base- Basic libraries




Text codecs for I/O



data BufferCodec from to state Source




encode :: Buffer from -> Buffer to -> IO (Buffer from, Buffer to)

The encode function translates elements of the buffer from to the buffer to. It should translate as many elements as possible given the sizes of the buffers, including translating zero elements if there is either not enough room in to, or from does not contain a complete multibyte sequence.

encode should raise an exception if, and only if, from begins with an illegal sequence, or the first element of from is not representable in the encoding of to. That is, if any elements can be successfully translated before an error is encountered, then encode should translate as much as it can and not throw an exception. This behaviour is used by the IO library in order to report translation errors at the point they actually occur, rather than when the buffer is translated.

close :: IO ()

Resources associated with the encoding may now be released. The encode function may not be called again after calling close.

getState :: IO state

Return the current state of the codec.

Many codecs are not stateful, and in these case the state can be represented as '()'. Other codecs maintain a state. For example, UTF-16 recognises a BOM (byte-order-mark) character at the beginning of the input, and remembers thereafter whether to use big-endian or little-endian mode. In this case, the state of the codec would include two pieces of information: whether we are at the beginning of the stream (the BOM only occurs at the beginning), and if not, whether to use the big or little-endian encoding.

setState :: state -> IO ()

data TextEncoding Source

A TextEncoding is a specification of a conversion scheme between sequences of bytes and sequences of Unicode characters.

For example, UTF-8 is an encoding of Unicode characters into a sequence of bytes. The TextEncoding for UTF-8 is utf8.


forall dstate estate . TextEncoding 


textEncodingName :: String

a string that can be passed to mkTextEncoding to create an equivalent TextEncoding.

mkTextDecoder :: IO (TextDecoder dstate)
mkTextEncoder :: IO (TextEncoder estate)


latin1 :: TextEncodingSource

The Latin1 (ISO8859-1) encoding. This encoding maps bytes directly to the first 256 Unicode code points, and is thus not a complete Unicode encoding. An attempt to write a character greater than '\255' to a Handle using the latin1 encoding will result in an error.

utf8 :: TextEncodingSource

The UTF-8 Unicode encoding

utf8_bom :: TextEncodingSource

The UTF-8 Unicode encoding, with a byte-order-mark (BOM; the byte sequence 0xEF 0xBB 0xBF). This encoding behaves like utf8, except that on input, the BOM sequence is ignored at the beginning of the stream, and on output, the BOM sequence is prepended.

The byte-order-mark is strictly unnecessary in UTF-8, but is sometimes used to identify the encoding of a file.

utf16 :: TextEncodingSource

The UTF-16 Unicode encoding (a byte-order-mark should be used to indicate endianness).

utf16le :: TextEncodingSource

The UTF-16 Unicode encoding (litte-endian)

utf16be :: TextEncodingSource

The UTF-16 Unicode encoding (big-endian)

utf32 :: TextEncodingSource

The UTF-32 Unicode encoding (a byte-order-mark should be used to indicate endianness).

utf32le :: TextEncodingSource

The UTF-32 Unicode encoding (litte-endian)

utf32be :: TextEncodingSource

The UTF-32 Unicode encoding (big-endian)

localeEncoding :: TextEncodingSource

The Unicode encoding of the current locale

mkTextEncoding :: String -> IO TextEncodingSource

Look up the named Unicode encoding. May fail with

  • isDoesNotExistError if the encoding is unknown

The set of known encodings is system-dependent, but includes at least:

  • UTF-8
  • UTF-16, UTF-16BE, UTF-16LE
  • UTF-32, UTF-32BE, UTF-32LE

On systems using GNU iconv (e.g. Linux), there is additional notation for specifying how illegal characters are handled:

  • a suffix of //IGNORE, e.g. UTF-8//IGNORE, will cause all illegal sequences on input to be ignored, and on output will drop all code points that have no representation in the target encoding.
  • a suffix of //TRANSLIT will choose a replacement character for illegal sequences or code points.

On Windows, you can access supported code pages with the prefix CP; for example, "CP1250".