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To: andrew@absentis.com
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Passing self to a new object
In-Reply-To: <20011028155625.A1303@alba.sw>
References: <20011028155625.A1303@alba.sw>
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Message-Id: <20011029180207D.garrigue@kurims.kyoto-u.ac.jp>
Date: Mon, 29 Oct 2001 18:02:07 +0900
From: Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>
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From: Andrew Lawson <andrew@absentis.com>

> 	I'm writing a gui program where the callback for a button
> creates a new object (also a gui) and needs to pass it a reference to
> itself in order that the new object can contact the original for
> information. The classes look something like this;
> 
> class xyz = 
> 	let top = ... in
> 	let ... = ... in
> 	object (self)
> 	  method btnNew  = ... ~command:(fun () = new abc self) top in
> 	...
> 	...
> 	end
> 
> and abc (parent:xyz) =
> 	object (self)
> 	var myparent = parent
> 	...
> 	end
> 
>My error is;
> .....
> Self type cannot escape its class

You've bumped into a painful quirk of ocaml's object system: self's
type is not xyz, and must be handled with care.

(By the way, your program seemed strange, since you were using self
and abc before defining them, but I suppose it was a copy error, and I
corrected it.)

The general solution to this is to first define a class type:
  class type xyz_t = object ('self)
     method 
       btnNew : ...
       ...
     end

Then change the code in btnNew to
  new abc (self :> xyz_t)

Remark that making xyz_t exact may be a bit lengthy, but you may leave
out methods you don't need to call from abc.

Hope this helps,

Jacques Garrigue
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