait Op[A]. Note that this name Op is the same in every @free-generated algebra.
  • For each smart constructor def foo(x: X, y: Y, ...): FS[Z], the @free generates a case class Foo, whose fields are the parameters of the constructor. For those with a background in object-oriented design, this is similar to the Command Pattern.
  • The parameter A in the root trait Op[A] describes the type of the expected result of that operation.


If the modified trait Users[F[_]] declares an interface for the algebra, the companion object Users provides an implementation for that interface.

The @free annotation adds into the companion object a class To[L[_]], which implements the Users GADT for the target type L[_] by using an InjK[Op, L] object. An InjK[F[_], G[_]] is a special type of tranformation, much like the FunctionK in cats, that allows to transform a F[A] into a G[A]. In the case of the To class, we need an InjK[Op, L], that allows transforming an object Req[A] in the Op ADT above, to a value L[A] in the target type L. The class To[L[_]] implements each operation of the algebra by just applying that InjK object to the instance of the Op ADT.

The InjK is based the Inject strategy from the Data types à-la-Carte article, which describes how to compose unrelated ADTs using the Coproduct (or EitherK in cats).

Dependency Injection

As you may have noticed when defining algebras with @free, there is no need to provide implicit evidence for the necessary Inject typeclasses that otherwise need to be manually provided to further evaluate your free monads when they are interleaved with other Free programs.

Beside providing the appropriate Inject evidences, Freestyle creates an implicit method that will enable implicit summoning of the smart constructors class implementation and an apply method that allows summoning instances of your smart constructors where needed. This effectively enables implicits based Dependency Injection where you may choose to override implementations using the implicits scoping rules to place different implementations where appropriate.

val users = Users[Users.Op]
// users: Users[Users.Op] = Users$To@58f46bf8

def myService[F[_]](implicit users: Users[F]) = ???
// myService: [F[_]](implicit users: Users[F])Nothing

def myService2[F[_]: Users] = ???
// myService2: [F[_]](implicit evidence$1: Users[F])Nothing

Composed Operations

The trait EffectLike[F[_]] mentioned above, apart from the type alias FS[_], also defines two type aliases FS.Seq[_] and FS.Par[].

trait EffectLike[F[_]] {
  final type FS[A] = FreeS.Par[F, A]
  final object FS {
    final type Seq[A] = FreeS[F, A]
    final type Par[A] = FreeS.Par[F, A]

These type aliases can be used to define some FS operations that are derived from other operations. , by combining other operations:

  • FS.Par[A] is an alias for FreeApplicative[F, A]. You can declare a derived operation of type FS.Par by applying the methods of the Functor and Applicative type classes.
  • FS.Seq[A] is an alias for Free[FreeApplicative[F, ?], A], that can be combined using the methods of the Monad type class.
import cats.syntax.apply._
// import cats.syntax.apply._

import cats.syntax.monad._
// import cats.syntax.monad._

@free trait X {
  def a: FS[Int]
  def b(i: Int): FS.Par[Int] = x => x + i)
  def c: FS.Par[Int] = (a,b(42)).mapN(_ + _)
  def d: FS.Seq[Int] = c.freeS.flatMap(x => b(x).freeS)
// defined trait X
// defined object X

We use the type aliae FS.Seq and FS.Par to indicate complex operations, which combine the algebra’s basic requests with the operations from the Monad (or Applicative) type class. The names follow the intuition that Applicative operations combine data-independent computations that can be run in parallel, and Monad operations combine data-dependent computations that need to be run in sequence.

Note that, although FS[_] and FS.Par[_] are equivalent, @free only allows using the former for abstract methods.

Convenient type aliases

As described above, @free always uses the name Op for the sealed trait at the root of the requests ADT. This allows you to access it uniformly, for instance to build a Coproduct type that helps you to parametrize you application code. Here is an example for this:

// import

@free trait Service1{
  def x(n: Int): FS[Int]
// defined trait Service1
// defined object Service1

@free trait Service2{
  def y(n: Int): FS[Int]
// defined trait Service2
// defined object Service2

@free trait Service3{
  def z(n: Int): FS[Int]
// defined trait Service3
// defined object Service3

type C1[A] = EitherK[Service1.Op, Service2.Op, A]
// defined type alias C1

type Module[A] = EitherK[Service3.Op, C1, A]
// defined type alias Module

This is obviously far from ideal, as building EitherK types by hand often results in bizarre compile errors when the types don’t align properly from being placed in the wrong order.

Combining @tagless and @free algebras

Freestyle allows us to compose @free and @tagless algebras. Let us consider the following @tagless algebras:

import freestyle.tagless._
// import freestyle.tagless._

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

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

For every @tagless algebra, there is also a free-based representation that is stack-safe by nature, and that can be used to lift @tagless algebras to a context mixing @free and @tagless algebras.

Let’s redefine program to support LoggingM which is a @free defined algebra of logging operations:


def program[F[_]]
   (implicit log: LoggingM[F], 
             validation : Validation.StackSafe[F], 
             interaction: Interaction.StackSafe[F]) = {

  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)).mapN(_ && _)
    _ <- if (valid)
            interaction.tell(s"$userInput is not valid")
    _ <- log.debug("Program finished")
  } yield ()
// program: [F[_]](implicit log:[F], implicit validation: Validation.StackSafe[F], implicit interaction: Interaction.StackSafe[F])[[β$0$][F,β$0$],Unit]

Since Validation and Interaction were @tagless algebras, we need their StackSafe representation in order to combine them with @free algebras.

Fear not. Freestyle provides a modular system to achieve Onion-style architectures and removes all the complexity from building EitherK types by hand and compose arbitrarily nested Modules containing Algebras.

Continue to Modules.