module Annotated_field:sig..end
An example is as follows:
module M = struct
  type t = {
    field1 : int;
    field2 : float;
    field3 : bool;
    field4 : string option;
  } with fields
  module A = Annotated_field
  let ann_fields = Fields.fold ~init:[]
    ~field1:(A.required ~doc:" documentation for field1")
    ~field2:(A.default 1.0 string_of_float ~doc:" documentation for field2")
    ~field3:(A.set ~doc:" documentation for field3")
    ~field4:(A.optional ~doc:" documentation for field4")
  let command = create
    ~summary:"summary"
    ~init:(fun () -> A.init ann_fields)
    ~usage_arg:""
    ~flags:(List.map ann_fields ~f:A.to_flag)
    ~final:(fun accum _anon_args ->
      let get of_string = A.get accum of_string in
      let get_opt of_string = A.get_opt accum of_string in
      Fields.map
        ~field1:(get int_of_string)
        ~field2:(get Float.of_string)
        ~field3:(get bool_of_string)
        ~field4:(get_opt ident)
    )
    ~main:(fun _ -> assert false)type 
type 
val required : ?name:string ->
       t list ->
       doc:string ->
       ('a, 'b) Fieldslib.Field.t -> t listval default : ?name:string ->
       'field ->
       ('field -> string) ->
       t list ->
       doc:string ->
       ('a, 'field) Fieldslib.Field.t -> t listval optional : ?name:string ->
       ?suppress_word_optional:bool ->
       t list ->
       doc:string ->
       ('a, 'b option) Fieldslib.Field.t ->
       t listval set : ?name:string ->
       t list ->
       doc:string ->
       ('a, bool) Fieldslib.Field.t -> t listval clear : ?name:string ->
       t list ->
       doc:string ->
       ('a, bool) Fieldslib.Field.t -> t listval list : ?name:string ->
       t list ->
       doc:string ->
       ('a, 'b list) Fieldslib.Field.t -> t listval init : t list ->
       accumval to_flag : t ->
       accum Deprecated_command.Flag.tval get : accum ->
       (string -> 'field) -> ('a, 'field) Fieldslib.Field.t -> 'fieldval get_opt : accum ->
       (string -> 'field) -> ('a, 'field option) Fieldslib.Field.t -> 'field optionval get_list : accum ->
       (string -> 'field) -> ('a, 'field list) Fieldslib.Field.t -> 'field list