[Haskell-cafe] Re: Data.Binary and little endian encoding
jwlato at gmail.com
Fri May 15 05:16:58 EDT 2009
> On Thu, May 14, 2009 at 8:46 PM, Don Stewart <dons at galois.com> wrote:
>> > On Thu, May 14, 2009 at 8:40 PM, Don Stewart <dons at galois.com> wrote:
>> > leimy2k:
>> > > I actually need little endian encoding... wondering if anyone else
>> > this
>> > > with Data.Binary. (because I'm working with Bell Lab's 9P protocol
>> > does
>> > > encode things on the network in little-endian order).
>> > >
>> > > Anyone got some "tricks" for this?
>> > Yes!
>> > There are big, little and host-endian primitives in the Get/Put
>> > Data-Binary-Put.html#v%3AputWord16le
>> > You can use these to build encoders directly.
>> > Cool... I just have to write my own encoder and decoder now.
>> > As a request could we get encodeLe decodeLe for a later version of this
>> > library? :-) That'd be totally awesome.
>> Oh, you mean entirely different instances for all the current ones, that
>> use LE encodings?
> Well the library is leaning towards "Network Byte Order" in that it has
> encode/decode that only encode/decode for Big Endian.
> Us folks who have to do little endian all now have to write our own
> encoding/decoding :-)
> I'm speaking specifically of the encode/decode functions. I have no idea
> how they're implemented.
The encode/decode functions just call the runGet/runPut functions for
whatever is being encoded or decoded to combined with the get/put
functions from the Binary instance. The endian-ness is entirely
determined by the Binary instance for the data type you're
encoding/decoding, not anything in the encode/decode functions
For data types you define, you can make the Binary instance LE
standard (although it may not be a good idea) by using the
little-endian primitives Don mentioned. For built-in types, if you
want to use a different endian-ness, you can make new get/put
functions then call runGet/runPut on those directly. You won't be
able to redefine the Binary instances, but if you'll be doing this
with a lot of different types you could make your own BinaryLE class
class BinaryLE t where
putLE :: t -> Put
getLE :: Get t
encodeLE :: BinaryLE a => a -> ByteString
encodeLE = runPut . putLE
decodeLE :: BinaryLE a => ByteString -> a
decodeLE = runGet getLE
I've done some work with Binary and little-endian, and this bias never
really bothered me (I think it's actually a host-endian bias, could be
mistaken). I was using my own data types so I had to make my own
custom Binary instances, which I would have had to do anyway. The
data format specified little-endian, so making the Binary instance
LE-standard seemed appropriate.
You'd only need to re-make custom get/put functions if you're using
types that already have Binary instances, and you intend to manipulate
the data outside of Haskell. Does that describe your situation?
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