Question

What is the generally accepted way to implement the main loop of a server that needs to wait on a heterogeneous set of events? That is the server should wait (not busywait) until one of the following occurs:

• new socket connection
• data available on an existing socket
• OS signal
• third-party library callbacks

Answer 1

I think you’re thinking in terms of a C paradigm with a single thread, nonblocking I/O, and a select() call.

You can manage to write something like that in Haskell, but Haskell has much more to offer:

• lightweight threads
• safe and efficient concurrent data primitives like Mvar and Chan
• the Big Gun: Software Transactional Memory

I recommend you fork a new thread for every separate point of contact with the outside world, and keep everything coordinated with STM.