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- Subject: Re: Google Summer of Code 2011
- From: David Kastrup <dak@...>
- Date: Tue, 22 Feb 2011 14:29:19 +0100
steve donovan <firstname.lastname@example.org> writes:
> On Tue, Feb 22, 2011 at 3:09 PM, Gunnar Zötl <email@example.com> wrote:
>> Anyways, call/cc would be so brilliant :)
> I've tried to understand Scheme continuations, but so far this brain
> has refused to make the jump.
Well, you presumably know the normal call/return mechanism in connection
with a call stack. Now you just don't use a separate call stack, but
just place it on the normal heap and let it be subject to
garbage-collection like anything else. So far so good.
A return is then not final as long as you keep a reference to the
associated continuation object. You can return again to the same place
in the logic of things.
In fact, the Chicken scheme compiler does just that: it never actually
returns, but if the return stack grows too large, it gets
Now Scheme is actually a rather pointless language for actually working
with continuations since most of its data _structures_ (like a list) are
static and not maintained inside of the call stack/heap.
That means that if you use a continuation, very little is actually
reliably being rewound to the state of the previous call/return.
call/cc may be brilliant, but it is basically only used as a low-level
tool for implementing more understandable constructs.