[I wrote initially, ...]
As I have migrated more of my application into Haskell, I find that I/O in one thread effectively blocks other threads.
Resolved - the SSL_read external I/O function needs to be "safe". My apologies for botching the mailing list threading, but I deleted the mail wherein someone helpfully asked whether I'd made my foreign calls "safe" or "unsafe", and I can't see that message on the archives. My initial take was, yes, of course I make external calls "safe", because they sure are going to trigger callbacks into the Haskell runtime and the documentation is quite clear about that. However, I'd made SSL_read "unsafe", since no callbacks there. Reversed that, and now my threads execute concurrently - SSL_read doesn't block Haskell execution in all the other threads. Either way, I can also report that it doesn't seem to matter whether SSL_read was called from a callback, or not. If unsafe, it blocks all the other threads either way, and if safe, it doesn't block them either way. So, thanks a lot, whoever you were! Henceforth I will play it safe and eschew unsafeness. I vaguely recall a discussion here a ways back, wherein the terms "safe" and "unsafe" were lamented and various confusing propositions were bandied about. I should dig that up if I can, and see if this practical example sheds some light on any of it. Donn Cave, donn@avvanta.com