Can compare return anything else than -1, 0 and 1?

Question

The docs for the Pervasives.compare function state that

compare x y returns 0 if x is equal to y, a negative integer if x is less than y, and a positive integer if x is greater than y.

This suggests it can return any negative or positive integer, not just -1 or 1, to represent greater- or lesser-ness. However, does this actually happen?

This would make writing code like

match String.compare key new_key with
  | 1  -> Node (left,                insert new_key right, key)
  | -1 -> Node (insert new_key left, right,                key)
  | _  -> Node (left,                right,                key)

much more difficult (using when, probably?).

I'm particularly interested in String.compare. Having a look at its implementation, it just forwards to Pervasives.compare, which in turn is implemented natively using external. No idea what it does.

Answer 1

An answer by yourself shows that no other values can be returned in the current implementation. I would be very careful, though, to rely on that. As long as compare is not documented as being three-valued, future versions of OCaml may change that behaviour.

[EDIT, answering a comment] In order to avoid clumsy case distinctions (as hinted in your original question), you can wrap compare into a function that returns a three-values type like so:

type comparison = Less | Equal | More

let my_compare a b = match compare a b with
| 0 -> Equal
| c when c>0 -> More
| _ -> Less