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

[David Leimbach]

This approach looks like it'd work just fine.

On Fri, May 15, 2009 at 2:16 AM, John Lato <jwlato at gmail.com> wrote:

> leimy2k:
> > On Thu, May 14, 2009 at 8:46 PM, Don Stewart <dons at galois.com> wrote:
> >
> >> leimy2k:
> >> >
> >> >
> >> > 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
> >> hit
> >> >     this
> >> >     > with Data.Binary. (because I'm working with Bell Lab's 9P
> protocol
> >> which
> >> >     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.
> >> >
> >> >
> >> http://hackage.haskell.org/packages/archive/binary/0.5.0.1/doc/html/
> >> >     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
> 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
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