Context.scala

import cats.data.ReaderT
import cats.implicits._
import cats.{Applicative, Functor}

trait ContextRead[F[_], Ctx] {

  def read: F[Ctx]

}

object ContextRead {

  def apply[F[_], Ctx](implicit context: ContextRead[F, Ctx]): ContextRead[F, Ctx] = implicitly

  def reader[F[_]: Applicative, Ctx]: ContextRead[ReaderT[F, Ctx, ?], Ctx] = new ReaderContextRead

  private class ReaderContextRead[F[_]: Applicative, Ctx] extends ContextRead[ReaderT[F, Ctx, ?], Ctx] {

    override def read: ReaderT[F, Ctx, Ctx] =
      ReaderT.ask[F, Ctx]

  }

  implicit def functorInstance[F[_]: Functor, Ctx]: Functor[ContextRead[F, ?]] = new ContextReadFunctor

  private class ContextReadFunctor[F[_]: Functor, Ctx] extends Functor[ContextRead[F, ?]] {
    override def map[A, B](fa: ContextRead[F, A])(f: A => B): ContextRead[F, B] =
      new ContextRead[F, B] {
        override def read: F[B] = fa.read.map(f)
      }
  }

}

trait ContextScope[F[_], Ctx] {

  def scope[T](f: Ctx => Ctx)(ft: F[T]): F[T]

}

object ContextScope {

  def apply[F[_], Ctx](implicit context: ContextScope[F, Ctx]): ContextScope[F, Ctx] = implicitly

  def reader[F[_]: Applicative, Ctx]: ContextScope[ReaderT[F, Ctx, ?], Ctx] = new ReaderContextScope

  private class ReaderContextScope[F[_]: Applicative, Ctx] extends ContextScope[ReaderT[F, Ctx, ?], Ctx] {

    override def scope[T](f: Ctx => Ctx)(ft: ReaderT[F, Ctx, T]): ReaderT[F, Ctx, T] =
      ReaderT.local(f)(ft)

  }

}

trait Context[F[_], Ctx] extends ContextRead[F, Ctx] with ContextScope[F, Ctx]

object Context {

  def apply[F[_], Ctx](implicit context: Context[F, Ctx]): Context[F, Ctx] = implicitly

  def reader[F[_]: Applicative, Ctx]: Context[ReaderT[F, Ctx, ?], Ctx] = new ReaderContext

  private class ReaderContext[F[_]: Applicative, Ctx] extends Context[ReaderT[F, Ctx, ?], Ctx] {

    override def read: ReaderT[F, Ctx, Ctx] =
      contextRead.read

    override def scope[T](f: Ctx => Ctx)(ft: ReaderT[F, Ctx, T]): ReaderT[F, Ctx, T] =
      contextScope.scope(f)(ft)

    private val contextRead = ContextRead.reader[F, Ctx]
    private val contextScope = ContextScope.reader[F, Ctx]

  }

}