[Haskell-beginners] Ffi, memory allocation and failure

[.]

Thanks Antoine and Edward!
I will try the exception in my first go.

Christian

On Wed, 2011-02-09 at 11:26 -0600, Antoine Latter wrote:
> On Wed, Feb 9, 2011 at 8:57 AM, C Gosch <ch.gosch at googlemail.com> wrote:
> > Hi Edward, hello to all list members,
> > thank you for your comments, I will keep that in mind. There will be
> > operations on matrices (and other objects) which modify the objects,
> > and those will be in IO or ST, I think.
> > However, I am planning to provide operations with matrices which guarantee
> > that each involved matrix is not changed, but which may return new matrices.
> > All matrices have to be allocated, and that allocation can fail ... how does
> > one usually go about this problem in Haskell?
> > Cheers,
> > Christian
> >
> 
> I would check for allocation failure in the C wrapper and return a
> status code or error message back to Haskell. Thus the prototype of
> the C wrapper might be:
> 
> > result_code_t c_create_matrix(t1 arg1, t2 arg2, c_matrix *result);
> 
> Then you could either throw a Haskell exception (which is allowed from
> pure code - but can only be caught in IO) or do everything under
> 'Maybe'.
> 
> If you expect allocation failure to be rare and exceptional, I would
> lean towards the exception. I haven't had the occasion to wrap C
> libraries were I expected this sort of thing.
> 
> Antoine
> 
> > 2011/2/9 Edward Z. Yang <ezyang at mit.edu>
> >>
> >> Hello Christian,
> >>
> >> Assuming that the foreign library is based off of mutation on matrices,
> >> there's not an easy way to export a nice pure interface to them, because
> >> they're not actually persistent: if another thread goes and modifies
> >> some pointer which you also have in pure code, referential transparency
> >> is broken.  So you'll have to think carefully about what sorts of
> >> guarantees
> >> you can (or want) to make to Haskell, and if you can't fulfill them, keep
> >> the code in IO (or create your own monad for manipulating matrices).
> >>
> >> Edward
> >
> >
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