Routes and HttpRoutes

http4s is a purely functional HTTP library for Scala, built on cats-effect and fs2 streams. Requests and responses are immutable values, and effects are captured in a polymorphic effect type F[_] such as IO.

Instead of mutating a servlet, you describe an HTTP service as a function from a request to an effectful optional response.

The core abstraction is HttpRoutes[F], an alias for Kleisli[OptionT[F, *], Request[F], Response[F]]. The OptionT models a route that may not match, returning no response.

You rarely write that signature by hand. Instead you build routes with the HttpRoutes.of constructor and a partial function.