类型参数不扩展给定类型 [英] Type parameter does not extend given type

问题描述

我想定义一个泛型，使其类型参数不扩展给定类型.

I would like to define a generic such that its type parameter does NOT extend a given type.

例如，

trait myTrait[T <: Throwable] {
  // ....
}

将定义一个 trait，其中它的类型参数扩展了 Throwable.我想要类似的东西(不是真正的 Scala 代码):

would define a trait where its type parameter extends Throwable. I want something like (not real Scala code):

trait myTrait[T Not(<:) Throwable] {
  // ....
}

type 类型参数不扩展 Throwable 的地方.有没有办法在 Scala 中构造这样的概念?

Where the type type parameter does NOT extend Throwable. Is there a way to construct such a notion in Scala?

推荐答案

你可以使用隐式来做这样的事情.这是 Miles Sabin 关于 Scala 语言的一个技巧:

You can do such a thing using implicits. Here's a trick from Miles Sabin on scala-language:

// Encoding for "A is not a subtype of B"
trait <:!<[A, B]

// Uses ambiguity to rule out the cases we're trying to exclude
implicit def nsub[A, B] : A <:!< B = null
implicit def nsubAmbig1[A, B >: A] : A <:!< B = null
implicit def nsubAmbig2[A, B >: A] : A <:!< B = null

// Type alias for context bound
type NOT[T] = {
 type Lambda[U] = U <:!< T
}

// foo does not accept T of type Unit
def foo[T : NOT[Unit]#Lambda](t : T) = t

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