Composing Functors

composing.scala

scala> val FL = Functor[List]
FL: scalaz.Functor[List] = scalaz.std.ListInstances$$anon$1@575f20e3

scala> val FF = Functor[Future]
FF: scalaz.Functor[scala.concurrent.Future] = scalaz.std.FutureInstance@678bc1a9

scala> val FF_FL = FF compose FL
FF_FL: scalaz.Functor[[α]scala.concurrent.Future[List[α]]] = scalaz.Functor$$anon$1@49487384

scala> val values = Future(List(1, 2, 3, 4, 5))
values: scala.concurrent.Future[List[Int]] = scala.concurrent.impl.Promise$DefaultPromise@16d8bdcc

scala> val mapped = FF_FL.map(values)(_ + 100)
mapped: scala.concurrent.Future[List[Int]] = scala.concurrent.impl.Promise$DefaultPromise@8ecd29

scala> mapped.foreach(println)
List(101, 102, 103, 104, 105)

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def functorCompose[F[_], G[_], A, B](value: F[G[A]])(mapping: A => B)(implicit functorF: Functor[F], functorG: Functor[G]): F[G[B]] = {
  (functorF compose functorG).map(value)(mapping)
}