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Re: Non-stack-copying call-with-current-continuation?
From: |
David Kastrup |
Subject: |
Re: Non-stack-copying call-with-current-continuation? |
Date: |
Sun, 04 Mar 2012 14:59:12 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.0.92 (gnu/linux) |
Andy Wingo <address@hidden> writes:
> Hi David,
>
> On Sun 04 Mar 2012 13:01, David Kastrup <address@hidden> writes:
>
>> The global symbol space is a different identity space than heap
>> equality, and it never gets garbage collected: the lifetime of a
>> gensym is eternal.
>
> This is not true in Guile, where symbols can be garbage collected.
The symbol name is not garbage collected. That is the difference
between gensym and make-symbol.
>> And frankly: the manual talks about prompts being composable and
>> gives an example which seems utterly wrong to me since it does not
>> actually abort a computation but rather half-finishes it. It is
>> unclear what part of the computation will finish and what will
>> complete.
>
> That is an interesting point. I guess there are two ways of answering
> it. One is to note that in Scheme, it's difficult in general to
> determine whether a computation is finished or will finish, because of
> call/cc.
>
> But you ask about a specific point, here: an abort to a prompt is
> basically boils down to a longjmp to the prompt's handler. The
> partial continuation is logically passed as an argument to the
> handler.
But where does the "partial continuation" start and where does it end?
If I am doing a "longjmp to the prompt's handler", how can it be that
the calling stack frame inside of the thunk that is supposed to be
exited can finish a calculation? Where is the difference between
(+ 34 (abort-to-prompt 'foo))
and
(let ((x (abort-to-prompt 'foo))) (+ 34 x)) ?
Why is the first allowed to complete and return a result, and the second
(presumably) not? Or _if_ the second is allowed to complete, what does
"abort" in "abort-to-prompt" even mean?
All this does not really make discernible sense to me. Whereas call/ec
has rather clear semantics and usage. The one thing that is not
self-evident is its behavior in case of misuse, namely when it is asked
to do a job only call/cc can.
--
David Kastrup
- Re: Non-stack-copying call-with-current-continuation?, (continued)
- Re: Non-stack-copying call-with-current-continuation?, David Kastrup, 2012/03/01
- Re: Non-stack-copying call-with-current-continuation?, Noah Lavine, 2012/03/01
- Re: Non-stack-copying call-with-current-continuation?, David Kastrup, 2012/03/01
- Re: Non-stack-copying call-with-current-continuation?, Noah Lavine, 2012/03/01
- Re: Non-stack-copying call-with-current-continuation?, David Kastrup, 2012/03/02
- Re: Non-stack-copying call-with-current-continuation?, Andreas Rottmann, 2012/03/03
- Re: Non-stack-copying call-with-current-continuation?, Andreas Rottmann, 2012/03/03
- Re: Non-stack-copying call-with-current-continuation?, Andy Wingo, 2012/03/03
- Re: Non-stack-copying call-with-current-continuation?, David Kastrup, 2012/03/04
- Re: Non-stack-copying call-with-current-continuation?, Andy Wingo, 2012/03/04
- Re: Non-stack-copying call-with-current-continuation?,
David Kastrup <=
- Re: Non-stack-copying call-with-current-continuation?, Andy Wingo, 2012/03/04
- Re: Non-stack-copying call-with-current-continuation?, Mark H Weaver, 2012/03/04
- Re: Non-stack-copying call-with-current-continuation?, David Kastrup, 2012/03/04
- Re: Non-stack-copying call-with-current-continuation?, Mark H Weaver, 2012/03/04
- Re: Non-stack-copying call-with-current-continuation?, David Kastrup, 2012/03/04
Re: Non-stack-copying call-with-current-continuation?, Nala Ginrut, 2012/03/01
Re: Non-stack-copying call-with-current-continuation?, Andy Wingo, 2012/03/03