scala – “应该启用更高的键控类型”警告

发布时间：2020-12-16 18:53:29 所属栏目：安全来源：网络整理

导读：在以下代码中(从 Functional Programming in Scala开始)： 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

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

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,我读了这篇post,但不明白.

解决方法

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.

出于某种原因,没有人开玩笑说：

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

…虽然它经常会在崩溃的早期终止.

那只是个玩笑.

（编辑：李大同）

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