GHC/IO/Encoding/Types.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE NoImplicitPrelude, ExistentialQuantification #-}
    {-# OPTIONS_GHC -funbox-strict-fields #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.IO.Encoding.Types
    -- Copyright   :  (c) The University of Glasgow, 2008-2009
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable
    --
    -- Types for text encoding/decoding
    --
    -----------------------------------------------------------------------------

    module GHC.IO.Encoding.Types (
        BufferCodec(..),
        TextEncoding(..),
        TextEncoder, TextDecoder,
        CodeBuffer, EncodeBuffer, DecodeBuffer,
        CodingProgress(..)
      ) where

    import GHC.Base
    import GHC.Word
    import GHC.Show
    -- import GHC.IO
    import GHC.IO.Buffer

    -- -----------------------------------------------------------------------------
    -- Text encoders/decoders

    data BufferCodec from to state = BufferCodec {
      encode :: CodeBuffer from 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.
       --
       -- If multiple CodingProgress returns are possible, OutputUnderflow must be
       -- preferred to InvalidSequence. This allows GHC's IO library to assume that
       -- if we observe InvalidSequence there is at least a single element available
       -- in the output buffer.
       --
       -- The fact that as many elements as possible are translated 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.

      recover :: Buffer from -> Buffer to -> IO (Buffer from, Buffer to),
       -- ^ The @recover@ function is used to continue decoding
       -- in the presence of invalid or unrepresentable sequences. This includes
       -- both those detected by @encode@ returning @InvalidSequence@ and those
       -- that occur because the input byte sequence appears to be truncated.
       --
       -- Progress will usually be made by skipping the first element of the @from@
       -- buffer. This function should only be called if you are certain that you
       -- wish to do this skipping and if the @to@ buffer has at least one element
       -- of free space. Because this function deals with decoding failure, it assumes
       -- that the from buffer has at least one element.
       --
       -- @recover@ may raise an exception rather than skipping anything.
       --
       -- Currently, some implementations of @recover@ may mutate the input buffer.
       -- In particular, this feature is used to implement transliteration.
       --
       -- @since 4.4.0.0

      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 ()
       -- restore the state of the codec using the state from a previous
       -- call to 'getState'.
     }

    type CodeBuffer from to = Buffer from -> Buffer to -> IO (CodingProgress, Buffer from, Buffer to)
    type DecodeBuffer = CodeBuffer Word8 Char
    type EncodeBuffer = CodeBuffer Char Word8

    type TextDecoder state = BufferCodec Word8 CharBufElem state
    type TextEncoder state = BufferCodec CharBufElem Word8 state

    -- | 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'.
    data TextEncoding
      = forall dstate estate . TextEncoding  {
            textEncodingName :: String,
                       -- ^ a string that can be passed to 'mkTextEncoding' to
                       -- create an equivalent 'TextEncoding'.
            mkTextDecoder :: IO (TextDecoder dstate),
                       -- ^ Creates a means of decoding bytes into characters: the result must not
                       -- be shared between several byte sequences or simultaneously across threads
            mkTextEncoder :: IO (TextEncoder estate)
                       -- ^ Creates a means of encode characters into bytes: the result must not
                       -- be shared between several character sequences or simultaneously across threads
      }

    -- | @since 4.3.0.0
    instance Show TextEncoding where
      -- | Returns the value of 'textEncodingName'
      show te = textEncodingName te

    -- | @since 4.4.0.0
    data CodingProgress = InputUnderflow  -- ^ Stopped because the input contains insufficient available elements,
                                          -- or all of the input sequence has been successfully translated.
                        | OutputUnderflow -- ^ Stopped because the output contains insufficient free elements
                        | InvalidSequence -- ^ Stopped because there are sufficient free elements in the output
                                          -- to output at least one encoded ASCII character, but the input contains
                                          -- an invalid or unrepresentable sequence
                        deriving (Eq, Show)