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Subject: Re: event variant types
To: briand at aracnet.com
Cc: lablgtk at kaba.or.jp
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Message-Id: <20040328191235E.garrigue at kurims.kyoto-u.ac.jp>
Date: Sun, 28 Mar 2004 19:12:35 +0900
From: Jacques Garrigue <garrigue at kurims.kyoto-u.ac.jp>
Lines: 26

From: briand at 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

Sorry, using variant polymorphism can get tricky if you want to keep
type safety.
There are two solutions:
* either replace "_" by "#Gdk.Tags.event_type" in the above
* or add the following line at the beginning of handle_event
     let event = (event :>  GdkEvent.any) in
  This is the most robust solution, as it will work even if you change
  handle_event.

You can of course add the same coercion at calls of handle_event, but
this is less clean.

Jacques