Recursive addition in F# using
let rec plus x y =
 match y with
 | 0 -> x;
 | succ(y) -> succ( plus(x y) );

let rec plus x y =
 match y with
 | 0 -> x
 | y -> 1 + (plus x (y - 1))

let (|Zero|Succ|) n =
  if n < 0 then failwith "Unexpected!"
  if n = 0 then Zero else Succ(n - 1)

let rec plus x y =
  match y with
  | Zero -> x
  | Succ y -> 1 + (plus x y)

type Number =
| Zero
| Succ of Number

let rec plus x y =
 match y with
 | Zero -> x
 | Succ(y1) -> Succ (plus x y1)

let rec (+) x y =
 match y with
 | Zero -> x
 | Succ(y1) -> Succ (x + y1)

type N = Zero | Succ of N

let rec NtoInt n =
  match n with
  | Zero -> 0
  | Succ x -> 1 + NtoInt x

let rec plus x y =
  match x with
  | Zero -> y
  | Succ n -> Succ (plus n y)

> plus (Succ (Succ Zero)) Zero |> NtoInt ;;
val it : int = 2
> plus (Succ (Succ Zero)) (Succ Zero) |> NtoInt ;;
val it : int = 3

let rec plus x y =
    match y with
    | 0 -> x
    | _ -> plus (x+1) (y-1)