RFD: deprecate permissions-related stuff in System.Directory

[Simon Marlow]

Malcolm Wallace wrote:
>>> I have a perfectly OK use of getPermissions: in hmake, there is a
>>> routine that scans the PATH variable looking for an executable with
>>> a given name.  Basically, it is a Haskell implementation of the unix
>>> 'which' command.  There is no actual use of the executable, just a
>>> test to report/reject the candidate pathname.
>> You wouldn't be losing much by using doesFileExist rather than 
>> getPermissions.
> 
> But on unix, it would be totally incorrect to return a file of the right
> name but with the wrong permissions.  This is what my Haskell `which`
> originally did, and it bit me hard on a couple of occasions, hence the
> extra check.
> 
>> On Windows it turns out to be really hard if not 
>> impossible to check for execute permission - the only reliable way is
>> to  actually try to execute the file.
> 
> Mounting a FAT filesystem on unix (or indeed any filesystem via Samba)
> also makes all files appear to be executable.  These are just broken
> platforms, and I don't think they should hold us hostage or dictate our
> preferred API.

Tempting as it is, I couldn't possibly label Windows as "broken" :-)

Since this is supposed to be a portable API, the fact that you can't 
implement it on certain platforms definitely *should* be an indication that 
it is the wrong API.

> I'm not saying that the current API is perfect - it could certainly be
> improved.  But I'm just pointing out that there are valid use-cases for
> what we have currently.  If you have concrete suggestions for a new API,
> I would certainly be interested.

One suggestion is to just get rid of this functionality - code that needs 
to do access-control stuff must use OS-specific APIs.  This is arguably the 
right thing.

What we have now is a "best-effort" API: it tries to do something 
reasonable, but may do something bogus.  Some people find this distasteful, 
but it can be useful at times (as you and others have pointed out).  I'm 
not really convinced one way or the other, which is why I started this RFD.

Cheers,
	Simon