Question

Let’s say I have the following discriminated union and value:

type DisUnion = 
    | One of string
    | Two of string
    | Three of string * string

let myVal = One("One")

I know I can use pattern matching to determine which case myVal belongs to, like this:

let whatever (x : DisUnion) = match x with
    | One(str) -> "It was One"
    | Two(str) - > "It was two"
    | Three(str, str) -> "It was three"

But I can’t seem to find an operator or method that allows me to determine the case identifier without pattern matching, like:

let isOne (x : DisUnion) = x :? One //I thought this would work, but it doesn't.

How would I do this? Any help is appreciated.

Answer 1

let isOne = function One(_) -> true | _ -> false

Note that this is equivalent to:

let isOne x = match x with One(_) -> true | _ -> false