Operator overloading / support discriminated unions in F#

Question

I have a discriminated union (tagged value) which is supposed to represent the degree of a polynomial

type Degree =
    |MinusInf
    |Fin of int

So I have a function which gives me the degree of a polynomial

>deg [1;2;3;4];;
val it : Degree = Fin 3

Now, my question is, how can I make a function which allows me to add degrees so that:

 Fin 2 + Fin 3 = Fin 5

All best

Answer 1

There's a fairly detailed article on MSDN. In short, you want to define a function like this on your Degree type:

type Degree =
    |MinusInf
    |Fin of int
    static member (+) (a: Degree, b: Degree) : Degree =
        match a, b with
        | Fin x, Fin y -> Fin (x+y)
        | _, _ -> MinusInf

let x = Fin 2 + Fin 3