/usr/lib/ocaml/atdgen/ag_json.ml is in libatdgen-ocaml-dev 1.3.1-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 | (*
  Mapping from ATD to JSON
*)
type json_float = [ `Float of int option (* max decimal places *)
                  | `Int ]
type json_list = [ `Array | `Object ]
type json_variant = { json_cons : string }
type json_field = {
  json_fname : string;
  json_unwrapped : bool
}
type json_repr =
    [
    | `Unit
    | `Bool
    | `Int
    | `Float of json_float
    | `String
    | `Sum
    | `Record
    | `Tuple
    | `List of json_list
    | `Option
    | `Nullable
    | `Shared
    | `Wrap (* should we add support for Base64 encoding of binary data? *)
    | `External
    | `Cell
    | `Field of json_field
    | `Variant of json_variant
    | `Def
    ]
let json_float_of_string s : [ `Float | `Int ] option =
  match s with
      "float" -> Some `Float
    | "int" -> Some `Int
    | _ -> None
let json_precision_of_string s =
  try Some (Some (int_of_string s))
  with _ -> None
let get_json_precision an =
  Atd_annot.get_field
    json_precision_of_string None ["json"] "precision" an
let get_json_float an : json_float =
  match
    Atd_annot.get_field json_float_of_string `Float ["json"] "repr" an
  with
      `Float -> `Float (get_json_precision an)
    | `Int -> `Int
let json_list_of_string s : json_list option =
  match s with
      "array" -> Some `Array
    | "object" -> Some `Object
    | _ -> None
let get_json_list an =
  Atd_annot.get_field json_list_of_string `Array ["json"] "repr" an
let get_json_cons default an =
  Atd_annot.get_field (fun s -> Some s) default ["json"] "name" an
let get_json_fname default an =
  Atd_annot.get_field (fun s -> Some s) default ["json"] "name" an
 |