Using common arguments with Command module of the Core library - Solved

cmd.ml

(* Build with:
  * corebuild cmd.byte *)


open Core.Std;;

(* With this code, we are trying to
 * Define a common set of arguments to be passed to all sub commands
 * Handle these common arguments all the same way
 * Define sub commands in a common, less verbose way *)

(* The program compiled could be used this way
 * cmd.byte sum 32 + 10 # Display 42
 * cmd.byte settings # Display all the settings
 * But we could use common arguments :
 * cmd.byte sum -c true 32 + 10 # Display 42 and a message about color
 * cmd.byte settings # Display all the settings
 * *)

(* Verbosity *)
let verb = ref 0;;
let color = ref false;;

(* A set of common flags *)
let shared_params =
  let open Command.Param in
  return (fun v c rc ->
    verb := v;
    color := c;
    rc)
  <*> flag "-v" (optional_with_default 0 int)
    ~doc:"n Set verbosity"
  <*> flag "-c" (optional_with_default false bool)
    ~doc:"bool Set color"
  <*> flag "--rc" (optional_with_default "" string)
    ~doc:"name Set configuration file"
;;

(* Two sub commands *)

(* Display the sum of the arguments *)
let sum =
  (
      "sum"
      ,
    Command.basic ~summary:""
      Command.Spec.(
        empty
        +> shared_params
        +> anon ("first_number" %: int)
        +> anon ("second_number" %: int)
      )
      (fun rc a b () -> (* XXX Strange arguments passed here *)
        (* We would like to get the numbers passed in arguments ("first
         * number"
         * and "second number" below) *)
        a + b |> printf "%i\n";
        (* Some code to use common arguments *)
        if !color then printf "Colored\n" else
          print_endline rc)
  )
;;

(* Print some settings and a number passed to the program *)
let settings =
  (
     "settings"
     ,
     Command.basic ~summary:"Display settings"
      Command.Spec.(
         empty
         +> shared_params
         +> anon (maybe ("a number" %: int))
      )
      (fun rc n () ->
        printf "n: %i\n" (Option.value ~default:0 n);
        printf "\nSettings\n";
        printf "Rc: %s\n" rc;
        printf "Color: %b\n" !color)
  )
;;

let () =
  let open Command in
  run begin
    group ~summary:"A program to test" [ sum ; settings ]
  end
;;

When compiling

$ corebuild cmd.byte
+ ocamlfind ocamlc -c -w A-4-33-40-41-42-43-34-44 -strict-sequence -g -bin-annot -short-paths -thread -syntax camlp4o -package bin_prot.syntax -package sexplib.syntax,comparelib.syntax,fieldslib.syntax,variantslib.syntax -package core -o cmd.cmo cmd.ml
File "cmd.ml", line 98, characters 5-44:
Warning 48: implicit elimination of optional argument ?extend
File "cmd.ml", line 49, characters 11-341:
Warning 27: unused variable summary.
File "cmd.ml", line 49, characters 20-341:
Warning 27: unused variable args.
Finished, 3 targets (0 cached) in 00:00:03.

Updated with solution given on the mailing list