In inetd and systemd type systems, it is possible for the system to bind a socket and launch the application with the socket already existing, for example to provide socket based service starting. I would like to take advantage of this functionality in one of my Haskell daemons.
The daemon currently calls, socket, bindSocket, listen to create a Socket object that I can later call accept on. To change this to an inetd type system I would need to use standard input as a Socket, but all I can find so far is stdin :: Handle, or fdToHandle :: CInt -> Handle - neither are quite what I need.
I can't seem to find anything that has type Handle -> Socket, nor anything that is like stdin :: Socket. The nearest I can find is mkSocket which is very low-level, and most other languages (ie, Ruby) provide a call to turn a file descriptor into a socket without having to specify various other parameters.
C applications have the luxury of having sd-daemon.h which handles socket passing automatically. In Haskell, this file has to be emulated manually.
You can use stdInput to get the stdin file descriptor. The handleToFd function can of course also be used. Since you want POSIX-specific behavior, you can't expect it to work on Windows, though.
Once you have the FD, you have no choice but to use the mkSocket function. Haskell can't guess what kind of socket that you want, so you have to specify it. You most probably want:
mkSocket fd AF_UNIX Stream defaultProtocol Listening
Please remember that this isn't necessarily how systemd passes file descriptors to your application. You must check the LISTEN_FDS and LISTEN_PID environment variables to see which file descriptors to use and whether its your job to even bind the sockets. The default file descriptor representing the default socket is FD 3, not FD 0 as you are assuming. systemd might also give you multiple sockets to use if the service file demands it.