[Haskell-cafe] Binary serialization, was Re: Abstraction leak
Donald Bruce Stewart
dons at cse.unsw.edu.au
Wed Jul 4 18:50:42 EDT 2007
> On Wed, Jul 04, 2007 at 09:44:13PM +1000, Donald Bruce Stewart wrote:
> >Binary instances are pretty easy to write. For a simple data type:
> > > instance Binary Exp where
> > > put (IntE i) = do put (0 :: Word8)
> > > put i
> > > put (OpE s e1 e2) = do put (1 :: Word8)
> > > put s
> > > put e1
> > > put e2
> > > get = do tag <- getWord8
> > > case tag of
> > > 0 -> liftM IntE get
> > > 1 -> liftM3 OpE get get get
> That's quite verbose! Plus I'm a bit concerned by the boxing implied
> by those IntE / OpE constructors in get. If you were using those
> values in a pattern match on the result of get, would the compiler be
> able to eliminate them and refer directly to the values in the source
Well, here's you're flattening a Haskell structure, so it has to get
reboxed. If it was bytestring chunks, or Ints, then you can avoid any
serious copying. The 'get' just tags a value.
> >The Data.Binary comes with one tool to derive these. The DrIFT preprocessor
> >also can, as can Stefan O'Rear's SYB deriver.
> >I just write them by hand, or use the tool that comes with the lib.
> >More docs here,
> > http://hackage.haskell.org/packages/archive/binary/0.3/doc/html/Data-Binary.html
> This doesn't seem to deal with endianness. Am I missing something?
That's the Haskell serialisation layer. Look at Data.Binary.Get/Put for
endian-primitives, to be used instead of 'get'. i.e. getWord16be
> >>>>world, you could operate on the packets in place in Haskell where
> >>>>possible and save the deserialization overhead...
> >>>Data.ByteString.* for this.
> Ah, does Data.Binary fuse with ByteString.* then?
They know about each other, and Binary avoids copying if you're reading
> >Hack those bytes! Quickly! :-)
> It's a shame the layout definition is so verbose. Erlang's is quite
> compact. I wonder if something could be done with template haskell to
> translate an Erlang-style data layout definition to the Data.Binary
Right, simple but a bit verbose. The Erlang bit syntax is a nice pattern
matching/layout syntax for bit/byte data. There's a couple of ports of
this to Haskell -- one using pattern guards, another using Template
Haskell. Look on hackage.haskell.org for bitsyntax if you're interested.
> (Bonus points for being able to parse ASN.1 and generate appropriate
> Haskell datatypes & serialization primitives automatically :-) )
I think there's at least an ASN.1 definition in the crypto library.
Dominic might be able to enlighten us on that.
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