OCaml Unbound Value

Question

I have to make a recursive function that counts the number of nodes of a binary tree of three diferent types, and I am supose to save them in a result with the following type int * int * int, I think my reasoning is right.

type dtree =
        Decision of string * int * dtree * string * int * dtree
      | Chance of string * int * dtree * string * int * dtree
      | Outcome of int
;;


let rec count dt =
    match dt with
          Decision(choiceL, costL, l, choiceR, costR, r) -> (x+1,y,z) count l count r
        | Chance(eventL, probL, l, eventR, probR, r) ->   (x,y+1,z) count l count r
        | Outcome value -> (x,y,z+1)

;;

Answer 1

I see many problems in your code, but it might be better if you asked a specific question. Just as a place to start, you're using the names x, y, and z without defining them anywhere.

The key to your problem, I think, is that you need to give names to the values returned by the recursive calls count l and count r. Once they have names you can use them in the result at the current level of the tree.

Update

Here's a function that adds up the values in a list of pairs. It has the same rough structure as what you're looking for:

let rec sumpairs pairs =
    match pairs with
    | [] -> (0, 0)
    | (x, y) :: tail ->
        let (subx, suby) = sumpairs tail in
        (subx + x, suby + y)