readerDistributive.scala

    implicit def readerDistributive[I]: Distributive[({type R[X]=Reader[I, X]})#R] = new Distributive[({type R[X]=Reader[I, X]})#R] {
      override def distributeImpl[G[_], A, B](fa: G[A])(f: (A) => Reader[I, B])(implicit gf: Functor[G]): Reader[I, G[B]] =
        Reader { i =>
          fa.map(a => f(a)(i))
        }

      override def map[A, B](fa: Reader[I, A])(f: (A) => B): Reader[I, B] = fa.map(f)
    }