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。
最佳答案
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。
关于scala - "Higher Kinded Type Should be Enabled"警告,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/21642271/