behaviour change in getDirectoryContents in GHC 7.2?

[John Millikin]

On Tue, Nov 1, 2011 at 11:43, Max Bolingbroke
<batterseapower at hotmail.com> wrote:
> Hi John,
>
> On 1 November 2011 17:14, John Millikin <jmillikin at gmail.com> wrote:
>> GHC 7.2 assumes Linux/BSD paths are text, which 1) silently breaks all
>> existing code and 2) makes it impossible to fix within the given API.
>
> Please can you give an example of code that is broken with the new
> behaviour? The PEP 383 mechanism will unavoidably break *some* code
> but I don't expect there to be much of it. One thing that most likely
> *will* be broken is code that attempts to reinterpret a String as a
> "byte string" - i.e. assuming that it was decoded using latin1, but I
> expect that such code can just be deleted when you upgrade to 7.2.

Examples of broken code are Darcs, my system-fileio, and likely
anything else which needs to open Unicode-named files in both 7.0 and
7.2.

As a quick example, consider the case of files with encodings
different from the user's locale. This is *very* common, especially
when interoperating with foreign Windows systems.

$ ghci-7.0.4
GHC> import System.Directory
GHC> createDirectory "path-test"
GHC> writeFile "path-test/\xA1\xA5" "hello\n"
GHC> writeFile "path-test/\xC2\xA1\xC2\xA5" "world\n"
GHC> ^D

$ ghci-7.2.1
GHC> import System.Directory
GHC> getDirectoryContents "path-test"
["\161\165","\61345\61349","..","."]
GHC> readFile "path-test/\161\165"
"world\n"
GHC> readFile "path-test/\61345\61349"
*** Exception: path-test/: openFile: does not exist (No such file or
directory)

> As I pointed out earlier in the thread you can recover the old
> behaviour if you really want it by manually reencoding the strings, so
> I would dispute the claim that it is "impossible to fix within the
> given API".

Please describe how I can, in GHC 7.2, read the contents of the file
"path-test/\xA1\xA5" without changing my locale.

As far as I can tell, there is no way to do this using the standard
libraries. I would have to fall back to the "unix" package, or even
FFI imports, to open that file.