问题描述
我知道在标准库中使用 co-和 contravariance （例如，集合和特征 Function ）我想知道 co-和 contravariance 如何在真实世界的业务应用程序。
I know about using co- and contravariance in the standard library (e.g. collections and trait Function) I wonder how co- and contravariance are used in design of "real world" business applications.
推荐答案
经典示例是函数，将Scala接口用于具有单个参数的函数：
The classic example is functions, taking the Scala interface for a function with a single argument:
trait Function1[-T1, +R]
对于该参数而言是逆变的（-），对于参数而言是协变的（ + ）返回类型。
Which is contravariant (the -) for the argument, and covariant (the +) for the return type.
为什么？
想象一下您有以下这些类：
Imagine you have these classes:
class Timelord { ... }
class Doctor extends Timelord { ... }
class Enemy { ... }
class Dalek extends Enemy { ... }
如果您有采取的方法，作为参数， Doctor =>敌人函数；那么可以提供 TimeLord =>的实例敌人。它仍然会接受 Doctor 的实例。
If you have a method that takes, as a parameter, a Doctor => Enemy function; then it's okay to supply an instance of TimeLord => Enemy. It'll still accept instances of Doctor.
所以 TimeLord =>敌人是 Doctor =>的子类。敌人，因为 TimeLord 是 Doctor 的超类，它是
So TimeLord => Enemy is a subclass of Doctor => Enemy because TimeLord is a superclass of Doctor, it's contravariant in that parameter.
同样，返回 Dalek 的函数在以下情况下有效您需要一个返回 Enemy 的函数，因为 Dalek 是- Enemy
Likewise, a function returning a Dalek is valid when you need a function returning some Enemy, because a Dalek is-an Enemy
所以 Doctor => Dalek 是 Doctor =>的子类。敌人是因为 Dalek 是 Enemy 的子类，该参数中的协变量。
So Doctor => Dalek is a subclass of Doctor => Enemy because Dalek is a subclass of Enemy, it's covariant in that parameter.
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