OCaml Module : include AND open? - module

Question

I'm fairly new with OCaml Module and I haven't managed to use my own module without combining both an "include" and an "open".
I've tried to put the signature in a separate .mli file, without success.
Below I'm indicated a minimum (not) working example, that I'm trying to compile with
ocamlc -o main Robot.ml main.ml
What to I need to do to only have to use "open", or only "include", but not both of them ?
File "Robot.ml" :
module type RobotSignature =
sig
val top: unit -> unit
end
module Robot =
struct
let top () =
begin
Printf.printf "top\n"
end
(* Should not be visible from the 'main' *)
let dummy () =
begin
Printf.printf "dummy\n"
end
end
File "main.ml" (not working) :
open Robot;;
top();
File "main.ml" (working) :
include Robot;;
open Robot;;
top();

Answer 1

You've got two levels of Robot. Since you explicitly called your module "Robot" within the file robot.ml you'll need to open Robot and then call Robot.top(). Anything in the robot.ml file is already put implicitly inside of a Robot module.
You could get rid of the extra 'module Robot' declaration in robot.ml.
robot.ml would become:
module type RobotSignature =
sig
val top: unit -> unit
end
let top () =
begin
Printf.printf "top\n"
end
Then it should work as you have it in your main.ml.
Update based on comment below: If you're concerned that everything in robot.ml will now be visible when you 'open Robot' you can define a robot.mli file which specifies the functions which are available externally. For example, let's say you add a function called helper in robot.ml:
let top () =
begin
Printf.printf "top\n"
end
let helper () =
Printf.printf "helper\n"
...and then you define your robot.mli as follows:
val top: unit -> unit
Then let's say you try to call helper from main.ml:
open Robot;;
top();
(* helper will not be visible here and you'll get a compile error*)
helper ()
Then when you try to compile you'll get an error:
$ ocamlc -o main robot.mli robot.ml main.ml
File "main.ml", line 4, characters 0-6:
Error: Unbound value helper

Answer 2

You have two ways to do this:
First, you can constrain your sub-structure to be of the right signature:
module Robot : RobotSignature = struct ... end
Then in main.ml, you can do open Robot.Robot: the first Robot means the compilation unit associated to robot.ml, the second Robot is the submodule you have defined inside robot.ml
You can also remove one level and create robot.mli containing:
val top: unit -> unit
and robot.ml containing:
let top () =
Printf.printf "top\n"
(* Should not be visible from the 'main' *)
let dummy () =
Printf.printf "dummy\n"
You can compile the modules using ocamlc -c robot.mli && ocamlc -c robot.ml and then in main.ml simply use open Robot.