Data.ByteString.Lazy.Builder

Builders are used to efficiently construct sequences of bytes from smaller parts. Typically, such a construction is part of the implementation of an encoding, i.e., a function for converting Haskell values to sequences of bytes. Examples of encodings are the generation of the sequence of bytes representing a HTML document to be sent in a HTTP response by a web application or the serialization of a Haskell value using a fixed binary format.

For an efficient implementation of an encoding, it is important that (a) little time is spent on converting the Haskell values to the resulting sequence of bytes and (b) that the representation of the resulting sequence is such that it can be consumed efficiently. Builders support (a) by providing an O(1) concatentation operation and efficient implementations of basic encodings for Chars, Ints, and other standard Haskell values. They support (b) by providing their result as a lazy ByteString, which is internally just a linked list of pointers to chunks of consecutive raw memory. Lazy ByteStrings can be efficiently consumed by functions that write them to a file or send them over a network socket. Note that each chunk boundary incurs expensive extra work (e.g., a system call) that must be amortized over the work spent on consuming the chunk body. Builders therefore take special care to ensure that the average chunk size is large enough. The precise meaning of large enough is application dependent. The current implementation is tuned for an average chunk size between 4kb and 32kb, which should suit most applications.

As a simple example of an encoding implementation, we show how to efficiently convert the following representation of mixed-data tables to an UTF-8 encoded Comma-Separated-Values (CSV) table.

data Cell = StringC String
          | IntC Int
          deriving( Eq, Ord, Show )

type Row   = [Cell]
type Table = [Row]

We use the following imports and abbreviate mappend to simplify reading.

import qualified Data.ByteString.Lazy               as L
import           Data.ByteString.Lazy.Builder
import           Data.ByteString.Lazy.Builder.ASCII (intDec)
import           Data.Monoid
import           Data.Foldable                        (foldMap)
import           Data.List                            (intersperse)

infixr 4 <>
(<>) :: Monoid m => m -> m -> m
(<>) = mappend

CSV is a character-based representation of tables. For maximal modularity, we could first render Tables as Strings and then encode this String using some Unicode character encoding. However, this sacrifices performance due to the intermediate String representation being built and thrown away right afterwards. We get rid of this intermediate String representation by fixing the character encoding to UTF-8 and using Builders to convert Tables directly to UTF-8 encoded CSV tables represented as lazy ByteStrings.

encodeUtf8CSV :: Table -> L.ByteString
encodeUtf8CSV = toLazyByteString . renderTable

renderTable :: Table -> Builder
renderTable rs = mconcat [renderRow r <> charUtf8 '\n' | r <- rs]

renderRow :: Row -> Builder
renderRow []     = mempty
renderRow (c:cs) =
    renderCell c <> mconcat [ charUtf8 ',' <> renderCell c' | c' <- cs ]

renderCell :: Cell -> Builder
renderCell (StringC cs) = renderString cs
renderCell (IntC i)     = intDec i

renderString :: String -> Builder
renderString cs = charUtf8 '"' <> foldMap escape cs <> charUtf8 '"'
  where
    escape '\\' = charUtf8 '\\' <> charUtf8 '\\'
    escape '\"' = charUtf8 '\\' <> charUtf8 '\"'
    escape c    = charUtf8 c

Note that the ASCII encoding is a subset of the UTF-8 encoding, which is why we can use the optimized function intDec to encode an Int as a decimal number with UTF-8 encoded digits. Using intDec is more efficient than stringUtf8 . show, as it avoids constructing an intermediate String. Avoiding this intermediate data structure significantly improves performance because encoding Cells is the core operation for rendering CSV-tables. See Data.ByteString.Lazy.Builder.BasicEncoding for further information on how to improve the performance of renderString.

We demonstrate our UTF-8 CSV encoding function on the following table.

strings :: [String]
strings =  ["hello", "\"1\"", "λ-wörld"]

table :: Table
table = [map StringC strings, map IntC [-3..3]]

The expression encodeUtf8CSV table results in the following lazy ByteString.

Chunk "\"hello\",\"\\\"1\\\"\",\"\206\187-w\195\182rld\"\n-3,-2,-1,0,1,2,3\n" Empty

We can clearly see that we are converting to a binary format. The 'λ' and 'ö' characters, which have a Unicode codepoint above 127, are expanded to their corresponding UTF-8 multi-byte representation.

We use the criterion library (http://hackage.haskell.org/package/criterion) to benchmark the efficiency of our encoding function on the following table.

import Criterion.Main     -- add this import to the ones above

maxiTable :: Table
maxiTable = take 1000 $ cycle table

main :: IO ()
main = defaultMain
  [ bench "encodeUtf8CSV maxiTable (original)" $
      whnf (L.length . encodeUtf8CSV) maxiTable
  ]

On a Core2 Duo 2.20GHz on a 32-bit Linux, the above code takes 1ms to generate the 22'500 bytes long lazy ByteString. Looking again at the definitions above, we see that we took care to avoid intermediate data structures, as otherwise we would sacrifice performance. For example, the following (arguably simpler) definition of renderRow is about 20% slower.

renderRow :: Row -> Builder
renderRow  = mconcat . intersperse (charUtf8 ',') . map renderCell

Similarly, using O(n) concatentations like ++ or the equivalent concat operations on strict and lazy ByteStrings should be avoided. The following definition of renderString is also about 20% slower.

renderString :: String -> Builder
renderString cs = charUtf8 $ "\"" ++ concatMap escape cs ++ "\""
  where
    escape '\\' = "\\"
    escape '\"' = "\\\""
    escape c    = return c

Apart from removing intermediate data-structures, encodings can be optimized further by fine-tuning their execution parameters using the functions in Data.ByteString.Lazy.Builder.Extras and their "inner loops" using the functions in Data.ByteString.Lazy.Builder.BasicEncoding.

The Builder type

data Builder

Builders denote sequences of bytes. They are Monoids where mempty is the zero-length sequence and mappend is concatenation, which runs in O(1).

Instances

Monoid Builder

Executing Builders

Internally, Builders are buffer-filling functions. They are executed by a driver that provides them with an actual buffer to fill. Once called with a buffer, a Builder fills it and returns a signal to the driver telling it that it is either done, has filled the current buffer, or wants to directly insert a reference to a chunk of memory. In the last two cases, the Builder also returns a continutation Builder that the driver can call to fill the next buffer. Here, we provide the two drivers that satisfy almost all use cases. See Data.ByteString.Lazy.Builder.Extras, for information about fine-tuning them.

toLazyByteString :: Builder -> ByteString

Execute a Builder and return the generated chunks as a lazy ByteString. The work is performed lazy, i.e., only when a chunk of the lazy ByteString is forced.

hPutBuilder :: Handle -> Builder -> IO ()

Output a Builder to a Handle. The Builder is executed directly on the buffer of the Handle. If the buffer is too small (or not present), then it is replaced with a large enough buffer.

It is recommended that the Handle is set to binary and BlockBuffering mode. See hSetBinaryMode and hSetBuffering.

This function is more efficient than hPut . toLazyByteString because in many cases no buffer allocation has to be done. Moreover, the results of several executions of short Builders are concatenated in the Handles buffer, therefore avoiding unnecessary buffer flushes.

Creating Builders

Binary encodings

byteString :: ByteString -> Builder

Create a Builder denoting the same sequence of bytes as a strict ByteString. The Builder inserts large ByteStrings directly, but copies small ones to ensure that the generated chunks are large on average.

lazyByteString :: ByteString -> Builder

Create a Builder denoting the same sequence of bytes as a lazy ByteString. The Builder inserts large chunks of the lazy ByteString directly, but copies small ones to ensure that the generated chunks are large on average.

int8 :: Int8 -> Builder

Encode a single signed byte as-is.

word8 :: Word8 -> Builder

Encode a single unsigned byte as-is.

Big-endian

int16BE :: Int16 -> Builder

Encode an Int16 in big endian format.

int32BE :: Int32 -> Builder

Encode an Int32 in big endian format.

int64BE :: Int64 -> Builder

Encode an Int64 in big endian format.

word16BE :: Word16 -> Builder

Encode a Word16 in big endian format.

word32BE :: Word32 -> Builder

Encode a Word32 in big endian format.

word64BE :: Word64 -> Builder

Encode a Word64 in big endian format.

floatBE :: Float -> Builder

Encode a Float in big endian format.

doubleBE :: Double -> Builder

Encode a Double in big endian format.

Little-endian

int16LE :: Int16 -> Builder

Encode an Int16 in little endian format.

int32LE :: Int32 -> Builder

Encode an Int32 in little endian format.

int64LE :: Int64 -> Builder

Encode an Int64 in little endian format.

word16LE :: Word16 -> Builder

Encode a Word16 in little endian format.

word32LE :: Word32 -> Builder

Encode a Word32 in little endian format.

word64LE :: Word64 -> Builder

Encode a Word64 in little endian format.

floatLE :: Float -> Builder

Encode a Float in little endian format.

doubleLE :: Double -> Builder

Encode a Double in little endian format.

Character encodings

ASCII (Char7)

The ASCII encoding is a 7-bit encoding. The Char7 encoding implemented here works by truncating the Unicode codepoint to 7-bits, prefixing it with a leading 0, and encoding the resulting 8-bits as a single byte. For the codepoints 0-127 this corresponds the ASCII encoding. In Data.ByteString.Lazy.Builder.ASCII, we also provide efficient implementations of ASCII-based encodings of numbers (e.g., decimal and hexadecimal encodings).

char7 :: Char -> Builder

Char7 encode a Char.

string7 :: String -> Builder

Char7 encode a String.

ISO/IEC 8859-1 (Char8)

The ISO/IEC 8859-1 encoding is an 8-bit encoding often known as Latin-1. The Char8 encoding implemented here works by truncating the Unicode codepoint to 8-bits and encoding them as a single byte. For the codepoints 0-255 this corresponds to the ISO/IEC 8859-1 encoding. Note that you can also use the functions from Data.ByteString.Lazy.Builder.ASCII, as the ASCII encoding and ISO/IEC 8859-1 are equivalent on the codepoints 0-127.

char8 :: Char -> Builder

Char8 encode a Char.

string8 :: String -> Builder

Char8 encode a String.

UTF-8

The UTF-8 encoding can encode all Unicode codepoints. We recommend using it always for encoding Chars and Strings unless an application really requires another encoding. Note that you can also use the functions from Data.ByteString.Lazy.Builder.ASCII for UTF-8 encoding, as the ASCII encoding is equivalent to the UTF-8 encoding on the Unicode codepoints 0-127.

charUtf8 :: Char -> Builder

UTF-8 encode a Char.

stringUtf8 :: String -> Builder

UTF-8 encode a String.