How does the yin-yang puzzle work?

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2022-01-22 03:30:06

I'm trying to grasp the semantics of call/cc in Scheme, and the Wikipedia page on continuations shows the yin-yang puzzle as an example:

First of all, I personally find the call/cc x to be harder to comprehend than the equivalent alternative, x get/cc. It still calls x, passing it the current continuation, but somehow is more amenable to being represented in my brain circuitry.

With that in mind, the construct (call-with-current-continuation (lambda (c) c)) becomes simply get-cc. We’re now down to this:

The next step is the body of the inner lambda. (display #\@) cc, in the more familiar syntax (to me, anyway) means print @; return cc;. While we’re at it, let’s also rewrite lambda (cc) body as function (arg) { body }, remove a bunch of parentheses, and change function calls to the C-like syntax, to get this:

It’s starting to make more sense now. It’s now a small step to rewrite this completely into C-like syntax (or JavaScript-like, if you prefer), to get this: