SelectiveFunctors.scala

import scalaz._
import Scalaz._
import scalaz.{Applicative, \/}

def selectA[F[_]: Applicative, A, B](s: F[A \/ B])(f: => F[A => B]): F[B] =
  (s |@| f)(_.leftMap(_).merge)

def selectM[F[_]: Monad, A, B](s: F[A \/ B])(f: => F[A => B]): F[B] =
  s flatMap {
    case -\/(a) => f.map(_(a))
    case \/-(b) => Monad[F].pure(b)
  }

val s: Option[Int \/ String] = Option(\/-("foo"))
def f: Option[Int => String] = {
  println("side-effect!")
  Option((i: Int) => i.toString)
}

val applicativeSelector = selectA(s) _
val monadSelector = selectM(s) _

println(applicativeSelector(f)) // prints "side-effect!"
println(monadSelector(f)) // doesn't print "side-effect!"