GNU bug report logs -
#21894
escape continuation doc wrong about reinvokability
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Reported by: Zefram <zefram <at> fysh.org>
Date: Thu, 12 Nov 2015 23:28:02 UTC
Severity: normal
Done: Andy Wingo <wingo <at> pobox.com>
Bug is archived. No further changes may be made.
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Your bug report
#21894: escape continuation doc wrong about reinvokability
which was filed against the guile package, has been closed.
The explanation is attached below, along with your original report.
If you require more details, please reply to 21894 <at> debbugs.gnu.org.
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21894: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=21894
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems
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On Fri 13 Nov 2015 00:27, Zefram <zefram <at> fysh.org> writes:
> # Escape continuations are delimited continuations whose
> # only use is to make a non-local exit--i.e., to escape from the current
> # continuation. Such continuations are invoked only once, and for this
> # reason they are sometimes called "one-shot continuations".
>
> O RLY?
:)
I removed that last sentence. By saying less it will be less possible
to enter into this kind of error :)
Thanks for the report,
Andy
BTW: You have submitted some really nice reports! If you would like to
assign copyright to the FSF so that you can get patches into Guile, let
me know and I can send you the docs. Cheers :)
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The manual says
# Escape continuations are delimited continuations whose
# only use is to make a non-local exit--i.e., to escape from the current
# continuation. Such continuations are invoked only once, and for this
# reason they are sometimes called "one-shot continuations".
O RLY?
scheme@(guile-user)> (use-modules (ice-9 control))
scheme@(guile-user)> (define cc #f)
scheme@(guile-user)> (list 'a (let/ec e (list 'b (e (call-with-current-continuation (lambda (c) (set! cc c) 0))))))
$1 = (a 0)
scheme@(guile-user)> (cc 1)
$2 = (a 1)
scheme@(guile-user)> (cc 2)
$3 = (a 2)
Clearly I have invoked this escape continuation, successfully, more
than once. The semantics here are perfectly sensible, it's just
the documentation that's off the mark, because it ignores how escape
continuations interact with other kinds of continuation. I suggest
changing "Such continuations are invoked only once" sentence to something
like
Such continuations can only be invoked from within the dynamic
extent of the call to which they will jump. Because the jump
ends that extent, if escape continuations are the only kind of
continuations being used it is only possible to invoke an escape
continuation at most once. For this reason they are sometimes
called "one-shot continuations", but that is a misnomer when other
kinds of continuations are also in use. Most kinds can reinstate a
dynamic extent that has been exited, and if the extent of an escape
continuation is reinstated then it can be invoked again to exit that
extent again. Conversely, an escape continuation cannot be invoked
from a separate thread that has its own dynamic state not including
the continuation's extent, even if the continuation's extent is
still in progress in its original thread and the continuation has
never been invoked.
-zefram
This bug report was last modified 9 years and 20 days ago.
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