There’s a great way to explain delimited continuations in the notes of Oleg’s Continuation Fest talk on using delimited continuations for CGI programming. Just so it doesn’t get overlooked, here it is:
I’m obsessed in pointing out that every programmer already knows and understands the delimited continuations; they might not know that word though. Everyone knows that when a process executes a system call like
read, it gets suspended. When the disk delivers the data, the process is resumed. That suspension of a process is its continuation. It is delimited: it is not the check-point of the whole OS, it is the check-point of a process only, from the invocation ofmain()up to the pointmain()returns. Normally these suspensions are resumed only once, but can be zero times (exit) or twice (fork).
I especially like the final part about exit and
fork, which drives home the notion that something more
subtle than returning from a typical function call is going on. If
anybody is confused over what suspended means, that last part
ought to clear things up.
The next time I need to explain delimited continuations, I know how I’m going to do it.