Scala 中类型不等式的类型约束 [英] Type constraint for type inequality in scala

问题描述

可能的重复:
强制类型差异

既然在scala =:= 中有一个强制相等的通用类型约束，那么是否有一个对类型强制执行不等于"的约束?基本上 != 但对于类型?

Since there is a generalized type constraint enforcing equality in scala =:=, is there one that enforces "not equals" for types? Basically != but for types?

编辑

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

Comment below points to an existing Q&A and the answer seems to be that (1) no, it's not in the standard library (2) yes, it's possible to define one.

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.

So I'll modify my question because of a thought that occurred to me after I saw the answer.

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，是否仍然是A =!= B?如果不是，是否可以修改解决方案，如果 A =!= B 那么A <: B 或 A >;: B?

If A <: B or A >: B, will it still be the case that A =!= B? If not, is it possible to modify the solution such that if A =!= B then it is not the case that A <: B or A >: B?

推荐答案

shapeless 定义了类型运算符 <代码>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，

shapeless defines the type operator A <:!< B (meaning A is not a subtype of B) using the same implicit ambiguity trick that's used for strict type inequality,

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 会话，

Sample 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]
                        ^

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