Re: Abstract FilePath Proposal

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).a... ) 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 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-Po... .

Best,

Mathieu

On 27 June 2015 at 12:12, Neil Mitchell 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 wrote:

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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

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depreciate the old one?

Niklas ________________________________ Från: Herbert Valerio Riedel Skickat: ‎2015-‎06-‎26 18:09 Till: libraries@haskell.org; ghc-devs@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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1

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