F#为什么我不能使用:? F#交互中的运算符? [英] F# Why can't I use the :? operator in F# interactive?

问题描述

我正在尝试检查变量是否具有某种特定类型，例如:

I am trying to check if a variable is of a certain type like so:

let s = "abc"
let isString = s :? string

但是在F#交互式中，出现以下错误:

but in F# interactive, I get the following error:

error FS0016: The type 'string' does not have any proper subtypes and cannot be used as the source of a type test or runtime coercion.

为什么会这样?我希望isString会是个笨蛋.

Why is this happening? I expect isString to be a bool.

推荐答案

因为您正在尝试使用密封类型.

Because you are trying with a sealed type.

尝试以下方法:

let s = box "abc"
let isString = s :? string

使用密封类型进行强制测试是没有意义的，因为它们不能具有任何子类型，这就是错误消息告诉您的内容.

There is no point in doing this coercion tests with sealed types since they can't have any subtype and that's what the error message is telling you.

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