A server is just a function: introduction to http4s

If you're going to write a modern web service, you have a wide range of frameworks and libraries to choose from. One of them is http4s, a functional library for writing HTTP services and clients in Scala. I'm going to show you why you should consider it for your next application, for reasons including but not limited to:
- type-safe request/response models that are uniform between client and server
- resource-safe servers, response/request bodies, and streaming
- framework-agnostic testing.

At the end of this talk, you should have a clear understanding of how http4s can make it easier to build modern, runtime-safe services in Scala.

Transcript

1. A SERVER IS JUST A FUNCTION
   A N I N T R O D U C T I O N TO H T T P 4 S
   J A K U B KO Z Ł O W S K I
   

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2. WHAT MAKES A SERVER?
   

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3. WHAT MAKES A FUNCTION?
   

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4. type Server = Request => Response?
   ?
   

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6. WHAT REALLY MAKES A SERVER?
   Streaming
   Shared resources
   State
   Errors
   Routing
   Auth
   Serialization
   
   

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7. HTTP4S
   Purely functional, streaming HTTP server/client
   https://http4s.org
   

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8. HTTP4S
   Purely functional, streaming HTTP server/client
   Multiple backends
   https://http4s.org
   

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9. HTTP4S
   Purely functional, streaming HTTP server/client
   Multiple backends
   Built on cats-effect and fs2
   https://http4s.org
   

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10. HTTP4S
    Purely functional, streaming HTTP server/client
    Multiple backends
    Built on cats-effect and fs2
    Supports websockets (server)
    https://http4s.org
    

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11. Function1[_, _]
    

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12. Function1[_, _]
    +
    IO[_]
    

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13. Function1[_, _]
    +
    IO[_]
    

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14. HttpApp = Request => IO[Response]
    HttpRoutes = Request => IO[Option[Response]]
    HTTP4S SERVER: INTUITION
    

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15. Kleisli
    

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17. ❌
    

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18. KLEISLI
    case class Kleisli[F[_], A, B](
    run: A => F[B]
    )
    

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19. KLEISLI
    case class Kleisli[F[_], A, B](
    run:
    )
    Function with effectful result
    A => F[B]
    

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20. KLEISLI
    val len: Kleisli[IO, Token, Int] = Kleisli { token =>
    IO.pure(token.length)
    }
    val cloned: Kleisli[IO, Token, String] = Kleisli { token =>
    IO.pure(token + token)
    }
    

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21. KLEISLI
    val f3 = for {
    a <- len
    b <- cloned
    } yield (a + b)
    val f4 = (len, cloned).tupled
    val len: Kleisli[IO, Token, Int] = Kleisli { token =>
    IO.pure(token.length)
    }
    val cloned: Kleisli[IO, Token, String] = Kleisli { token =>
    IO.pure(token + token)
    }
    

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22. OptionT
    

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23. OPTIONT
    case class OptionT[F[_], A](
    value: F[Option[A]]
    )
    

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24. OPTIONT
    Option nested in an effect
    case class OptionT[F[_], A](
    value:
    )
    F[Option[A]]
    

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25. OPTIONT
    val num: OptionT[IO, Int] = OptionT(IO.pure(42.some))
    def text(num: Int): OptionT[IO, String] = num match {
    case n if n % 2 === 0 => OptionT(IO.pure("foo".some))
    case _ => OptionT.none
    }
    

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26. OPTIONT
    val num: OptionT[IO, Int] = OptionT(IO.pure(42.some))
    def text(num: Int): OptionT[IO, String] = num match {
    case n if n % 2 === 0 => OptionT(IO.pure("foo".some))
    case _ => OptionT.none
    }
    val result = for {
    n <- num
    s <- text(n)
    s2 <- text(n + 1)
    s3 <- text(n + 2)
    } yield s + s2 + s3
    val unwrapped: IO[Option[String]] = result.value
    

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27. HttpApp = Kleisli[IO, Request, Response]
    HttpRoutes = Kleisli[OptionT[IO, ?], Request, Response]
    HTTP4S SERVER: THE REAL THING
    (ALMOST)
    

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28. HTTP4S SERVER: THE REAL REAL THING
    HttpApp[F[_]] = Kleisli[F, Request[F], Response[F]]
    HttpRoutes[F[_]] = Kleisli[OptionT[F, ?], Request[F], Response[F]]
    Effect type of entity body stream
    

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29. HTTP4S SERVER: THE REAL REAL THING
    Http[F[_], G[_]] = Kleisli[F, Request[G], Response[G]]
    HttpApp[F[_]] = Http[F, F]
    HttpRoutes[F[_]] = Http[OptionT[F, ?], F]
    

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30. ALL THIS ABSTRACTION... WHY?
    (STAY TUNED)
    Spoiler: DRY
    

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31. ROUTING DSL
    

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32. object Main extends IOApp {
    val routes = HttpRoutes.of[IO] {
    case GET -> Root / "hello" => Ok("Hello, world!")
    }
    def run(args: List[String]): IO[ExitCode] =
    BlazeServerBuilder[IO]
    .withHttpApp(routes.orNotFound)
    .bindHttp(port = 8080)
    .resource
    .use(_ => IO.never)
    }
    

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33. object Main extends IOApp {
    val routes = HttpRoutes.of[IO] {
    case GET -> Root / "hello" => Ok("Hello, world!")
    }
    def run(args: List[String]): IO[ExitCode] =
    BlazeServerBuilder[IO]
    .withHttpApp( )
    .bindHttp(port = 8080)
    .resource
    .use(_ => IO.never)
    }
    routes.orNotFound HttpRoutes => HttpApp
    

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34. object Main extends IOApp {
    val routes = HttpRoutes.of[IO] {
    case GET -> Root / "hello" => Ok("Hello, world!")
    }
    def run(args: List[String]): IO[ExitCode] =
    BlazeServerBuilder[IO]
    .withHttpApp(routes.orNotFound)
    .bindHttp(port = 8080)
    .resource
    .use(_ => IO.never)
    }
    

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35. HttpRoutes.of[IO] {
    case GET -> Root / "hello" => Ok("Hello, world!")
    }
    

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36. HttpRoutes.of[IO] {
    case -> Root / "hello" => Ok("Hello, world!")
    }
    GET
    

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37. HttpRoutes.of[IO] {
    case GET -> => Ok("Hello, world!")
    }
    Root / "hello"
    

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38. Ok("Hello, world!")
    HttpRoutes.of[IO] {
    case GET -> Root / "hello" =>
    }
    

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39. HttpRoutes.of[IO] {
    case GET -> Root / "hello" => Ok("Hello, world!")
    }
    

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40. HttpRoutes.of[IO] {
    case request @ POST -> Root / "echo" =>
    Ok(request.body)
    }
    

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41. HttpRoutes.of[IO] {
    case request @ POST -> Root / "echo" =>
    Ok( )
    }
    request.body
    

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42. HttpRoutes.of[IO] {
    case request @ POST -> Root / "echo" =>
    Ok( )
    }
    request.body
    Literally Stream[IO, Byte]
    

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43. CLIENT
    

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44. val request = Request[IO](uri = uri"https://http4s.org")
    val remoteCall: IO[String] =
    client.expect[String](request)
    

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45. val request = Request[IO](uri = uri"https://http4s.org")
    val remoteCall: IO[String] =
    client.expect[String](request)
    remoteCall
    BlazeClientBuilder[IO](executionContext).resource.use { client =>
    }
    

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46. case GET -> Root / "remote" / "stream" =>
    val response = client.stream(request).flatMap(_.body)
    Ok(response)
    

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47. trait Client[F[_]] {
    def run(req: Request[F]): Resource[F, Response[F]]
    //+ a bunch of other convenient methods
    }
    

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48. trait Client[F[_]] {
    def run(req: Request[F]): Resource[F, Response[F]]
    }
    def apply[F[_]](f:
    Request[F] => Resource[F, Response[F]]
    ): Client[F]
    

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49. trait Client[F[_]] {
    def run(req: Request[F]): Resource[F, Response[F]]
    }
    def apply[F[_]](f:
    Request[F] => Resource[F, Response[F]]
    ): Client[F]
    

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50. trait Client[F[_]] {
    def run(req: Request[F]): Resource[F, Response[F]]
    }
    def fromHttpApp[F[_]](app: HttpApp[F]): Client[F]
    def apply[F[_]](f:
    Request[F] => Resource[F, Response[F]]
    ): Client[F]
    

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51. T E S T I N G R E S O U R C E
    S A F E T Y
    E X T E N S I B I L I T Y
    

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52. T E S T I N G R E S O U R C E
    S A F E T Y
    E X T E N S I B I L I T Y
    

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53. TESTING
    

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54. TESTING
    Server routes
    

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55. TESTING
    Server routes Server HTTP interface
    

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56. TESTING
    Server routes Server HTTP interface Client calls
    

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57. TESTING ROUTES
    def routes(client: Client[IO]): HttpRoutes[IO] = {
    HttpRoutes.of[IO] {
    //...
    case GET -> Root / "remote" =>
    val remoteCall: IO[String] =
    client.expect[String](request)
    for {
    result <- remoteCall
    response <- Ok(result)
    } yield response
    //...
    }
    }
    

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58. val remoteClient = Client.fromHttpApp(HttpApp.notFound[IO])
    TESTING ROUTES
    

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59. val remoteClient = Client.fromHttpApp(HttpApp.notFound[IO])
    val routes = Main.routes(remoteClient)
    TESTING ROUTES
    

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60. val body = Json.obj("foo" -> Json.fromString("bar"))
    val request =
    Request[IO](method = Method.POST, uri = uri"/echo").withEntity(body)
    val remoteClient = Client.fromHttpApp(HttpApp.notFound[IO])
    val routes = Main.routes(remoteClient)
    httsps://http4s.org
    TESTING ROUTES
    

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61. routes.run(request).value.flatMap(_.value.as[Json]).map(_ shouldBe body)
    val remoteClient = Client.fromHttpApp(HttpApp.notFound[IO])
    val routes = Main.routes(remoteClient)
    val body = Json.obj("foo" -> Json.fromString("bar"))
    val request =
    Request[IO](method = Method.POST, uri = uri"/echo").withEntity(body)
    TESTING ROUTES
    

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62. routes.run(request).value.flatMap(_.value.as[Json]).map(_ shouldBe body)
    val remoteClient = Client.fromHttpApp(HttpApp.notFound[IO])
    val routes = Main.routes(remoteClient)
    val body = Json.obj("foo" -> Json.fromString("bar"))
    val request =
    Request[IO](method = Method.POST, uri = uri"/echo").withEntity(body)
    TESTING ROUTES
    Client.fromHttpApp(routes.orNotFound).expect[Json](request).map(_ shouldBe body)
    or
    

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63. TESTING HTTP
    val blazeClient = BlazeClientBuilder[IO](ExecutionContext.global).resource
    

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64. TESTING HTTP
    val blazeClient = BlazeClientBuilder[IO](ExecutionContext.global).resource
    Main.server.use { server =>
    }
    

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65. TESTING HTTP
    val blazeClient = BlazeClientBuilder[IO](ExecutionContext.global).resource
    Main.server.use { server =>
    blazeClient.use { client =>
    }
    }
    

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66. TESTING HTTP
    val blazeClient = BlazeClientBuilder[IO](ExecutionContext.global).resource
    Main.server.use { server =>
    blazeClient.use { client =>
    }
    }
    client.expect[String](server.baseUri / "hello")
    .map(_ shouldBe "Hello world!")
    

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67. TESTING CLIENTS
    class TodoClient(client: Client[IO]) {
    def getTodo(id: Int): IO[Todo] =
    client.expect[Todo](Request[IO](uri = uri"/todos" / id.toString))
    }
    

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68. TESTING CLIENTS
    class TodoClient(client: Client[IO]) {
    def getTodo(id: Int): IO[Todo] =
    client.expect[Todo](Request[IO](uri = uri"/todos" / id.toString))
    }
    //in test
    val mockServer = HttpRoutes
    .of[IO] {
    case GET -> Root / "todos" / IntVar(id) =>
    Ok(Todo(id, false))
    }
    .orNotFound
    

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69. TESTING CLIENTS
    val mockServer = HttpRoutes
    .of[IO] {
    case GET -> Root / "todos" / IntVar(id) =>
    Ok(Todo(id, false))
    }
    .orNotFound
    

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70. TESTING CLIENTS
    val clientRaw = Client.fromHttpApp(mockServer)
    val todoClient = new TodoClient(clientRaw)
    val mockServer = HttpRoutes
    .of[IO] {
    case GET -> Root / "todos" / IntVar(id) =>
    Ok(Todo(id, false))
    }
    .orNotFound
    

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71. TESTING CLIENTS
    val mockServer = HttpRoutes
    .of[IO] {
    case GET -> Root / "todos" / IntVar(id) =>
    Ok(Todo(id, false))
    }
    .orNotFound
    val clientRaw = Client.fromHttpApp(mockServer)
    val todoClient = new TodoClient(clientRaw)
    todoClient.getTodo(1).map(_ shouldBe Todo(1, false))
    

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72. T E S T I N G R E S O U R C E
    S A F E T Y
    E X T E N S I B I L I T Y
    

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73. T E S T I N G R E S O U R C E
    S A F E T Y
    E X T E N S I B I L I T Y
    

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74. CATS.EFFECT.RESOURCE
    

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75. RESOURCE
    abstract class Resource[F[_], A] {
    def use[B](f: A => F[B]): F[B]
    }
    object Resource {
    def make[F[_], A](acquire: F[A])
    (release: A => F[Unit]): Resource[F, A]
    }
    

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76. RESOURCE
    Resource.make(IO(createPool()))(pool => IO(pool.close()))
    

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77. RESOURCE
    db <- Resource.make(IO(createPool()))(pool => IO(pool.close()))
    

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78. RESOURCE
    val server = for {
    db <- Resource.make(IO(createPool()))(pool => IO(pool.close()))
    server <- BlazeServerBuilder[IO].withHttpApp(routes(db))
    } yield server
    server.use(_ => IO.sleep(10.seconds))
    

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79. RESOURCE
    val server = for {
    db <- Resource.make(IO(createPool()))(pool => IO(pool.close()))
    server <- BlazeServerBuilder[IO].withHttpApp(routes(db))
    } yield server
    server.use(_ => IO.sleep(10.seconds))
    1. Create pool
    

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80. RESOURCE
    val server = for {
    db <- Resource.make(IO(createPool()))(pool => IO(pool.close()))
    server <- BlazeServerBuilder[IO].withHttpApp(routes(db))
    } yield server
    server.use(_ => IO.sleep(10.seconds))
    1. Create pool
    2. Start server
    

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81. RESOURCE
    val server = for {
    db <- Resource.make(IO(createPool()))(pool => IO(pool.close()))
    server <- BlazeServerBuilder[IO].withHttpApp(routes(db))
    } yield server
    server.use(_ => IO.sleep(10.seconds))
    1. Create pool
    2. Start server
    3. Sleep 10 seconds
    

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82. RESOURCE
    val server = for {
    db <- Resource.make(IO(createPool()))(pool => IO(pool.close()))
    server <- BlazeServerBuilder[IO].withHttpApp(routes(db))
    } yield server
    server.use(_ => IO.sleep(10.seconds))
    1. Create pool
    2. Start server
    3. Sleep 10 seconds
    4. Stop server
    

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83. RESOURCE
    val server = for {
    db <- Resource.make(IO(createPool()))(pool => IO(pool.close()))
    server <- BlazeServerBuilder[IO].withHttpApp(routes(db))
    } yield server
    server.use(_ => IO.sleep(10.seconds))
    1. Create pool
    2. Start server
    3. Sleep 10 seconds
    4. Stop server
    5. Close pool
    

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84. FS2.STREAM[F[_], A]
    - 0 to ∞ values
    - Can have effects in F
    - Supports resource acquisition/cleanup
    

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85. FS2.STREAM[F[_], A]
    - 0 to ∞ values
    - Can have effects in F
    - Supports resource acquisition/cleanup
    Stream(1,2,3)
    

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86. FS2.STREAM[F[_], A]
    - 0 to ∞ values
    - Can have effects in F
    - Supports resource acquisition/cleanup
    Stream(1,2,3)
    Stream.eval(IO(util.Random.nextInt))
    

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87. FS2.STREAM[F[_], A]
    - 0 to ∞ values
    - Can have effects in F
    - Supports resource acquisition/cleanup
    Stream(1,2,3)
    Stream.eval(IO(util.Random.nextInt))
    Stream.awakeDelay[IO](1.second)
    

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88. FS2.STREAM[F[_], A]
    - 0 to ∞ values
    - Can have effects in F
    - Supports resource acquisition/cleanup
    Stream(1,2,3)
    Stream.eval(IO(util.Random.nextInt))
    Stream.awakeDelay[IO](1.second).compile.toList: IO[List[FiniteDuration]]
    

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89. FS2.STREAM[F[_], A]
    Stream(1,2,3)
    Stream.eval(IO(util.Random.nextInt))
    Stream.awakeDelay[IO](1.second).compile.toList: IO[List[FiniteDuration]]
    

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90. FS2.STREAM[F[_], A]
    Stream(1,2,3)
    Stream.eval(IO(util.Random.nextInt))
    Stream.awakeDelay[IO](1.second).compile.toList: IO[List[FiniteDuration]]
    + Stream.bracket
    + Stream.resource
    

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91. RESOURCE SAFETY IN HTTP4S
    

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92. RESOURCE SAFETY IN HTTP4S
    

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93. RESOURCE SAFETY IN HTTP4S
    BlazeServerBuilder[IO].resource: Resource[IO, Server[IO]]
    

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94. RESOURCE SAFETY IN HTTP4S
    BlazeServerBuilder[IO].resource: Resource[IO, Server[IO]]
    BlazeClientBuilder[IO](ec).resource: Resource[IO, Client[IO]]
    

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95. RESOURCE SAFETY IN HTTP4S
    BlazeServerBuilder[IO].resource: Resource[IO, Server[IO]]
    BlazeClientBuilder[IO](ec).resource: Resource[IO, Client[IO]]
    (request: Request[IO]).body: fs2.Stream[IO, Byte]
    

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96. RESOURCE SAFETY IN HTTP4S
    BlazeServerBuilder[IO].resource: Resource[IO, Server[IO]]
    BlazeClientBuilder[IO](ec).resource: Resource[IO, Client[IO]]
    (request: Request[IO]).body: fs2.Stream[IO, Byte]
    (response: Response[IO]).body: fs2.Stream[IO, Byte]
    

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97. RESOURCE SAFETY IN HTTP4S
    BlazeServerBuilder[IO].resource: Resource[IO, Server[IO]]
    BlazeClientBuilder[IO](ec).resource: Resource[IO, Client[IO]]
    (request: Request[IO]).body: fs2.Stream[IO, Byte]
    (response: Response[IO]).body: fs2.Stream[IO, Byte]
    ...and then there's Client
    

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98. trait Client[F[_]] {
    def run(req: Request[F]) : Resource[F, Response[F]]
    def stream(req: Request[F]): Stream[F, Response[F]]
    }
    

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99. trait Client[F[_]] {
    def run(req: Request[F]) : Resource[F, Response[F]]
    def stream(req: Request[F]): Stream[F, Response[F]]
    def fetch[A](req: Request[F])(f: Response[F] => F[A]) : F[A]
    def expect[A](req: Request[F])(implicit d: EntityDecoder[F, A]): F[A]
    }
    (these have like 10 overloads*)
    *some are about to be removed
    

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100. trait Client[F[_]] {
     def run(req: Request[F]) : Resource[F, Response[F]]
     def stream(req: Request[F]): Stream[F, Response[F]]
     def fetch[A](req: Request[F])(f: Response[F] => F[A]) : F[A]
     def expect[A](req: Request[F])(implicit d: EntityDecoder[F, A]): F[A]
     def status(req: Request[F]). : F[Status]
     def successful(req: Request[F]): F[Boolean]
     }
     

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101. trait Client[F[_]] {
     def run(req: Request[F]) : Resource[F, Response[F]]
     def stream(req: Request[F]): Stream[F, Response[F]]
     def fetch[A](req: Request[F])(f: Response[F] => F[A]) : F[A]
     def expect[A](req: Request[F])(implicit d: EntityDecoder[F, A]): F[A]
     def status(req: Request[F]). : F[Status]
     def successful(req: Request[F]): F[Boolean]
     def toKleisli[A](f: Response[F] => F[A]): Kleisli[F, Request[F], A]
     //...
     }
     

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102. trait Client[F[_]] {
     def run(req: Request[F]) : Resource[F, Response[F]]
     def stream(req: Request[F]): Stream[F, Response[F]]
     def fetch[A](req: Request[F])(f: Response[F] => F[A]) : F[A]
     def expect[A](req: Request[F])(implicit d: EntityDecoder[F, A]): F[A]
     def status(req: Request[F]). : F[Status]
     def successful(req: Request[F]): F[Boolean]
     def toKleisli[A](f: Response[F] => F[A]): Kleisli[F, Request[F], A]
     //...
     //here be dragons - don't use unless you know what you're doing
     def toHttpApp: HttpApp[F]
     }
     

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103. T E S T I N G R E S O U R C E
     S A F E T Y
     E X T E N S I B I L I T Y
     

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104. T E S T I N G R E S O U R C E
     S A F E T Y
     E X T E N S I B I L I T Y
     

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105. EXTENSIBILITY
     

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106. EXTENSIBILITY
     If a server/client is still a function...
     

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107. EXTENSIBILITY
     If a server/client is still a function...
     what can we do with functions?
     

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108. case class Data(x: Int)
     val f: Data => Int = _.x
     

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109. case class Data(x: Int)
     val f: Data => Int = _.x
     def wrapF(f1: Data => Int): Data => Int =
     data =>
     f1(data.copy(x = data.x + 1)) * 2
     

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110. case class Data(x: Int)
     val f: Data => Int = _.x
     def wrapF(f1: Data => Int): Data => Int =
     data =>
     f1(data.copy(x = data.x + 1)) * 2
     val f2: Data => Int = wrapF(f)
     

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111. case class Data(x: Int)
     val f: Data => Int = _.x
     val f2: Data => Int = wrapF(f)
     def wrapF(f1: Data => Int): Data => Int =
     data =>
     f1( ) * 2
     data.copy(x = data.x + 1)
     

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115. HttpApp[F[_]] = Kleisli[F, Request[F], Response[F]]
     

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116. SERVER MIDDLEWARE
     http: HttpApp[F]
     

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117. SERVER MIDDLEWARE
     def apply[F[_]](
     http: HttpApp[F]): HttpApp[F]
     

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118. SERVER MIDDLEWARE: RESPONSE TIMING
     object ResponseTiming {
     def apply[F[_]](
     http: HttpApp[F],
     timeUnit: TimeUnit = MILLISECONDS,
     headerName: CaseInsensitiveString = CaseInsensitiveString("X-Response-Time"))(
     implicit F: Sync[F],
     clock: Clock[F]): HttpApp[F] =
     Kleisli { req =>
     for {
     before <- clock.monotonic(timeUnit)
     resp <- http(req)
     after <- clock.monotonic(timeUnit)
     header = Header(headerName.value, s"${after - before}")
     } yield resp.putHeaders(header)
     }
     }
     

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119. SERVER MIDDLEWARE: RESPONSE TIMING
     object ResponseTiming {
     def apply[F[_]](
     http: HttpApp[F],
     timeUnit: TimeUnit = MILLISECONDS,
     headerName: CaseInsensitiveString = CaseInsensitiveString("X-Response-Time"))(
     implicit F: Sync[F],
     clock: Clock[F]): HttpApp[F] =
     Kleisli { req =>
     for {
     before <- clock.monotonic(timeUnit)
     after <- clock.monotonic(timeUnit)
     header = Header(headerName.value, s"${after - before}")
     } yield
     }
     }
     Kleisli { req =>
     resp <- http(req)
     resp.putHeaders(header)
     }
     

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120. SERVER MIDDLEWARE: HEADER ECHO
     object HeaderEcho {
     def apply[F[_]: Functor, G[_]: Functor]
     (echoHeadersWhen: CaseInsensitiveString => Boolean)
     (http: Http[F, G]): Http[F, G] =
     Kleisli { req: Request[G] =>
     val headersToEcho = req.headers.filter(h => echoHeadersWhen(h.name))
     http(req).map(_.putHeaders(headersToEcho.toList: _*))
     }
     }
     

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121. SERVER MIDDLEWARE: HEADER ECHO
     object HeaderEcho {
     def apply[F[_]: Functor, G[_]: Functor]
     (echoHeadersWhen: CaseInsensitiveString => Boolean)
     =
     Kleisli { req: Request[G] =>
     val headersToEcho = req.headers.filter(h => echoHeadersWhen(h.name))
     http(req).map(_.putHeaders(headersToEcho.toList: _*))
     }
     }
     (http: Http[F, G]): Http[F, G]
     

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122. CLIENT MIDDLEWARE
     client: Client[F]
     

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123. CLIENT MIDDLEWARE
     def apply[F[_]](
     client: Client[F]
     ): Client[F]
     

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124. CLIENT MIDDLEWARE: BASE URL
     object BaseUrl {
     def apply[F[_]](base: Uri)(
     client: Client[F]
     )(implicit F: Bracket[F, Throwable]): Client[F] = {
     Client.apply[F] { req =>
     client.run(req.withUri(base.resolve(req.uri)))
     }
     }
     }
     

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125. SERVER CLIENT
     

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126. LEARN MORE
     http4s.org
     gitter.im/http4s/http4s
     gitter.im/typelevel/cats-effect
     

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127. ATTRIBUTION
     natural power by Icon Island from the Noun Project
     testing by tom from the Noun Project
     modules by mikicon from the Noun Project
     

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128. THANK YOU
     Slides: bit.ly/2WaFCOM
     Code: git.io/fjs4v
     Twitter: @kubukoz
     My blog: blog.kubukoz.com
     

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129. BONUS SLIDE: WEBSOCKETS
     case GET -> Root / "ws" / "echo" =>
     Queue.bounded[IO, WebSocketFrame](100).flatMap { q =>
     WebSocketBuilder[IO].build(
     send = q.dequeue,
     receive = q.enqueue
     )
     }
     

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130. BONUS SLIDE: WEBSOCKETS
     case GET -> Root / "ws" / "echo" =>
     Queue.bounded[IO, WebSocketFrame](100).flatMap { q =>
     WebSocketBuilder[IO].build(
     )
     }
     send = q.dequeue,
     receive = q.enqueue
     

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131. BONUS SLIDE: SWAGGER (BECAUSE SOMEONE ALWAYS ASKS)
     

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132. BONUS SLIDE: SWAGGER (BECAUSE SOMEONE ALWAYS ASKS)
     

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