On Sun, Jun 08, 2003 at 09:06:07PM +1000, Manuel M T Chakravarty wrote:
Ross Paterson <ross@soi.city.ac.uk> wrote,
On Wed, Jun 04, 2003 at 11:09:43PM +1000, Manuel M T Chakravarty wrote:
StablePtr are used to export references to Haskell values to C, where they are treated as abstract data. In C one traditionally uses (void *) for that purpose (see "man qsort(3)"). We want to make sure HsStablePtr is not to wide to be passed as an argument to C functions expecting such abstract types (such as qsort(3)).
How does HsStablePtr differ from HsDouble in this respect? (except that the C side can do even less with HsStablePtr.)
HsDouble is guaranteed to be one of C's floating types; hence, all operations on floating types can be applied on values of type HsDouble.
I meant to say that you can't pass an HsDouble to a "polymorphic" C function expecting (void *), so why should you be able to pass an HsStablePtr? Maybe you're arguing that otherwise it couldn't be passed to any legacy C function, whereas doubles can, though to different ones.
You know it somehow corresponds to a pointer on the Haskell side, but that doesn't seem relevant.
There is no guarantee whatsoever that HsStablePtr is a pointer on the Haskell side. In GHC, stable pointers are integral indexes into a table of stable values.
Same in Hugs (though that's not a coincidence), which makes it all the stranger that they're treated as pointers.