base- Core data structures and operations
Copyright(c) The University of Glasgow 1994-2009
Licensesee libraries/base/LICENSE
Safe HaskellSafe



Basic types for the implementation of IO Handles.

The API of this module is unstable and not meant to be consumed by the general public. If you absolutely must depend on it, make sure to use a tight upper bound, e.g., base < 4.X rather than base < 5, because the interface can change rapidly without much warning.



data Handle Source #

Haskell defines operations to read and write characters from and to files, represented by values of type Handle. Each value of this type is a handle: a record used by the Haskell run-time system to manage I/O with file system objects. A handle has at least the following properties:

  • whether it manages input or output or both;
  • whether it is open, closed or semi-closed;
  • whether the object is seekable;
  • whether buffering is disabled, or enabled on a line or block basis;
  • a buffer (whose length may be zero).

Most handles will also have a current I/O position indicating where the next input or output operation will occur. A handle is readable if it manages only input or both input and output; likewise, it is writable if it manages only output or both input and output. A handle is open when first allocated. Once it is closed it can no longer be used for either input or output, though an implementation cannot re-use its storage while references remain to it. Handles are in the Show and Eq classes. The string produced by showing a handle is system dependent; it should include enough information to identify the handle for debugging. A handle is equal according to == only to itself; no attempt is made to compare the internal state of different handles for equality.


Instances details
Show Handle

@since base-

Instance details

Defined in GHC.Internal.IO.Handle.Types

Eq Handle

@since base-

Instance details

Defined in GHC.Internal.IO.Handle.Types

data Handle__ Source #


(RawIO dev, IODevice dev, BufferedIO dev, Typeable dev) => Handle__ 


data BufferMode Source #

Three kinds of buffering are supported: line-buffering, block-buffering or no-buffering. These modes have the following effects. For output, items are written out, or flushed, from the internal buffer according to the buffer mode:

  • line-buffering: the entire output buffer is flushed whenever a newline is output, the buffer overflows, a hFlush is issued, or the handle is closed.
  • block-buffering: the entire buffer is written out whenever it overflows, a hFlush is issued, or the handle is closed.
  • no-buffering: output is written immediately, and never stored in the buffer.

An implementation is free to flush the buffer more frequently, but not less frequently, than specified above. The output buffer is emptied as soon as it has been written out.

Similarly, input occurs according to the buffer mode for the handle:

  • line-buffering: when the buffer for the handle is not empty, the next item is obtained from the buffer; otherwise, when the buffer is empty, characters up to and including the next newline character are read into the buffer. No characters are available until the newline character is available or the buffer is full.
  • block-buffering: when the buffer for the handle becomes empty, the next block of data is read into the buffer.
  • no-buffering: the next input item is read and returned. The hLookAhead operation implies that even a no-buffered handle may require a one-character buffer.

The default buffering mode when a handle is opened is implementation-dependent and may depend on the file system object which is attached to that handle. For most implementations, physical files will normally be block-buffered and terminals will normally be line-buffered.



buffering is disabled if possible.


line-buffering should be enabled if possible.

BlockBuffering (Maybe Int)

block-buffering should be enabled if possible. The size of the buffer is n items if the argument is Just n and is otherwise implementation-dependent.

data BufferCodec from to state Source #




  • 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 -> State# RealWorld -> (# State# RealWorld, 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 base-

  • 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 ()

Bundled Patterns

pattern BufferCodec :: CodeBuffer from to -> (Buffer from -> Buffer to -> IO (Buffer from, Buffer to)) -> IO () -> IO state -> (state -> IO ()) -> BufferCodec from to state 

data NewlineMode Source #

Specifies the translation, if any, of newline characters between internal Strings and the external file or stream. Haskell Strings are assumed to represent newlines with the '\n' character; the newline mode specifies how to translate '\n' on output, and what to translate into '\n' on input.




data Newline Source #

The representation of a newline in the external file or stream.




Instances details
Read Newline

@since base-

Instance details

Defined in GHC.Internal.IO.Handle.Types

Show Newline

@since base-

Instance details

Defined in GHC.Internal.IO.Handle.Types

Eq Newline

@since base-

Instance details

Defined in GHC.Internal.IO.Handle.Types

Ord Newline

@since base-

Instance details

Defined in GHC.Internal.IO.Handle.Types

nativeNewline :: Newline Source #

The native newline representation for the current platform: LF on Unix systems, CRLF on Windows.

universalNewlineMode :: NewlineMode Source #

Map '\r\n' into '\n' on input, and '\n' to the native newline representation on output. This mode can be used on any platform, and works with text files using any newline convention. The downside is that readFile >>= writeFile might yield a different file.

universalNewlineMode  = NewlineMode { inputNL  = CRLF,
                                      outputNL = nativeNewline }

noNewlineTranslation :: NewlineMode Source #

Do no newline translation at all.

noNewlineTranslation  = NewlineMode { inputNL  = LF, outputNL = LF }

nativeNewlineMode :: NewlineMode Source #

Use the native newline representation on both input and output

nativeNewlineMode  = NewlineMode { inputNL  = nativeNewline
                                   outputNL = nativeNewline }