Re: Non-stack-copying call-with-current-continuation?

[Andreas Rottmann <a.rottmann@gmx.at>]

David Kastrup <dak@gnu.org> writes:

> Noah Lavine <noah.b.lavine@gmail.com> writes:
>
>> Hello,
>>
>>> Sure, but things like gensym and make-prompt-tag (and (list '()) for
>>> creating an eq?-unique object) are artificial hygiene coming at a cost
>>> in symbol table and symbol generation time rather than "lexical"
>>> hygiene.  They need _extra_ work, whereas the
>>> call-with-current-continuation approach needed _less_ work.  Basically I
>>> want something like call-with-single-continuation that will only allow
>>> one return (and any dynwind out counts and should work if it is the
>>> first, so it is not exactly equivalent to using
>>> with-continuation-barrier) and come without the stack-copying cost of
>>> call-with-current-continuation.
>>
>> I agree that it's not pretty. We have hygienic macros so we don't have
>> to use gensym, after all. But I don't know of a better way.
>
> Well, to wrap this up: the manual (not current) states
>
>     It is traditional in Scheme to implement exception systems using
>     `call-with-current-continuation'.  Continuations (*note
>     Continuations::) are such a powerful concept that any other control
>     mechanism -- including `catch' and `throw' -- can be implemented in
>     terms of them.
>
> [...]
>
>        The more targeted mechanism provided by `catch' and `throw' does not
>     need to save and restore the C stack because the `throw' always jumps
>     to a location higher up the stack of the code that executes the
>     `throw'.  Therefore Guile implements the `catch' and `throw' primitives
>     independently of `call-with-current-continuation', in a way that takes
>     advantage of this _upwards only_ nature of exceptions.
>
>
> I think that using something like "call-with-single-continuation" as the
> underlying primitive would make Guile quite more similar to
> "traditional" systems in the code base.  It would also provide a
> minimally-invasive tool for tuning existing code based on
> call-with-current-continuation in case that the stack copying semantics
> are _not_ required.  Definitely more Schemeish than stuff like, uh,
> prompts?
>
Just to throw my two cents in: Racket (and probably other Schemes)
provide this primitive under the name call-with-escape-continuation
(call/ec):

http://docs.racket-lang.org/reference/cont.html?q=call/ec#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29

Regards, Rotty
-- 
Andreas Rottmann -- <http://rotty.yi.org/>