Scala:为什么 Seq.contains 采用 Any 参数，而不是序列类型的参数? [英] Scala: Why does Seq.contains take an Any argument, instead of an argument of the sequence type?

问题描述

例如，为什么 List(1,2,3,4).contains("wtf") 甚至可以编译?如果编译器拒绝这个不是很好吗?

So for example why does List(1,2,3,4).contains("wtf") even compile? Wouldn't it be nice if the compiler rejected this?

推荐答案

很多有趣的答案，但这是我自己的理论:如果 contains 没有收到 Any，那么 Seq 不能是协变的.

Lots of interesting answers, but here's my own theory: if contains did not receive an Any, then Seq could not be co-variant.

参见，例如，Set，它不是协变的，它的 contains 使用 A 而不是 Any.

See, for instance, Set, which is not co-variant and whose contains take an A instead of an Any.

原因留给读者作为练习.;-) 但这里有一个提示:

The reasons for that is left as an exercise to the reader. ;-) But here is a hint:

scala> class Container[+A](elements: A*) {                         
     |   def contains(what: A): Boolean = elements exists (what ==)
     | }
<console>:7: error: covariant type A occurs in contravariant position in type A of value what
         def contains(what: A): Boolean = elements exists (what ==)
                      ^

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