Main.scala

case class Endo[A](run: A => A) {
    def apply(a: A): A = run(a)
    
    def compose(other: Endo[A]): Endo[A] = {
        Endo.endo(run compose other.run)   
    }
    
    def andThen(other: Endo[A]): Endo[A] = {
        other compose this
    }
    
    def ~>(other: Endo[A]): Endo[A] = compose(other)
}

object Endo {
    
    def endo[A](f: A => A): Endo[A] = new Endo(f)
    
    def idEndo[A]: Endo[A] = endo[A](a => a)
    
}

object Main extends App{
    // Int
    {
        val f = Endo.endo[Int](_ + 1)
        val g = Endo.endo[Int](_ * 2)
        
        val fg = f ~> g
        val gf = g ~> f
        
        println(s"fg(3): ${fg(3)}")
        println(s"gf(3): ${gf(3)}")
    }

    // Int => Int
    {
        val f = Endo.endo[Int => Int](_f => i => _f(i + 1))
        val g = Endo.endo[Int => Int](_g => i => _g(i * 2))

        val fg = f ~> g
        val gf = g ~> f
        
        println(s"fg(identity)(3): ${fg(identity)(3)}")
        println(s"gf(identity)(3): ${gf(identity)(3)}")
    }
}