bytestring-0.10.0.2: Fast, compact, strict and lazy byte strings with a list interface

Portability | GHC
Maintainer | Simon Meier <iridcode@gmail.com>
Safe Haskell | None

Data.ByteString.Lazy.Builder

Contents

- The Builder type
- Executing Builders
- Creating Builders
  - Binary encodings
    - Big-endian
    - Little-endian
  - Character encodings
    - ASCII (Char7)
    - ISO/IEC 8859-1 (Char8)
    - UTF-8

Description

`Builder`s 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. `Builder`s support (a) by providing an O(1) concatentation operation and efficient implementations of basic encodings for `Char`s, `Int`s, 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 `ByteString`s 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. `Builder`s 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 `Table`s as `String`s 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 `Builder`s to convert `Table`s directly to UTF-8 encoded CSV tables represented as lazy `ByteString`s.

    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 `Cell`s 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 `ByteString`s 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.

Synopsis

- data Builder
- toLazyByteString :: Builder -> ByteString
- hPutBuilder :: Handle -> Builder -> IO ()
- byteString :: ByteString -> Builder
- lazyByteString :: ByteString -> Builder
- int8 :: Int8 -> Builder
- word8 :: Word8 -> Builder
- int16BE :: Int16 -> Builder
- int32BE :: Int32 -> Builder
- int64BE :: Int64 -> Builder
- word16BE :: Word16 -> Builder
- word32BE :: Word32 -> Builder
- word64BE :: Word64 -> Builder
- floatBE :: Float -> Builder
- doubleBE :: Double -> Builder
- int16LE :: Int16 -> Builder
- int32LE :: Int32 -> Builder
- int64LE :: Int64 -> Builder
- word16LE :: Word16 -> Builder
- word32LE :: Word32 -> Builder
- word64LE :: Word64 -> Builder
- floatLE :: Float -> Builder
- doubleLE :: Double -> Builder
- char7 :: Char -> Builder
- string7 :: String -> Builder
- char8 :: Char -> Builder
- string8 :: String -> Builder
- charUtf8 :: Char -> Builder
- stringUtf8 :: String -> Builder

# The Builder type

data Builder

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

Instances

Monoid Builder |

# Executing Builders

Internally, `Builder`s 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 `Builder`s are concatenated in the `Handle`s 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 `ByteString`s 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 `Char`s and `String`s 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`.