Free monads on top of http4s


Free monads on top of http4s

The main goal is to write your API's using free monads + interpreters


Extract from HttpfsFreeSpec test:

import cats.effect.{IO, Sync, Timer}
import cats.{Functor, ~>}
import io.freemonads.http.api._
import io.freemonads.http.resource._
import org.http4s._
import org.http4s.circe.CirceEntityCodec._
import org.http4s.dsl.Http4sDsl

// some model
case class Mock(id: Option[String], name: String, age: Int)

// our routes need the Algebra dsls + interpreters so we can write free monads logic
def mockRoutes[F[_] : Sync : Timer : Functor, Algebra[_], Serializer[_], Deserializer[_]](
    implicit http4sFreeDsl: Http4sFreeDsl[Algebra],
    resourceDsl: ResourceDsl[Algebra, Serializer, Deserializer],
    interpreters: Algebra ~> F): HttpRoutes[F] = {

  val dsl = new Http4sDsl[F] {}
  import dsl._
  import http4sFreeDsl._
  import resourceDsl._

  HttpRoutes.of[F] {
    case r @ GET -> Root / "mock" / id =>
      for {
        mock <- fetch[Mock](r.uri) //
      } yield mock.ok[F]

    case r @ POST -> Root / "mock" =>
      for {
        mockRequest <- parseRequest[F, Mock](r)
        savedResource <- store[Mock](r.uri, mockRequest)
      } yield savedResource.created[F]

API model

The API has next main types:

  type ApiResult[R] = Either[ApiError, R]
  type ApiFree[F[_], R] = EitherT[Free[F, *], ApiError, R]
  • ApiResult represents the result of an API call, which can be successful (and retrieves the model) or an error (more on this below)
  • ApiFree represents a program that is a composition of different Api calls (free monad composition)

Errors supported (mapped to HTTP error codes on API response):

  sealed trait ApiError
  final case class RequestFormatError(request: Any, details: Any, cause: Option[Throwable] = None) extends ApiError
  final case class NonAuthorizedError(resource: Option[Any]) extends ApiError
  final case class ResourceNotFoundError(id: Option[String] = None, cause: Option[Throwable] = None) extends ApiError
  final case class NotImplementedError(method: String) extends ApiError
  final case class ResourceAlreadyExistError(id: String, cause: Option[Throwable] = None) extends ApiError
  final case class RuntimeError(message: String, cause: Option[Throwable] = None) extends ApiError

This allows the business logic to give enough detail on the type of error to the API controller.

Algebras, DSLs and interpreters

The code is organized around Algebras, DSL's and interpreters:

  • Algebras define a set of functions in a domain (for example store and fetch for resource algebra)
  • DSL's are used in your monadic compositions to create programs
  • interpreters implement the logic inside the Algebras

Rest Algebra (Http4sAlgebra)

Basic REST functions:

  • parseRequest[F, R](request: Request[F]): parse request to a type R, retrieving the model or format error



  for {
    mockRequest <- parseRequest[F, Mock](r)
  } yield Created(mockRequest)

Resource Algebra

Http resource management:

  • store[R](id: Uri, r: R): stores a resource R using it's id as an uri, returns saved resource or error
  • fetch[R](id: Uri): retrieves a resource by id (uri) or not found error


import io.freemonads.http.resource._

def storeProgram[F[_]](id: Uri, mock: Mock)(implicit resourceDsl: ResourceDsl[F, Serializer, Deserializer]) =
  for {
    _ <- validate(mock) // example where you can implement some kind of validation
    mock <-[Mock](id, mock)
  } yield mock
def fetchProgram[F[_]](id: Uri)(implicit resourceDsl: ResourceDsl[F, Serializer, Deserializer]) =
  for {
    mock <- resourceDsl.fetch[Mock](id)
  } yield mock


Free Monad interpreters are just a natural transformation from our Free Monad context Free[_] into the effects context we want to use F[_]: FlatMap.

Example to instance a Cats IO interpreter:

import io.freemonads._

implicit val interpreters = http4sInterpreter[IO]


Test Http DSLs

You can use Http4FreeIOMatchers or Http4FreeIdMatchers to test API dsls on IO[_] or Id[_].


  • resultOk(expectedValue) --> match a succesfull API call
  • resultError[ErrorType] --> match an specific error type when calling an API
  • resultErrorNotFound, resultErrorConflict --> instances of resultError


class TestSpec extends Specification with specs2.Http4FreeIdMatchers { def is: SpecStructure =

      ApiResource should: <br/>
      Store a resource                                $store
      Fetch an existing resource                      $fetchFound
      Return not found error for nonexistent resource $fetchNotFound
  implicit val interpreter: ResourceAlgebra ~> Id = ???
  implicit val dsl = instance[ResourceAlgebra, Encoder, Decoder]

  def store: MatchResult[Any] =[Mock](newMockIdUri, newMock).map(_.body) must resultOk(newMock)
  def fetchFound: MatchResult[Any] = dsl.fetch[Mock](existingUri).map(_.body) must resultOk(existingMock)
  def fetchNotFound: MatchResult[Any] = dsl.fetch[Mock](nonexistingUri).map(_.body) must resultErrorNotFound

IT Resource DSL with ArangoDB

You can use DockerKitConfigWithArango as a Docker configuration kit with Arango for integration tests.

Example with Specs2:

class ArangoServiceIT(env: Env)
    extends Specification
        with DockerKitSpotify
        with DockerKitConfigWithArango
        with DockerTestKit

You can configure the Docker on your application.conf file:

# docker containers
docker {

  arango {
    image-name = "arangodb/arangodb:3.7.10"
    memory = 536870912 # 512MB
    memory-reservation = 268435456 # 256MB
    environmental-variables = ["ARANGO_ROOT_PASSWORD=rootpassword"]
    ready-checker {
      http-response-code {
        port = 8529
        path = "/"
        within = 100
        looped {
          attempts = 20
          delay = 1250
    port-maps {
      default-arango-port {
        external = 18529
        internal = 8529