Sttp: An Extensible API client

25 Oct 2022

Sttp: An Extensible API client

5 minute read

A while back I blogged about how to write an api client using ZIO and http4s client. Revisiting it is interesting, so much changed since then! The post was a nice example at the time, but thinking about that solution as a real world implementation, it suffers from a couple of problems:

  • it’s not extensible;
  • it’s highly coupled - what is not necessarily a problem, when it’s part of the domain. But it would be nice to have.

Simply making changes according to these two topics wouldn’t evolve the idea that much though, so here’s the new proposition: let’s implement an extensible client, properly!

What’s wrong with the previous client?

Having client.getThis and client.getThat implicates in changing the client itself whenever we need to add a new request. Besides, the request / response strategy could be better and shapeless should be removed.

Let’s start with a base definition for our new requests:


This is a brilliant approach that I “borrowed” from sttp, where the request carries the information about the response. In principle, we could define a trait with one single method:

def send[A](request: Request[A]): F[A]

For the new, modern, super-duper awesome client, let’s keep the effect system generic. Just for fun :wink:

What is the plan?

We need a public API to consume, so we will build a client to the If you haven’t watched Pokemon, don’t tell me. That would make me feel old!

Let’s use the already mentioned sttp to send requests, and for JSON parsing we’ll be using zio-json.

Let’s catch them all!

First things first, an api host:

import sttp.model.Uri
import sttp.model.Uri.UriContext

final case class ApiHost private (uri: Uri)

object ApiHost:
  final val default = ApiHost(uri"")

Now let’s define the generic request to consume the Pokemon Api. I know it’s not so obvious, but what we need is the…

PokeRequest (Best. Name. Ever.)

Most resources available via can be requested using the following pattern:{resource}/{id or name}/

Let’s abstract the path parameters and make it easy for the user to create a request in the following format, so it can be sent via sttp:

Request[Either[ResponseException[String, String], A], Any]

A request that can fail with a ResponseException or succeed with A… 🤔 this is not that readable, is it? I think a couple of type aliases will be handy here:

type FailureResponse = ResponseException[String, String]
type SttpRequest[A]  = Request[Either[FailureResponse, A], Any]

That’s better, with that we can have a makeRequest that returns a SttpRequest[A]. It will receive the complete uri and a JsonDecoder[A] implicitly:

import sttp.client3.ziojson.asJson
import sttp.model.{ MediaType, Uri }
import zio.json.JsonDecoder

def makeRequest[A](uri: Uri)(using JsonDecoder[A]): SttpRequest[A] =

The complete way to build requests using sttp is very well documented here. makeRequest returns the description of a simple GET request that parses the result into json using JsonDecoder.

Now we have all we need to create the trait PokeRequest:

trait PokeRequest[A](id: String | Long):
  val resource: String

  def sttpRequest(host: ApiHost)(using JsonDecoder[A]): SttpRequest[A] =
    makeRequest(host.uri.addPath(resource, id.toString))

The new trait is decoupled from the client, and easily customizable, well done us! Before creating a concrete request we need the response well defined (the generic A in the request), so let’s define a Berry:

import zio.json.{ jsonField, DeriveJsonDecoder, JsonDecoder }

final case class Berry(
    id: Int,
    name: String,
    @jsonField("growth_time") growthTime: Int,
    @jsonField("max_harvest") maxHarvest: Int,
    @jsonField("natural_gift_power") naturalGiftPower: Int,
    size: Int,
    smoothness: Int,
    @jsonField("soil_dryness") soilDryness: Int,
    firmness: NamedAPIResource,
    flavors: List[BerryFlavorMap],
    item: NamedAPIResource,
    @jsonField("natural_gift_type") naturalGiftType: NamedAPIResource

object Berry:
  given JsonDecoder[Berry] = DeriveJsonDecoder.gen

The pokeapi response has all its field names in snake case, so jsonField is used when the names don’t match. The most important part is the creation of the JsonDecoder using the code generator. All in place to write BerryRequest:

final case class BerryRequest(id: String | Long) extends PokeRequest[Berry](id):
  val resource = "berry"

Having the response, writting the request is that simple! What do we need now to send request? You won’t see that coming, dear reader! We need a…


I am a master naming stuff

Cool, now let’s get to business. Sttp gives us the generic structures we are looking for, we just need to adapt it to our domain, making it more restrict. Our PokeApiClient will be defined using an SttpBackend:

import sttp.client3.*
import sttp.monad.MonadError

case class PokeApiClient[F[_], +P](host: ApiHost)(using
    backend: SttpBackend[F, P]
  given monadError: MonadError[F] = backend.responseMonad

The backend wraps the real http client, F is the effect type in use (ZIO, Future, etc) and P indicates extra capabilities like streaming or websockets, so don’t worry about it, we are not using any of those. The last important bit is the given monadError: it’s the way to inform if the request was successful or not.

The client will have only one public method send that receives the request, the JsonDecoder and return a F[A]. However, splitting it in two makes it easier to understand:

def send[A](request: PokeRequest[A])(using JsonDecoder[A]): F[A] =
  doSend(request).flatMap {
    case Right(value) => monadError.unit(value)
    case Left(error) =>  monadError.error(error)

private def doSend[A](
    request: PokeRequest[A]
)(using JsonDecoder[A]): F[Either[FailureResponse, A]] =

doSend provides the request with a host and effectively sends the request using the backend, returning the response body. send will then flatMap it and use the appropriate channels of monadError to return the result, unit for success and error for failure.

This last line some complexity in it, but honestly, don’t think about it. Let’s focus on what’s matter: we have a generic, extensible api client!

Using the PokeApiClient

All the sttp backends published for Scala 3 are supported. An example with Futures:

given backend: SttpBackend[Future, Any] = HttpClientFutureBackend()
val client = PokeApiClient()

client.send(ContestTypeRequest(1)).onComplete {
    case Success(contest) => println(contest.names)
    case Failure(t)       => println(s"Failed with: $t")

or with ZIO:

val client = AsyncHttpClientZioBackend().map(implicit backend => PokeApiClient())

val zio = client.flatMap(_.send(PokemonRequest("bulbasaur")))
val pokemon = Unsafe.unsafeCompat { implicit u =>

You can find more examples using different backends, and the complete code in the Github repo.


Revisiting our own code is a stimulating experience - always good to see how the perspective changed, and what we’ve learned. Sttp is a powerful tool, and the knowledge acquired studying it made it easy to create an extensible api client, that allows the user to decide which effect system they want to use.

The final outcome is in the list of wrapper libraries of the PokeApi. Can you see possible improvements? Just let me know!