From: Paul Brannan <pbrannan@...>
Date: 2003-07-28T23:53:24+09:00
Subject: Re: Throwing exceptions into continuations.

On Sun, Jul 27, 2003 at 10:53:42AM +0900, Christopher Armstrong wrote:
> Hi all. I'm playing around with Ruby because I like experimenting with
> concurrency systems, and found out it has continuations.
> 
> I'm going to be calling user-defined code inside of a callcc block,
> and the user-defined code will either return a value or raise an
> error, and I want that value or error to be passed on to the
> continuation.

Is there a reason why you are using continuations for this?  Perhaps
some other construct like procs or threads would work better.

> I can't find any way to throw into the continuation. If there is no
> way, can I request that a #throw method be added, or something like
> that?

I'm not entirely sure what you mean by "throw into".  Do you want to
cause an exception to occur in the middle of the user-defined code?

There is a Thread#raise, if you want to use a thread.  You can stop the
thread if you want it to stop running for a while.

> The only alternative I could think of is to have special code on the
> calling side of the callcc that will watch the return value to see if
> it is a array of two elements, the first being
> :special-internal-throw-exception-bit, and if so, throwing the second
> element of the array. This isn't what I want, though, especially
> because it won't maintain the stack trace from the other side.

You might be interested in what I did here:
  http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/rubycollections/rubycollections/rbc/iterator/continuation_iterator.rb?rev=1.17&content-type=text/vnd.viewcvs-markup

There are two continuations: one doing the iterating and one waiting for
the next item from the iteration.  The iteration loop is wrapped with a
begin/rescue block, and if I get the exception, it gets re-raised by the
caller with a backtrace that includes both the backtrace of the
iteration block and the backtrace of the code that called #next.  Not
perfect, but it gives me all the information I need when I'm debugging.

Paul