An exercise to implement Monad in Scala 3

monad.sc

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

trait Monad[F[_]] extends Functor[F]:
  def unit[A](a: A): F[A]
  def flatMap[A, B](fa: F[A])(f: A => F[B]): F[B]
  def map[A, B](fa: F[A])(f: A => B): F[B] = flatMap(fa)(x => unit(f(x)))

object Monad:
  def apply[F[_]](using ev: Monad[F]): Monad[F] = ev

object MonadSyntax:
  extension[F[_], A](fa: F[A])(using ev: Monad[F])
    def flatMap[B](f: A => F[B]): F[B] = ev.flatMap(fa)(f)
    def map[B](f: A => B): F[B] = ev.map(fa)(f)

trait MyBox[+A]
case class FilledBox[A](x: A) extends MyBox[A]
case object EmptyBox extends MyBox[Nothing]

object MyBox:
  def apply[A](a: A): MyBox[A] = FilledBox(a)
  def empty: MyBox[Nothing] = EmptyBox

given Monad[MyBox] with
  override def unit[A](a: A) = FilledBox(a)
  override def flatMap[A, B](fa: MyBox[A])(f: A => MyBox[B]): MyBox[B] =
    fa match
      case FilledBox(x) => f(x)
      case EmptyBox => EmptyBox

import MonadSyntax.*

def showMyBox[A](box: MyBox[A]): String =
  box match
    case FilledBox(x) => s"The result is $x"
    case _ => "Nothing there"

val result = for {
  a <- MyBox(1)
  b <- MyBox(2)
} yield a + b

println(showMyBox(result))

val empty = for {
  a <- MyBox(1)
  _ <- MyBox.empty
} yield a

println(showMyBox(empty))