zipper: returns the sublist starting at focus position | but why when n > 0?

Question

I found this type zipper and get functions defined like this:

type 'a zipper = 'a list * int
exception Empty

let empty = ([], 0)

(* get: returns the sublist starting at focus position *)
let rec get lz = 
  match lz with
  | (l, 0) -> l 
  | (h::t, n) when n > 0 -> get (t, n-1)
  | _ -> raise Empty

But that when n > 0 is not clear to me, that is not necessary, because assuming we will provide argument n > 0, it will always be captured by the first match, isn't this correct? Why that when n > 0?

Answer 1

But you can't assume that the value of n will either be 0 or a positive value, because int is a signed type.

Nothing would stop you some writing:

get ([1;2;3;4;5;6], -1)

Though Empty is a very odd choice of names for an exception in this situation. It would make much more sense to use either Not_found or even better, Invalid_argument.

I would further add that the type alias here seems rather non-idiomatic. Rather:

let rec get i lst =
  match lst, i with
  | l, 0 -> l
  | h::t, n when n > 0 -> get (i-1) t
  | _ -> invalid_arg "Negative index"