| Copyright | Lennart Kolmodin |
|---|---|
| License | BSD3-style (see LICENSE) |
| Maintainer | Lennart Kolmodin <kolmodin@gmail.com> |
| Stability | unstable |
| Portability | portable to Hugs and GHC. Requires the FFI and some flexible instances. |
| Safe Haskell | Trustworthy |
| Language | Haskell98 |
Data.Binary
Contents
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 further processed (for example, compressed with gzip).
The binary package is notable in that it provides both pure, and high performance serialisation.
Values encoded using the Binary class are always encoded in network order (big endian) form, and encoded data should be portable across machine endianness, word size, or compiler version. For example, data encoded using the Binary class could be written on any machine, and read back on any another.
If the specifics of the data format is not important to you, for example, you are more interested in serializing and deserializing values than in which format will be used, it is possible to derive Binary instances using the generic support. See GBinaryGet and GBinaryPut.
If you have specific requirements about the encoding format, you can use the encoding and decoding primitives directly, see the modules Data.Binary.Get and Data.Binary.Put.
Synopsis
- classBinary t where
- classGBinaryGet f where
- classGBinaryPut f where
- dataGet a
- typePut = PutM ()
- putWord8 :: Word8 -> Put
- getWord8 :: GetWord8
- encode :: Binary a => a -> ByteString
- decode :: Binary a => ByteString -> a
- decodeOrFail :: Binary a => ByteString -> Either (ByteString, ByteOffset, String) (ByteString, ByteOffset, a)
- encodeFile :: Binary a => FilePath -> a -> IO ()
- decodeFile :: Binary a => FilePath -> IO a
- decodeFileOrFail :: Binary a => FilePath -> IO (Either (ByteOffset, String) a)
- module Data.Word
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 decoding 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
Encode a value in the Put monad.
Decode a value in the Get monad
Encode a list of values in the Put monad. The default implementation may be overridden to be more efficient but must still have the same encoding format.
put :: (Generic t, GBinaryPut (Rep t)) => t -> PutSource#
Encode a value in the Put monad.
get :: (Generic t, GBinaryGet (Rep t)) => Get t Source#
Decode a value in the Get monad
Instances
Example
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
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
Generic support
Beginning with GHC 7.2, it is possible to use binary serialization without writing any instance boilerplate code.
{-# LANGUAGE DeriveGeneric #-} import Data.Binary import GHC.Generics (Generic) data Foo = Foo deriving (Generic) -- GHC will automatically fill out the instance instance Binary FooThis mechanism makes use of GHC's efficient built-in generics support.
classGBinaryGet f whereSource#
Minimal complete definition
Instances
| GBinaryGet (V1 :: * -> *)Source# | |
| GBinaryGet (U1 :: * -> *)Source# | |
| Binary a => GBinaryGet (K1 i a :: * -> *)Source# | |
| (GSumGet a, GSumGet b, SumSize a, SumSize b) => GBinaryGet (a :+: b :: * -> *)Source# | |
| (GBinaryGet a, GBinaryGet b) => GBinaryGet (a :*: b :: * -> *)Source# | |
| GBinaryGet a => GBinaryGet (M1 i c a :: * -> *)Source# | |
classGBinaryPut f whereSource#
Minimal complete definition
Instances
| GBinaryPut (V1 :: * -> *)Source# | |
| GBinaryPut (U1 :: * -> *)Source# | |
| Binary a => GBinaryPut (K1 i a :: * -> *)Source# | |
| (GSumPut a, GSumPut b, SumSize a, SumSize b) => GBinaryPut (a :+: b :: * -> *)Source# | |
| (GBinaryPut a, GBinaryPut b) => GBinaryPut (a :*: b :: * -> *)Source# | |
| GBinaryPut a => GBinaryPut (M1 i c a :: * -> *)Source# | |
The Get and Put monads
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 -> a Source#
Decode a value from a lazy ByteString, reconstructing the original structure.
decodeOrFail :: Binary a => ByteString -> Either (ByteString, ByteOffset, String) (ByteString, ByteOffset, a) Source#
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 a Source#
Decode a value from a file. In case of errors, error will be called with the error message.
Since: 0.7.0.0
decodeFileOrFail :: Binary a => FilePath -> IO (Either (ByteOffset, String) a) Source#
Decode a value from a file. In case of success, the value will be returned in Right. In case of decoder errors, the error message together with the byte offset will be returned.
module Data.Word
