package ru.tinkoff.travel.utils

import cats.{Monoid, MonoidK}
import shapeless.tag
import shapeless.tag.@@


object monoids {
  type Endo[X] = Endo.T[X]

  object Endo {
    abstract class Tag private()
    type T[X] = (X => X) @@ Tag

    def apply[X](f: X => X): T[X] = tag[Tag].apply[X => X](f)

    implicit val endoMonoidK = new MonoidK[T] {
      def empty[A]: T[A] = Endo(identity[A])
      def combineK[A](x: T[A], y: T[A]): T[A] = Endo(x.compose(y))
    }

    implicit def endoMonoid[A] = endoMonoidK.algebra[A]
  }
}