ByteString provides a faster and more memory efficient data type than
[Word8] for processing raw bytes. By creating a Unicode data type similar to
ByteString that deals in units of characters instead of units of bytes we can achieve similar performance improvements over
String for text processing. A Unicode data type also removes the error prone process of keeping track of strings encoded as raw bytes stored in
ByteStrings. Using functions such as
length on a Unicode string just works even though different encodings use different numbers of bytes to represent a character.
A new module,
Text.Unicode, defines the efficient Unicode string data type:
Functions to encode and decode Unicode strings to and from
ByteStrings are provided together with
Data.List like functions.
data Encoding = Ascii | Utf8 | Utf16 | Iso88591 decode :: Encoding -> ByteString -> UnicodeString encode :: Encoding -> UnicodeString -> ByteString
ByteString is decoded using the wrong codec several error handling strategies are possible:
- An exception is raised using
error. This may be fine for many cases.
- Unknown byte sequences are replaced with some character (e.g.
'?'). This is useful for debugging, etc. where some input/output is better than none.
- The decode function returns values of type
Either CodecError UnicodeStringwhere
CodecErrorcontains some useful error information.
The final API should provide at least a few error handling strategies of different sophistication.
One example in this design space for error handling is this iconv library: http://haskell.org/~duncan/iconv/ It provides a most general conversion function with type:
:: EncodingName -- ^ Name of input string encoding -> EncodingName -- ^ Name of output string encoding -> L.ByteString -- ^ Input text -> [Span] data Span = -- | An ordinary output span in the target encoding Span !S.ByteString -- | An error in the conversion process. If this occurs it will be the -- last span. | ConversionError !ConversionError
Then the other simpler error handling strategies are wrappers over this interface. One converts strictly and returns Either L.ByteString ConversionError, the other converts lazily and uses exceptions. There is also a fuzzy mode where conversion errors are ignored or transliterated, using similar replacement characters or
Several I/O functions that deal with
UnicodeStrings might be needed. All text based I/O should require an explicit encoding or use the default encoding (as set by the user's locale). Example:
readFile :: Encoding -> FilePath -> UnicodeString
Data.List API is already duplicated in large parts in
Data.ByteString. It will be duplicated again here. Will keeping the APIs in sync be a huge pain in the future?
New I/O library
How many new I/O functions are needed? Would it be enough to use
ByteString's I/O interface:
import qualified Data.ByteString as B echo = do content <- decode Utf8 <$> B.readFile "myfile.txt" B.putStrLn $ encode Utf8 content
Should the encoding used to represent Unicode code points be included in the type?
data Encoding e => UnicodeString e
This might save some recoding as opposed to always using the same internal encoding for
UnicodeString. It's necessary that UnicodeString can be used between different text processing libraries. Is this possible or will any library end up specifying a particular value for
Encoding e and thus make it harder to interact with that library?
This approach is used by
Python 3000 will see an overhaul of their Unicode approach, including a new
bytes type, a merge of
unicode and a new I/O library. This proposals takes many ideas from that overhaul. The relevant PEPs are: