您可以为 None 指定类型参数或告诉编译器它是一个 Option[String] 吗? [英] Can you specify type argument for None or tell compiler that it's an Option[String]?

问题描述

我想知道我是否可以在我的代码中写这样的东西:

I wonder if I can write something like this in my code:

None[String]

推荐答案

我很惊讶没有人提到 Option.empty 的存在:

I am surprised that nobody mentioned the existence of Option.empty:

scala> Option.empty[String]
res0: Option[String] = None

请注意，在许多情况下，只需在需要 Option[String] 的地方使用 None 即可正常工作.或者换句话说，(如 Aleksey Izmailov 所示)，以下是正确的:

Note that in many cases simply using None where an Option[String] is expected will work fine. Or in other words, (as shown by Aleksey Izmailov), the following is corrrect:

def f(o: Option[String]) = ...; f(None)

这是因为 None 扩展了 Option[Nothing]，所以凭借 Option 是协变的(并且 Nothing> 作为所有其他类型的子类型)，对于任何 T，None 始终与 Option[T] 兼容.

This is because None extends Option[Nothing], so by virtue of Option being covariant (and Nothing being a sub-type of every other type), None is always a compatible with Option[T] for any T.

这也是为什么类型归属也是一个很好的选择(对于您确实需要明确选项类型的情况，例如如果需要驱动类型推断):

This is also why type ascription is also a fine alternative (for the cases where you do need to be explicit on the type of options, by example if it is needed to drive type inference):

scala> None: Option[String]
res0: Option[String] = None

这篇关于您可以为 None 指定类型参数或告诉编译器它是一个 Option[String] 吗?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！