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

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?

Two things: (1) Your function takes two arguments instead of one (strictly speaking it has two argument lists, since the context bound is an implicit parameter). (2) I don't think it is possible to use context bounds with functors, see also stackoverflow.com/questions/10849142/…. — devkat
– devkat, Commented Jul 11, 2016 at 8:06

Peter Neyens · Accepted Answer · 2016-07-11 08:42:20Z

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)

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