behaviour change in getDirectoryContents in GHC 7.2?

Simon Marlow marlowsd at
Mon Nov 7 18:02:32 CET 2011

On 06/11/2011 16:56, John Millikin wrote:
> 2011/11/6 Max Bolingbroke<batterseapower at>:
>> On 6 November 2011 04:14, John Millikin<jmillikin at>  wrote:
>>> For what it's worth, on my Ubuntu system, Nautilus ignores the locale
>>> and just treats all paths as either UTF8 or invalid.
>>> To me, this seems like the most reasonable option; the concept of
>>> "locale encoding" is entirely vestigal, and should only be used in
>>> certain specialized cases.
>> Unfortunately non-UTF8 locale encodings are seen in practice quite
>> often. I'm not sure about Linux, but certainly lots of Windows systems
>> are configured with a locale encoding like GBK or Big5.
> This doesn't really matter for file paths, though. The Win32 file API
> uses wide-character functions, which ought to work with Unicode text
> regardless of what the user set their locale to.
>>> Paths as text is what *Windows* programmers expect. Paths as bytes is
>>> what's expected by programmers on non-Windows OSes, including Linux
>>> and OS X.
>> IIRC paths on OS X are guaranteed to be valid UTF-8. The only platform
>> that uses bytes for paths (that we care about) is Linux.
> UTF-8 is bytes. It can be treated as text in some cases, but it's
> better to think about it as bytes.
>>> I'm not saying one is inherently better than the other, but
>>> considering that various UNIX  and UNIX-like operating systems have
>>> been using byte-based paths for near on forty years now, trying to
>>> abolish them by redefining the type is not a useful action.
>> We have to:
>>   1. Provide an API that makes sense on all our supported OSes
>>   2. Have getArgs :: IO [String]
>>   3. Have it such that if you go to your console and write
>> (./MyHaskellProgram 你好) then getArgs tells you ["你好"]
>> Given these constraints I don't see any alternative to PEP-383 behaviour.
> Requirement #1 directly contradicts #2 and #3.
>>> If you're going to make all the System.IO stuff use text, at least
>>> give us an escape hatch. The "unix" package is ideally suited, as it's
>>> already inherently OS-specific. Something like this would be perfect:
>> You can already do this with the implemented design. We have:
>> openFile :: FilePath ->  IO Handle
>> The FilePath will be encoded in the fileSystemEncoding. On Unix this
>> will have PEP383 roundtripping behaviour. So if you want openFile' ::
>> [Byte] ->  IO Handle you can write something like this:
>> escape = map (\b ->  if b<  128 then chr b else chr (0xEF00 + b))
>> openFile = openFile' . escape
>> The bytes that reach the API call will be exactly the ones you supply.
>> (You can also implement "escape" by just encoding the [Byte] with the
>> fileSystemEncoding).
>> Likewise, if you have a String and want to get the [Byte] we decoded
>> it from, you just need to encode the String again with the
>> fileSystemEncoding.
>> If this is not enough for you please let me know, but it seems to me
>> that it covers all your use cases, without any need to reimplement the
>> FFI bindings.
> This is not enough, since these strings are still being passed through
> the potentially (and in 7.2.1, actually) broken path encoder.

I think you might be misunderstanding how the new API works.  Basically, 
imagine a reversible transformation:

   encode :: String -> [Word8]
   decode :: [Word8] -> String

this transformation is applied in the appropriate direction by the IO 
library to translate filesystem paths into FilePath and vice versa.  No 
information is lost; furthermore you can apply the transformation 
yourself in order to recover the original [Word8] from a String, or to 
inject your own [Word8] file path.


All this does is mean that the common case where you want to interpret 
file system paths as text works with no fuss, without breaking anything 
in the case when the file system paths are not actually text.

It would probably be better to have an abstract FilePath type and to 
keep the original bytes, decoding on demand.  But that is a big change 
to the API and would break much more code.  One day we'll do this 
properly; for now we have this, which I think is a pretty reasonble 


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