+ - 签署Scala中的通用声明 [英] +- Signs in Generic Declaration in Scala

问题描述

我在这个链接中查看了PartialFunction的文档：

  trait PartialFunction [-A，+ B]扩展（A）⇒B 
  

也许有人可以帮助澄清泛型声明中加号和减号的意义吗？

解决方案

+和 - 的意思是协变和逆变类型。总之，这意味着：
$ b preartialFunction [-A1，+ B1] <： PartialFunction [-A2，+ B2] 仅当 A1：> A2 和 B1 <：B2 ，其中<：是子类型关系。

- 通常用于输入参数，+用于输出 - 在C＃中，它们甚至在 >和 out 。在存在类型上构建的还有一些更原始的 Java中的通用差异支持 - 实际上你可以用 _< ;: SomeType （协方差）或抽象类型成员类型T< ;: SomeType 在Scala中。

没有修饰符 PartialFunction [A1，B1] 与 PartialFunction [A2，B2] （换句话说，它将是不变的）。

这种类型也有一些限制，例如协变（+）类型不能处于逆向位置（只能从方法返回），反之亦然。这是为了支持 Liskov替代原则，可以通过in/out自然理解，解释。

另外，值得注意的是 A => B （语法糖用于 Function1 ）本身使用了co- / contra-variance：

  trait Function1 [-T1，+ R]扩展AnyRef 
  

I was looking at the documentation for PartialFunction in this link:

trait PartialFunction[-A, +B] extends (A) ⇒ B

Maybe someone can help clarify the significance of the plus and minus signs in the generic declaration?

解决方案

"+" and "-" mean covariant and contravariant types respectively. In short, it means that:

PartialFunction[-A1, +B1] <: PartialFunction[-A2, +B2] only if A1 :> A2 and B1 <: B2, where <: is subtyping relationship.

"-" usually applied for input parameters, "+" for output - in C# they even use respective keywords in and out. There is also some more primitive generic variance support in Java built up on existential types - actually you can do it using _ <: SomeType (covariance) or abstract type members type T <: SomeType in Scala as well.

Without modifiers PartialFunction[A1, B1] would have no direct relationship to a PartialFunction[A2, B2] (in other words, it would be invariant).

P.S. There are also some restrictions applied to such types, like covariant("+") type can't be in contravariant position (you can only return it from a method) and vice-versa. This is done in order to support Liskov Substitution Principle and naturally understandable by "in"/"out" interpretation.

Also, it worth noticing that A => B (syntax sugar for Function1) itself is using co-/contra-variance:

 trait Function1 [-T1, +R] extends AnyRef

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