为什么Equals [A ：B，B]无法编译(Why Equals[A : B, B] doesn't compile)

为什么Equals [A <：B>：B，B]无法编译(Why Equals[A <: B >: B, B] doesn't compile)

我使用case class Equals[A >: B <: B, B]来检查类型等价，例如，

scala> Equals[Int, Int] res0: Equals[Int,Int] = Equals() scala> Equals[Int, String] <console>:10: error: type arguments [Int,String] do not conform to method apply's type parameter bounds [A >: B <: B,B] Equals[Int, String]

但是当我重新声明Equals案例类时：

scala> case class Equals[A <: B >: B, B] <console>:1: error: ']' expected but '>:' found. case class Equals[A <: B >: B, B]

我想知道为什么[A >: B <:B, B]有效，但[A <: B >: B, B]没有。

I use case class Equals[A >: B <: B, B] to check type equivalency, e.g.,

scala> Equals[Int, Int] res0: Equals[Int,Int] = Equals() scala> Equals[Int, String] <console>:10: error: type arguments [Int,String] do not conform to method apply's type parameter bounds [A >: B <: B,B] Equals[Int, String]

But when I re-declare the Equals case class as:

scala> case class Equals[A <: B >: B, B] <console>:1: error: ']' expected but '>:' found. case class Equals[A <: B >: B, B]

I'd like to know why [A >: B <:B, B] works , but [A <: B >: B, B] doesn't.

最满意答案

根据Scala参考规范项4.4类型参数 ：

TypeParam :: =（id |'_'）[TypeParamClause] ['>：'Type] ['<：'Type] ['：'Type]

绑定序列是A >: Lover <: Upper但不是A <: Upper >: Lover

According to Scala Reference Specification item 4.4 Type Parameters:

TypeParam ::= (id | ‘_’) [TypeParamClause] [‘>:’ Type] [‘<:’ Type] [‘:’ Type]

Bound sequence is A >: Lover <: Upper but not A <: Upper >: Lover