On 01/10/2011 12:31, Volker Wysk wrote:
Hello
The function to extract a file descriptor from a handle, handleToFd, closes the handle as a side effect. This appears to be necessary in the general case, otherwise the implementers wouldn't have made it this way.
But there are cases in which it isn't necessary to close the handle.
For instance, this little function needs to extract the file descriptor, but there is no need to close, or change, the handle.
isatty :: Handle -- ^ Handle to check -> IO Bool -- ^ Whether the handle is connected to a terminal isatty h = do fd<- handleToFd_noclose h isterm<- {# call isatty as hssh_c_isatty #} -- this is c2hs ((fromIntegral fd) :: CInt) return (isterm /= (0::CInt))
handleToFd_noclose :: Handle -> IO Fd -- to be implemented
The isatty above can't be implemented with things the way they are. To me, this is a serious drawback.
The cases, in which the handle needs to be closed, should be identified, and documented. Then, a version of handleToFd, which doesn't close the handle can be implemented.
There's another danger. A Handle has an associated finalizer that closes the file descriptor when the Handle is no longer needed by the program, so you can't make a handleToFd_noClose because the Handle might be closed anyway. What you can do is make a withHandleFD: withHandleFD :: Handle -> (FD -> IO a) -> IO a it's still quite dodgy, depending on what you do with the FD. Perhaps it should be called unsafeWithHandleFD. Anyway, patches gratefully accepted... Cheers, Simon