From: skaller <skaller@users.sourceforge.net>

> I just tried making Event.event covariant but it didn't seem to work,
> perhaps I did something wrong here?
> 
> (* event.ml *)
> type +'a event =
>     Communication of 'a behavior
>   | Choose of 'a event list
>   | WrapAbort of 'a event * (unit -> unit)
>   | Guard of (unit -> 'a event)

The + above is not needed (it is implied by the definition)

> (* event.mli *)
> type +'a event

That + is sufficient.

> Rebuilt and reinstalled Ocaml 3.09.0 ..  changed example to:
> 
>       List.map (fun w ->
>         (* Event.wrap (receive w.control) (fun (w,e) -> w, (e:> ec)) *)
>          (receive w.control) 
>       )
>       [] (* !windows .. *)
> 
> eliminating the Event.wrap and hoping this would fix it .. 
> 
> skaller@rosella:/work/felix/flx$ ocamlc -thread r.ml
> File "r.ml", line 28, characters 6-143:
> This expression has type (window * event) Event.event list
> but is here used with type ws_event list
> Type (window * event) Event.event is not compatible with type
>   ws_event = (window * ec) Event.event
> Type event = [ `Repaint ] is not compatible with type
>   ec = [ `New_window | `Repaint ]
> The first variant type does not allow tag(s) `New_window

You still need to coerce the resulting list:
(List.map (fun w -> receive w.control) !windows :> ws_event list)

Jacques