:: a typeCtrl KGHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05
Modulebinary-0.8.9.3Haskell2010
The Get monad. A monad for efficiently building structures from encoded lazy ByteStrings.
Primitives are available to decode words of various sizes, both big and little endian.
Let's decode binary data representing illustrated here. In this example the values are in little endian.
+------------------+--------------+-----------------+
| 32 bit timestamp | 32 bit price | 16 bit quantity |
+------------------+--------------+-----------------+A corresponding Haskell value looks like this:
data Trade = Trade
{ timestamp :: !Word32
, price :: !Word32
, qty :: !Word16
} deriving (Show)
The fields in Trade are marked as strict (using !) since we don't need
laziness here. In practise, you would probably consider using the UNPACK
pragma as well.
https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/pragmas.html#unpack-pragma
Now, let's have a look at a decoder for this format.
getTrade :: Get Trade
getTrade = do
timestamp <- getWord32le
price <- getWord32le
quantity <- getWord16le
return $! Trade timestamp price quantity
Or even simpler using applicative style:
getTrade' :: Get Trade
getTrade' = Trade <$> getWord32le <*> getWord32le <*> getWord16le
There are two kinds of ways to execute this decoder, the lazy input method and the incremental input method. Here we will use the lazy input method.
Let's first define a function that decodes many Trades.
getTrades :: Get [Trade]
getTrades = do
empty <- isEmpty
if empty
then return []
else do trade <- getTrade
trades <- getTrades
return (trade:trades)
Finally, we run the decoder:
lazyIOExample :: IO [Trade]
lazyIOExample = do
input <- BL.readFile "trades.bin"
return (runGet getTrades input)
This decoder has the downside that it will need to read all the input before it can return. On the other hand, it will not return anything until it knows it could decode without any decoder errors.
You could also refactor to a left-fold, to decode in a more streaming fashion, and get the following decoder. It will start to return data without knowing that it can decode all input.
incrementalExample :: BL.ByteString -> [Trade]
incrementalExample input0 = go decoder input0
where
decoder = runGetIncremental getTrade
go :: Decoder Trade -> BL.ByteString -> [Trade]
go (Done leftover _consumed trade) input =
trade : go decoder (BL.chunk leftover input)
go (Partial k) input =
go (k . takeHeadChunk $ input) (dropHeadChunk input)
go (Fail _leftover _consumed msg) _input =
error msg
takeHeadChunk :: BL.ByteString -> Maybe BS.ByteString
takeHeadChunk lbs =
case lbs of
(BL.Chunk bs _) -> Just bs
_ -> Nothing
dropHeadChunk :: BL.ByteString -> BL.ByteString
dropHeadChunk lbs =
case lbs of
(BL.Chunk _ lbs') -> lbs'
_ -> BL.Empty
The lazyIOExample uses lazy I/O to read the file from the disk, which is
not suitable in all applications, and certainly not if you need to read
from a socket which has higher likelihood to fail. To address these needs,
use the incremental input method like in incrementalExample.
For an example of how to read incrementally from a Handle,
see the implementation of decodeFileOrFail.
Monad GetDefined in binary-0.8.9.3 · Data.Binary.Get.InternalFunctor GetDefined in binary-0.8.9.3 · Data.Binary.Get.InternalMonadFail GetDefined in binary-0.8.9.3 · Data.Binary.Get.InternalApplicative GetDefined in binary-0.8.9.3 · Data.Binary.Get.InternalAlternative GetDefined in binary-0.8.9.3 · Data.Binary.Get.InternalMonadPlus GetDefined in binary-0.8.9.3 · Data.Binary.Get.InternalThe lazy interface consumes a single lazy ByteString. It's the easiest interface to get started with, but it doesn't support interleaving I/O and parsing, unless lazy I/O is used.
There is no way to provide more input other than the initial data. To be able to incrementally give more data, see the incremental input interface.
An offset, counted in bytes.
The incremental interface gives you more control over how input is provided during parsing. This lets you e.g. interleave parsing and I/O.
The incremental interface consumes a strict ByteString at a time, each being part of the total amount of input. If your decoder needs more input to finish it will return a Partial with a continuation. If there is no more input, provide it Nothing.
Fail will be returned if it runs into an error, together with a message, the position and the remaining input. If it succeeds it will return Done with the resulting value, the position and the remaining input.
A decoder procuced by running a Get monad.
Fail !ByteString !ByteOffset StringThe decoder ran into an error. The decoder either used fail or was not provided enough input. Contains any unconsumed input and the number of bytes consumed.
Partial (Maybe ByteString -> Decoder a)Done !ByteString !ByteOffset aThe decoder has successfully finished. Except for the output value you also get any unused input as well as the number of bytes consumed.
Run a Get monad. See Decoder for what to do next, like providing input, handling decoder errors and to get the output value. Hint: Use the helper functions pushChunk, pushChunks and pushEndOfInput.
Feed a Decoder with more input. If the Decoder is Done or Fail it will add the input to ByteString of unconsumed input.
runGetIncremental myParser `pushChunk` myInput1 `pushChunk` myInput2
Feed a Decoder with more input. If the Decoder is Done or Fail it will add the input to ByteString of unconsumed input.
runGetIncremental myParser `pushChunks` myLazyByteString
Skip ahead n bytes. Fails if fewer than n bytes are available.
Test whether all input has been consumed, i.e. there are no remaining undecoded bytes.
Get the total number of bytes read to this point.
Run the given decoder, but without consuming its input. If the given decoder fails, then so will this function.
Label a decoder. If the decoder fails, the label will be appended on a new line to the error message string.
An efficient get method for strict ByteStrings. Fails if fewer than n
bytes are left in the input. If n <= 0 then the empty string is returned.
An efficient get method for lazy ByteStrings. Fails if fewer than n
bytes are left in the input.
Get a lazy ByteString that is terminated with a NUL byte. The returned string does not contain the NUL byte. Fails if it reaches the end of input without finding a NUL.
Get the remaining bytes as a lazy ByteString. Note that this can be an expensive function to use as it forces reading all input and keeping the string in-memory.
Read a Word8 from the monad state
Read a Word16 in big endian format
Read a Word32 in big endian format
Read a Word64 in big endian format
Read a Word16 in little endian format
Read a Word32 in little endian format
Read a Word64 in little endian format
O(1). Read a single native machine word. The word is read in host order, host endian form, for the machine you're on. On a 64 bit machine the Word is an 8 byte value, on a 32 bit machine, 4 bytes.
O(1). Read a 2 byte Word16 in native host order and host endianness.
O(1). Read a Word32 in native host order and host endianness.
O(1). Read a Word64 in native host order and host endianess.
Read an Int8 from the monad state
Read an Int16 in big endian format.
Read an Int32 in big endian format.
Read an Int64 in big endian format.
Read an Int16 in little endian format.
Read an Int32 in little endian format.
Read an Int64 in little endian format.
O(1). Read a single native machine word in native host order. It works in the same way as getWordhost.
O(1). Read a 2 byte Int16 in native host order and host endianness.
O(1). Read an Int32 in native host order and host endianness.
O(1). Read an Int64 in native host order and host endianess.
Read a Float in big endian IEEE-754 format.
Read a Float in little endian IEEE-754 format.
Read a Float in IEEE-754 format and host endian.
Read a Double in big endian IEEE-754 format.
Read a Double in little endian IEEE-754 format.
Read a Double in IEEE-754 format and host endian.
Deprecated. Use runGetIncremental instead. This function will be removed.
DEPRECATED. Provides compatibility with previous versions of this library. Run a Get monad and return a tuple with three values. The first value is the result of the decoder. The second and third are the unused input, and the number of consumed bytes.
Deprecated. This will force all remaining input, don't use it.
DEPRECATED. Get the number of bytes of remaining input. Note that this is an expensive function to use as in order to calculate how much input remains, all input has to be read and kept in-memory. The decoder keeps the input as a strict bytestring, so you are likely better off by calculating the remaining input in another way.
Deprecated. Use getByteString instead of getBytes.
DEPRECATED. Same as getByteString.