Minimum boilerplate lib and pattern for Free/Coproduct/Inject based applications
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ComposeFree is a small lib inspired by Runars Composable Application Architecture which minimizes the boilerplate required to build an application based on a Coproduct of Free DSLs.

To use it, include in build.sbt

resolvers += Resolver.bintrayRepo("bondlink", "composefree")

libraryDependencies += "bondlink" %% "composefree" % "1.1.0"

Basic use is a pared down version of the manual process, with the following high level steps:

  • Define DSLs as sealed families of case classes
  • Define interpreters for DSLs as NaturalTransformations
  • Define the application Coproduct type for your composed DSLs
  • Create an instance of ComposeFree[YourApplicationType] and import it

For example, let's say we wanted to combine a simple Console DSL with the Pure DSL provided in the ComposeFree lib.

First we define an ADT for our Console operations, and pattern match it in a NaturalTransformation to an effectful monad.

import scalaz.Id.Id
import scalaz.~>

sealed trait ConsoleOps[A]
case class print(s: String) extends ConsoleOps[Unit]

object RunConsole extends (ConsoleOps ~> Id) {
  def apply[A](op: ConsoleOps[A]) = op match {
    case print(s) => println(s)

Now we need a NaturalTransformation for the Pure dsl.

import composefree.puredsl._

object RunPure extends (PureOp ~> Id) {
  def apply[A](op: PureOp[A]) = op match {
    case pure(a) => a

Then we can define the Coproduct type for our application, and obtain our ComposeFree instance.

import composefree.ComposeFree
import scalaz.Coproduct

object Program {
  type Program[A] = Coproduct[ConsoleOps, PureOp, A]

object compose extends ComposeFree[Program.Program]

Last we will create an interpreter for our program type by combining our individual interpreters.

import composefree.syntax._

val interp = RunConsole |: RunPure

And finally we can define a program and execute it.

val prog: compose.Composed[Unit] = {
  import compose._
  for {
    s <- pure("Hello world!").as[PureOp]
    // use of .as[T] helper to cast as super type is
    // required for operations with type parameters that cannot be
    // implicitly converted to the correct coproduct member type
    _ <- print(s)
  } yield ()
// prog: compose.Composed[Unit] = Gosub(Suspend(Coproduct(\/-(Coproduct(\/-(pure(Hello world!)))))),<function1>)

// Hello world!
// res6: scalaz.Id.Id[Unit] = ()

Composite commands can be defined in individual DSLs and mixed into larger programs as follows.

object PureComposite {
  import compose.lift._
  import composefree.syntax._
  import scalaz.Free

  def makeTuple(s1: String, s2: String): Free[PureOp, (String, String)] =
    for {
      a <- pure(s1).as[PureOp]
      b <- pure(s2).as[PureOp]
    } yield (a, b)
// defined object PureComposite

import compose._
// import compose._

import Program._
// import Program._

val prog = for {
  s <- PureComposite.makeTuple("Hello", "World!").as[Program].op
  _ <- print(s._1)
  _ <- print(s._2)
} yield ()
// prog: scalaz.Free[compose.RecNode,Unit] = Gosub(Gosub(Suspend(Coproduct(\/-(Coproduct(\/-(pure(Hello)))))),<function1>),<function1>)

// Hello
// World!
// res7: scalaz.Id.Id[Unit] = ()