Now, some of the most common operations used in dynamic web pages relate to directory listing/manipulation. I was happily thinking that System.Directory would provide the needed functionality. Indeed it does, but unfortunately setCurrentDirectory breaks the thread abstraction.
What I mean is that if one page wants to change directory using setCurrentDirectory, this change affects all other (lightweight) threads as well, which is not how "ordinary" system threads works.
AFAIK, this _is_ how "ordinary" system threads work. See, for example: http://www.uwm.edu/cgi-bin/IMT/wwwman?topic=chdir(2)&msection=3f, and http://msdn.microsoft.com/library/default.asp?url=/library/en-us/ fileio/base/setcurrentdirectory.asp
Also it is clearly not what one would want in the kind of application I'm writing.
Is this behavior of setCurrentDirectory intended? I can't see a situation in which you could take advantage of it, but that doesn't mean there can't be any. =)
If it is not intended, is there any hope of it being "fixed"?
The behaviour is intended because it is the way the underlying operating systems work. It's hard to work around that; while it would be definitely possible (but tedious) to fix this for Haskell's IO library, we'd run into serious trouble with FFI calls (if you call two C functions from different Haskell threads, which will be the current directory of the process while both functions execute simultaneously?) So I fear that the only thing you can do is not to change the current directory in multi-threaded programs. Cheers, Wolfgang