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Message-ID: <16487.13929.266175.311338 at soggy.deldotd.com>
Date: Sun, 28 Mar 2004 12:32:41 -0800
To: Jacques Garrigue <garrigue at kurims.kyoto-u.ac.jp>
Cc: lablgtk at kaba.or.jp
Subject: Re: event variant types
In-Reply-To: <20040328191235E.garrigue at kurims.kyoto-u.ac.jp>
References: <16483.50834.898498.682875 at soggy.deldotd.com>
	<16483.63571.455639.187324@akasha.ijm.jussieu.fr>
	<16486.17988.561211.516751@soggy.deldotd.com>
	<20040328191235E.garrigue@kurims.kyoto-u.ac.jp>
From: briand at aracnet.com

>>>>> "Jacques" == Jacques Garrigue <garrigue at kurims.kyoto-u.ac.jp> writes:

  Jacques> From: briand@aracnet.com
  >> I've created a simpler test program.
  >> 
  >> here is some relevant code:
  >> 
  >> let handle_event event =
  >> match GdkEvent.get_type event with
  >> (* #GdkEvent.Button.types -> handle_button_event event; true *)
  >> `KEY_PRESS -> Printf.printf "key press\n"; true
  >> | `MOTION_NOTIFY -> Printf.printf "motion\n"; true
  >> | _ -> false

  Jacques> Sorry, using variant polymorphism can get tricky if you
  Jacques> want to keep type safety.

Thanks you for your continued help.  I'm new to ocaml/ml it's taking a
while to get used to the type system.

  Jacques> There are two solutions:
  Jacques> * either replace "_" by "#Gdk.Tags.event_type" in the above
  Jacques> * or add the following line at the beginning of handle_event
  Jacques>      let event = (event :>  GdkEvent.any) in

  Jacques>   This is the most robust solution, as it will work even if
  Jacques>   you change handle_event.

yes - the coercion works well.  So the above code is now clean.

However, we would now like to use the information to obtain keycodes
and such :

    match GdkEvent.get_type event with
        (* #GdkEvent.Button.types -> handle_button_event event; true *)
        `KEY_PRESS ->     
          let key_code = GdkEvent.Key.keyval event in
            handle_key_event key_code; true
          (*    | `BUTTON_PRESS -> Printf.printf "button press\n"; true *)
      | `MOTION_NOTIFY -> Printf.printf "motion\n"; true
      | _ -> false

The error message, which is somewhat lengthy is included below.  Now
I'm even more confused.  Since the key_code expression is more
"narrow" than the event type, I would not expect ocaml to complain.

So it seems that I need to coerce event once again to extract
event-specific information. Unfortunately my lack of type
understanding is a problem.  I've tried this via several methods most
of which result in syntax errors or other errors.

So here I am on the list.  BTW, would it be simpler to add the "types"
variable which is present in the button press event to all of the
events ?  I' d be happy to implement this and send patches, it would
be little work.

Thanks

Brian

The error message:

This expression has type GdkEvent.any = Gdk.Tags.event_type Gdk.event
but is here used with type
  GdkEvent.Key.t = [ `KEY_PRESS | `KEY_RELEASE ] Gdk.event
Type
  Gdk.Tags.event_type =
    [ `BUTTON_PRESS
    | `BUTTON_RELEASE
    | `CLIENT_EVENT
    | `CONFIGURE
    | `DELETE
    | `DESTROY
    | `DRAG_ENTER
    | `DRAG_LEAVE
    | `DRAG_MOTION
    | `DRAG_STATUS
    | `DROP_FINISHED
    | `DROP_START
    | `ENTER_NOTIFY
    | `EXPOSE
    | `FOCUS_CHANGE
    | `KEY_PRESS
    | `KEY_RELEASE
    | `LEAVE_NOTIFY
    | `MAP
    | `MOTION_NOTIFY
    | `NOTHING
    | `NO_EXPOSE
    | `PROPERTY_NOTIFY
    | `PROXIMITY_IN
    | `PROXIMITY_OUT
    | `SCROLL
    | `SELECTION_CLEAR
    | `SELECTION_NOTIFY
    | `SELECTION_REQUEST
    | `SETTING
    | `THREE_BUTTON_PRESS
    | `TWO_BUTTON_PRESS
    | `UNMAP
    | `VISIBILITY_NOTIFY
    | `WINDOW_STATE ]
is not compatible with type [ `KEY_PRESS | `KEY_RELEASE ]