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.

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.

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.

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.

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.

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:

• isPermissionError if a system resource limit would be exceeded.

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:

• isEOFError if the end of file has been reached.

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.

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

This operation may fail with:

• System.IO.Error.isEOFError if the end of file has been reached.

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.

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.

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.

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.

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.

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.

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:

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

The construct try comp exposes IO errors which occur within a computation, and which are not fully handled.

Non-I/O exceptions are not caught by this variant; to catch all exceptions, use Control.Exception.try from Control.Exception.

The bracket function captures a common allocate, compute, deallocate idiom in which the deallocation step must occur even in the case of an error during computation. This is similar to try-catch-finally in Java.

This version handles only IO errors, as defined by Haskell 98. The version of bracket in Control.Exception handles all exceptions, and should be used instead.

A variant of bracket where the middle computation doesn't want x.

This version handles only IO errors, as defined by Haskell 98. The version of bracket_ in Control.Exception handles all exceptions, and should be used instead.

The Haskell 98 type for exceptions in the IO monad. Any I/O operation may raise an IOError instead of returning a result. For a more general type of exception, including also those that arise in pure code, see Control.Exception.Exception.

In Haskell 98, this is an opaque type.

Construct an IOError value with a string describing the error. The fail method of the IO instance of the Monad class raises a userError, thus:

The catch function establishes a handler that receives any IOError raised in the action protected by catch. An IOError is caught by the most recent handler established by catch. These handlers are not selective: all IOErrors are caught. Exception propagation must be explicitly provided in a handler by re-raising any unwanted exceptions. For example, in

the function f returns [] when an end-of-file exception (cf. isEOFError) occurs in g; otherwise, the exception is propagated to the next outer handler.

When an exception propagates outside the main program, the Haskell system prints the associated IOError value and exits the program.

Non-I/O exceptions are not caught by this variant; to catch all exceptions, use Control.Exception.catch from Control.Exception.

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:

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.