您不能编写 def append（x：Sequence [A]）：Sequence [A ]} ，因为A在 append 变量中处于协变位置，同时是协变的。

You can't write def append(x: Sequence[A]): Sequence[A]}, as A is in contravariant position in the append argument, while being covariant.

您应该这样写：

abstract class Sequence[+A] {
   def append[B >: A](x: Sequence[B]): Sequence[B]
}

您在示例为何不进行编译，又（共，对和和-内）方差如何工作？

简而言之：

+ A表示将A转换为A的超类型是安全的上下文（狗可能会转换为动物）。如果您编写 append [A]（x：Sequence [A]），则表示x只能是A或A的子类型（狗，yorsay等），但决不能一个超类型（例如Animal），因此这与+ A注释矛盾，并且在编译时失败。使用签名 append [B&gt ;: A]（x：Sequence [B]），可以通过避免在函数参数中命名A来解决此问题。

+A states that it is safe to convert this A to a supertype of A in all context ( A Dog maybe converted to an Animal). If you write append[A](x: Sequence[A]) you are stating that x can only be A or subtypes of A (Dog, yorsay etc), but never a supertype (like Animal), so this contradicts the +A annotation and fails at compilation time. With the signature append[B >: A](x: Sequence[B]) you fix it by avoiding naming A in the function arguments.

因此， [B&gt ;: A] 定义了B的下限，指出B必须是A的超类型，或者A，但从不在层次结构中低于A的任何类中，因此符合+ A签名。

So, [B >: A] is defining a lower bound for B, stating that B must be a supertype of A, or A, but never any class below A in the hierarchy, and hence complying with +A signature.

我知道协方差和协变是复杂的概念，很难理解，我也会不时感到困惑。

I know covariance and contravariance are complex concepts and hard to understand, I also get confused from time to time.