[Caml-list] Passing self to a new object

[Caml-list] Passing self to a new object

[Andrew Lawson]

Hi all
	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
	let btnNew  = ... ~command:(fun () = new abc self) top in
	object (self)
	...
	...
	end

class abc (parent:xyz) =
	object (self)
	var myparent = parent
	...
	end

	My error is;

This expression has type
  < btnDelete : 'a; btnEdit : 'b; btnExit : 'c; btnFind : 'd;
    btnNew : Widget.button Widget.widget; get_config : 'e -> 'f;
    list_record_directory : 'g; new_filename : 'h; read_configuration :
'i;
    self : 'j; set_config : 'k -> 'l -> 'm; topLevel : 'n; .. >
but is here used with type 'j
Self type cannot escape its class

	And I have no clue what I am doing wrong :)

		All suggestions appreciated

			Andrew


-- 

   Andrew Lawson
andrew@absentis.com
-------------------
Bug reports: http://caml.inria.fr/bin/caml-bugs  FAQ: http://caml.inria.fr/FAQ/
To unsubscribe, mail caml-list-request@inria.fr  Archives: http://caml.inria.fr

Re: [Caml-list] Passing self to a new object

[Jacques Garrigue]

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
-------------------
Bug reports: http://caml.inria.fr/bin/caml-bugs  FAQ: http://caml.inria.fr/FAQ/
To unsubscribe, mail caml-list-request@inria.fr  Archives: http://caml.inria.fr

Re: [Caml-list] Passing self to a new object

[Andrew Lawson]

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
-------------------
Bug reports: http://caml.inria.fr/bin/caml-bugs  FAQ: http://caml.inria.fr/FAQ/
To unsubscribe, mail caml-list-request@inria.fr  Archives: http://caml.inria.fr

Re: [Caml-list] Passing self to a new object

[Jacques Garrigue]

From: Andrew Lawson <andrew@absentis.com>

> > 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?

Well, this was delayed for a very long time, but I'm starting to think
that we should really do something about it.

> > 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 :)

No, this is a coercion. This is perfectly type-safe (and checked at
compile time).  This is needed for type inference. Languages with
explicit type information, like Java, do it silently.

Cheers,

Jacques Garrigue
-------------------
Bug reports: http://caml.inria.fr/bin/caml-bugs  FAQ: http://caml.inria.fr/FAQ/
To unsubscribe, mail caml-list-request@inria.fr  Archives: http://caml.inria.fr