getting a Binary module into the standard libs

Hal Daume III hdaume@ISI.EDU
Tue, 10 Dec 2002 10:03:24 -0800 (PST)

Hi again all,

I've got a somewhat functioning new Binary module for GHC which supports
bit writes.  I've written a small test script to make sure simple things essentially looks like this:

data BinItem = ByteItem Word8
             | BitItem  Int Word8   -- num_bits bits
             | FlushItem            -- flush byte
             | forall a . (Eq a, Binary a, Show a) => BinaryItem a  --
some arbi
trary data
             | forall a . (Eq a, Show a) => PutItem a (BinHandle -> a ->
IO ()) 
(BinHandle -> IO a)

on which Eq is defined (I use unsafePerformIO to cast the forall'd

then we have test functions which essentially open up a binMem or a binIO
and write a list of BinItems to it, then read them back.  We can then
check if what we read back is the same as what we wrote.

The tests I have run are:

byteTest = map ByteItem [1,2,3,4,5]

bitTest1 = [BitItem 3 6, FlushItem]
bitTest2 = [BitItem 3 6, BitItem 4 9, BitItem 1 0, FlushItem]
bitTest3 = [BitItem 6 10, BitItem 6 10, FlushItem]

flushTest1 = [BitItem 3 6, FlushItem, BitItem 4 9, BitItem 1 0, FlushItem]
flushTest2 = [ByteItem 1, FlushItem, FlushItem, FlushItem, FlushItem,
ByteItem 2
, FlushItem]

comboTest1 = [ByteItem 5, BitItem 3 6, FlushItem, ByteItem 9, BitItem 4 9,
em 1 0, FlushItem, ByteItem 84, BitItem 3 2, FlushItem]
comboTest2 = [ByteItem 5, BitItem 3 6, ByteItem 9, BitItem 7 9, BitItem 4
0, Byt
eItem 84, BitItem 3 2, FlushItem]

maybeTest1 = [BinaryItem (Just 5::Maybe Int), BinaryItem (Nothing::Maybe
Int), B
inaryItem (Just 0::Maybe Int), BinaryItem (Just 1::Maybe Int), BinaryItem
ng :: Maybe Int), FlushItem]
maybeTest2 = map (\i -> PutItem i putMaybeInt getMaybeInt) [Just 5,
Nothing, Jus
t 0, Just 1, Nothing] ++ [FlushItem]

Which all work fine.  putMaybeInt uses a bit to put the Just/Nothing,
while the normal Binary instance for Maybe uses a byte.

...this brings up another question...since we don't have named instances,
I'm thinking of separating the actual instance definitions into two
modules, one which uses bit-based instances and one which uses byte-based
instances.  You could then import whichever makes more sense for your
application.  The problem with this is that sometimes it might make sense
to use bytes in one aspect and bits in another.

Another solution would be to extend the Binary class to:

class Binary a where
    put_   :: BinHandle -> a -> IO ()
    put    :: BinHandle -> a -> IO (Bin a)
    get    :: BinHandle -> IO a
    putWithBits_ :: BinHandle -> a -> IO ()
    putWithBits  :: BinHandle -> a -> IO (Bin a)
    getWithBits  :: BinHandle -> IO a

so that each instance specifies how to write itself both with bits and
bytes.  of course they could default off eachother, so the user only has
to write one put and one get if they're lazy.


 - Hal