Abstract FilePath Proposal

[David Turner]

On 28 June 2015 at 02:02, Yitzchak Gale <gale at sefer.org> wrote:

> OK, based on what David and Brandon wrote, I guess
> that representing paths as bytestrings does make
> some low-level sense on all platforms. Although
> for Windows we would still need some way to deal
> with the requirement that the bytestring have an even
> length.
>

I would guess this could just be done by making the type abstract so you
can't easily get to the underlying bytes. Windows will only ever give you
even-length bytestrings (in directory listings or similar) and all the
other ways of synthesizing paths from strings could be set up to preserve
the evenness.

If you end up passing an odd-length bytestring to Windows as a path then
Bad Things could certainly happen, but no worse than mucking around with
other unsafe APIs like Data.ByteString.Internal.



On 28 June 2015 at 02:02, Yitzchak Gale <gale at sefer.org> wrote:

> OK, based on what David and Brandon wrote, I guess
> that representing paths as bytestrings does make
> some low-level sense on all platforms. Although
> for Windows we would still need some way to deal
> with the requirement that the bytestring have an even
> length.
>
> We will need platform-dependent coercions of
> paths to and from String/Text. Those might sometimes
> be partial functions. We need a notion of the coercions
> for the current platform, and we also need it to be
> possible to access the coercions for all platforms.
>
>
> On Sun, Jun 28, 2015 at 12:28 AM David Turner <
> dct25-561bs at mythic-beasts.com> wrote:
>
>> Hi,
>>
>> I'm +1 on the general idea of this proposal. Using String for filenames
>> has caused me all sorts of trouble, particularly when I've had to deal with
>> a bunch of files whose names don't all use the same encoding.
>>
>> However, be careful about the exact semantics of filenames on Windows.
>> Quoting MSDN:
>>
>>
>> There is no need to perform any Unicode normalization on path and file
>> name strings for use by the Windows file I/O API functions because* the
>> file system treats path and file names as an opaque sequence of WCHARs*.
>> Any normalization that your application requires should be performed with
>> this in mind, external of any calls to related Windows file I/O API
>> functions.
>>
>>
>> (from
>> https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx,
>> emphasis mine)
>>
>> Thus FilePath = String (or Text) doesn't really seem correct on Windows
>> either (although it'll be pretty close as long as you stay within the BMP).
>>
>> By my reckoning, when you get down to brass tacks, all filesystems on all
>> platforms name files with sequences of bytes. There are various interesting
>> ways to represent these bytes to human beings as sequences of characters,
>> but aiming for FilePath = ByteString everywhere and dealing with the
>> conversion to characters elsewhere seems more correct.
>>
>> Cheers,
>>
>> David
>>
>>
>>
>> On 27 June 2015 at 22:02, Brandon Allbery <allbery.b at gmail.com> wrote:
>> > On Sat, Jun 27, 2015 at 4:50 PM, Yitzchak Gale <gale at sefer.org> wrote:
>> >>
>> >> On Mac OS X, it's normalized Unicode. The important
>> >> point is *normalized* - if you create a FilePath from two
>> >> different Unicode strings that have the same normalized
>> >> form, the result FilePaths must be equal on Mac OS X.
>> >
>> >
>> > This is only true for higher level OS X APIs. ghc normally operates in
>> the
>> > BSD layer, which mostly follows POSIX semantics; in particular,
>> filesystem
>> > paths are bytestrings in the BSD layer, and only normalized in Cocoa
>> APIs.
>> > (Which, among other things, means you can make a GUI application dump
>> core
>> > by trying to use a file dialog in a directory containing a filename
>> created
>> > using the BSD API which does not use a UTF8 encoding.)
>> >
>> > --
>> > brandon s allbery kf8nh                               sine nomine
>> associates
>> > allbery.b at gmail.com
>> ballbery at sinenomine.net
>> > unix, openafs, kerberos, infrastructure, xmonad
>> http://sinenomine.net
>> >
>> > _______________________________________________
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>> > Libraries at haskell.org
>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>> >
>>
>>
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