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Message-ID: <16488.41762.711651.270386 at soggy.deldotd.com>
Date: Mon, 29 Mar 2004 14:28:50 -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: <20040329172014U.garrigue at kurims.kyoto-u.ac.jp>
References: <16487.13929.266175.311338 at soggy.deldotd.com>
	<20040329112224Z.garrigue@kurims.kyoto-u.ac.jp>
	<16487.52466.85193.17461@soggy.deldotd.com>
	<20040329172014U.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
  >> 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

  Jacques> This is a FAQ: your definition of p1 breaks the value
  Jacques> restriction, and as a result the function is no longer
  Jacques> polymorphic. If you try to apply it to two events of
  Jacques> different types, you get an error.  Just keep p1 outside of
  Jacques> your function.

OK - I think I found this in the FAQ, is it :

My program is not polymorphic (enough)?

I'm still unclear on how the introduction of p1 "forces" event to be
monomorphic, but I'll think harder :-)

Thank You very much for your help.

Brian