Copyright | (c) The University of Glasgow, 1994-2009 |
---|---|
License | see libraries/base/LICENSE |
Maintainer | [email protected] |
Stability | provisional |
Portability | non-portable |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
External API for GHC's Handle implementation
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:
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.
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:
hFlush
is issued, or the handle is closed.hFlush
is issued, or the handle is closed.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:
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.
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 |
:: (IODevice dev, BufferedIO dev, Typeable dev) | |
=> dev | the underlying IO device, which must support |
-> FilePath | a string describing the |
-> IOMode | |
-> Maybe TextEncoding | |
-> NewlineMode | |
-> IO Handle |
makes a new Handle
mkDuplexHandle :: (IODevice dev, BufferedIO dev, Typeable dev) => dev -> FilePath -> Maybe TextEncoding -> NewlineMode -> IO Handle Source
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 Source
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 Bool Source
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 Source
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 () 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
hdl
is writable, the buffer is flushed as for hFlush
;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 char8
, 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 Handle Source
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 = Integer Source
data HandlePosn Source
HandlePosn Handle HandlePosition |
hGetPosn :: Handle -> IO HandlePosn Source
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:
isPermissionError
if a system resource limit would be exceeded.A mode that determines the effect of hSeek
hdl mode i
.
AbsoluteSeek | the position of |
RelativeSeek | the position of |
SeekFromEnd | the position of |
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:
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 Source
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 Source
hIsClosed :: Handle -> IO Bool Source
hIsReadable :: Handle -> IO Bool Source
hIsWritable :: Handle -> IO Bool Source
hGetBuffering :: Handle -> IO BufferMode Source
Computation hGetBuffering
hdl
returns the current buffering mode for hdl
.
hIsSeekable :: Handle -> IO Bool Source
hSetEcho :: Handle -> Bool -> IO () Source
Set the echoing status of a handle connected to a terminal.
hGetEcho :: Handle -> IO Bool Source
Get the echoing status of a handle connected to a terminal.
hIsTerminalDevice :: Handle -> IO Bool Source
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.
The representation of a newline in the external file or stream.
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.
NewlineMode | |
nativeNewline :: Newline Source
The native newline representation for the current platform: LF
on Unix systems, CRLF
on Windows.
noNewlineTranslation :: NewlineMode Source
Do no newline translation at all.
noNewlineTranslation = NewlineMode { inputNL = LF, outputNL = LF }
universalNewlineMode :: NewlineMode Source
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 Source
Use the native newline representation on both input and output
nativeNewlineMode = NewlineMode { inputNL = nativeNewline outputNL = nativeNewline }
hShow :: Handle -> IO String Source
hShow
is in the IO
monad, and gives more comprehensive output than the (pure) instance of Show
for Handle
.
hWaitForInput :: Handle -> Int -> IO Bool Source
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.NOTE for GHC users: unless you use the -threaded
flag, hWaitForInput hdl 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 Source
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 Source
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 Source
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:
hClose
is applied to it;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 () 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:
isFullError
if the device is full; orisPermissionError
if another system resource limit would be exceeded.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:
isFullError
if the device is full; orisPermissionError
if another system resource limit would be exceeded.hGetBuf :: Handle -> Ptr a -> Int -> IO Int Source
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 Source
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 () 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 Int Source
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/base-4.8.2.0/GHC-IO-Handle.html