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Date: -- (:)
From: Andrew Lawson <andrew@a...>
Subject: Re: [Caml-list] Passing self to a new object
On Mon, Oct 29, 2001 at 06:02:07PM +0900, Jacques Garrigue wrote:
> 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.
 
	Ouch, I knew that even such an otherwise nice language would turn
out to have some fuzzy bits someday :)
 
> (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.)

	I admit this was a quick example of the problem, not the program
itself, which has many more hairy bits.
 
> The general solution to this is to first define a class type:
>   class type xyz_t = object ('self)
>      method 
>        btnNew : ...
>        ...
>      end

	Well this gives me an excuse to start playing with types and
interfaces  anyway.  Is this a 'known problem, will be sorted
eventually' or a 'just live with it' sort of thing?

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

	 Casting! and here was I giving a C++ programmer some abuse
about this last week :)

> 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.

	Well, thats not so bad then ... I suppose good style should ahve
me doing this anyway.

> Hope this helps,

	Most definitely

> Jacques Garrigue

		Thanks very much

			Andrew

-- 

   Andrew Lawson
andrew@absentis.com
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