Arc Forumnew | comments | leaders | submitlogin
2 points by conanite 5359 days ago | link | parent

call/cc, (or ccc in arc) isn't actually used very much in the official distribution, except to provide escape routines, à la

  (catch
    (each x xs 
      (if (foo x) 
          (throw x))))
so you don't need to go all the way to the end of a loop.

The last time I looked, ccc isn't used to capture and re-use first-class continuation objects except for this escape-clause purpose.

Here's a fine article on the subject - http://www.intertwingly.net/blog/2005/04/13/Continuations-fo...

And here's a long-winded, verbose, java-oriented article I wrote last year in an attempt to explain ccc in java terms - http://www.fnargs.com/2008/05/continuations-arc-vs-java.html



1 point by conanite 5359 days ago | link

On top of that, while we're on the topic, I don't know what happens if you jump into another continuation in the following circumstances:

1. you're inside atomic-invoke, call-w/stdout, call-w/stdin, or anything that creates context that needs to be undone on the way out. The problem is that a continuation from in here can be exited multiple times although it was entered only once.

2. similarly, you're inside protect, (equivalent to "finally" in java) - is the "after"-fn called, and if so, only once or for each re-entered continuation?

3. the continuation you jump into was originally running on another thread.

I should write tests to figure out what happens in all these cases, but if anybody knows the answers I can live with being lazy :)

With all these questions, I can only echo fallintothis' remark that continuations are notoriously difficult to explain, and while they make for some great intellectual amusement (see Mondo Bizarro - http://arcfn.com/2008/03/arc-continuation-puzzle.html and http://arcfn.com/2008/03/continuations-made-difficult.html ), they're not practical for everyday coding.

-----

1 point by tumult 5357 days ago | link

This is what dynamic-wind is for. http://docs.plt-scheme.org/reference/cont.html#(def._((quote...)

(dynamic-wind is one of the coolest things ever)

-----