An example how to compile a higher-level language into a lower-level one with parameterised argument types in Free monads way

tagless-final-kv-store.scala

package scalagroup
import cats.Functor
import cats.instances.function._
import cats.instances.string._
import cats.syntax.eq._
import cats.syntax.functor._
import simulacrum.typeclass

object KVStore {
  def main(args: Array[String]): Unit = {

    trait KVStoreApi[Key, Value, F[_]] {
      def get(key: Key): F[Option[Value]]
    }

    final case class User(username: String, password: String)

    @typeclass trait UserApi[F[_]] {
      def getUser(username: String, password: String): F[Option[User]]
    }

    implicit def userApiToKvInterpreter[F[_]: Functor](implicit kv: KVStoreApi[String, String, F]): UserApi[F] =
      (username: String, password: String) =>
        for (pass <- kv.get(username)) yield pass.filter(_ === password).map(User(username, _))

    val store = Map("some_user" -> "some_password")

    implicit val userKvToIdInterpreter: KVStoreApi[String, String, Map[String, String] => ?] =
      key => map => map.get(key)

    println(UserApi[Map[String, String] => ?].getUser("some_user", "some_password")(store))
  }
}