Function combination

conf.sc

interp.configureCompiler(_.settings.YpartialUnification.value = true)

fcomb.sc

import $file.`conf`
import $ivy.`org.typelevel::cats-core:1.1.0`
import cats.{Endo, Semigroup, SemigroupK}
import cats.instances.function._
import cats.instances.option._
import cats.syntax.semigroup._
import cats.syntax.semigroupk._

implicit def endoSemigroup[T]: Semigroup[Endo[T]] = new Semigroup[Endo[T]] {
  override def combine(x: Endo[T], y: Endo[T]) = x andThen y
}

val f1: Option[Int => Int] = Some(_ + 1)
val f2: Option[Int => Int] = Some(_ + 2)
val f3: Option[Int => Int] = None

val fx = (f3 |+| f3).getOrElse(identity[Int](_))

println(fx(0))