impl Functor use PolyFunction

readme.md

• https://twitter.com/xuwei_k/status/1405043274671263744
• scala/scala3#17548

build.sbt

scalaVersion := "3.4.0-RC1-bin-20230727-ac69c66-NIGHTLY"

crossScalaVersions += "3.3.0"

FunctorPolyFunction.scala

package example

trait Functor[F[_]] {
  def map[A, B](fa: F[A])(f: A => B): F[B]
}

object Functor {
  def fromPolyFunction[F[_]](x: [A, B] => (fa: F[A]) => (f: A => B) => F[B]): Functor[F] =
    new Functor[F] {
      def map[A, B](fa: F[A])(f: A => B): F[B] = x(fa)(f)
    }

  implicit val list: Functor[List] = 
    fromPolyFunction([A, B] => (fa: List[A]) => (f: A => B) => fa.map(f))

  implicit val vector: Functor[Vector] = 
    fromPolyFunction([A, B] => (fa: Vector[A]) => (f: A => B) => fa.map(f))

  implicit val option: Functor[Option] = 
    fromPolyFunction([A, B] => (fa: Option[A]) => (f: A => B) => fa.map(f))

  implicit def either[L]: Functor[[a] =>> Either[L, a]] = 
    fromPolyFunction([A, B] => (fa: Either[L, A]) => (f: A => B) => fa.map(f))
}

object FunctorExplicitInstances {

  val list: Functor[List] = 
    new Functor[List] {
      def map[A, B](fa: List[A])(f: A => B) = fa map f
    }

  val vector: Functor[Vector] = 
    new Functor[Vector] {
      def map[A, B](fa: Vector[A])(f: A => B) = fa map f
    }

  val option: Functor[Option] = 
    new Functor[Option] {
      def map[A, B](fa: Option[A])(f: A => B) = fa map f
    }

  def either[L]: Functor[[a] =>> Either[L, a]] = 
    new Functor[[a] =>> Either[L, a]] {
      def map[A, B](fa: Either[L, A])(f: A => B) = fa map f
    }

}