The http4s DSL

Recall from earlier that an HttpService[F] is just a type alias for Kleisli[F, Request[F], Response[F]]. This provides a minimal foundation for declaring services and executing them on blaze or a servlet container. While this foundation is composeable, it is not highly productive. Most service authors will seek a higher level DSL.

Add the http4s-dsl to your build

One option is the http4s-dsl. It is officially supported by the http4s team, but kept separate from core in order to encourage multiple approaches for different needs.

This tutorial assumes that http4s-dsl is on your classpath. Add the following to your build.sbt:

libraryDependencies ++= Seq(
  "org.http4s" %% "http4s-dsl" % http4sVersion,

All we need is a REPL to follow along at home:

$ sbt console

The simplest service

We’ll need the following imports to get started:

import cats.effect._
// import cats.effect._

import org.http4s._,, org.http4s.implicits._
// import org.http4s._
// import
// import org.http4s.implicits._

The central concept of http4s-dsl is pattern matching. An HttpService[F] is declared as a simple series of case statements. Each case statement attempts to match and optionally extract from an incoming Request[F]. The code associated with the first matching case is used to generate a F[Response[F]].

The simplest case statement matches all requests without extracting anything. The right hand side of the request must return a F[Response[F]].

In the following we use cats.effect.IO as the effect type F.

val service = HttpService[IO] {
  case _ =>
// service: org.http4s.HttpService[cats.effect.IO] = Kleisli(org.http4s.HttpService$$$Lambda$60483/1348313016@2d2cb885)

Testing the Service

One beautiful thing about the HttpService[F] model is that we don’t need a server to test our route. We can construct our own request and experiment directly in the REPL.

scala> val getRoot = Request[IO](Method.GET, uri("/"))
getRoot: org.http4s.Request[cats.effect.IO] = Request(method=GET, uri=/, headers=Headers())

scala> val io =
io: cats.effect.IO[org.http4s.Response[cats.effect.IO]] = <function1>

Where is our Response[F]? It hasn’t been created yet. We wrapped it in an IO. In a real service, generating a Response[F] is likely to be an asynchronous operation with side effects, such as invoking another web service or querying a database, or maybe both. Operating in a F gives us control over the sequencing of operations and lets us reason about our code like good functional programmers. It is the HttpService[F]’s job to describe the task, and the server’s job to run it.

But here in the REPL, it’s up to us to run it:

scala> val response = io.unsafeRunSync
response: org.http4s.Response[cats.effect.IO] = Response(status=200, headers=Headers())


Generating responses

We’ll circle back to more sophisticated pattern matching of requests, but it will be a tedious affair until we learn a more succinct way of generating F[Response]s.

Status codes

http4s-dsl provides a shortcut to create an F[Response] by applying a status code:

scala> val okIo = Ok()
okIo: cats.effect.IO[org.http4s.Response[cats.effect.IO]] = IO(Response(status=200, headers=Headers(Content-Length: 0)))

scala> val ok = okIo.unsafeRunSync
ok: org.http4s.Response[cats.effect.IO] = Response(status=200, headers=Headers(Content-Length: 0))

This simple Ok() expression succinctly says what we mean in a service:

HttpService[IO] {
  case _ => Ok()
// res0: org.http4s.Response[cats.effect.IO] = Response(status=200, headers=Headers(Content-Length: 0))

This syntax works for other status codes as well. In our example, we don’t return a body, so a 204 No Content would be a more appropriate response:

HttpService[IO] {
  case _ => NoContent()
// res1: org.http4s.Response[cats.effect.IO] = Response(status=204, headers=Headers())


http4s adds a minimum set of headers depending on the response, e.g:

scala> Ok("Ok response.").unsafeRunSync.headers
res2: org.http4s.Headers = Headers(Content-Type: text/plain; charset=UTF-8, Content-Length: 12)

Extra headers can be added using putHeaders, for example to specify cache policies:

import org.http4s.headers.`Cache-Control`
// import org.http4s.headers.Cache$minusControl

import org.http4s.CacheDirective.`no-cache`
// import$minuscache

// import
scala> Ok("Ok response.", `Cache-Control`(NonEmptyList(`no-cache`(), Nil))).unsafeRunSync.headers
res3: org.http4s.Headers = Headers(Content-Type: text/plain; charset=UTF-8, Cache-Control: no-cache, Content-Length: 12)

http4s defines all the well known headers directly, but sometimes you need to define custom headers, typically prefixed by an X-. In simple cases you can construct a Header instance by hand

scala> Ok("Ok response.", Header("X-Auth-Token", "value")).unsafeRunSync.headers
res4: org.http4s.Headers = Headers(Content-Type: text/plain; charset=UTF-8, X-Auth-Token: value, Content-Length: 12)


http4s has special support for Cookie headers using the Cookie type to add and invalidate cookies. Adding a cookie will generate the correct Set-Cookie header:

scala> Ok("Ok response.").map(_.addCookie(Cookie("foo", "bar"))).unsafeRunSync.headers
res5: org.http4s.Headers = Headers(Content-Type: text/plain; charset=UTF-8, Content-Length: 12, Set-Cookie: foo=bar)

Cookie can be further customized to set, e.g., expiration, the secure flag, httpOnly, flag, etc

scala> Ok("Ok response.").map(_.addCookie(Cookie("foo", "bar", expires = Some(, httpOnly = true, secure = true))).unsafeRunSync.headers
res6: org.http4s.Headers = Headers(Content-Type: text/plain; charset=UTF-8, Content-Length: 12, Set-Cookie: foo=bar; Expires=Thu, 17 May 2018 16:41:46 GMT; Secure; HttpOnly)

To request a cookie to be removed on the client, you need to set the cookie value to empty. http4s can do that with removeCookie

scala> Ok("Ok response.").map(_.removeCookie("foo")).unsafeRunSync.headers
res7: org.http4s.Headers = Headers(Content-Type: text/plain; charset=UTF-8, Content-Length: 12, Set-Cookie: foo=; Expires=Thu, 01 Jan 1970 00:00:00 GMT; Max-Age=0)

Responding with a body

Simple bodies

Most status codes take an argument as a body. In http4s, Request[F] and Response[F] bodies are represented as a fs2.Stream[F, ByteVector]. It’s also considered good HTTP manners to provide a Content-Type and, where known in advance, Content-Length header in one’s responses.

All of this hassle is neatly handled by http4s’ EntityEncoders. We’ll cover these in more depth in another tut. The important point for now is that a response body can be generated for any type with an implicit EntityEncoder in scope. http4s provides several out of the box:

scala> Ok("Received request.").unsafeRunSync
res8: org.http4s.Response[cats.effect.IO] = Response(status=200, headers=Headers(Content-Type: text/plain; charset=UTF-8, Content-Length: 17))

scala> import java.nio.charset.StandardCharsets.UTF_8
import java.nio.charset.StandardCharsets.UTF_8

scala> Ok("binary".getBytes(UTF_8)).unsafeRunSync
res9: org.http4s.Response[cats.effect.IO] = Response(status=200, headers=Headers(Content-Type: application/octet-stream, Content-Length: 6))

Per the HTTP specification, some status codes don’t support a body. http4s prevents such nonsense at compile time:

scala> NoContent("does not compile")
<console>:29: error: type mismatch;
 found   : String("does not compile")
 required: org.http4s.Header
       NoContent("does not compile")

Asynchronous responses

While http4s prefers F[_]: Effect, you may be working with libraries that use standard library [Future]s. Some relevant imports:

import scala.concurrent.Future
// import scala.concurrent.Future

// import

You can respond with a Future of any type that has an EntityEncoder by lifting it into IO or any F[_] that suspends future. Note: unlike IO, wrapping a side effect in Future does not suspend it, and the resulting expression would still be side effectful, unless we wrap it in IO.

scala> val io = Ok(IO.fromFuture(IO(Future {
     |   println("I run when the future is constructed.")
     |   "Greetings from the future!"
     | })))
io: cats.effect.IO[org.http4s.Response[cats.effect.IO]] = IO$552725983

scala> io.unsafeRunSync
I run when the future is constructed.
res11: org.http4s.Response[cats.effect.IO] = Response(status=200, headers=Headers(Content-Type: text/plain; charset=UTF-8, Content-Length: 26))

As good functional programmers who like to delay our side effects, we of course prefer to operate in [F]s:

scala> val io = Ok(IO {
     |   println("I run when the IO is run.")
     |   "Mission accomplished!"
     | })
io: cats.effect.IO[org.http4s.Response[cats.effect.IO]] = IO$652950487

scala> io.unsafeRunSync
I run when the IO is run.
res12: org.http4s.Response[cats.effect.IO] = Response(status=200, headers=Headers(Content-Type: text/plain; charset=UTF-8, Content-Length: 21))

Note that in both cases, a Content-Length header is calculated. http4s waits for the Future or F to complete before wrapping it in its HTTP envelope, and thus has what it needs to calculate a Content-Length.

Streaming bodies

Streaming bodies are supported by returning a fs2.Stream. Like Futures and IOs, the stream may be of any type that has an EntityEncoder.

An intro to Stream is out of scope, but we can glimpse the power here. This stream emits the elapsed time every 100 milliseconds for one second:

val drip: fs2.Stream[IO, String] =
  fs2.Scheduler[IO](2).flatMap { s =>
    import scala.concurrent.duration._
// drip: fs2.Stream[cats.effect.IO,String] = Stream(..)

We can see it for ourselves in the REPL:

scala> val dripOutIO = drip.through(fs2.text.lines).through(_.evalMap(s => {IO{println(s); s}})).compile.drain
dripOutIO: cats.effect.IO[Unit] = IO$1437544639

scala> dripOutIO.unsafeRunSync
132771202 nanoseconds217263133 nanoseconds317118082 nanoseconds417261831 nanoseconds518412938 nanoseconds617233907 nanoseconds717194870 nanoseconds818064829 nanoseconds916982726 nanoseconds1017131023 nanoseconds

When wrapped in a Response[F], http4s will flush each chunk of a Stream as they are emitted. Note that a stream’s length can’t generally be anticipated before it runs, so this triggers chunked transfer encoding:

scala> Ok(drip).unsafeRunSync
res14: org.http4s.Response[cats.effect.IO] = Response(status=200, headers=Headers(Content-Type: text/plain; charset=UTF-8, Transfer-Encoding: chunked))

Matching and extracting requests

A Request is a regular case class - you can destructure it to extract its values. By extension, you can also match/case it with different possible destructurings. To build these different extractors, you can make use of the DSL.

Most often, you extract the Request into a HTTP Method (verb) and the path, via the -> object. On the left side, you’ll have the HTTP Method, on the other side the path. Naturally, _ is a valid matcher too, so any call to /api can be blocked, regardless of Method:

scala> HttpService[IO] {
     |   case _ -> Root / "api" => Forbidden()
     | }
res15: org.http4s.HttpService[cats.effect.IO] = Kleisli(org.http4s.HttpService$$$Lambda$60483/1348313016@7884fc4e)

To also block all subcalls /api/..., you’ll need /:, which is right associative, and matches everything after, and not just the next element:

scala> HttpService[IO] {
     |   case _ -> "api" /: _ => Forbidden()
     | }
res16: org.http4s.HttpService[cats.effect.IO] = Kleisli(org.http4s.HttpService$$$Lambda$60483/1348313016@49013d98)

For matching more than one Method, there’s |:

scala> HttpService[IO] {
     |   case (GET | POST) -> Root / "api"  => ???
     | }
res17: org.http4s.HttpService[cats.effect.IO] = Kleisli(org.http4s.HttpService$$$Lambda$60483/1348313016@66801508)

Honorable mention: ~, for matching file extensions.

scala> HttpService[IO] {
     |   case GET -> Root / file ~ "json" => Ok(s"""{"response": "You asked for $file"}""")
     | }
res18: org.http4s.HttpService[cats.effect.IO] = Kleisli(org.http4s.HttpService$$$Lambda$60483/1348313016@3c3a9b1e)

Handling path parameters

Path params can be extracted and converted to a specific type but are Strings by default. There are numeric extractors provided in the form of IntVar and LongVar.

def getUserName(userId: Int): IO[String] = ???
// getUserName: (userId: Int)cats.effect.IO[String]

val usersService = HttpService[IO] {
  case GET -> Root / "users" / IntVar(userId) =>
// usersService: org.http4s.HttpService[cats.effect.IO] = Kleisli(org.http4s.HttpService$$$Lambda$60483/1348313016@3030fe46)

If you want to extract a variable of type T, you can provide a custom extractor object which implements def unapply(str: String): Option[T], similar to the way in which IntVar does it.

import java.time.LocalDate
// import java.time.LocalDate

import scala.util.Try
// import scala.util.Try

// import

object LocalDateVar {
  def unapply(str: String): Option[LocalDate] = {
    if (!str.isEmpty)
// defined object LocalDateVar

def getTemperatureForecast(date: LocalDate): IO[Double] = IO(42.23)
// getTemperatureForecast: (date: java.time.LocalDate)cats.effect.IO[Double]

val dailyWeatherService = HttpService[IO] {
  case GET -> Root / "weather" / "temperature" / LocalDateVar(localDate) =>
    Ok(getTemperatureForecast(localDate).map(s"The temperature on $localDate will be: " + _))
// dailyWeatherService: org.http4s.HttpService[cats.effect.IO] = Kleisli(org.http4s.HttpService$$$Lambda$60483/1348313016@27aa3465)

// Response(status=200, headers=Headers(Content-Type: text/plain; charset=UTF-8, Content-Length: 44))

Handling query parameters

A query parameter needs to have a QueryParamDecoderMatcher provided to extract it. In order for the QueryParamDecoderMatcher to work there needs to be an implicit QueryParamDecoder[T] in scope. QueryParamDecoders for simple types can be found in the QueryParamDecoder object. There are also QueryParamDecoderMatchers available which can be used to return optional or validated parameter values.

In the example below we’re finding query params named country and year and then parsing them as a String and java.time.Year.

import java.time.Year
// import java.time.Year

// import

object CountryQueryParamMatcher extends QueryParamDecoderMatcher[String]("country")
// defined object CountryQueryParamMatcher

implicit val yearQueryParamDecoder: QueryParamDecoder[Year] =
// yearQueryParamDecoder: org.http4s.QueryParamDecoder[java.time.Year] = org.http4s.QueryParamDecoder$$anon$8@33e368a

object YearQueryParamMatcher extends QueryParamDecoderMatcher[Year]("year")
// defined object YearQueryParamMatcher

def getAverageTemperatureForCountryAndYear(country: String, year: Year): IO[Double] = ???
// getAverageTemperatureForCountryAndYear: (country: String, year: java.time.Year)cats.effect.IO[Double]

val averageTemperatureService = HttpService[IO] {
  case GET -> Root / "weather" / "temperature" :? CountryQueryParamMatcher(country) +& YearQueryParamMatcher(year)  =>
    Ok(getAverageTemperatureForCountryAndYear(country, year).map(s"Average temperature for $country in $year was: " + _))
// averageTemperatureService: org.http4s.HttpService[cats.effect.IO] = Kleisli(org.http4s.HttpService$$$Lambda$60483/1348313016@7df3e8ac)