bytestring-0.10.8.1: Fast, compact, strict and lazy byte strings with a list interface

Copyright (c) 2010 Jasper Van der Jeugt(c) 2010 - 2011 Simon Meier BSD3-style (see LICENSE) Simon Meier GHC Trustworthy Haskell98

Data.ByteString.Builder

Description

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.Builder
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.Builder.Prim 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.Builder.Extra and their "inner loops" using the functions in Data.ByteString.Builder.Prim.

Synopsis

# The Builder type

data Builder Source #

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

 # Methodsstimes :: Integral b => b -> Builder -> Builder Source # # Methodsmconcat :: [Builder] -> Builder Source #

# 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.Builder.Extra, for information about fine-tuning them.

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.

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

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.

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.

Construct a Builder that copies the ShortByteString.

Encode a single signed byte as-is.

Encode a single unsigned byte as-is.

### Big-endian

Encode an Int16 in big endian format.

Encode an Int32 in big endian format.

Encode an Int64 in big endian format.

Encode a Word16 in big endian format.

Encode a Word32 in big endian format.

Encode a Word64 in big endian format.

Encode a Float in big endian format.

Encode a Double in big endian format.

### Little-endian

Encode an Int16 in little endian format.

Encode an Int32 in little endian format.

Encode an Int64 in little endian format.

Encode a Word16 in little endian format.

Encode a Word32 in little endian format.

Encode a Word64 in little endian format.

Encode a Float in little endian format.

Encode a Double in little endian format.

## Character encodings

Conversion from Char and String into Builders in various 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.

Char7 encode a Char.

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.

Char8 encode a Char.

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.

UTF-8 encode a Char.

UTF-8 encode a String.

## Formatting numbers as text

Formatting of numbers as ASCII text.

Note that you can also use these functions for the ISO/IEC 8859-1 and UTF-8 encodings, as the ASCII encoding is equivalent on the codepoints 0-127.

### Decimal numbers

Decimal encoding of numbers using ASCII encoded characters.

Decimal encoding of an Int8 using the ASCII digits.

e.g.

toLazyByteString (int8Dec 42)   = "42"
toLazyByteString (int8Dec (-1)) = "-1"

Decimal encoding of an Int16 using the ASCII digits.

Decimal encoding of an Int32 using the ASCII digits.

Decimal encoding of an Int64 using the ASCII digits.

Decimal encoding of an Int using the ASCII digits.

Decimal encoding of an Integer using the ASCII digits.

Decimal encoding of a Word8 using the ASCII digits.

Decimal encoding of a Word16 using the ASCII digits.

Decimal encoding of a Word32 using the ASCII digits.

Decimal encoding of a Word64 using the ASCII digits.

Decimal encoding of a Word using the ASCII digits.

Currently slow. Decimal encoding of an IEEE Float.

Currently slow. Decimal encoding of an IEEE Double.

Encoding positive integers as hexadecimal numbers using lower-case ASCII characters. The shortest possible representation is used. For example,

>>> toLazyByteString (word16Hex 0x0a10)
Chunk "a10" Empty


Shortest hexadecimal encoding of a Word8 using lower-case characters.

Shortest hexadecimal encoding of a Word16 using lower-case characters.

Shortest hexadecimal encoding of a Word32 using lower-case characters.

Shortest hexadecimal encoding of a Word64 using lower-case characters.

Shortest hexadecimal encoding of a Word using lower-case characters.

Encode a Int8 using 2 nibbles (hexadecimal digits).

Encode a Int16 using 4 nibbles.

Encode a Int32 using 8 nibbles.

Encode a Int64 using 16 nibbles.

Encode a Word8 using 2 nibbles (hexadecimal digits).

Encode a Word16 using 4 nibbles.

Encode a Word32 using 8 nibbles.

Encode a Word64 using 16 nibbles.

Encode an IEEE Float using 8 nibbles.

Encode an IEEE Double using 16 nibbles.

Encode each byte of a ByteString using its fixed-width hex encoding.

Encode each byte of a lazy ByteString using its fixed-width hex encoding.

# Orphan instances

 # Methods