    {-# OPTIONS_GHC -XNoImplicitPrelude -XBangPatterns #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.IO.Device
    -- Copyright   :  (c) The University of Glasgow, 1994-2008
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable
    --
    -- Type classes for I/O providers.
    --
    -----------------------------------------------------------------------------

    module GHC.IO.Device (
        RawIO(..),
        IODevice(..),
        IODeviceType(..),
        SeekMode(..)
      ) where

    #ifdef __GLASGOW_HASKELL__
    import GHC.Base
    import GHC.Word
    import GHC.Arr
    import GHC.Enum
    import GHC.Read
    import GHC.Show
    import GHC.Ptr
    import Data.Maybe
    import GHC.Num
    import GHC.IO
    import {-# SOURCE #-} GHC.IO.Exception ( unsupportedOperation )
    #endif
    #ifdef __NHC__
    import Foreign
    import Ix
    import Control.Exception.Base
    unsupportedOperation = userError "unsupported operation"
    #endif

    -- | A low-level I/O provider where the data is bytes in memory.
    class RawIO a where
      -- | Read up to the specified number of bytes, returning the number
      -- of bytes actually read.  This function should only block if there
      -- is no data available.  If there is not enough data available,
      -- then the function should just return the available data. A return
      -- value of zero indicates that the end of the data stream (e.g. end
      -- of file) has been reached.
      read                :: a -> Ptr Word8 -> Int -> IO Int

      -- | Read up to the specified number of bytes, returning the number
      -- of bytes actually read, or 'Nothing' if the end of the stream has
      -- been reached.
      readNonBlocking     :: a -> Ptr Word8 -> Int -> IO (Maybe Int)

      -- | Write the specified number of bytes.
      write               :: a -> Ptr Word8 -> Int -> IO ()

      -- | Write up to the specified number of bytes without blocking.  Returns
      -- the actual number of bytes written.
      writeNonBlocking    :: a -> Ptr Word8 -> Int -> IO Int

    -- | I/O operations required for implementing a 'Handle'.
    class IODevice a where
      -- | @ready dev write msecs@ returns 'True' if the device has data
      -- to read (if @write@ is 'False') or space to write new data (if
      -- @write@ is 'True').  @msecs@ specifies how long to wait, in
      -- milliseconds.
      --
      ready :: a -> Bool -> Int -> IO Bool

      -- | closes the device.  Further operations on the device should
      -- produce exceptions.
      close :: a -> IO ()

      -- | returns 'True' if the device is a terminal or console.
      isTerminal :: a -> IO Bool
      isTerminal _ = return False

      -- | returns 'True' if the device supports 'seek' operations.
      isSeekable :: a -> IO Bool
      isSeekable _ = return False

      -- | seek to the specified position in the data.
      seek :: a -> SeekMode -> Integer -> IO ()
      seek _ _ _ = ioe_unsupportedOperation

      -- | return the current position in the data.
      tell :: a -> IO Integer
      tell _ = ioe_unsupportedOperation

      -- | return the size of the data.
      getSize :: a -> IO Integer
      getSize _ = ioe_unsupportedOperation

      -- | change the size of the data.
      setSize :: a -> Integer -> IO ()
      setSize _ _ = ioe_unsupportedOperation

      -- | for terminal devices, changes whether characters are echoed on
      -- the device.
      setEcho :: a -> Bool -> IO ()
      setEcho _ _ = ioe_unsupportedOperation

      -- | returns the current echoing status.
      getEcho :: a -> IO Bool
      getEcho _ = ioe_unsupportedOperation

      -- | some devices (e.g. terminals) support a "raw" mode where
      -- characters entered are immediately made available to the program.
      -- If available, this operations enables raw mode.
      setRaw :: a -> Bool -> IO ()
      setRaw _ _ = ioe_unsupportedOperation

      -- | returns the 'IODeviceType' corresponding to this device.
      devType :: a -> IO IODeviceType

      -- | duplicates the device, if possible.  The new device is expected
      -- to share a file pointer with the original device (like Unix @dup@).
      dup :: a -> IO a
      dup _ = ioe_unsupportedOperation

      -- | @dup2 source target@ replaces the target device with the source
      -- device.  The target device is closed first, if necessary, and then
      -- it is made into a duplicate of the first device (like Unix @dup2@).
      dup2 :: a -> a -> IO a
      dup2 _ _ = ioe_unsupportedOperation

    ioe_unsupportedOperation :: IO a
    ioe_unsupportedOperation = throwIO unsupportedOperation

    -- | Type of a device that can be used to back a
    -- 'GHC.IO.Handle.Handle' (see also 'GHC.IO.Handle.mkFileHandle'). The
    -- standard libraries provide creation of 'GHC.IO.Handle.Handle's via
    -- Posix file operations with file descriptors (see
    -- 'GHC.IO.Handle.FD.mkHandleFromFD') with FD being the underlying
    -- 'GHC.IO.Device.IODevice' instance.
    --
    -- Users may provide custom instances of 'GHC.IO.Device.IODevice'
    -- which are expected to conform the following rules:

    data IODeviceType
      = Directory -- ^ The standard libraries do not have direct support
                  -- for this device type, but a user implementation is
                  -- expected to provide a list of file names in
                  -- the directory, in any order, separated by @'\0'@
                  -- characters, excluding the @"."@ and @".."@ names. See
                  -- also 'System.Directory.getDirectoryContents'.  Seek
                  -- operations are not supported on directories (other
                  -- than to the zero position).
      | Stream    -- ^ A duplex communications channel (results in
                  -- creation of a duplex 'GHC.IO.Handle.Handle'). The
                  -- standard libraries use this device type when
                  -- creating 'GHC.IO.Handle.Handle's for open sockets.
      | RegularFile -- ^ A file that may be read or written, and also
                    -- may be seekable.
      | RawDevice -- ^ A "raw" (disk) device which supports block binary
                  -- read and write operations and may be seekable only
                  -- to positions of certain granularity (block-
                  -- aligned).
      deriving (Eq)

    -- -----------------------------------------------------------------------------
    -- SeekMode type

    -- | A mode that determines the effect of 'hSeek' @hdl mode i@.
    data SeekMode
      = AbsoluteSeek        -- ^ the position of @hdl@ is set to @i@.
      | RelativeSeek        -- ^ the position of @hdl@ is set to offset @i@
                            -- from the current position.
      | SeekFromEnd         -- ^ the position of @hdl@ is set to offset @i@
                            -- from the end of the file.
        deriving (Eq, Ord, Ix, Enum, Read, Show)