Monad Laws

trait Monad[F[_]]:
  def pure[A](a: A): F[A]
  def flatMap[A, B](fa: F[A])(f: A → F[B]): F[B]

pure(a).flatMap(f)        = f(a)
m.flatMap(pure)           = m
m.flatMap(g).flatMap(h)   = m.flatMap(b => g(b).flatMap(h))