Scala DI wth implicits and Cats

scala_cats_di.scala

//> using toolkit typelevel:default

import cats.effect.*

object Hello extends IOApp.Simple:
  def run =
    val server = for {
      given Database <- Postgres.build
      given UserRepo <- DefaultUserRepo.build
      given UserService <- DefaultUserService.build
      given UserController <- DefaultUserController.build
      httpServer <- HttpServer.build
    } yield httpServer

    server.use(_.run)

// DB

trait Database

object Postgres:
  def build: Resource[IO, Postgres] = ???

class Postgres extends Database {}

// Repo

trait UserRepo

object DefaultUserRepo:
  def build(using db: Database): Resource[IO, DefaultUserRepo] =
    Resource.pure(DefaultUserRepo(db))

class DefaultUserRepo(db: Database) extends UserRepo {}

// Service

trait UserService

object DefaultUserService:
  def build(using repo: UserRepo): Resource[IO, DefaultUserService] =
    Resource.pure(DefaultUserService(repo))

class DefaultUserService(repo: UserRepo) extends UserService {}

// Controller

trait UserController

object DefaultUserController:
  def build(using service: UserService): Resource[IO, DefaultUserController] =
    Resource.pure(DefaultUserController(service))

class DefaultUserController(service: UserService) extends UserController {}

// Http Server

object HttpServer:
  def build(using controller: UserController): Resource[IO, HttpServer] =
    Resource.pure(HttpServer(controller))

class HttpServer(controller: UserController):
  def run: IO[Unit] = ???