https://gcc.gnu.org/onlinedocs/gcc-4.4.3/gcc/Empty-Structures.html#Empty-Str...
GCC permits a C structure to have no members:
struct empty { };
The structure will have size zero. In C++, empty structures are part of the language. G++ treats empty structures as if they had a single member of type char.
so it's either GCC is absurd, or it isn't. Standard C also supports zero-width members (to force alignment), whether to use () for that or not in modelling Haskell-Storable structs. Could be useful.https://stackoverflow.com/questions/13802728/what-is-zero-width-bit-field - Oleg On 5.1.2022 10.01, Harendra Kumar wrote:
The Storable instance of () is defined in the "Foreign.Storable" module of the "base" package as follows:
instance Storable () where sizeOf _ = 0 alignment _ = 1 peek _ = return () poke _ _ = return ()
The size of () is defined as 0. It sounds absurd for a Storable to have a size of 0? This means that we can read an infinite number of () type values out of nothing (no memory location required) or store an infinite number of () type values without even requiring a memory location to write to.
This is causing a practical problem in our Storable array implementation. The array is constrained to a Storable type. Since () has a Storable instance, one can store () in the Storable array. But it causes a problem because we determine the array element size using sizeOf on the type. For () type it turns out to be 0. Essentially, the array of () would always be of size 0. Now, we cannot determine the length of the array from its byte length as you could store infinite such elements in an empty array. The Storable instance of () seems to be an oddity and makes us use a special case everywhere in the code to handle this, and this special casing makes it highly prone to errors when we change code.
Can this be fixed? Is there a compelling argument to keep it like this? A possible fix could be to represent it by a single byte in memory which can be discarded when reading or writing. Another alternative is to not provide a Storable instance for it at all. Let the users write their own if they need it.
If you think this does not have a problem, can you suggest how to elegantly handle the array implementation problem as I described above?
Thanks, Harendra _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries