> By the way, I have some difficulties to simply add a new element to the same
> list, each time I enter in a while-loop:
>
> let count = ref 3 ;;    (* number of iteration *)
> let list = [] in
>
> while (!count > 0)  do
>  decr count;
>  let list = list@[!count] in
>  Printf.printf "The 1st element is  %i \n" (List.hd list) ;
> done;
>
> Printf.printf "list contains %i elements \n" (List.length list) ;;

The let in the function is local, so is completely different to the
global list value. You'd want to use a ref.

let list = ref [];;

while ... do
  list := !list @ [!count];
done

(* btw, the output proves you're not adding to the global list, as the
head of the list should never change, as you're concatenating to the
end of the list *)

(* as a further aside, concatenating two lists takes time proportional
to the length of the first argument (your global list). prepending is
faster, though ends up with a reversed list, as in: list := !count ::
!list *)

> I try to declare "list" as a global variable:
>
> let count = ref 3 ;;
> let list = [] ;;
>
> while (!count > 0)  do
>  decr count;
>  list = [!count]@list ; ***
>  Printf.printf "The 1st element is  %i \n" (List.hd list) ;
> done;

*** 'list = [!count] @ list' is a boolean equation. It in fact tests
if list is structurally equivalent to [!count] @ list, and generates
the value false, which is then discarded. Since list is never
modified, it is always empty, thus List.hd will raise the exception
you have witnessed.

> But this time I get a "Warning S: this expression (list = [!count]@list ;)
> should have type unit."

Yes, this is what I meant by the boolean equation -- it's return type
is bool, not unit.

Jonathan