Running as an http4s server

To expose an endpoint as an http4s server, first add the following dependency:

"com.softwaremill.tapir" %% "tapir-http4s-server" % "0.7.1"

and import the package:

import tapir.server.http4s._

This adds two extension methods to the Endpoint type: toRoutes and toRoutesRecoverErrors. This first requires the logic of the endpoint to be given as a function of type:

I => F[Either[E, O]]

where F[_] is the chosen effect type. The second recovers errors from failed effects, and hence requires that E is a subclass of Throwable (an exception); it expects a function of type I => F[O]. For example:

import tapir._
import tapir.server.http4s._
import cats.effect.IO
import org.http4s.HttpRoutes
import cats.effect.ContextShift

// will probably come from somewhere else
implicit val cs: ContextShift[IO] = 

def countCharacters(s: String): IO[Either[Unit, Int]] = 
  IO.pure(Right[Unit, Int](s.length))

val countCharactersEndpoint: Endpoint[String, Unit, Int, Nothing] =[Int])
val countCharactersRoutes: HttpRoutes[IO] = 
  countCharactersEndpoint.toRoutes(countCharacters _)

Note that these functions take one argument, which is a tuple of type I. This means that functions which take multiple arguments need to be converted to a function using a single argument using .tupled:

def logic(s: String, i: Int): IO[Either[Unit, String]] = ???
val anEndpoint: Endpoint[(String, Int), Unit, String, Nothing] = ??? 
val aRoute: Route = anEndpoint.toRoute((logic _).tupled)

The created HttpRoutes are the usual http4s Kleisli-based transformation of a Request to a Response, and can be further composed using http4s middlewares or request-transforming functions. The tapir-generated HttpRoutes captures from the request only what is described by the endpoint.

It’s completely feasible that some part of the input is read using a http4s wrapper function, which is then composed with the tapir endpoint descriptions. Moreover, “edge-case endpoints”, which require some special logic not expressible using tapir, can be always implemented directly using http4s.


The http4s interpreter accepts streaming bodies of type Stream[F, Byte], which can be used both for sending response bodies and reading request bodies. Usage: streamBody[Stream[F, Byte]](schema, mediaType).


The interpreter can be configured by providing an implicit Http4sServerOptions value and status mappers, see common server configuration for details.

The http4s options also includes configuration for the blocking execution context to use, and the io chunk size.

Defining an endpoint together with the server logic

It’s also possible to define an endpoint together with the server logic in a single, more concise step. See common server configuration for details.