So throw serves to "point back" to the current continuation, and optionally continues with a return value. (This seems to be the behavior of a simple (ccc (throw) ...) without the let; I'm not sure why it's defined this way.) The above loop returns 5 -- the continuation captured by ccc is outside of the loop, so its action is to return whatever value we say the while expression returns and continue evaluating anything after it.
This way, Arc gives you a decorator for return values. You can use this pattern to decorate any portion of your loop, to give different effects. Outside of the loop, it's like a break. Inside of the loop, it's like a continue due to how while is expanded.
As you can see, the continuation is captured around the body, but the test properly resides outside of the body, so it'll still be executed even if you continue (because, again, the default continuation is to keep on chuggin' along).
You could have a while macro encapsulate these (similar to catch) so you don't have to write
(point break
(while test
(point continue
body)))
I've been trying to think of good names for such a loop. The most descriptive words are long (e.g., continuable-while, breakable-while), but we usually recognize "esc" as "escape", so it's a short yet descriptive option. Something like
To make code show up here properly, put two spaces before each line of code and paste it here. As in:
;I used this function to add spaces so I could put it here!
(def add-spaces (xt)
(no:map [prn " " _] lines.xt))
This assumes you have it indented already. If you don't, find a text editor that indents Lisp code for you (I recommend DrRacket). The 'ppr function works reasonably well, too.
ccc is certainly way to go, but just in case: mzscheme does not use continuation passing style so the ccc can work much much slower than just a function call because it copies the stack.
Actually I didn't do the profiling so maybe there is nothing to worry about.
Ah, that helps understand ccc. I've been mulling ccc for months without having a handle on it. But seeing a simple solution to a simple problem I can relate to helps immeasurably.
