MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <16487.52466.85193.17461 at soggy.deldotd.com>
Date: Sun, 28 Mar 2004 23:14:58 -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: <20040329112224Z.garrigue at kurims.kyoto-u.ac.jp>
References: <16486.17988.561211.516751 at soggy.deldotd.com>
	<20040328191235E.garrigue@kurims.kyoto-u.ac.jp>
	<16487.13929.266175.311338@soggy.deldotd.com>
	<20040329112224Z.garrigue@kurims.kyoto-u.ac.jp>
From: briand at aracnet.com

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

  Jacques>   let key_code = GdkEvent.Key.keyval (GdkEvent.Key.cast event) in

And this works ! Thank You.

The code now looks like :

let handle_event event =
  (* type coercion I think... *)
  let event = (event :> GdkEvent.any) in
  match GdkEvent.get_type event with
      #GdkEvent.Button.types -> Printf.printf "button press\n"; true
    | `KEY_PRESS -> 
        let key_code = GdkEvent.Key.keyval (GdkEvent.Key.cast event) in
          true
(*    | `BUTTON_PRESS -> Printf.printf "button press\n"; true *)
    | `MOTION_NOTIFY -> Printf.printf "motion\n"; true
    | _ -> false

  Jacques> Of course the above pattern-matching guarantees that event is a key
  Jacques> event, but the type system is not strong enough to see that.
  Jacques> Thinking of this, this should be possible to do it with private types
  Jacques> and variant dispatch, but this is getting complicated...

???  I don't know what this means.  But if it does not make things simpler ...

I have one last question, at it is maybe a put ocaml question. It is
VERY strange.

The following code WORKS

let p1 = ref { x=0.; y=0.; z=0. };;

let draw_polyline_interactively = 
  fun area drawv event ->
  let event = (event :> GdkEvent.any) in
  ...

However when I try to establish p1 as a closure variable :

let draw_polyline_interactively = 
  let p1 = ref { x=0.; y=0.; z=0. } in
    fun area drawv event ->
      let event = (event :> GdkEvent.any) in
      ...

I get an error at the _calling_ location (not in the function declaration) :

This expression has type GdkEvent.Button.t = GdkEvent.Button.types Gdk.event
but is here used with type
  GdkEvent.Key.t = [ `KEY_PRESS | `KEY_RELEASE ] Gdk.event
Type
  GdkEvent.Button.types =
    [ `BUTTON_PRESS
    | `BUTTON_RELEASE
    | `THREE_BUTTON_PRESS
    | `TWO_BUTTON_PRESS ]
is not compatible with type [ `KEY_PRESS | `KEY_RELEASE ] 
These two variant types have no intersection

How could the (seemingly unrelated) local variable possibly impact the
treatment of event ???  The error ONLY occurs when I try to use the p1
variable, any other declaration has no effect.  I simply need to move
p1 outside, into the top level, changing nothing else and everything
is fine.

Sorry to be such a pest.  I seem to have picked a difficult topic to
start with in ocaml :-(


Brian