我是scala编程的新手，有没有人能解释这一点，为什么这个想法显示了这些错误，但效果很好？

case class FlatMap[A, B](sub: IO[A], f: A => IO[B]) extends IO[B]

@annotation.tailrec
  def run[A](io: IO[A]): A = io match {
    case Pure(a) => a
    case Suspend(r) => r()
    case FlatMap(sub, k) => sub match {
      case Pure(a) => run(k(a `Type mismatch, Required Nothing, Found Any`))
      case Suspend(r) => run(k(r() `same as above`))
      case FlatMap(y, g) => run(y flatMap (a => g(a `same as above`) flatMap k `Required Any => IO[NotInferredB], Found: Nothing => IO[A]`))
    }
  }

我想了解更多关于函数式编程和scala中的单子，
有什么好的资源吗？谢谢！