GHC.IO.Handle

base-4.3.1.0: Basic libraries

Portability | non-portable
Stability | provisional
Maintainer | libraries@haskell.org

GHC.IO.Handle

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

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

Eq Handle |
Show Handle |
Typeable Handle |

data BufferMode

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

NoBuffering | buffering is disabled if possible.
LineBuffering | 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.

Instances

Eq BufferMode |
Ord BufferMode |
Read BufferMode |
Show BufferMode |

mkFileHandle

Arguments

:: (IODevice dev, BufferedIO dev, Typeable dev) |
=> dev | the underlying IO device, which must support `IODevice`, `BufferedIO` and `Typeable`
-> FilePath | a string describing the `Handle`, e.g. the file path for a file. Used in error messages.
-> IOMode |
-> Maybe TextEncoding |
-> NewlineMode |
-> IO Handle |

makes a new `Handle`

mkDuplexHandle :: (IODevice dev, BufferedIO dev, Typeable dev) => dev -> FilePath -> Maybe TextEncoding -> NewlineMode -> IO Handle

like `mkFileHandle`, except that a `Handle` is created with two independent buffers, one for reading and one for writing. Used for full-duplex streams, such as network sockets.

hFileSize :: Handle -> IO Integer

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 ()

`hSetFileSize` `hdl` `size` truncates the physical file with handle `hdl` to `size` bytes.

hIsEOF :: Handle -> IO Bool

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 has to attempt to read from the stream to determine whether there is any more data to be read.

hLookAhead :: Handle -> IO Char

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:

- `isEOFError` if the end of file has been reached.

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

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 ()

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 ()

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)

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 ()

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 ()

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 Handle

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 ()

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 ()

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)

type HandlePosition = Integer

data HandlePosn

Constructors

HandlePosn Handle HandlePosition |

Instances

Eq HandlePosn |
Show HandlePosn |

hGetPosn :: Handle -> IO HandlePosn

Computation `hGetPosn` `hdl` returns the current I/O position of `hdl` as a value of the abstract type `HandlePosn`.

hSetPosn :: HandlePosn -> IO ()

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:

- `isPermissionError` if a system resource limit would be exceeded.

data SeekMode

A mode that determines the effect of `hSeek` `hdl mode i`.

Constructors

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.

Instances

Enum SeekMode |
Eq SeekMode |
Ord SeekMode |
Read SeekMode |
Show SeekMode |
Ix SeekMode |

hSeek :: Handle -> SeekMode -> Integer -> IO ()

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:

- `isIllegalOperationError` if the Handle is not seekable, or does not support the requested seek mode.
- `isPermissionError` if a system resource limit would be exceeded.

hTell :: Handle -> IO Integer

Computation `hTell` `hdl` returns the current position of the handle `hdl`, as the number of bytes from the beginning of the file. The value returned may be subsequently passed to `hSeek` to reposition the handle to the current position.

This operation may fail with:

- `isIllegalOperationError` if the Handle is not seekable.

hIsOpen :: Handle -> IO Bool

hIsClosed :: Handle -> IO Bool

hIsReadable :: Handle -> IO Bool

hIsWritable :: Handle -> IO Bool

hGetBuffering :: Handle -> IO BufferMode

Computation `hGetBuffering` `hdl` returns the current buffering mode for `hdl`.

hIsSeekable :: Handle -> IO Bool

hSetEcho :: Handle -> Bool -> IO ()

Set the echoing status of a handle connected to a terminal.

hGetEcho :: Handle -> IO Bool

Get the echoing status of a handle connected to a terminal.

hIsTerminalDevice :: Handle -> IO Bool

Is the handle connected to a terminal?

hSetNewlineMode :: Handle -> NewlineMode -> IO ()

Set the `NewlineMode` on the specified `Handle`. All buffered data is flushed first.

data Newline

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

Constructors

LF | '\n'
CRLF | '\r\n'

Instances

Eq Newline |
Ord Newline |
Read Newline |
Show Newline |

data NewlineMode

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 |
Fields inputNL :: Newline the representation of newlines on input outputNL :: Newline the representation of newlines on output

Instances

Eq NewlineMode |
Ord NewlineMode |
Read NewlineMode |
Show NewlineMode |

nativeNewline :: Newline

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

noNewlineTranslation :: NewlineMode

Do no newline translation at all.

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

universalNewlineMode :: NewlineMode

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 :: NewlineMode

Use the native newline representation on both input and output

     nativeNewlineMode  = NewlineMode { inputNL  = nativeNewline
                                        outputNL = nativeNewline }

hShow :: Handle -> IO String

`hShow` is in the `IO` monad, and gives more comprehensive output than the (pure) instance of `Show` for `Handle`.

hWaitForInput :: Handle -> Int -> IO Bool

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. Note that `hWaitForInput` waits until one or more full characters are available, which means that it needs to do decoding, and hence may fail with a decoding error.

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

This operation may fail with:

- `isEOFError` if the end of file has been reached.
- a decoding error, if the input begins with an invalid byte sequence in this Handle's encoding.

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 Char

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:

- `isEOFError` if the end of file has been reached.

hGetLine :: Handle -> IO String

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 String

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:

- `isEOFError` if the end of file has been reached.

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

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:

- `isFullError` if the device is full; or
- `isPermissionError` if another system resource limit would be exceeded.

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

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

This operation may fail with:

- `isFullError` if the device is full; or
- `isPermissionError` if another system resource limit would be exceeded.

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

`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` 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 Int

`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`.

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.

NOTE: on Windows, this function does not work correctly; it behaves identically to `hGetBuf`.

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

`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 Int