Lifting a function which takes implicit parameter using functor (Scalaz7) - scala

Question

Just started learning Scalaz. Here is my code
trait Monoid[A] {
def mappend(a1: A, a2: A): A
def mzero: A
}
object Monoid {
implicit val IntMonoid: Monoid[Int] = new Monoid[Int] {
def mappend(a1: Int, a2: Int): Int = a1 + a2
def mzero: Int = 0
}
implicit val StringMonoid: Monoid[String] = new Monoid[String] {
def mappend(a1: String, a2: String): String = a1 + a2
def mzero: String = ""
}
}
trait MonoidOp[A] {
val F: Monoid[A]
val value: A
def |+|(a2: A): A = F.mappend(value, a2)
}
object MonoidOp{
implicit def toMonoidOp[A: Monoid](a: A): MonoidOp[A] = new MonoidOp[A]{
val F = implicitly[Monoid[A]]
val value = a
}
}
I have defined a function (just for the sake of it)
def addXY[A: Monoid](x: A, y: A): A = x |+| y
I want to lift it so that it could be used using Containers like Option, List, etc. But when I do this
def addXYOptioned = Functor[Option].lift(addXY)
It says error: could not find implicit value for evidence parameter of type scalaz.Monoid[A]
def addOptioned = Functor[Option].lift(addXY)
How to lift such functions?

Answer 1

Your method addXY needs a Monoid[A] but there is no Monoid[A] in scope when used in addXYOptioned, so you also need to add the Monoid constraint to addXYOptioned.
The next problem is that Functor.lift only lifts a function A => B, but we can use Apply.lift2 to lift a function (A, B) => C.
Using the Monoid from Scalaz itself :
import scalaz._, Scalaz._
def addXY[A: Monoid](x: A, y: A): A = x |+| y
def addXYOptioned[A: Monoid] = Apply[Option].lift2(addXY[A] _)
We could generalize addXYOptioned to make it possible to lift addXY into any type constructor with an Apply instance :
def addXYApply[F[_]: Apply, A: Monoid] = Apply[F].lift2(addXY[A] _)
addXYApply[List, Int].apply(List(1,2), List(3,4))
// List[Int] = List(4, 5, 5, 6)
addXYApply[Option, Int].apply(1.some, 2.some)
// Option[Int] = Some(3)