example.fsx

// differentiate the kind of payload you want to take
type AdditionPayload =
    | Integers of int * int
    | Strings of string * string

module Operations =
    // handle your integers
    let private addInts a b = a + b

    // handle your strings
    let private addStrings (a: string) (b: string) = (int a) + (int b)

    // an operation that takes a union and based on the payload
    // provides the correct operation
    let sum (values: AdditionPayload) =
        match values with 
        | Integers (a, b) -> addInts a b
        | Strings (a, b) -> addStrings a b 


Operations.sum (Integers (1, 2))
Operations.sum (Strings ("10", "20"))