“应启用更高级的类型"警告 [英] "Higher Kinded Type Should be Enabled" Warning

问题描述

在以下代码中(来自 Scala 中的函数式编程):

In the following code (from 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[_]] {

这是怎么回事?请注意，我阅读了这篇帖子，但不明白.

What's going on here? Note that I read this post, but didn't understand it.

推荐答案

请参阅 higherKinds.

只有在启用此标志的情况下，才能写入更高级的类型.

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

这些设计模式所隐含的抽象级别通常是Scala 代码库新手理解的障碍.

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:

For some reason, no one has joked about:

Scala 中更高级的类型导致了图灵完备的类型系统，不再保证编译器终止.

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

...虽然它通常会因崩溃而提前终止.

...though often it will just terminate early with a crash.

这只是个玩笑.

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