Scala 泛型中的协变（covariant）与逆变(contravariant)符号

  看到这个问题《+- Signs in Generic Declaration in Scala》下面有一个很有意思的答案：

“+” 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.

  简而言之，-意味着逆变成立，+意味着协变成立。