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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Modulererebase-1.21.2Haskell2010

Data.ByteString.Builder.Extra

  • 3 types
  • 23 values
datadata Next
#

After running a BufferWriter action there are three possibilities for what comes next:

Constructors

  • Done

    This means we're all done. All the builder data has now been written.

  • More !Int BufferWriter

    This indicates that there may be more data to write. It gives you the next BufferWriter action. You should call that action with an appropriate buffer. The int indicates the minimum buffer size required by the next BufferWriter action. That is, if you call the next action you must supply it with a buffer length of at least this size.

  • Chunk !StrictByteString BufferWriter

    In addition to the data that has just been written into your buffer by the BufferWriter action, it gives you a pre-existing chunk of data as a StrictByteString. It also gives you the following BufferWriter action. It is safe to run this following action using a buffer with as much free space as was left by the previous run action.

valuedefaultChunkSize :: Int
#

The chunk size used for I/O. Currently set to 32k, less the memory management overhead

valuesmallChunkSize :: Int
#

The recommended chunk size. Currently set to 4k, less the memory management overhead

valueflush :: Builder
#

Flush the current buffer. This introduces a chunk boundary.

valuebyteStringThreshold :: Int -> StrictByteString -> Builder
#

Construct a Builder that copies the StrictByteStrings, if it is smaller than the treshold, and inserts it directly otherwise.

For example, byteStringThreshold 1024 copies StrictByteStrings whose size is less or equal to 1kb, and inserts them directly otherwise. This implies that the average chunk-size of the generated LazyByteString may be as low as 513 bytes, as there could always be just a single byte between the directly inserted 1025 byte, StrictByteStrings.

valuesafeStrategy
  1. :: Int

    Size of first buffer

  2. -> Int

    Size of successive buffers

  3. -> AllocationStrategy

    An allocation strategy that guarantees that at least half of the allocated memory is used for live data

#

Use this strategy for generating LazyByteStrings whose chunks are likely to survive one garbage collection. This strategy trims buffers that are filled less than half in order to avoid spilling too much memory.

valuetoLazyByteStringWith
  1. :: AllocationStrategy

    Buffer allocation strategy to use

  2. -> LazyByteString

    LazyByteString to use as the tail of the generated lazy LazyByteString

  3. -> Builder

    Builder to execute

  4. -> LazyByteString

    Resulting LazyByteString

#

Heavy inlining. Execute a Builder with custom execution parameters.

This function is inlined despite its heavy code-size to allow fusing with the allocation strategy. For example, the default Builder execution function toLazyByteString is defined as follows.

{-# NOINLINE toLazyByteString #-}
toLazyByteString =
  toLazyByteStringWith (safeStrategy smallChunkSize defaultChunkSize) L.Empty

where L.Empty is the zero-length LazyByteString.

In most cases, the parameters used by toLazyByteString give good performance. A sub-performing case of toLazyByteString is executing short (<128 bytes) Builders. In this case, the allocation overhead for the first 4kb buffer and the trimming cost dominate the cost of executing the Builder. You can avoid this problem using

toLazyByteStringWith (safeStrategy 128 smallChunkSize) L.Empty

This reduces the allocation and trimming overhead, as all generated LazyByteStrings fit into the first buffer and there is no trimming required, if more than 64 bytes and less than 128 bytes are written.

valueuntrimmedStrategy
  1. :: Int

    Size of the first buffer

  2. -> Int

    Size of successive buffers

  3. -> AllocationStrategy

    An allocation strategy that does not trim any of the filled buffers before converting it to a chunk

#

Use this strategy for generating LazyByteStrings whose chunks are discarded right after they are generated. For example, if you just generate them to write them to a network socket.

valuefloatHost :: Float -> Builder
#

Encode a Float in native host order. Values encoded this way are not portable to different endian machines, without conversion.

valueintHost :: Int -> Builder
#

Encode a single native machine Int. The Int is encoded in host order, host endian form, for the machine you're on. On a 64 bit machine the Int is an 8 byte value, on a 32 bit machine, 4 bytes. Values encoded this way are not portable to different endian or int sized machines, without conversion.

valuewordHost :: Word -> Builder
#

Encode a single native machine Word. The Word is encoded 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. Values encoded this way are not portable to different endian or word sized machines, without conversion.

typetype BufferWriter = Ptr Word8 -> Int -> IO (Int, Next)
#

A BufferWriter represents the result of running a Builder. It unfolds as a sequence of chunks of data. These chunks come in two forms:

  • an IO action for writing the Builder's data into a user-supplied memory buffer.

  • a pre-existing chunks of data represented by a StrictByteString

While this is rather low level, it provides you with full flexibility in how the data is written out.

The BufferWriter itself is an IO action: you supply it with a buffer (as a pointer and length) and it will write data into the buffer. It returns a number indicating how many bytes were actually written (which can be 0). It also returns a Next which describes what comes next.