Mysterious "open Core.Std" effect. (taken from Real World OCaml PDF edition, with mod) I want to know what is done under the hood.

rwo_nestedmod_ng.ml

(* open Core.Std
   When I opened Core.Std here, no problem occured. But...
*)

module type ID = sig
  type t
  val of_string : string -> t
  val to_string : t -> string
end

module String_id = struct
  type t = string
  let of_string x = x
  let to_string x = x
end

(* When I moved "open Core.Std" here...*)
open Core.Std

module Hostname : sig
  include ID
  val mine : unit -> t
end = struct
  include String_id
  let mine = Unix.gethostname
end

(* ...I got this error:

File "nestedmod1_ng.ml", line 23, characters 6-66:
Error: Signature mismatch:
       ...
       Values do not match:
         val mine : unit -> string
       is not included in
         val mine : unit -> t
       File "nestedmod1_ng.ml", line 22, characters 2-22:
         Expected declaration
       File "nestedmod1_ng.ml", line 25, characters 6-10: Actual declaration


  Actually I had to rewrite above as:
   let mine () = of_string (Unix.gethostname ())

  But it seems to me 'Unix.gethostname' has exactly same type of 'Core.Std.Unix.gethostname':

utop # Unix.gethostname;;
- : unit -> string = <fun>

utop # Core.Std.Unix.gethostname;;
- : unit -> string = <fun>


  So now, I'm very curious about what "open Core.Std" actually does.
  Do that change meanings of 'string' or any other OCaml constructs 
  like 'module', '()'...?
*)

https://twitter.com/camloeba/status/423337850231201792

Thank you!

To detect this kind of overwriting errors, we should add 'warn_A' flag to _tags file, like this:

true: warn_A