The Reader effect is used to request values from an “environment”. The main method is ask to get the current environment (or “configuration” if you prefer to see it that way) and you can run an effect stack containing a Reader effect by providing a value for the environment with the runReader method.

You can also inject a “local” reader into a “bigger” one:

import org.atnos.eff._, all._, syntax.all._

      case class Conf(host: String, port: Int)

      type R1[A] = Reader[Int, A]
      type R2[A] = Reader[Conf, A]

      type S = Fx.fx2[R1, R2]

      def getPort[R](implicit r: Reader[Int, *] |= R): Eff[R, String] = for {
        p1 <- ask[R, Int]
      } yield "the port is " + p1

      getPort[S].translateReader((_: Conf).port).runReader(Conf("prod", 80)).run

> the port is 80