Data.Binary

ghc-binary-0.5.0.2: Binary serialisation for Haskell values using lazy ByteStrings

Portability | portable to Hugs and GHC. Requires the FFI and some flexible instances
Stability | unstable
Maintainer | Lennart Kolmodin <kolmodin@dtek.chalmers.se>

Data.Binary

Contents

- The Binary class
- The Get and Put monads
- Useful helpers for writing instances
- Binary serialisation
- IO functions for serialisation

Description

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.

Synopsis

- class Binary t where
  - put :: t -> Put
  - get :: Get t
- data Get a
- type Put = PutM ()
- putWord8 :: Word8 -> Put
- getWord8 :: Get Word8
- encode :: Binary a => a -> ByteString
- decode :: Binary a => ByteString -> a
- encodeFile :: Binary a => FilePath -> a -> IO ()
- decodeFile :: Binary a => FilePath -> IO a
- module Data.Word

# The Binary class

class Binary t where

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.

Methods

put :: t -> Put

Encode a value in the Put monad.

get :: Get t

Decode a value in the Get monad

Instances

Binary Bool |
Binary Char |
Binary Double |
Binary Float |
Binary Int |
Binary Int8 |
Binary Int16 |
Binary Int32 |
Binary Int64 |
Binary Integer |
Binary Ordering |
Binary Word |
Binary Word8 |
Binary Word16 |
Binary Word32 |
Binary Word64 |
Binary () |
Binary ByteString |
Binary ByteString |
Binary IntSet |
Binary a => Binary [a] |
(Binary a, Integral a) => Binary (Ratio a) |
Binary a => Binary (Maybe a) |
Binary e => Binary (Tree e) |
Binary e => Binary (Seq e) |
Binary e => Binary (IntMap e) |
(Ord a, Binary a) => Binary (Set a) |
(Binary a, Binary b) => Binary (Either a b) |
(Binary a, Binary b) => Binary (a, b) |
(Binary i, Ix i, Binary e, IArray UArray e) => Binary (UArray i e) |
(Binary i, Ix i, Binary e) => Binary (Array i e) |
(Ord k, Binary k, Binary e) => Binary (Map k e) |
(Binary a, Binary b, Binary c) => Binary (a, b, c) |
(Binary a, Binary b, Binary c, Binary d) => Binary (a, b, c, d) |
(Binary a, Binary b, Binary c, Binary d, Binary e) => Binary (a, b, c, d, e) |
(Binary a, Binary b, Binary c, Binary d, Binary e, Binary f) => Binary (a, b, c, d, e, f) |
(Binary a, Binary b, Binary c, Binary d, Binary e, Binary f, Binary g) => Binary (a, b, c, d, e, f, g) |
(Binary a, Binary b, Binary c, Binary d, Binary e, Binary f, Binary g, Binary h) => Binary (a, b, c, d, e, f, g, h) |
(Binary a, Binary b, Binary c, Binary d, Binary e, Binary f, Binary g, Binary h, Binary i) => Binary (a, b, c, d, e, f, g, h, i) |
(Binary a, Binary b, Binary c, Binary d, Binary e, Binary f, Binary g, Binary h, Binary i, Binary j) => Binary (a, b, c, d, e, f, g, h, i, j) |

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

data Get a

The Get monad is just a State monad carrying around the input ByteString We treat it as a strict state monad.

Instances

Monad Get |
Functor Get |
MonadFix Get |
Applicative Get |

type Put = PutM ()

Put merely lifts Builder into a Writer monad, applied to ().

# Useful helpers for writing instances

putWord8 :: Word8 -> Put

Efficiently write a byte into the output buffer

getWord8 :: Get Word8

Read a Word8 from the monad state

# Binary serialisation

encode :: Binary a => a -> ByteString

Encode a value using binary serialisation to a lazy ByteString.

decode :: Binary a => ByteString -> a

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

# IO functions for serialisation

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

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 a

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