Data.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 string 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. It is about 40% faster than groupBy (==)

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

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

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

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

O(n) Concatenate a list of ByteStrings.

Map a function over a ByteString and concatenate the results

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

O(n) notElem is the inverse of elem

Similar to dropWhileEnd, drops the longest (possibly empty) suffix of elements satisfying the predicate and returns the remainder.

dropWhileEnd p is equivalent to reverse . dropWhile p . reverse.

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).

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).

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

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

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

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

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.

O(n) 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.

Check whether one string is a substring of another.

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

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

However, the real implementation uses memcmp to compare the end of the string only, with no reverse required..

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)

O(1) Convert a Word8 into a ByteString

O(n) Sort a ByteString efficiently, using counting sort.

O(n) The stripPrefix function takes two ByteStrings and returns Just the remainder of the second iff the first is its prefix, and otherwise Nothing.

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

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

O(n), where n is the length of the result. 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 the next byte in the string, and b is the seed value for further production.

Examples:

   unfoldr (\x -> if x <= 5 then Just (x, x + 1) else Nothing) 0
== pack [0, 1, 2, 3, 4, 5]

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.

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.

O(1) ByteString index, starting from 0, that returns Just if:

0 <= n < length bs

Similar to break, returns the longest (possibly empty) prefix of elements which do not satisfy the predicate and the remainder of the string.

break p is equivalent to span (not . p) and to (takeWhile (not . p) &&& dropWhile (not . p)).

Under GHC, a rewrite rule will transform break (==) into a call to the specialised breakByte:

break ((==) x) = breakByte x
break (==x) = breakByte x

O(1) drop n xs returns the suffix of xs after the first n elements, or empty if n > length xs.

Similar to dropWhile, drops the longest (possibly empty) prefix of elements satisfying the predicate and returns the remainder.

foldl1' is like foldl1, but strict in the accumulator. An exception will be thrown in the case of an empty ByteString.

O(1) Extract the first element of a ByteString, which must be non-empty. An exception will be thrown in the case of an empty ByteString.

This is a partial function, consider using uncons instead.

O(1) Returns all the elements of a ByteString except the last one. An exception will be thrown in the case of an empty ByteString.

This is a partial function, consider using unsnoc instead.

O(1) Extract the last element of a ByteString, which must be finite and non-empty. An exception will be thrown in the case of an empty ByteString.

This is a partial function, consider using unsnoc instead.

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

replicate w c = fst (unfoldrN w (\u -> Just (u,u)) c)

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

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

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

Note that

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

scanl1 is a variant of scanl that has no starting value argument.

scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]

scanr is similar to foldr, but returns a list of successive reduced values from the right.

scanr f z [..., x{n-1}, xn] == [..., x{n-1} `f` (xn `f` z), xn `f` z, z]

Note that

head (scanr f z xs) == foldr f z xs
last (scanr f z xs) == z

scanr1 is a variant of scanr that has no starting value argument.

Similar to span, returns the longest (possibly empty) prefix of elements satisfying the predicate and the remainder of the string.

span p is equivalent to break (not . p) and to (takeWhile p &&& dropWhile p).

O(1) splitAt n xs is equivalent to (take n xs, drop n xs).

O(1) Extract the elements after the head of a ByteString, which must be non-empty. An exception will be thrown in the case of an empty ByteString.

This is a partial function, consider using uncons instead.

O(1) take n, applied to a ByteString xs, returns the prefix of xs of length n, or xs itself if n > length xs.

Similar to takeWhile, returns the longest (possibly empty) prefix of elements satisfying the predicate.

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

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

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

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.

Append a ByteString to a file.

getContents. Read stdin strictly. Equivalent to hGetContents stdin The Handle is closed after the contents have been read.

Read a line from stdin.

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.

Write a ByteString to stdout.

Read an entire file strictly into a ByteString.

Write a ByteString to a file.

O(1) The empty ByteString

O(n) elem is the ByteString membership predicate.

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' is like foldl, but strict in the accumulator.

foldl1 is a variant of foldl that has no starting value argument, and thus must be applied to non-empty ByteStrings. An exception will be thrown in the case of an empty ByteString.

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.

foldr' is like foldr, but strict in the accumulator.

foldr1 is a variant of foldr that has no starting value argument, and thus must be applied to non-empty ByteStrings An exception will be thrown in the case of an empty ByteString.

O(1) length returns the length of a ByteString as an Int.

O(n) maximum returns the maximum value from a ByteString An exception will be thrown in the case of an empty ByteString.

O(n) minimum returns the minimum value from a ByteString An exception will be thrown in the case of an empty ByteString.

O(1) Test whether a ByteString is empty.

O(1) ByteString index (subscript) operator, starting from 0.

This is a partial function, consider using indexMaybe instead.

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

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.

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

Outputs a ByteString to the specified Handle.

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

O(n) Append two ByteStrings

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

For applications with large numbers of string literals, pack can be a bottleneck. In such cases, consider using unsafePackAddress (GHC only).

O(n) Like unfoldr, unfoldrN builds a ByteString from a seed value. However, the length of the result is limited by the first argument to unfoldrN. This function is more efficient than unfoldr when the maximum length of the result is known.

The following equation relates unfoldrN and unfoldr:

fst (unfoldrN n f s) == take n (unfoldr f s)

O(n) cons is analogous to (:) for lists, but of different complexity, as it requires making a copy.

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

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.

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.

O(1) dropEnd n xs is equivalent to take (length xs - n) xs. Drops n elements from end of bytestring.

>>> dropEnd 3 "abcdefg""abcd">>> dropEnd 0 "abcdefg""abcdefg">>> dropEnd 4 "abc"""

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

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

split 10  "a\nb\nd\ne" == ["a","b","d","e"]   -- fromEnum '\n' == 10
split 97  "aXaXaXa"    == ["","X","X","X",""] -- fromEnum 'a' == 97
split 120 "x"          == ["",""]             -- fromEnum 'x' == 120
split undefined ""     == []                  -- and not [""]

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.

O(n) The stripSuffix function takes two ByteStrings and returns Just the remainder of the second iff the first is its suffix, and otherwise Nothing.

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

O(1) takeEnd n xs is equivalent to drop (length xs - n) xs. Takes n elements from end of bytestring.

>>> takeEnd 3 "abcdefg""efg">>> takeEnd 0 "abcdefg""">>> takeEnd 4 "abc""abc"

Returns the longest (possibly empty) suffix of elements satisfying the predicate.

takeWhileEnd p is equivalent to reverse . takeWhile p . reverse.

O(1) Convert a StrictByteString into a LazyByteString.

O(n) Convert a LazyByteString into a StrictByteString.

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

foldr1' is a variant of foldr1, but is strict in the accumulator.

Type synonym for the strict flavour of ByteString.

Returns the longest (possibly empty) suffix of elements which do not satisfy the predicate and the remainder of the string.

breakEnd p is equivalent to spanEnd (not . p) and to (dropWhileEnd (not . p) &&& takeWhileEnd (not . p)).

Break a string on a substring, returning a pair of the part of the string prior to the match, and the rest of the string.

The following relationships hold:

break (== c) l == breakSubstring (singleton c) l

For example, to tokenise a string, dropping delimiters:

tokenise x y = h : if null t then [] else tokenise x (drop (length x) t)
    where (h,t) = breakSubstring x y

To skip to the first occurrence of a string:

snd (breakSubstring x y)

To take the parts of a string before a delimiter:

fst (breakSubstring x y)

Note that calling `breakSubstring x` does some preprocessing work, so you should avoid unnecessarily duplicating breakSubstring calls with the same pattern.

O(1) ByteString index, starting from 0, that returns Just if:

0 <= n < length bs

O(n) Check whether a ByteString represents valid UTF-8.

O(n). Construct a new ByteString from a CString. The resulting ByteString is an immutable copy of the original CString, and is managed on the Haskell heap. The original CString must be null terminated.

O(n). Construct a new ByteString from a CStringLen. The resulting ByteString is an immutable copy of the original CStringLen. The ByteString is a normal Haskell value and will be managed on the Haskell heap.

Returns the longest (possibly empty) suffix of elements satisfying the predicate and the remainder of the string.

spanEnd p is equivalent to breakEnd (not . p) and to (dropWhileEnd p &&& takeWhileEnd p).

We have

spanEnd (not . isSpace) "x y z" == ("x y ", "z")

and

spanEnd (not . isSpace) ps
   ==
let (x, y) = span (not . isSpace) (reverse ps) in (reverse y, reverse x)

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 (==97) "aabbaca" == ["","","bb","c",""] -- fromEnum 'a' == 97
splitWith undefined ""     == []                  -- and not [""]

O(n) construction Use a ByteString with a function requiring a null-terminated CString. The CString is a copy and will be freed automatically; it must not be stored or used after the subcomputation finishes.

O(n) construction Use a ByteString with a function requiring a CStringLen. As for useAsCString this function makes a copy of the original ByteString. It must not be stored or used after the subcomputation finishes.

Beware that this function is not required to add a terminating NUL byte at the end of the CStringLen it provides. If you need to construct a pointer to a null-terminated sequence, use useAsCString (and measure length independently if desired).

Read a handle's entire contents strictly into a ByteString.

This function reads chunks at a time, increasing the chunk size on each read. The final string is then reallocated to the appropriate size. For files > half of available memory, this may lead to memory exhaustion. Consider using readFile in this case.

The Handle is closed once the contents have been read, or if an exception is thrown.

Read a line from a handle

A synonym for hPut, for compatibility

O(n) The elemIndexEnd function returns the last index of the element in the given ByteString which is equal to the query element, or Nothing if there is no such element. The following holds:

elemIndexEnd c xs = case elemIndex c (reverse xs) of
  Nothing -> Nothing
  Just i  -> Just (length xs - 1 - i)

O(n) The findIndexEnd function takes a predicate and a ByteString and returns the index of the last element in the ByteString satisfying the predicate.

Convert a FilePath to a ByteString.

The FilePath type is expected to use the file system encoding as reported by getFileSystemEncoding. This encoding allows for round-tripping of arbitrary data on platforms that allow arbitrary bytes in their paths. This conversion function does the same thing that openFile would do when decoding the FilePath.

This function is in IO because the file system encoding can be changed. If the encoding can be assumed to be constant in your use case, you may invoke this function via unsafePerformIO.

Read a ByteString directly from the specified Handle. This is far more efficient than reading the characters into a String and then using pack. First argument is the Handle to read from, and the second is the number of bytes to read. It returns the bytes read, up to n, or empty if EOF has been reached.

hGet is implemented in terms of hGetBuf.

If the handle is a pipe or socket, and the writing end is closed, hGet will behave as if EOF was reached.

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.

Like hGet, except that a shorter ByteString may be returned if there are not enough bytes immediately available to satisfy the whole request. hGetSome only blocks if there is no data available, and EOF has not yet been reached.

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.

A specialised version of zipWith for the common case of a simultaneous map over two ByteStrings, to build a 3rd.

Convert a ByteString to a FilePath.

This function uses the file system encoding, and resulting FilePaths can be safely used with standard IO functions and will reference the correct path in the presence of arbitrary non-UTF-8 encoded paths.

This function is in IO because the file system encoding can be changed. If the encoding can be assumed to be constant in your use case, you may invoke this function via unsafePerformIO.