This is yet another "insufficient polymorphism" question that I just
can't work out ...

I have a signature like this:

newt.mli:

  type 'a component

  val form_add_component :
    [> `Form ] component -> [> `Component ] component -> unit
  val form_add_components :
    [> `Form ] component -> [> `Component ] component list -> unit

An implementation like this:

newt.ml:

  type 'a component = {
    co : newtComponent;
    mutable in_form : bool;
  }

  let form_add_component form co =
    if co.in_form then failwith "component can only be added to a single form";
    newtFormAddComponent form.co co.co;
    co.in_form <- true

  let form_add_components form components =
    List.iter (fun co -> form_add_component form co) components

And my test program is:

04_form.ml:

  let text = textbox 1 1 15 5 [WRAP; SCROLL] in
  textbox_set_text text "This is some text in a reflowed text box.";
  let b1 = button 5 8 "  OK  " in
  let b2 = button 5 13 "Cancel" in
  let form = form None None [] in
  form_add_components form [text; b1; b2];
                                  ^^

File "examples/04_form.ml", line 17, characters 33-35:
This expression has type [ `Button | `Component ] Newt.component
but is here used with type [ `Component | `Textbox ] Newt.component
The second variant type does not allow tag(s) `Button
make: *** [examples/04_form.cmo] Error 2

This all worked until I added the newt.mli file.  (I previously just
had a newt.ml file, with explicit types on all the functions).

Rich.

-- 
Richard Jones
Red Hat