So I'm the one user in a thousand that will want to provide my own I/O functions, for example. In the old world, I guess I would be looking for some extended API where my I/O functions are parameters to the "open" or "init" function, and the IMAP functions take over from there. In a more pure functional oriented model, could it be an extended API that exposes the IMAP functionality as operations on data, and leaves it to me to deal with the I/O?
I believe a typeclass could solve this for you. The typeclass member functions serve as your interface definition. For example, say "auth" was a member function. Then you could implement instances which authorized using NTLM, HTTP Basic, etc. It's similar to how you'd do the same thing in java with interfaces, in fact. Justin