Question

I’m trying to write a simple recursive function that look over list and return a pair of integer. This is easy to write in c/c++/java but i’m new to ocaml so somehow hard to find out the solution due to type conflict

it should goes like ..

let rec test p l = ... ;;
val separate : (’a -> bool) -> ’a list -> int * int = <fun>
test (fun x -> x mod 2 = 0) [-3; 5; 2; -6];; 
- : int * int = (2, 2)

so the problem is how can i recursively return value on tuple ..

Answer 1

Been away from OCaml for a bit, but I think this will do the trick in regards to REALFREE’s description in the comment

let rec test l = 
  match l with 
      [] -> (0,0) 
    | x::xs -> 
        if x > 0 then match (test xs) with (x,y) -> (x+1, y)
        else  match (test xs) with (x,y) -> (x, y+1);;

You can used the nested match statements to pull out pieces of the tuple to modify

EDIT:
I didn’t know about the syntax Pascal Cuoq mentioned in his comment below, here’s the code like that, it’s neater and a little shorter:

let rec test l = 
  match l with 
      [] -> (0,0) 
    | x::xs -> 
    if x > 0 then let (x,y) = test xs in (x+1, y)
    else let (x,y) = test xs in (x, y+1);;

But the accepted answer is still much better, especially with the tail recursion ;).