Bigarray question

Bigarray question

[Thomas Fischbacher]

As far as I can see from the documentation, it is not explicitly permitted 
to change a once allocated bigarray data structure in such a way that one 
replaces the data pointer by some other data pointer (while retaining 
size and type). This would, however, be quite useful in some situations.

Could this be changed?

-- 
regards,               tf@cip.physik.uni-muenchen.de              (o_
 Thomas Fischbacher -  http://www.cip.physik.uni-muenchen.de/~tf  //\
(lambda (n) ((lambda (p q r) (p p q r)) (lambda (g x y)           V_/_
(if (= x 0) y (g g (- x 1) (* x y)))) n 1))                  (Debian GNU)

Re: [Caml-list] Bigarray question

[Dmitry Bely]

Thomas Fischbacher <Thomas.Fischbacher@Physik.Uni-Muenchen.DE> writes:

> As far as I can see from the documentation, it is not explicitly permitted 
> to change a once allocated bigarray data structure in such a way that one 
> replaces the data pointer by some other data pointer (while retaining 
> size and type). This would, however, be quite useful in some situations.

Is your bigarray a wrapper around some externally allocated data, created
via alloc_bigarray() C call? If yes, you can safely replace the data
pointer with another one via Caml-C interface. If not, why do you need that?

- Dmitry Bely

Re: [Caml-list] Bigarray question

[Thomas Fischbacher]

On Fri, 25 Nov 2005, Dmitry Bely wrote:

> Thomas Fischbacher <Thomas.Fischbacher@Physik.Uni-Muenchen.DE> writes:
> 
> > As far as I can see from the documentation, it is not explicitly permitted 
> > to change a once allocated bigarray data structure in such a way that one 
> > replaces the data pointer by some other data pointer (while retaining 
> > size and type). This would, however, be quite useful in some situations.
> 
> Is your bigarray a wrapper around some externally allocated data, created
> via alloc_bigarray() C call?

It's a somewhat special case of just that situation, yes.

I supposed I may just do so, however, the specification (documentation) 
does not give me any guarantee that this is valid, so, at least the 
documentation has to be changed before I can rely on this.

There is a subtle difference between "just accidentally works" 
and "works because it is guaranteed by specification".

-- 
regards,               tf@cip.physik.uni-muenchen.de              (o_
 Thomas Fischbacher -  http://www.cip.physik.uni-muenchen.de/~tf  //\
(lambda (n) ((lambda (p q r) (p p q r)) (lambda (g x y)           V_/_
(if (= x 0) y (g g (- x 1) (* x y)))) n 1))                  (Debian GNU)

Re: [Caml-list] Bigarray question

[Dmitry Bely]

Thomas Fischbacher <Thomas.Fischbacher@Physik.Uni-Muenchen.DE> writes:

>> > As far as I can see from the documentation, it is not explicitly permitted 
>> > to change a once allocated bigarray data structure in such a way that one 
>> > replaces the data pointer by some other data pointer (while retaining 
>> > size and type). This would, however, be quite useful in some situations.
>> 
>> Is your bigarray a wrapper around some externally allocated data, created
>> via alloc_bigarray() C call?
>
> It's a somewhat special case of just that situation, yes.
>
> I supposed I may just do so, however, the specification (documentation) 
> does not give me any guarantee that this is valid, so, at least the 
> documentation has to be changed before I can rely on this.

1. For the current implementation it's safe (use Data_bigarray_val(v) to
change the data pointer). I don't see any reason why it could become unsafe.
2. It is up to you whether to use this feature or not.

> There is a subtle difference between "just accidentally works"
> and "works because it is guaranteed by specification".

OCaml is a developing language, so IMHO nobody guarantees that nothing will
change in the future.

- Dmitry Bely

Re: [Caml-list] Bigarray question

[Thomas Fischbacher]

On Mon, 28 Nov 2005, Dmitry Bely wrote:

> 1. For the current implementation it's safe (use Data_bigarray_val(v) to
> change the data pointer). I don't see any reason why it could become unsafe.
> 2. It is up to you whether to use this feature or not.

I will neither use undocumented programming techniques, nor recommend 
doing so to other programmers, no matter what kind of language. That would 
- quite in general - be a very bad habit that eventually must lead to bugs 
that are hard to impossible to diagnose and debug.

Assigning to Data_bigarray_val is not explicitly permitted.

Nevertheless, being allowed to do so would be helpful. So, a simple change 
to the documentation would suffice to make me happy.

-- 
regards,               tf@cip.physik.uni-muenchen.de              (o_
 Thomas Fischbacher -  http://www.cip.physik.uni-muenchen.de/~tf  //\
(lambda (n) ((lambda (p q r) (p p q r)) (lambda (g x y)           V_/_
(if (= x 0) y (g g (- x 1) (* x y)))) n 1))                  (Debian GNU)

Bigarray question

[Thomas Fischbacher]

Hello everybody,

as far as I can tell, it is easy (but not explicitly allowed in the
documentation) to allocate a Caml bigarray and change the data
pointer inside that bigarray afterwards.

What I am into right now is writing a function which allows me to
walk a caml function over the entries of a sparse matrix operator
managed by a low-level sparse matrix linear algebra C library.
Therefore, it would be rather elegant to allocate bigarrays for
index and data entries only once and pass them on to the ML
callback for every row of the matrix.

There is no problem in implementing this, technically speaking.
But this would require a change to the ML documentation stating
that it is explicitly considered as permissible to change the
data pointer in a bigarray. Can I get that, please?

-- 
best regards,
Thomas Fischbacher
t.fischbacher@soton.ac.uk