Re: So, what the heck is a continuation anyway?

[Dan Sugalski <address@hidden>]

At 03:11 PM 12/10/2001 -0500, Shriram Krishnamurthi wrote:
>Dan, here's an explanation that should make sense to an assembly hacker:
>
>One simple way to think of a continuation is
>
>   It's the stack.
>
>At some point, while my program is running, I look at the stack and
>think, "Hey, this is a useful computation -- maybe I'd like to come
>back and do this later".  I "capture" it.  Think of that as making a
>copy of the stack.

Sounds like a warped closure. (Or a closure's a warped continuation. Or both)

>Later I say, "Hmm, that was a useful computation -- I'd like to do it
>now".  At that point, I use the copied stack.
>
>Scheme asks, why use it only once?  Use it as many times as you want.
>So each time I use it, I have to copy it again.

Is the copying fully required, and does anything that happens after the 
snapshot's taken affect the snapshot?

For example, if I have:

    {
       int i = 0;
       make_continuation();
       print i;
       i = 3;
    }

Will that print 0 or 3 when the continuation is invoked? And if I invoke 
that continuation a second time will it print 0 or 3?

>There are lots of papers on how to implement continuations MUCH more
>efficiently than this, but explaining them won't help you understand
>the operation much more easily.

Given the rather odd internals of Parrot, I'm not sure how much help 
they'll be, but I'll dig in once I get a good grip on this.

>So, what's an example of a computation that totters on from stack to
>stack?  Think Web programming.  See my lecture notes for more:
>
>   http://www.cs.brown.edu/courses/cs173/2001/Lectures/
>
>Specifically, the lectures for
>
>   2001-10-01    Web Programming
>   2001-10-03    Converting Programs for the Web
>   2001-10-10-12 Continuations
>
>Don't worry, the parens don't bite. (-:

I've survived the horror that is SelfGOL. (It's a perl program that's 
self-replicating, makes other perl programs self-replicating, plays the 
game of life, and displays a rotating banner. Without loops. In less than 
1200 characters of source) Parens are *nothing* compared to that. :)

					Dan

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Dan Sugalski                          even samurai
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