Oleg's great way of explaining delimited continuations

Tom Moertel — Mon, 05 May 2008 09:58:00 -0400

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 of main() up to the point main() 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.

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Oleg's great way of explaining delimited continuations