phil:
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 data?
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....
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 ByteStrings.
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 form?
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. -- Don