Re: [Haskell-cafe] Re: Data.Binary and little endian encoding

This approach looks like it'd work just fine. On Fri, May 15, 2009 at 2:16 AM, John Lato wrote:

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On Thu, May 14, 2009 at 8:46 PM, Don Stewart wrote:

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leimy2k:

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On Thu, May 14, 2009 at 8:40 PM, Don Stewart wrote:

leimy2k: > I actually need little endian encoding... wondering if anyone

else hit

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this > with Data.Binary. (because I'm working with Bell Lab's 9P

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which

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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

monads.

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http://hackage.haskell.org/packages/archive/binary/0.5.0.1/doc/html/

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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

leimy2k: protocol this

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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 themselves.

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 like this:

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?

John _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe