Portability | portable to Hugs and GHC. Requires the FFI and some flexible instances |
---|---|

Stability | unstable |

Maintainer | Lennart Kolmodin <kolmodin@dtek.chalmers.se> |

Binary serialisation of Haskell values to and from lazy ByteStrings.
The Binary library provides methods for encoding Haskell values as
streams of bytes directly in memory. The resulting `ByteString`

can
then be written to disk, sent over the network, or futher processed
(for example, compressed with gzip).

The `Binary`

package is notable in that it provides both pure, and
high performance serialisation.

Values are always encoded in network order (big endian) form, and encoded data should be portable across machine endianess, word size, or compiler version. For example, data encoded using the Binary class could be written from GHC, and read back in Hugs.

# The Binary class

The `Binary`

class provides `put`

and `get`

, methods to encode and
decode a Haskell value to a lazy ByteString. It mirrors the Read and
Show classes for textual representation of Haskell types, and is
suitable for serialising Haskell values to disk, over the network.

For parsing and generating simple external binary formats (e.g. C structures), Binary may be used, but in general is not suitable for complex protocols. Instead use the Put and Get primitives directly.

Instances of Binary should satisfy the following property:

decode . encode == id

That is, the `get`

and `put`

methods should be the inverse of each
other. A range of instances are provided for basic Haskell types.

Encode a value in the Put monad.

Decode a value in the Get monad

To serialise a custom type, an instance of Binary for that type is required. For example, suppose we have a data structure:

data Exp = IntE Int | OpE String Exp Exp deriving Show

We can encode values of this type into bytestrings using the following instance, which proceeds by recursively breaking down the structure to serialise:

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 t <- get :: Get Word8 case t of 0 -> do i <- get return (IntE i) 1 -> do s <- get e1 <- get e2 <- get return (OpE s e1 e2)

Note how we write an initial tag byte to indicate each variant of the data type.

We can simplify the writing of `get`

instances using monadic
combinators:

get = do tag <- getWord8 case tag of 0 -> liftM IntE get 1 -> liftM3 OpE get get get

The generation of Binary instances has been automated by a script using Scrap Your Boilerplate generics. Use the script here: http://darcs.haskell.org/binary/tools/derive/BinaryDerive.hs.

To derive the instance for a type, load this script into GHCi, and bring your type into scope. Your type can then have its Binary instances derived as follows:

$ ghci -fglasgow-exts BinaryDerive.hs *BinaryDerive> :l Example.hs *Main> deriveM (undefined :: Drinks) instance Binary Main.Drinks where put (Beer a) = putWord8 0 >> put a put Coffee = putWord8 1 put Tea = putWord8 2 put EnergyDrink = putWord8 3 put Water = putWord8 4 put Wine = putWord8 5 put Whisky = putWord8 6 get = do tag_ <- getWord8 case tag_ of 0 -> get >>= \a -> return (Beer a) 1 -> return Coffee 2 -> return Tea 3 -> return EnergyDrink 4 -> return Water 5 -> return Wine 6 -> return Whisky

To serialise this to a bytestring, we use `encode`

, which packs the
data structure into a binary format, in a lazy bytestring

> let e = OpE "*" (IntE 7) (OpE "/" (IntE 4) (IntE 2)) > let v = encode e

Where `v`

is a binary encoded data structure. To reconstruct the
original data, we use `decode`

> decode v :: Exp OpE "*" (IntE 7) (OpE "/" (IntE 4) (IntE 2))

The lazy ByteString that results from `encode`

can be written to
disk, and read from disk using Data.ByteString.Lazy IO functions,
such as hPutStr or writeFile:

> writeFile "/tmp/exp.txt" (encode e)

And read back with:

> readFile "/tmp/exp.txt" >>= return . decode :: IO Exp OpE "*" (IntE 7) (OpE "/" (IntE 4) (IntE 2))

We can also directly serialise a value to and from a Handle, or a file:

> v <- decodeFile "/tmp/exp.txt" :: IO Exp OpE "*" (IntE 7) (OpE "/" (IntE 4) (IntE 2))

And write a value to disk

> encodeFile "/tmp/a.txt" v

# The Get and Put monads

The Get monad is just a State monad carrying around the input ByteString

# Useful helpers for writing instances

# Binary serialisation

encode :: Binary a => a -> ByteStringSource

Encode a value using binary serialisation to a lazy ByteString.

decode :: Binary a => ByteString -> aSource

Decode a value from a lazy ByteString, reconstructing the original structure.

# IO functions for serialisation

encodeFile :: Binary a => FilePath -> a -> IO ()Source

Lazily serialise a value to a file

This is just a convenience function, it's defined simply as:

encodeFile f = B.writeFile f . encode

So for example if you wanted to compress as well, you could use:

B.writeFile f . compress . encode

decodeFile :: Binary a => FilePath -> IO aSource

Lazily reconstruct a value previously written to a file.

This is just a convenience function, it's defined simply as:

decodeFile f = return . decode =<< B.readFile f

So for example if you wanted to decompress as well, you could use:

return . decode . decompress =<< B.readFile f

After contructing the data from the input file, `decodeFile`

checks
if the file is empty, and in doing so will force the associated file
handle closed, if it is indeed empty. If the file is not empty,
it is up to the decoding instance to consume the rest of the data,
or otherwise finalise the resource.

module Data.Word