Ben Rudiak-Gould wrote:
On Mon, 28 Jul 2003, Simon Marlow wrote:
[...] lookupFileByPathname must open the file, without knowing whether the file will be used for reading or writing in the future.
I know; I'm hoping against hope that this isn't an insurmountable problem.
Well, I fear it is, at least on POSIX...
If the OS provides a "reopen" function which is like open except that it takes a file handle instead of a pathname,
On POSIX, I'm not aware of anything like that, only dup/dup2, but you can't change the access mode after duplicating the fd (at least fcntl on Linux is not capable of doing it).
[...] a File contains a handle with minimal access permissions and maximal sharing permissions,
The next problem: How should one get a file descriptor on POSIX without knowing the access mode in advance? If the file is not readable O_RDONLY will fail, if it is only writeable O_WRONLY will fail, O_RDWR is even worse... OK, we could stat the file first, but there is no guarantee that the file permissions are still the same when we later want to "reopen" it.
[...] If there's a way to open files by unique ID instead of pathname, that would also work.
I'm not aware of this on POSIX (open a file by inode/fs?).
[...] All we need here is a way to change the access and sharing rights on an already-open handle. I find it hard to believe that after decades of use by millions of people, the UNIX file API provides no way to do this safely.
Personally, I think this is a sign that one is heading towards the wrong direction... :-)
[...] What are the practical problems with relying on finalizers? As far as I can see, the "no more filehandles available" problem is completely solved by forcing a major GC and trying again when it occurs.
But on quite a few systems there is an upper limit on the *global* number of open files, so you would be a "bad citizen" for such a system.
How did you intend text encodings to work? I see several possibilities:
textDecode :: TextEncoding -> [Octet] -> [Char]
or
decodeInputStream :: TextEncoding -> InputStream -> TextInputStream getChar :: TextInputStream -> IO Char etc.
or
setInputStreamCoding :: InputStream -> TextEncoding -> IO () getChar :: InputStream -> IO Char
I was thinking of the second. It could easily be implemented as the third under the hood. But I was hoping someone else would worry about it. :-)
In the non-IO versions you have a problem if the encoder/decoder encounters an error because of a malformed InputStream. In the IO case one can simply raise an IO exception. And using "Maybe TextInputStream" won't help, because this would essentially make the encoder/decoder strict in its InputStream argument. Cheers, S.