相关疑难解决方法(0)

在以下代码中(来自Scala中的Functional Programming):

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

trait Monad[F[_]] {
  def unit[A](a: => A): F[A]
  def flatMap[A,B](ma: F[A])(f: A => F[B]): F[B]
  def apply[A](a: => A): F[A]
}

我看到以下警告:

[warn] C:\...\Monad.scala:3: higher-kinded type should be enabled
[warn] by making the implicit value scala.language.higherKinds visible.
[warn] This can be achieved by adding the import clause 'import scala.language.higherKinds'
[warn] or by setting the compiler option -language:higherKinds.
[warn] See the Scala docs for value scala.language.higherKinds …