base-4.2.0.0: Basic librariesSource codeContentsIndex
GHC.IO.Handle
Portabilitynon-portable
Stabilityprovisional
Maintainerlibraries@haskell.org
Description
External API for GHC's Handle implementation
Synopsis
data Handle
data BufferMode
= NoBuffering
| LineBuffering
| BlockBuffering (Maybe Int)
mkFileHandle :: (IODevice dev, BufferedIO dev, Typeable dev) => dev -> FilePath -> IOMode -> Maybe TextEncoding -> NewlineMode -> IO Handle
mkDuplexHandle :: (IODevice dev, BufferedIO dev, Typeable dev) => dev -> FilePath -> Maybe TextEncoding -> NewlineMode -> IO Handle
hFileSize :: Handle -> IO Integer
hSetFileSize :: Handle -> Integer -> IO ()
hIsEOF :: Handle -> IO Bool
hLookAhead :: Handle -> IO Char
hSetBuffering :: Handle -> BufferMode -> IO ()
hSetBinaryMode :: Handle -> Bool -> IO ()
hSetEncoding :: Handle -> TextEncoding -> IO ()
hGetEncoding :: Handle -> IO (Maybe TextEncoding)
hFlush :: Handle -> IO ()
hFlushAll :: Handle -> IO ()
hDuplicate :: Handle -> IO Handle
hDuplicateTo :: Handle -> Handle -> IO ()
hClose :: Handle -> IO ()
hClose_help :: Handle__ -> IO (Handle__, Maybe SomeException)
type HandlePosition = Integer
data HandlePosn = HandlePosn Handle HandlePosition
hGetPosn :: Handle -> IO HandlePosn
hSetPosn :: HandlePosn -> IO ()
data SeekMode
= AbsoluteSeek
| RelativeSeek
| SeekFromEnd
hSeek :: Handle -> SeekMode -> Integer -> IO ()
hTell :: Handle -> IO Integer
hIsOpen :: Handle -> IO Bool
hIsClosed :: Handle -> IO Bool
hIsReadable :: Handle -> IO Bool
hIsWritable :: Handle -> IO Bool
hGetBuffering :: Handle -> IO BufferMode
hIsSeekable :: Handle -> IO Bool
hSetEcho :: Handle -> Bool -> IO ()
hGetEcho :: Handle -> IO Bool
hIsTerminalDevice :: Handle -> IO Bool
hSetNewlineMode :: Handle -> NewlineMode -> IO ()
data Newline
= LF
| CRLF
data NewlineMode = NewlineMode {
inputNL :: Newline
outputNL :: Newline
}
nativeNewline :: Newline
noNewlineTranslation :: NewlineMode
universalNewlineMode :: NewlineMode
nativeNewlineMode :: NewlineMode
hShow :: Handle -> IO String
hWaitForInput :: Handle -> Int -> IO Bool
hGetChar :: Handle -> IO Char
hGetLine :: Handle -> IO String
hGetContents :: Handle -> IO String
hPutChar :: Handle -> Char -> IO ()
hPutStr :: Handle -> String -> IO ()
hGetBuf :: Handle -> Ptr a -> Int -> IO Int
hGetBufNonBlocking :: Handle -> Ptr a -> Int -> IO Int
hPutBuf :: Handle -> Ptr a -> Int -> IO ()
hPutBufNonBlocking :: Handle -> Ptr a -> Int -> IO Int
Documentation
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.

GHC note: a Handle will be automatically closed when the garbage collector detects that it has become unreferenced by the program. However, relying on this behaviour is not generally recommended: the garbage collector is unpredictable. If possible, use explicit an explicit hClose to close Handles when they are no longer required. GHC does not currently attempt to free up file descriptors when they have run out, it is your responsibility to ensure that this doesn't happen.

show/hide Instances
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 System.IO.hFlush is issued, or the handle is closed.
  • block-buffering: the entire buffer is written out whenever it overflows, a System.IO.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 System.IO.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.

Constructors
NoBufferingbuffering is disabled if possible.
LineBufferingline-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.
show/hide Instances
mkFileHandleSource
:: (IODevice dev, BufferedIO dev, Typeable dev)
=> deva string describing the Handle, e.g. the file path for a file. Used in error messages.
-> FilePath
-> IOMode
-> Maybe TextEncoding
-> NewlineMode
-> IO Handle
makes a new Handle
mkDuplexHandle :: (IODevice dev, BufferedIO dev, Typeable dev) => dev -> FilePath -> Maybe TextEncoding -> NewlineMode -> IO HandleSource
like mkFileHandle, except that a Handle is created with two independent buffers, one for reading and one for writing. Used for full-dupliex streams, such as network sockets.
hFileSize :: Handle -> IO IntegerSource
For a handle hdl which attached to a physical file, hFileSize hdl returns the size of that file in 8-bit bytes.
hSetFileSize :: Handle -> Integer -> IO ()Source
hSetFileSize hdl size truncates the physical file with handle hdl to size bytes.
hIsEOF :: Handle -> IO BoolSource

For a readable handle hdl, hIsEOF hdl returns True if no further input can be taken from hdl or for a physical file, if the current I/O position is equal to the length of the file. Otherwise, it returns False.

NOTE: hIsEOF may block, because it is the same as calling hLookAhead and checking for an EOF exception.

hLookAhead :: Handle -> IO CharSource

Computation hLookAhead returns the next character from the handle without removing it from the input buffer, blocking until a character is available.

This operation may fail with:

hSetBuffering :: Handle -> BufferMode -> IO ()Source

Computation hSetBuffering hdl mode sets the mode of buffering for handle hdl on subsequent reads and writes.

If the buffer mode is changed from BlockBuffering or LineBuffering to NoBuffering, then

  • if hdl is writable, the buffer is flushed as for hFlush;
  • if hdl is not writable, the contents of the buffer is discarded.

This operation may fail with:

  • isPermissionError if the handle has already been used for reading or writing and the implementation does not allow the buffering mode to be changed.
hSetBinaryMode :: Handle -> Bool -> IO ()Source

Select binary mode (True) or text mode (False) on a open handle. (See also openBinaryFile.)

This has the same effect as calling hSetEncoding with latin1, together with hSetNewlineMode with noNewlineTranslation.

hSetEncoding :: Handle -> TextEncoding -> IO ()Source

The action hSetEncoding hdl encoding changes the text encoding for the handle hdl to encoding. The default encoding when a Handle is created is localeEncoding, namely the default encoding for the current locale.

To create a Handle with no encoding at all, use openBinaryFile. To stop further encoding or decoding on an existing Handle, use hSetBinaryMode.

hSetEncoding may need to flush buffered data in order to change the encoding.

hGetEncoding :: Handle -> IO (Maybe TextEncoding)Source

Return the current TextEncoding for the specified Handle, or Nothing if the Handle is in binary mode.

Note that the TextEncoding remembers nothing about the state of the encoder/decoder in use on this Handle. For example, if the encoding in use is UTF-16, then using hGetEncoding and hSetEncoding to save and restore the encoding may result in an extra byte-order-mark being written to the file.

hFlush :: Handle -> IO ()Source

The action hFlush hdl causes any items buffered for output in handle hdl to be sent immediately to the operating system.

This operation may fail with:

  • isFullError if the device is full;
  • isPermissionError if a system resource limit would be exceeded. It is unspecified whether the characters in the buffer are discarded or retained under these circumstances.
hFlushAll :: Handle -> IO ()Source

The action hFlushAll hdl flushes all buffered data in hdl, including any buffered read data. Buffered read data is flushed by seeking the file position back to the point before the bufferred data was read, and hence only works if hdl is seekable (see hIsSeekable).

This operation may fail with:

  • isFullError if the device is full;
  • isPermissionError if a system resource limit would be exceeded. It is unspecified whether the characters in the buffer are discarded or retained under these circumstances;
  • isIllegalOperation if hdl has buffered read data, and is not seekable.
hDuplicate :: Handle -> IO HandleSource
Returns a duplicate of the original handle, with its own buffer. The two Handles will share a file pointer, however. The original handle's buffer is flushed, including discarding any input data, before the handle is duplicated.
hDuplicateTo :: Handle -> Handle -> IO ()Source

Makes the second handle a duplicate of the first handle. The second handle will be closed first, if it is not already.

This can be used to retarget the standard Handles, for example:

 do h <- openFile "mystdout" WriteMode
    hDuplicateTo h stdout
hClose :: Handle -> IO ()Source
Computation hClose hdl makes handle hdl closed. Before the computation finishes, if hdl is writable its buffer is flushed as for hFlush. Performing hClose on a handle that has already been closed has no effect; doing so is not an error. All other operations on a closed handle will fail. If hClose fails for any reason, any further operations (apart from hClose) on the handle will still fail as if hdl had been successfully closed.
hClose_help :: Handle__ -> IO (Handle__, Maybe SomeException)Source
type HandlePosition = IntegerSource
data HandlePosn Source
Constructors
HandlePosn Handle HandlePosition
show/hide Instances
hGetPosn :: Handle -> IO HandlePosnSource
Computation hGetPosn hdl returns the current I/O position of hdl as a value of the abstract type HandlePosn.
hSetPosn :: HandlePosn -> IO ()Source

If a call to hGetPosn hdl returns a position p, then computation hSetPosn p sets the position of hdl to the position it held at the time of the call to hGetPosn.

This operation may fail with:

data SeekMode Source
A mode that determines the effect of hSeek hdl mode i, as follows:
Constructors
AbsoluteSeekthe position of hdl is set to i.
RelativeSeekthe position of hdl is set to offset i from the current position.
SeekFromEndthe position of hdl is set to offset i from the end of the file.
show/hide Instances
hSeek :: Handle -> SeekMode -> Integer -> IO ()Source

Computation hSeek hdl mode i sets the position of handle hdl depending on mode. The offset i is given in terms of 8-bit bytes.

If hdl is block- or line-buffered, then seeking to a position which is not in the current buffer will first cause any items in the output buffer to be written to the device, and then cause the input buffer to be discarded. Some handles may not be seekable (see hIsSeekable), or only support a subset of the possible positioning operations (for instance, it may only be possible to seek to the end of a tape, or to a positive offset from the beginning or current position). It is not possible to set a negative I/O position, or for a physical file, an I/O position beyond the current end-of-file.

This operation may fail with:

hTell :: Handle -> IO IntegerSource
hIsOpen :: Handle -> IO BoolSource
hIsClosed :: Handle -> IO BoolSource
hIsReadable :: Handle -> IO BoolSource
hIsWritable :: Handle -> IO BoolSource
hGetBuffering :: Handle -> IO BufferModeSource
Computation hGetBuffering hdl returns the current buffering mode for hdl.
hIsSeekable :: Handle -> IO BoolSource
hSetEcho :: Handle -> Bool -> IO ()Source
Set the echoing status of a handle connected to a terminal.
hGetEcho :: Handle -> IO BoolSource
Get the echoing status of a handle connected to a terminal.
hIsTerminalDevice :: Handle -> IO BoolSource
Is the handle connected to a terminal?
hSetNewlineMode :: Handle -> NewlineMode -> IO ()Source
Set the NewlineMode on the specified Handle. All buffered data is flushed first.
data Newline Source
The representation of a newline in the external file or stream.
Constructors
LF'\n'
CRLF'\r\n'
show/hide Instances
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.
Constructors
NewlineMode
inputNL :: Newlinethe representation of newlines on input
outputNL :: Newlinethe representation of newlines on output
show/hide Instances
nativeNewline :: NewlineSource
The native newline representation for the current platform: LF on Unix systems, CRLF on Windows.
noNewlineTranslation :: NewlineModeSource

Do no newline translation at all.

 noNewlineTranslation  = NewlineMode { inputNL  = LF, outputNL = LF }
universalNewlineMode :: NewlineModeSource

Map '\r\n' into '\n' on input, and '\n' to the native newline represetnation 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 }
nativeNewlineMode :: NewlineModeSource

Use the native newline representation on both input and output

 nativeNewlineMode  = NewlineMode { inputNL  = nativeNewline
                                    outputNL = nativeNewline }
hShow :: Handle -> IO StringSource
hShow is in the IO monad, and gives more comprehensive output than the (pure) instance of Show for Handle.
hWaitForInput :: Handle -> Int -> IO BoolSource

Computation hWaitForInput hdl t waits until input is available on handle hdl. It returns True as soon as input is available on hdl, or False if no input is available within t milliseconds.

If t is less than zero, then hWaitForInput waits indefinitely.

This operation may fail with:

NOTE for GHC users: unless you use the -threaded flag, hWaitForInput t where t >= 0 will block all other Haskell threads for the duration of the call. It behaves like a safe foreign call in this respect.

hGetChar :: Handle -> IO CharSource

Computation hGetChar hdl reads a character from the file or channel managed by hdl, blocking until a character is available.

This operation may fail with:

hGetLine :: Handle -> IO StringSource

Computation hGetLine hdl reads a line from the file or channel managed by hdl.

This operation may fail with:

  • isEOFError if the end of file is encountered when reading the first character of the line.

If hGetLine encounters end-of-file at any other point while reading in a line, it is treated as a line terminator and the (partial) line is returned.

hGetContents :: Handle -> IO StringSource

Computation hGetContents hdl returns the list of characters corresponding to the unread portion of the channel or file managed by hdl, which is put into an intermediate state, semi-closed. In this state, hdl is effectively closed, but items are read from hdl on demand and accumulated in a special list returned by hGetContents hdl.

Any operation that fails because a handle is closed, also fails if a handle is semi-closed. The only exception is hClose. A semi-closed handle becomes closed:

  • if hClose is applied to it;
  • if an I/O error occurs when reading an item from the handle;
  • or once the entire contents of the handle has been read.

Once a semi-closed handle becomes closed, the contents of the associated list becomes fixed. The contents of this final list is only partially specified: it will contain at least all the items of the stream that were evaluated prior to the handle becoming closed.

Any I/O errors encountered while a handle is semi-closed are simply discarded.

This operation may fail with:

hPutChar :: Handle -> Char -> IO ()Source

Computation hPutChar hdl ch writes the character ch to the file or channel managed by hdl. Characters may be buffered if buffering is enabled for hdl.

This operation may fail with:

hPutStr :: Handle -> String -> IO ()Source

Computation hPutStr hdl s writes the string s to the file or channel managed by hdl.

This operation may fail with:

hGetBuf :: Handle -> Ptr a -> Int -> IO IntSource

hGetBuf hdl buf count reads data from the handle hdl into the buffer buf until either EOF is reached or count 8-bit bytes have been read. It returns the number of bytes actually read. This may be zero if EOF was reached before any data was read (or if count is zero).

hGetBuf ignores whatever TextEncoding the Handle is currently using, and reads bytes directly from the underlying IO device.

hGetBuf never raises an EOF exception, instead it returns a value smaller than count.

If the handle is a pipe or socket, and the writing end is closed, hGetBuf will behave as if EOF was reached.

hGetBuf ignores the prevailing TextEncoding and NewlineMode on the Handle, and reads bytes directly.

hGetBufNonBlocking :: Handle -> Ptr a -> Int -> IO IntSource

hGetBufNonBlocking hdl buf count reads data from the handle hdl into the buffer buf until either EOF is reached, or count 8-bit bytes have been read, or there is no more data available to read immediately.

hGetBufNonBlocking is identical to hGetBuf, except that it will never block waiting for data to become available, instead it returns only whatever data is available. To wait for data to arrive before calling hGetBufNonBlocking, use hWaitForInput.

hGetBufNonBlocking ignores whatever TextEncoding the Handle is currently using, and reads bytes directly from the underlying IO device.

If the handle is a pipe or socket, and the writing end is closed, hGetBufNonBlocking will behave as if EOF was reached.

hGetBufNonBlocking ignores the prevailing TextEncoding and NewlineMode on the Handle, and reads bytes directly.

hPutBuf :: Handle -> Ptr a -> Int -> IO ()Source

hPutBuf hdl buf count writes count 8-bit bytes from the buffer buf to the handle hdl. It returns ().

hPutBuf ignores any text encoding that applies to the Handle, writing the bytes directly to the underlying file or device.

hPutBuf ignores the prevailing TextEncoding and NewlineMode on the Handle, and writes bytes directly.

This operation may fail with:

  • ResourceVanished if the handle is a pipe or socket, and the reading end is closed. (If this is a POSIX system, and the program has not asked to ignore SIGPIPE, then a SIGPIPE may be delivered instead, whose default action is to terminate the program).
hPutBufNonBlocking :: Handle -> Ptr a -> Int -> IO IntSource
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