Data.ByteString.Lazy

A time and space-efficient implementation of lazy byte vectors using lists of packed Word8 arrays, suitable for high performance use, both in terms of large data quantities, or high speed requirements. Lazy ByteStrings are encoded as lazy lists of strict chunks of bytes.

A key feature of lazy ByteStrings is the means to manipulate large or unbounded streams of data without requiring the entire sequence to be resident in memory. To take advantage of this you have to write your functions in a lazy streaming style, e.g. classic pipeline composition. The default I/O chunk size is 32k, which should be good in most circumstances.

Some operations, such as concat, append, reverse and cons, have better complexity than their Data.ByteString equivalents, due to optimisations resulting from the list spine structure. For other operations lazy ByteStrings are usually within a few percent of strict ones.

The recomended way to assemble lazy ByteStrings from smaller parts is to use the builder monoid from Data.ByteString.Builder.

This module is intended to be imported qualified, to avoid name clashes with Prelude functions. eg.

 import qualified Data.ByteString.Lazy as B

Original GHC implementation by Bryan O'Sullivan. Rewritten to use UArray by Simon Marlow. Rewritten to support slices and use ForeignPtr by David Roundy. Rewritten again and extended by Don Stewart and Duncan Coutts. Lazy variant by Duncan Coutts and Don Stewart.

The ByteString type

data ByteString

A space-efficient representation of a Word8 vector, supporting many efficient operations.

A lazy ByteString contains 8-bit bytes, or by using the operations from Data.ByteString.Lazy.Char8 it can be interpreted as containing 8-bit characters.

Instances

Eq ByteString

Data ByteString

Ord ByteString

Read ByteString

Show ByteString

Typeable ByteString

IsString ByteString

Monoid ByteString

NFData ByteString

Introducing and eliminating ByteStrings

empty :: ByteString

O(1) The empty ByteString

singleton :: Word8 -> ByteString

O(1) Convert a Word8 into a ByteString

pack :: [Word8] -> ByteString

O(n) Convert a '[Word8]' into a ByteString.

unpack :: ByteString -> [Word8]

O(n) Converts a ByteString to a '[Word8]'.

fromStrict :: ByteString -> ByteString

O(1) Convert a strict ByteString into a lazy ByteString.

toStrict :: ByteString -> ByteString

O(n) Convert a lazy ByteString into a strict ByteString.

Note that this is an expensive operation that forces the whole lazy ByteString into memory and then copies all the data. If possible, try to avoid converting back and forth between strict and lazy bytestrings.

fromChunks :: [ByteString] -> ByteString

O(c) Convert a list of strict ByteString into a lazy ByteString

toChunks :: ByteString -> [ByteString]

O(c) Convert a lazy ByteString into a list of strict ByteString

foldrChunks :: (ByteString -> a -> a) -> a -> ByteString -> a

Consume the chunks of a lazy ByteString with a natural right fold.

foldlChunks :: (a -> ByteString -> a) -> a -> ByteString -> a

Consume the chunks of a lazy ByteString with a strict, tail-recursive, accumulating left fold.

Basic interface

cons :: Word8 -> ByteString -> ByteString

O(1) cons is analogous to '(:)' for lists.

cons' :: Word8 -> ByteString -> ByteString

O(1) Unlike cons, 'cons\'' is strict in the ByteString that we are consing onto. More precisely, it forces the head and the first chunk. It does this because, for space efficiency, it may coalesce the new byte onto the first 'chunk' rather than starting a new 'chunk'.

So that means you can't use a lazy recursive contruction like this:

 let xs = cons\' c xs in xs

You can however use cons, as well as repeat and cycle, to build infinite lazy ByteStrings.

snoc :: ByteString -> Word8 -> ByteString

O(n\c)/ Append a byte to the end of a ByteString

append :: ByteString -> ByteString -> ByteString

O(n\c)/ Append two ByteStrings

head :: ByteString -> Word8

O(1) Extract the first element of a ByteString, which must be non-empty.

uncons :: ByteString -> Maybe (Word8, ByteString)

O(1) Extract the head and tail of a ByteString, returning Nothing if it is empty.

unsnoc :: ByteString -> Maybe (ByteString, Word8)

O(n\c)/ Extract the init and last of a ByteString, returning Nothing if it is empty.

• It is no faster than using init and last

last :: ByteString -> Word8

O(n\c)/ Extract the last element of a ByteString, which must be finite and non-empty.

tail :: ByteString -> ByteString

O(1) Extract the elements after the head of a ByteString, which must be non-empty.

init :: ByteString -> ByteString

O(n\c)/ Return all the elements of a ByteString except the last one.

null :: ByteString -> Bool

O(1) Test whether a ByteString is empty.

length :: ByteString -> Int64

O(n\c)/ length returns the length of a ByteString as an Int64

Transforming ByteStrings

map :: (Word8 -> Word8) -> ByteString -> ByteString

O(n) map f xs is the ByteString obtained by applying f to each element of xs.

reverse :: ByteString -> ByteString

O(n) reverse xs returns the elements of xs in reverse order.

intersperse :: Word8 -> ByteString -> ByteString

The intersperse function takes a Word8 and a ByteString and `intersperses' that byte between the elements of the ByteString. It is analogous to the intersperse function on Lists.

intercalate :: ByteString -> [ByteString] -> ByteString

O(n) The intercalate function takes a ByteString and a list of ByteStrings and concatenates the list after interspersing the first argument between each element of the list.

transpose :: [ByteString] -> [ByteString]

The transpose function transposes the rows and columns of its ByteString argument.

Reducing ByteStrings (folds)

foldl :: (a -> Word8 -> a) -> a -> ByteString -> a

foldl, applied to a binary operator, a starting value (typically the left-identity of the operator), and a ByteString, reduces the ByteString using the binary operator, from left to right.

foldl' :: (a -> Word8 -> a) -> a -> ByteString -> a

'foldl\'' is like foldl, but strict in the accumulator.

foldl1 :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8

foldl1 is a variant of foldl that has no starting value argument, and thus must be applied to non-empty ByteStrings.

foldl1' :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8

'foldl1\'' is like foldl1, but strict in the accumulator.

foldr :: (Word8 -> a -> a) -> a -> ByteString -> a

foldr, applied to a binary operator, a starting value (typically the right-identity of the operator), and a ByteString, reduces the ByteString using the binary operator, from right to left.

foldr1 :: (Word8 -> Word8 -> Word8) -> ByteString -> Word8

foldr1 is a variant of foldr that has no starting value argument, and thus must be applied to non-empty ByteStrings

Special folds

concat :: [ByteString] -> ByteString

O(n) Concatenate a list of ByteStrings.

concatMap :: (Word8 -> ByteString) -> ByteString -> ByteString

Map a function over a ByteString and concatenate the results

any :: (Word8 -> Bool) -> ByteString -> Bool

O(n) Applied to a predicate and a ByteString, any determines if any element of the ByteString satisfies the predicate.

all :: (Word8 -> Bool) -> ByteString -> Bool

O(n) Applied to a predicate and a ByteString, all determines if all elements of the ByteString satisfy the predicate.

maximum :: ByteString -> Word8

O(n) maximum returns the maximum value from a ByteString

minimum :: ByteString -> Word8

O(n) minimum returns the minimum value from a ByteString

Building ByteStrings

Scans

scanl :: (Word8 -> Word8 -> Word8) -> Word8 -> ByteString -> ByteString

scanl is similar to foldl, but returns a list of successive reduced values from the left. This function will fuse.

 scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]

Note that

 last (scanl f z xs) == foldl f z xs.

Accumulating maps

mapAccumL :: (acc -> Word8 -> (acc, Word8)) -> acc -> ByteString -> (acc, ByteString)

The mapAccumL function behaves like a combination of map and foldl; it applies a function to each element of a ByteString, passing an accumulating parameter from left to right, and returning a final value of this accumulator together with the new ByteString.

mapAccumR :: (acc -> Word8 -> (acc, Word8)) -> acc -> ByteString -> (acc, ByteString)

The mapAccumR function behaves like a combination of map and foldr; it applies a function to each element of a ByteString, passing an accumulating parameter from right to left, and returning a final value of this accumulator together with the new ByteString.

Infinite ByteStrings

repeat :: Word8 -> ByteString

repeat x is an infinite ByteString, with x the value of every element.

replicate :: Int64 -> Word8 -> ByteString

O(n) replicate n x is a ByteString of length n with x the value of every element.

cycle :: ByteString -> ByteString

cycle ties a finite ByteString into a circular one, or equivalently, the infinite repetition of the original ByteString.

iterate :: (Word8 -> Word8) -> Word8 -> ByteString

iterate f x returns an infinite ByteString of repeated applications of f to x:

 iterate f x == [x, f x, f (f x), ...]

Unfolding ByteStrings

unfoldr :: (a -> Maybe (Word8, a)) -> a -> ByteString

O(n) The unfoldr function is analogous to the List 'unfoldr'. unfoldr builds a ByteString from a seed value. The function takes the element and returns Nothing if it is done producing the ByteString or returns Just (a,b), in which case, a is a prepending to the ByteString and b is used as the next element in a recursive call.

Substrings

Breaking strings

take :: Int64 -> ByteString -> ByteString

O(n\c)/ take n, applied to a ByteString xs, returns the prefix of xs of length n, or xs itself if n > length xs.

drop :: Int64 -> ByteString -> ByteString

O(n\c)/ drop n xs returns the suffix of xs after the first n elements, or [] if n > length xs.

splitAt :: Int64 -> ByteString -> (ByteString, ByteString)

O(n\c)/ splitAt n xs is equivalent to (take n xs, drop n xs).

takeWhile :: (Word8 -> Bool) -> ByteString -> ByteString

takeWhile, applied to a predicate p and a ByteString xs, returns the longest prefix (possibly empty) of xs of elements that satisfy p.

dropWhile :: (Word8 -> Bool) -> ByteString -> ByteString

dropWhile p xs returns the suffix remaining after takeWhile p xs.

span :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)

span p xs breaks the ByteString into two segments. It is equivalent to (takeWhile p xs, dropWhile p xs)

break :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)

break p is equivalent to span (not . p).

group :: ByteString -> [ByteString]

The group function takes a ByteString and returns a list of ByteStrings such that the concatenation of the result is equal to the argument. Moreover, each sublist in the result contains only equal elements. For example,

 group "Mississippi" = ["M","i","ss","i","ss","i","pp","i"]

It is a special case of groupBy, which allows the programmer to supply their own equality test.

groupBy :: (Word8 -> Word8 -> Bool) -> ByteString -> [ByteString]

The groupBy function is the non-overloaded version of group.

inits :: ByteString -> [ByteString]

O(n) Return all initial segments of the given ByteString, shortest first.

tails :: ByteString -> [ByteString]

O(n) Return all final segments of the given ByteString, longest first.

Breaking into many substrings

split :: Word8 -> ByteString -> [ByteString]

O(n) Break a ByteString into pieces separated by the byte argument, consuming the delimiter. I.e.

 split '\n' "a\nb\nd\ne" == ["a","b","d","e"]
 split 'a'  "aXaXaXa"    == ["","X","X","X",""]
 split 'x'  "x"          == ["",""]

and

 intercalate [c] . split c == id
 split == splitWith . (==)

As for all splitting functions in this library, this function does not copy the substrings, it just constructs new ByteStrings that are slices of the original.

splitWith :: (Word8 -> Bool) -> ByteString -> [ByteString]

O(n) Splits a ByteString into components delimited by separators, where the predicate returns True for a separator element. The resulting components do not contain the separators. Two adjacent separators result in an empty component in the output. eg.

 splitWith (=='a') "aabbaca" == ["","","bb","c",""]
 splitWith (=='a') []        == []

Predicates

isPrefixOf :: ByteString -> ByteString -> Bool

O(n) The isPrefixOf function takes two ByteStrings and returns True iff the first is a prefix of the second.

isSuffixOf :: ByteString -> ByteString -> Bool

O(n) The isSuffixOf function takes two ByteStrings and returns True iff the first is a suffix of the second.

The following holds:

 isSuffixOf x y == reverse x `isPrefixOf` reverse y

Search for arbitrary substrings

Searching ByteStrings

Searching by equality

elem :: Word8 -> ByteString -> Bool

O(n) elem is the ByteString membership predicate.

notElem :: Word8 -> ByteString -> Bool

O(n) notElem is the inverse of elem

Searching with a predicate

find :: (Word8 -> Bool) -> ByteString -> Maybe Word8

O(n) The find function takes a predicate and a ByteString, and returns the first element in matching the predicate, or Nothing if there is no such element.

 find f p = case findIndex f p of Just n -> Just (p ! n) ; _ -> Nothing

filter :: (Word8 -> Bool) -> ByteString -> ByteString

O(n) filter, applied to a predicate and a ByteString, returns a ByteString containing those characters that satisfy the predicate.

partition :: (Word8 -> Bool) -> ByteString -> (ByteString, ByteString)

O(n) The partition function takes a predicate a ByteString and returns the pair of ByteStrings with elements which do and do not satisfy the predicate, respectively; i.e.,

 partition p bs == (filter p xs, filter (not . p) xs)

Indexing ByteStrings

index :: ByteString -> Int64 -> Word8

O(c) ByteString index (subscript) operator, starting from 0.

elemIndex :: Word8 -> ByteString -> Maybe Int64

O(n) The elemIndex function returns the index of the first element in the given ByteString which is equal to the query element, or Nothing if there is no such element. This implementation uses memchr(3).

elemIndices :: Word8 -> ByteString -> [Int64]

O(n) The elemIndices function extends elemIndex, by returning the indices of all elements equal to the query element, in ascending order. This implementation uses memchr(3).

findIndex :: (Word8 -> Bool) -> ByteString -> Maybe Int64

The findIndex function takes a predicate and a ByteString and returns the index of the first element in the ByteString satisfying the predicate.

findIndices :: (Word8 -> Bool) -> ByteString -> [Int64]

The findIndices function extends findIndex, by returning the indices of all elements satisfying the predicate, in ascending order.

count :: Word8 -> ByteString -> Int64

count returns the number of times its argument appears in the ByteString

 count = length . elemIndices

But more efficiently than using length on the intermediate list.

Zipping and unzipping ByteStrings

zip :: ByteString -> ByteString -> [(Word8, Word8)]

O(n) zip takes two ByteStrings and returns a list of corresponding pairs of bytes. If one input ByteString is short, excess elements of the longer ByteString are discarded. This is equivalent to a pair of unpack operations.

zipWith :: (Word8 -> Word8 -> a) -> ByteString -> ByteString -> [a]

zipWith generalises zip by zipping with the function given as the first argument, instead of a tupling function. For example, zipWith (+) is applied to two ByteStrings to produce the list of corresponding sums.

unzip :: [(Word8, Word8)] -> (ByteString, ByteString)

O(n) unzip transforms a list of pairs of bytes into a pair of ByteStrings. Note that this performs two pack operations.

Ordered ByteStrings

Low level conversions

Copying ByteStrings

copy :: ByteString -> ByteString

O(n) Make a copy of the ByteString with its own storage. This is mainly useful to allow the rest of the data pointed to by the ByteString to be garbage collected, for example if a large string has been read in, and only a small part of it is needed in the rest of the program.

I/O with ByteStrings

Standard input and output

getContents :: IO ByteString

getContents. Equivalent to hGetContents stdin. Will read lazily

putStr :: ByteString -> IO ()

Write a ByteString to stdout

putStrLn :: ByteString -> IO ()

Deprecated: Use Data.ByteString.Lazy.Char8.putStrLn instead. (Functions that rely on ASCII encodings belong in Data.ByteString.Lazy.Char8)

Write a ByteString to stdout, appending a newline byte

interact :: (ByteString -> ByteString) -> IO ()

The interact function takes a function of type ByteString -> ByteString as its argument. The entire input from the standard input device is passed to this function as its argument, and the resulting string is output on the standard output device.

Files

readFile :: FilePath -> IO ByteString

Read an entire file lazily into a ByteString. The Handle will be held open until EOF is encountered.

writeFile :: FilePath -> ByteString -> IO ()

Write a ByteString to a file.

appendFile :: FilePath -> ByteString -> IO ()

Append a ByteString to a file.

I/O with Handles

hGetContents :: Handle -> IO ByteString

Read entire handle contents lazily into a ByteString. Chunks are read on demand, using the default chunk size.

Once EOF is encountered, the Handle is closed.

Note: the Handle should be placed in binary mode with hSetBinaryMode for hGetContents to work correctly.

hGet :: Handle -> Int -> IO ByteString

Read n bytes into a ByteString, directly from the specified Handle.

hGetNonBlocking :: Handle -> Int -> IO ByteString

hGetNonBlocking is similar to hGet, except that it will never block waiting for data to become available, instead it returns only whatever data is available. If there is no data available to be read, hGetNonBlocking returns empty.

Note: on Windows and with Haskell implementation other than GHC, this function does not work correctly; it behaves identically to hGet.

hPut :: Handle -> ByteString -> IO ()

Outputs a ByteString to the specified Handle.

hPutNonBlocking :: Handle -> ByteString -> IO ByteString

Similar to hPut except that it will never block. Instead it returns any tail that did not get written. This tail may be empty in the case that the whole string was written, or the whole original string if nothing was written. Partial writes are also possible.

Note: on Windows and with Haskell implementation other than GHC, this function does not work correctly; it behaves identically to hPut.

hPutStr :: Handle -> ByteString -> IO ()

A synonym for hPut, for compatibility