Documentation

Wraps hPutChar

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

Wraps hPutStr

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

Wraps hPutStrLn

The same as hPutStr', but adds a newline character

Wraps hPrint

Computation hPrint' hdl t writes the string representation of t given by the shows function to the file or channel managed by hdl and appends a newline.

This operation may fail with:

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

Wraps hPutBuf

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).

Wraps hPutBufNonBlocking

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'.

Wraps hWaitForInput

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.

Wraps hGetChar

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.

Wraps hGetLine

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.

Wraps hLookAhead

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.

Wraps hGetContents

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.

Wraps hGetBuf

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.

Wraps hGetBufSome

hGetBufSome hdl buf count reads data from the handle hdl into the buffer buf. If there is any data available to read, then hGetBufSome returns it immediately; it only blocks if there is no data to be 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).

hGetBufSome 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, hGetBufSome will behave as if EOF was reached.

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

Wraps hGetBufNonBlocking

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.

Wraps hClose

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.

Wraps hFileSize

For a handle hdl which attached to a physical file, hFileSize hdl returns the size of that file in 8-bit bytes.

Wraps hSetFileSize

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

Wraps hIsEOF

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.

Wraps hSetBuffering

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.

Wraps hGetBuffering

Computation hGetBuffering' hdl returns the current buffering mode

Wraps hFlush

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.

Wraps hGetPosn

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

Wraps hSetPosn

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.

Wraps hSeek

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.

Wraps hTell

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.

Wraps hIsOpen

Wraps hIsClosed

Wraps hIsReadable

Wraps hIsWritable

Wraps hIsSeekable

Wraps hIsTerminalDevice

Is the handle connected to a terminal?

Wraps hSetEcho

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

Wraps hGetEcho

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

Wraps hShow

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

Wraps hReady

Computation hReady' hdl indicates whether at least one item is available for input from handle hdl.

This operation may fail with:

• isEOFError if the end of file has been reached.

Wraps hSetEncoding

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.

Wraps hGetEncoding

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.

Wraps hSetNewlineMode

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

Wraps getChar

Read a character from the standard input device (same as hGetChar' stdin).

Wraps getLine

Read a line from the standard input device (same as hGetLine' stdin).

Wraps getContents

The getContents operation returns all user input as a single string, which is read lazily as it is needed (same as hGetContents' stdin).

Wraps readLn

The readLn' function combines getLine' and readIO.

Wraps interact

The interact' function takes a function of type String->String as its argument. The entire input from the standard input device is passed to this function as its argument, and the resulting string is output on the standard output device.

Wraps putChar

Write a character to the standard output device (same as hPutChar' stdout).

Wraps putStr

Write a string to the standard output device (same as hPutStr' stdout).

Wraps putStrLn

The same as putStr', but adds a newline character.

Wraps print

The print' function outputs a value of any printable type to the standard output device. Printable types are those that are instances of class Show; print' converts values to strings for output using the show operation and adds a newline.

For example, a program to print' the first 20 integers and their powers of 2 could be written as:

main = print' ([(n, 2^n) | n <- [0..19]])

Wraps isEOF

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.

Wraps readFile

The readFile' function reads a file and returns the contents of the file as a string. The file is read lazily, on demand, as with getContents'.

Wraps writeFile

The computation writeFile' file str function writes the string str, to the file file.

Wraps appendFile

The computation appendFile' file str function appends the string str, to the file file.

Note that writeFile' and appendFile' write a literal string to a file. To write a value of any printable type, as with print', use the show function to convert the value to a string first.

main = appendFile' "squares" (show [(x,x*x) | x <- [0,0.1..2]])

Wraps withFile

withFile' name mode act opens a file using openFile' and passes the resulting handle to the computation act. The handle will be closed on exit from withFile', whether by normal termination or by raising an exception. If closing the handle raises an exception, then this exception will be raised by withFile' rather than any exception raised by act.

Wraps openFile

Computation openFile' file mode allocates and returns a new, open handle to manage the file file. It manages input if mode is ReadMode, output if mode is WriteMode or AppendMode, and both input and output if mode is ReadWriteMode.

If the file does not exist and it is opened for output, it should be created as a new file. If mode is WriteMode and the file already exists, then it should be truncated to zero length. Some operating systems delete empty files, so there is no guarantee that the file will exist following an openFile' with mode WriteMode unless it is subsequently written to successfully. The handle is positioned at the end of the file if mode is AppendMode, and otherwise at the beginning (in which case its internal position is 0). The initial buffer mode is implementation-dependent.

This operation may fail with:

• isAlreadyInUseError if the file is already open and cannot be reopened;
• isDoesNotExistError if the file does not exist; or
• isPermissionError if the user does not have permission to open the file.

Note: if you will be working with files containing binary data, you'll want to be using openBinaryFile'.

Wraps withBinaryFile

withBinaryFile' name mode act opens a file using openBinaryFile' and passes the resulting handle to the computation act. The handle will be closed on exit from withBinaryFile', whether by normal termination or by raising an exception.

Wraps openBinaryFile

Like openFile', but open the file in binary mode. On Windows, reading a file in text mode (which is the default) will translate CRLF to LF, and writing will translate LF to CRLF. This is usually what you want with text files. With binary files this is undesirable; also, as usual under Microsoft operating systems, text mode treats control-Z as EOF. Binary mode turns off all special treatment of end-of-line and end-of-file characters. (See also hSetBinaryMode'.)

Wraps openTempFile

The function creates a temporary file in ReadWrite mode. The created file isn't deleted automatically, so you need to delete it manually.

The file is creates with permissions such that only the current user can read/write it.

With some exceptions (see below), the file will be created securely in the sense that an attacker should not be able to cause openTempFile' to overwrite another file on the filesystem using your credentials, by putting symbolic links (on Unix) in the place where the temporary file is to be created. On Unix the O_CREAT and O_EXCL flags are used to prevent this attack, but note that O_EXCL is sometimes not supported on NFS filesystems, so if you rely on this behaviour it is best to use local filesystems only.

Wraps openBinaryTempFile

Like openTempFile', but opens the file in binary mode. See openBinaryFile' for more comments.

Wraps openTempFileWithDefaultPermissions

Like openTempFile', but uses the default file permissions

Wraps openBinaryTempFileWithDefaultPermissions

Like openBinaryTempFile', but uses the default file permissions

Wraps mkTextEncoding

Look up the named Unicode encoding. May fail with

• isDoesNotExistError if the encoding is unknown

The set of known encodings is system-dependent, but includes at least:

• UTF-8

• UTF-16, UTF-16BE, UTF-16LE
• UTF-32, UTF-32BE, UTF-32LE

On systems using GNU iconv (e.g. Linux), there is additional notation for specifying how illegal characters are handled:

• a suffix of //IGNORE, e.g. UTF-8//IGNORE, will cause all illegal sequences on input to be ignored, and on output will drop all code points that have no representation in the target encoding.
• a suffix of //TRANSLIT will choose a replacement character for illegal sequences or code points.

On Windows, you can access supported code pages with the prefix CP; for example, "CP1250".