openFile file mode allocates and returns a new, open handle to manage the file fff. 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 fff 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
I/O position is 0). The initial buffer mode is implementation-dependent.
( the openFile computation 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. )