Abstract FilePath Proposal

Sun Jun 28 12:09:10 UTC 2015

Hi,

I think it'd be more robust to handle normalisation when converting from
String/Text to FilePath (and combining things with (</>) and so on) rather
than in the underlying representation.

It's absolutely crucial that you can ask the OS for a filename (which it
gives you as a sequence of bytes) and then pass that exact same sequence of
bytes back to the OS without any normalisation or other useful alterations
having taken place.

You can do some deeply weird stuff in Windows by starting an absolute path
with \\?\, including apparently using '.' and '..' as the name of a
filesystem component:

Because it turns off automatic expansion of the path string, the "\\?\"
prefix also allows the use of ".." and "." in the path names, which can be
useful if you are attempting to perform operations on a file with these
otherwise reserved relative path specifiers as part of the fully qualified
path.

(from
https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
)

I don't fancy shaking all the corner cases out of this. An explicit
'normalise' function seems ok, but baking normalisation into the type
itself seems bad.

Cheers,

David

On 28 June 2015 at 11:03, Boespflug, Mathieu <m at tweag.io> wrote:

> Hi Neil,
>
> why does the proposal *not* include normalization?
>
> There are four advantages that I see to making FilePath a datatype:
>
> 1. it makes it possible to implement the correct semantics for some
> systems (including POSIX),
> 2. it allows for information hiding, which in turn helps modularity,
> 3. the type is distinct from any other type, hence static checks are
> stronger,
> 4. it becomes possible to quotient values over some arbitrary set of
> identities that makes sense. i.e. in the case of FilePath, arguably
> "foo/bar//baz" *is* "foo/bar/baz" *is* "foo//bar/baz" for all intents
> and purposes, so it is not useful to distinguish these three ways of
> writing down the same path (and in fact in practice distinguishing
> them leads to subtle bugs). That is, the Eq instance compares
> FilePath's modulo a few laws.
>
> Do you propose to forego (4)? If so why so?
>
> If we're going through a deprecation process, could we do so once, by
> getting the notion of path equality we want right the first time?
> Contrary to type indexing FilePath, it seems to me that the design
> space for path identities is much smaller. Essentially, exactly the
> ones here:
> https://hackage.haskell.org/package/filepath-1.1.0.2/docs/System-FilePath-Posix.html#v:normalise
> .
>
> Best,
>
> Mathieu
>
>
> On 27 June 2015 at 12:12, Neil Mitchell <ndmitchell at gmail.com> wrote:
> > Hi Niklas,
> >
> > The function writeFile takes a FilePath. We could fork base or tell
> everyone
> > to use writeFile2, but in practice everyone will keep using writeFile,
> and
> > this String for FilePath. This approach is the only thing we could figure
> > that made sense.
> >
> > Henning: we do not propose normalisation on initialisation. For ASCII
> > strings fromFilePath . toFilePath will be id. It might also be for
> unicode
> > on some/all platforms. Of course, you can write your own FilePath creator
> > that does normalisation on construction.
> >
> > Thanks, Neil
> >
> >
> > On Saturday, 27 June 2015, Niklas Larsson <metaniklas at gmail.com> wrote:
> >>
> >> Hi!
> >>
> >> Instead of trying to minimally patch the existing API and still breaking
> >> loads of code, why not make a new API that doesn't have to compromise
> and
> >> depreciate the old one?
> >>
> >> Niklas
> >> ________________________________
> >> Från: Herbert Valerio Riedel
> >> Skickat: ‎2015-‎06-‎26 18:09
> >> Till: libraries at haskell.org; ghc-devs at haskell.org
> >> Ämne: Abstract FilePath Proposal
> >>
> >> -----BEGIN PGP SIGNED MESSAGE-----
> >> Hash: SHA1
> >>
> >> Hello *,
> >>
> >> What?
> >> =====
> >>
> >> We (see From: & CC: headers) propose, plain and simple, to turn the
> >> currently defined type-synonym
> >>
> >>   type FilePath = String
> >>
> >> into an abstract/opaque data type instead.
> >>
> >> Why/How/When?
> >> =============
> >>
> >> For details (including motivation and a suggested transition scheme)
> >> please consult
> >>
> >>   https://ghc.haskell.org/trac/ghc/wiki/Proposal/AbstractFilePath
> >>
> >>
> >>
> >> Suggested discussion period: 4 weeks
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> >> =lg0o
> >> -----END PGP SIGNATURE-----
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