"应启用更高的金属类型"警告

Question

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 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,我读过这篇文章,但不明白.

Answer 1

请参阅doc for higherKinds.

只有在启用此标志的位置,才能写入更高级的类型.

这些设计模式隐含的抽象级别通常是理解Scala代码库新手的障碍.

出于某种原因,没有人开玩笑说:

Scala中较高的kinded类型导致Turing-complete类型系统,不再保证编译器终止.

...虽然它经常会在崩溃的早期终止.

那只是个玩笑.

Answer 2

如果您想取消此警告，只需添加到您的导入部分：

import scala.language.higherKinds