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Re: Scheme mistakes (was Re: nil)
> From: "Alan Bawden" <Alan@lcs.mit.edu>
> > Consider evaluating:
> > (throw (f x) 42)
> > Since you know that the expression `(f)' must yield a continuation,
> > old continuation can be discarded (and perhaps reclaimed by the
> > collector) -before- you call the function `f'.
> From: "Joe Marshall" <email@example.com>
> Date: Thu, 14 Aug 2003 16:09:20 -0400
> (throw (call-with-current-continuation (lambda (c) ...)) 42)
> What about it? In this case `c' will be a continuation that expects
> another continuation, which will in turn be passed `42'. So:
> (throw (call-with-current-continuation (lambda (c) (throw c k))) 42)
> is the same as:
> (throw k 42)
Ok. I didn't give the example I thought I had.
In Scheme now (without discarding continuations) all continuations are part
of a tree rooted at some null continuation established when Scheme booted.
I was concerned that if throw disconnects its continuation before receiving
the new one, that you'd be able to detach part of the tree. But on further
thought, I can see how you could do this safely. Rather than have throw
discard its continuation via some magic side effect, it can discard its
by invoking a continuation captured near the root. So you can do this
in `regular' scheme.