OCaml Functor Demo1

demo1.ml

(* Functor example *)

module type Dog = sig
    val barks : bool
end;;

module type Animal = sig (* Shouldn't `Animal` module/functor have some parameter in it's signature??? *)
    val approach : unit -> string
end;;

module ADog : Dog = struct
    let barks = true
end;;

module AnAnimal (D: Dog): Animal = struct (* I've set a `sort of parameter` here... but never defined it in types. Is it expected to work this way? *)
    let approach () =
        match D.barks with
        | true -> "BARKS!"
        | false -> "is silent."
        ;;
end;;

module Hanna = AnAnimal(ADog);;

let () = print_endline ("Hanna " ^ (Hanna.approach ()))

@Octachron Today at 5:28 PM

The module type Animal is the module type of the result.
You need to name the module parameter in order to use it. You can define the functor type, but this is more verbose for little gain

module type f = functor(X:sig val x: int end) -> sig val y : int end
module F: f = functor (X:sig val x:int end) -> struct let y = X.x + 1 end