scala - scala 中类型不等式的类型约束

Question

这个问题已经有答案了:

已关闭 11 年前。

Possible Duplicate:
Enforce type difference

由于 scala 中存在强制相等的广义类型约束 =:= ，是否有一个强制类型“不等于”的方法？基本上!=但对于类型？

编辑

下面的评论指向现有的 Q&A答案似乎是(1)不，它不在标准库中(2)是的，可以定义一个。

所以我会修改我的问题，因为我看到答案后想到了一个想法。

鉴于现有的解决方案:

sealed class =!=[A,B]

trait LowerPriorityImplicits {
  implicit def equal[A]: =!=[A, A] = sys.error("should not be called")
}
object =!= extends LowerPriorityImplicits {
  implicit def nequal[A,B](implicit same: A =:= B = null): =!=[A,B] = 
    if (same != null) sys.error("should not be called explicitly with same type")
    else new =!=[A,B]
}     

case class Foo[A,B](a: A, b: B)(implicit e: A =!= B)

如果A <: B或A >: B ，还会是A =!= B吗？ ？如果不是，是否可以修改解决方案，使得 A =!= B那么情况就不是 A <: B或A >: B ？

Answer 1

shapeless定义类型运算符A <:!< B (意味着 A 不是 B 的子类型)使用与严格类型不等式相同的隐式歧义技巧，

trait <:!<[A, B]

implicit def nsub[A, B] : A <:!< B = new <:!<[A, B] {}
implicit def nsubAmbig1[A, B >: A] : A <:!< B = sys.error("Unexpected call")
implicit def nsubAmbig2[A, B >: A] : A <:!< B = sys.error("Unexpected call")

示例 REPL session ，

scala> import shapeless.TypeOperators._
import shapeless.TypeOperators._

scala> implicitly[Int <:!< String]
res0: shapeless.TypeOperators.<:!<[Int,String] =
  shapeless.TypeOperators$$anon$2@200fde5c

scala> implicitly[Int <:!< Int]
<console>:11: error: ambiguous implicit values:
 both method nsubAmbig1 in object TypeOperators of type
   [A, B >: A]=> shapeless.TypeOperators.<:!<[A,B]
 and method nsubAmbig2 in object TypeOperators of type
   [A, B >: A]=> shapeless.TypeOperators.<:!<[A,B]
 match expected type shapeless.TypeOperators.<:!<[Int,Int]
              implicitly[Int <:!< Int]
                        ^

scala> class Foo ; class Bar extends Foo
defined class Foo
defined class Bar

scala> implicitly[Foo <:!< Bar]
res2: shapeless.TypeOperators.<:!<[Foo,Bar] =
  shapeless.TypeOperators$$anon$2@871f548

scala> implicitly[Bar <:!< Foo]
<console>:13: error: ambiguous implicit values:
 both method nsubAmbig1 in object TypeOperators of type
   [A, B >: A]=> shapeless.TypeOperators.<:!<[A,B]
 and method nsubAmbig2 in object TypeOperators of type
   [A, B >: A]=> shapeless.TypeOperators.<:!<[A,B]
 match expected type shapeless.TypeOperators.<:!<[Bar,Foo]
              implicitly[Bar <:!< Foo]
                        ^