Unset or remove an OCaml callback registration

Unset or remove an OCaml callback registration

[Hezekiah M. Carty]

Is it possible to unregister a function or other value registered with
Callback.register?  I use (Callback.register "foo" some_func) to
register coordinate transforms for a C library.  If no callback is
registered for "foo" then a default transform is used by the C
library.  I would like to be able to set and then remove the
association of some_func to the name "foo" at various points in my
program so that the C code will fall back on the default transform
(which does not require a callback and is much faster) when the named
callback "foo" is undefined or has been unregistered.

Is this possible, either from the C or OCaml side without making the
callback associate with "foo" an option type (use (Callback.register
"foo" (Some some_func)) to set a callback and (Callback.register "foo"
None) to clear it)?

Thanks,
Hez

-- 
Hezekiah M. Carty
Graduate Research Assistant
University of Maryland
Department of Atmospheric and Oceanic Science

Re: [Caml-list] Unset or remove an OCaml callback registration

[Jacques Garrigue]

From: "Hezekiah M. Carty" <hcarty@atmos.umd.edu>

> Is it possible to unregister a function or other value registered with
> Callback.register?  I use (Callback.register "foo" some_func) to
> register coordinate transforms for a C library.  If no callback is
> registered for "foo" then a default transform is used by the C
> library.  I would like to be able to set and then remove the
> association of some_func to the name "foo" at various points in my
> program so that the C code will fall back on the default transform
> (which does not require a callback and is much faster) when the named
> callback "foo" is undefined or has been unregistered.
> 
> Is this possible, either from the C or OCaml side without making the
> callback associate with "foo" an option type (use (Callback.register
> "foo" (Some some_func)) to set a callback and (Callback.register "foo"
> None) to clear it)?

Since Callback.register has type: string -> 'a -> unit
you are not limited by the ocaml type system.
So you can reset your value with
  Callback.register "foo" 0
and check for equality with Val_int(0) on the C side.
(Note that you must initialize the value to 0 at program startup,
because the default for an unitialized value is 0 which is not
Val_int(0))

Jacques Garrigue

Re: [Caml-list] Unset or remove an OCaml callback registration

[Hezekiah M. Carty]

On Thu, 03 Apr 2008, Jacques Garrigue wrote:

> From: "Hezekiah M. Carty" <hcarty@atmos.umd.edu>
> > Is this possible, either from the C or OCaml side without making the
> > callback associate with "foo" an option type (use (Callback.register
> > "foo" (Some some_func)) to set a callback and (Callback.register "foo"
> > None) to clear it)?
> 
> Since Callback.register has type: string -> 'a -> unit
> you are not limited by the ocaml type system.
> So you can reset your value with
>   Callback.register "foo" 0
> and check for equality with Val_int(0) on the C side.
> (Note that you must initialize the value to 0 at program startup,
> because the default for an unitialized value is 0 which is not
> Val_int(0))

If I wrap the C interface in an OCaml module, would it be enough to
include:

let () = Callback.register "foo" 0

in the top level of the module to have this command execute when the
module is loaded?  Or does the module user have to do some explicit
initialization?

Thank you very much for the response,
Hez

Re: [Caml-list] Unset or remove an OCaml callback registration

[Gordon Henriksen]

On Apr 3, 2008, at 10:07, Hezekiah M. Carty wrote:

> On Thu, 03 Apr 2008, Jacques Garrigue wrote:
>
>> From: "Hezekiah M. Carty" <hcarty@atmos.umd.edu>
>>> Is this possible, either from the C or OCaml side without making  
>>> the callback associate with "foo" an option type (use  
>>> (Callback.register "foo" (Some some_func)) to set a callback and  
>>> (Callback.register "foo" None) to clear it)?
>>
>> Since Callback.register has type: string -> 'a -> unit you are not  
>> limited by the ocaml type system. So you can reset your value with
>>  Callback.register "foo" 0
>> and check for equality with Val_int(0) on the C side. (Note that  
>> you must initialize the value to 0 at program startup, because the  
>> default for an unitialized value is 0 which is not Val_int(0))
>
> If I wrap the C interface in an OCaml module, would it be enough to  
> include:
>
> let () = Callback.register "foo" 0
>
> in the top level of the module to have this command execute when the  
> module is loaded?  Or does the module user have to do some explicit  
> initialization?


I've used this technique, although I did have initialization ordering  
problems. The outer module is not necessarily initialized before the  
inner one. In my case, the initialization was idempotent, so I simply  
repeated it in each submodule.

— Gordon