[m-dev.] Porposal: Support reading/writing either dos/unix file endings on any platform

[Paul Bone]

io.open_input/4 and io.open_output/4 open files for text input.  This means
that line endings are automatically translated between platforms.  If a
UNIX-style OS is used the file is assumed to be in the UNIX format, if a
DOS-style OS is used then the file is assumed to be in the DOS format.
However these assumptions aren't always correct.

I share files with developers who use Windows (while I use Linux).  If a
Mercury program such as the error program opens a file created by a
developer who uses windows, then it incorrectly assumes the file uses the
UNIX file endings, and after modifying and saving the file it uses a mix of
both endings.

Rather than awkwardly patch the error program, every other program that
should behave like this, it'd be better to add support for this in Mercury's
standard library.  I propose that:

    + An input stream's line ending style is automatically detected from the
      file's contents, not from the host OS.
    + An output stream's line ending style can be specified by a new
      io.open_output/5 predicate.  io.open_output/4 should redirect to this
      and provide a default behaviour (use the host OS's preferred line
      ending).
    + A new pair of predicates input_stream_line_ending_style/2 and
      output_stream_line_ending_style/2 be created to retrieve the line
      ending style of a file handle.  This can be used to open an output
      file in the same style as an input file.

:- type line_ending_style
    --->    unix_line_endings
    ;       dos_line_endings.

:- type maybe_line_ending_style
    --->    host_os_line_endings
    ;       unix_line_endings
    ;       dos_line_endings.

    % Maybe these should take the IO state pair?
    %
:- pred input_stream_line_ending_style(input_stream::in,
    line_ending_style::out) is det.

:- pred output_stream_line_ending_style(output_stream::in,
    line_ending_style::out) is det.

    % open_output(Filename, LineEndingStyle, Result, !IO),
    %
:- pred open_output(string::in, maybe_line_ending_style::in,
    res(output_stream)::out, io::di, io::uo) is det.

Thoughts?

Thanks.


-- 
Paul Bone
http://www.bone.id.au