Why this ocaml code emits warning 6 for String.sub? (labels were omitted in the application of this function.)

why_labels_omitted.ml

(* -*- coding: utf-8 -*- *)
open Core.Std

module Strrange = struct
  let (//) opt def = match opt with Some x -> x | None -> def
  let min x y = if x < y then x else y

  type t = {base: string; mutable start: int; mutable finish: int}

  let create ?(start=0) ?finish ?(empty=false) str =
    {base = str; start = start;
     finish =
	if empty then start
	else min (finish // String.length str) (String.length str)}

  let of_string str = create str

  let to_string ?(offset=0) {base; start; finish} =
    let start = start + offset in
    String.sub base start (finish - start)

end

Here is the warning from ocaml:

File "why_labels_omitted.ml", line 20, characters 4-14:
Warning 6: labels were omitted in the application of this function.

Finally I found Core.Std.String.sub "foobar" has following type:

# Core.Std.String.sub "foobar";;
- : pos:int -> len:int -> string = <fun>

So, changing line 20 as following cleared this warning:

    String.sub base ~pos:start ~len:(finish - start)

Umm... bit frustrated.

Thank you so much, so the problem is about the types not being identical ?

Yes and No. The problem of my code is type mismatching as you said. The problem I (indirectly) criticized here was that how hard it was to understand one warning message, which was caused by the split situation of the learning environment of OCaml between its standard and Core (which Real World OCaml is based on) at that time.

Oh, okay