| Copyright | (c) Duncan Coutts 2012-2013 |
|---|---|
| License | BSD-style |
| Maintainer | duncan@community.haskell.org |
| Stability | stable |
| Portability | ghc only |
| Safe Haskell | Trustworthy |
| Language | Haskell98 |
Data.ByteString.Short
Contents
Description
A compact representation suitable for storing short byte strings in memory.
In typical use cases it can be imported alongside Data.ByteString, e.g.
import qualified Data.ByteString as B
import qualified Data.ByteString.Short as B
(ShortByteString, toShort, fromShort)Other ShortByteString operations clash with Data.ByteString or Prelude
functions however, so they should be imported qualified with a different
alias e.g.
import qualified Data.ByteString.Short as B.Short
Synopsis
- data ShortByteString
- toShort :: ByteString -> ShortByteString
- fromShort :: ShortByteString -> ByteString
- pack :: [Word8] -> ShortByteString
- unpack :: ShortByteString -> [Word8]
- empty :: ShortByteString
- null :: ShortByteString -> Bool
- length :: ShortByteString -> Int
- index :: ShortByteString -> Int -> Word8
- packCString :: CString -> IO ShortByteString
- packCStringLen :: CStringLen -> IO ShortByteString
- useAsCString :: ShortByteString -> (CString -> IO a) -> IO a
- useAsCStringLen :: ShortByteString -> (CStringLen -> IO a) -> IO a
The ShortByteString type
data ShortByteString Source #
A compact representation of a Word8 vector.
It has a lower memory overhead than a ByteString and does not
contribute to heap fragmentation. It can be converted to or from a
ByteString (at the cost of copying the string data). It supports very few
other operations.
It is suitable for use as an internal representation for code that needs
to keep many short strings in memory, but it should not be used as an
interchange type. That is, it should not generally be used in public APIs.
The ByteString type is usually more suitable for use in interfaces; it is
more flexible and it supports a wide range of operations.
Instances
Memory overhead
With GHC, the memory overheads are as follows, expressed in words and in bytes (words are 4 and 8 bytes on 32 or 64bit machines respectively).
ByteStringunshared: 9 words; 36 or 72 bytes.ByteStringshared substring: 5 words; 20 or 40 bytes.ShortByteString: 4 words; 16 or 32 bytes.
For the string data itself, both ShortByteString and ByteString use
one byte per element, rounded up to the nearest word. For example,
including the overheads, a length 10 ShortByteString would take
16 + 12 = 28 bytes on a 32bit platform and 32 + 16 = 48 bytes on a
64bit platform.
These overheads can all be reduced by 1 word (4 or 8 bytes) when the
ShortByteString or ByteString is unpacked into another constructor.
For example:
data ThingId = ThingId {-# UNPACK #-} !Int
{-# UNPACK #-} !ShortByteStringThis will take 1 + 1 + 3 words (the ThingId constructor +
unpacked Int + unpacked ShortByteString), plus the words for the
string data.
Heap fragmentation
With GHC, the ByteString representation uses pinned memory,
meaning it cannot be moved by the GC. This is usually the right thing to
do for larger strings, but for small strings using pinned memory can
lead to heap fragmentation which wastes space. The ShortByteString
type (and the Text type from the text package) use unpinned memory
so they do not contribute to heap fragmentation. In addition, with GHC,
small unpinned strings are allocated in the same way as normal heap
allocations, rather than in a separate pinned area.
Conversions
toShort :: ByteString -> ShortByteString Source #
O(n). Convert a ByteString into a ShortByteString.
This makes a copy, so does not retain the input string.
fromShort :: ShortByteString -> ByteString Source #
O(n). Convert a ShortByteString into a ByteString.
pack :: [Word8] -> ShortByteString Source #
O(n). Convert a list into a ShortByteString
unpack :: ShortByteString -> [Word8] Source #
O(n). Convert a ShortByteString into a list.
Other operations
empty :: ShortByteString Source #
O(1). The empty ShortByteString.
null :: ShortByteString -> Bool Source #
O(1) Test whether a ShortByteString is empty.
length :: ShortByteString -> Int Source #
O(1) The length of a ShortByteString.
index :: ShortByteString -> Int -> Word8 Source #
O(1) ShortByteString index (subscript) operator, starting from 0.
Low level conversions
Packing CStrings and pointers
packCString :: CString -> IO ShortByteString Source #
O(n). Construct a new ShortByteString from a CString. The
resulting ShortByteString is an immutable copy of the original
CString, and is managed on the Haskell heap. The original
CString must be null terminated.
Since: 0.10.10.0
packCStringLen :: CStringLen -> IO ShortByteString Source #
O(n). Construct a new ShortByteString from a CStringLen. The
resulting ShortByteString is an immutable copy of the original CStringLen.
The ShortByteString is a normal Haskell value and will be managed on the
Haskell heap.
Since: 0.10.10.0
Using ByteStrings as CStrings
useAsCString :: ShortByteString -> (CString -> IO a) -> IO a Source #
O(n) construction. Use a ShortByteString 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.
Since: 0.10.10.0
useAsCStringLen :: ShortByteString -> (CStringLen -> IO a) -> IO a Source #
O(n) construction. Use a ShortByteString with a function requiring a CStringLen.
As for useAsCString this function makes a copy of the original ShortByteString.
It must not be stored or used after the subcomputation finishes.
Since: 0.10.10.0