F# Why can't I use the :? operator in F# interactive?

Question

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

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

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.

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

Answer 1

Because you are trying with a sealed type.

Try instead this:

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

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.