> Does a type representing unsigned 32-bits integer exist in Caml ?

Yes: it's called int32.  Think about it: being "unsigned" or "signed"
is not a property of the representation (it will be 32 binary digits
in both cases), it's just that some operations (division, modulus and
comparisons) interpret those bits differently.

> All I'd need is an abstract type defining these integers, plus some
> functions to go from strings to 32-bits unsigned integers and vice-versa.

Here you are:

module UInt32 = struct
  type t = int32
  let of_string = Int32.of_string
  let to_string n = Printf.sprintf "%lu" n
end

You can throw in some arithmetic operations as well:

  let add = Int32.add
  let sub = Int32.sub
  let mul = Int32.mul

etc.  As I said above, the only operations that need to be treated
specially are comparisons and division/modulus.

Hope this helps,

- Xavier Leroy

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