Alternative.sc

import cats.Alternative
import cats.syntax.apply._
import cats.syntax.semigroupk._
import cats.syntax.option._
import cats.instances.option._
import cats.syntax.traverse._
import cats.instances.list._

trait Div[A] {
  def div(n: Int): Option[A]
}
object Div {
  def apply(m: Int, s: String): Div[String] = n => if (n % m == 0) s.some else None
  val Default:                 Div[String] = n => n.toString.some
}
implicit val dividesAlternative = new Alternative[Div] {
  def empty[A]: Div[A] = _ => None
  def combineK[A](x: Div[A], y: Div[A]): Div[A] = in => x.div(in) orElse y.div(in)
  def pure[A](x: A): Div[A] = _ => x.some
  def ap[A, B](ff: Div[A => B])(fa: Div[A]): Div[B] =
    in => ff.div(in) ap fa.div(in)
}
import Div.Default
val fizzbuzz = Div(15, "FizzBuzz") <+> Div(3, "Fizz") <+> Div(5, "Buzz") <+> Default
(1 to 100).map(fizzbuzz.div).toList.sequence[Option, String]