Tomasz Zielonka wrote:
The general problem to solve is "How to we convert things to and from a binary format, so they can be efficiently written to and from disk".
Possibly the best way to handle this would be to extend the Storable class, e.g.
class Storable a where ... hPutObj :: Handle -> a -> IO () hGetObj :: Handle -> IO a toByteList :: a -> [Word8] fromByteList :: [Word8] -> a toByteArray :: a -> Array Int Word8 fromByteArray :: Array Int Word8 -> a
Hmmm, I thought that Foreign.Storable.Storable is meant for different purposes, namely for accessing C data in memory and for marshalling between Haskell and C worlds. Are you sure that these functionalities must overlap?
Extending Storable would require to extend its every existing instance. It might be better to introduce some subclass, but I'm not sure that every type supporting marshalling to and from binary format can support operations from Storable.
It's quite easy to write value of type [[Int]] to disk, but how to define alignment and sizeOf methods for this type?
Sorry, I'd overlooked the fact that Storable was limited to fixed-size
types.
In any case, reading/writing data to memory and to disk aren't all
that different; the existence of mmap() relies upon that fact. And,
mmap() aside, on most OSes, all I/O comes down to reading and writing
blocks of memory (i.e. read() and write()); everything else is just
additional layers.
I still think that we need a superclass of Storable, as any instance
of Storable can be read and written with e.g.:
hPutObj :: (Storable a) => Handle -> a -> IO ()
hPutObj h x = do
alloca $ \ptr -> do
poke ptr x
fd <- handleToFd h
let bptr = castPtr ptr :: Ptr CChar
let blen = fromIntegral $ sizeOf x
write fd bptr blen
hGetObj' :: (Storable a) => Handle -> a -> IO a
hGetObj' h dummy = do
alloca $ \ptr -> do
let blen = fromIntegral $ sizeOf dummy
let bptr = castPtr ptr :: Ptr CChar
fd <- handleToFd h
read fd bptr blen
x <- peek ptr
return x
hGetObj :: (Storable a) => Handle -> IO a
hGetObj h = hGetObj' h undefined
A similar process can be used to convert instances of Storable to or
from a list (or array) of bytes.
--
Glynn Clements