为什么Iterator在Scala 2.8中有一个contains方法但是Iterable没有？ [英] Why does Iterator have a contains method but Iterable does not, in Scala 2.8?

问题描述

我想在我的Iterables上调用'contains'： - ）

I would like to call 'contains' on my Iterables :-)

推荐答案

原因 Iterable 没有包含方法，因为它的定义方式会对方差产生直接影响。基本上，有两种类型的签名对它有意义：

The reason Iterable does not have a contains method is because the way it is defined can have direct consequences on variance. Basically, there are two type signatures that make sense for it:

def contains(v: Any): Boolean
def contains(v: A): Boolean

第二个定义增加了类型安全性。但是， A （它是集合的类型参数）出现在反变量位置，这会强制集合不变。 可以定义如下：

The second definition has increased type safety. However, A, which is the type parameter of collection, appears in a contra-variant position, which forces the collection to be invariant. It could be defined like this:

def contains[B >: A](v: B): Boolean

但是使用<$ c，这不会比第一个签名提供任何改进$ c>任何。

因此，你会看到 immutable.Seq 是共变体并使用第一个签名，而 immutable.Set 是不变的并且使用第二个签名。

As a consequence of this, you'll see that immutable.Seq is co-variant and uses the first signature, while immutable.Set is invariant and uses the second signature.

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