scala - "Higher Kinded Type Should be Enabled"警告

Question

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

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 for a discussion
[warn] why the feature should be explicitly enabled.
[warn] trait Functor[F[_]] {
[warn]               ^
[warn] C:\...\Monad.scala:7: higher-kinded type should be enabled
[warn] by making the implicit value scala.language.higherKinds visible.
[warn] trait Monad[F[_]] {

这里发生了什么？请注意，我阅读了此 post，但听不懂。

Answer 1

请参阅higherKinds的文档。

Only where this flag is enabled, higher-kinded types can be written.

The level of abstraction implied by these design patterns is often a barrier to understanding for newcomers to a Scala codebase.

由于某些原因，没有人开玩笑:

Higher kinded types in Scala lead to a Turing-complete type system, where compiler termination is no longer guaranteed.

...尽管通常它会因崩溃而提前终止。

那只是个玩笑。