traverse.sc

//import cats.Applicative
//import cats.implicits.*

/*
def sequence[A, F[_], G[_]](fga: F[G[A]])(using Traverse[F], Applicative[G]): G[F[A]] =
  traverse(fga)(identity)

trait Traverse[F[_]]:
  def traverse[A, B, G[_]](fa: F[A])(f: A => G[B])(using Applicative[G]): G[F[B]]
*/  

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

trait Applicative[F[_]] extends Functor[F] {
  override def map[A, B](fa: F[A])(f: A => B): F[B] = ???

  def apply[A, B](fa: F[A])(ff: F[A => B]): F[B] = ???

  def map2[A, B, C](fa: F[A])(fb: F[B])(f: (A, B) => C)(implicit ev: Functor[F]): F[C] =
    apply(fb)(ev.map(fa)(a => (b => f(a, b))))
}

/*
val foo: Either[Nothing, List[Int]] = List(Right(1), Right(2)).traverse(r => r.asInstanceOf[Either[Nothing, Int]])
println(foo)
*/