Documentation

A handle managing input from the Haskell program's standard input channel.

A handle managing output to the Haskell program's standard output channel.

A handle managing output to the Haskell program's standard error channel.

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 (on POSIX systems) is a FIFO without a reader and WriteMode was requested; or
• isPermissionError if the user does not have permission to open the file.

On POSIX systems, openFile is an interruptible operation as described in Control.Exception.

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

withFile name mode act opens a file like 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.

@since base-4.16.0.0

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.)

A version of openBinaryFile that takes an action to perform with the handle. If an exception occurs in the action, then the file will be closed automatically. The action should close the file when finished with it so the file does not remain open until the garbage collector collects the handle.

@since base-4.16.0.0

Like openFile, but opens the file in ordinary blocking mode. This can be useful for opening a FIFO for writing: if we open in non-blocking mode then the open will fail if there are no readers, whereas a blocking open will block until a reader appear.

Note: when blocking happens, an OS thread becomes tied up with the processing, so the program must have at least another OS thread if it wants to unblock itself. By corollary, a non-threaded runtime will need a process-external trigger in order to become unblocked.

On POSIX systems, openFileBlocking is an interruptible operation as described in Control.Exception.

@since base-4.4.0.0

withFileBlocking name mode act opens a file like openFileBlocking and passes the resulting handle to the computation act. The handle will be closed on exit from withFileBlocking, 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.

@since base-4.16.0.0

Turn an existing file descriptor into a Handle. This is used by various external libraries to make Handles.

Makes a binary Handle. This is for historical reasons; it should probably be a text Handle with the default encoding and newline translation instead.

Old API kept to avoid breaking clients

Turn an existing Handle into a file descriptor. This function throws an IOError if the Handle does not reference a file descriptor.

@since base-4.10.0.0