Hi!

Thanks for the question.

The model of SRFI 248 (which is deliberately simple and basic but rich
enough to build more complex abstractions on top of it) is that there
is basically just one type of effect. Whatever effect is raised, the
innermost handler is called (and can move control to the next
enclosing handler by reraising the effect), so one can detect every
unwinding of the stack through
‘with-unwind-handler’/‘raise-continuable’ and thus implement a version
of ‘dynamic-wind’ that works for delimited continuations.

With delimited continuations, ‘call/cc’ is not really necessary any
more (except for backward compatibility). As the unwinding occurring
from instantiating a continuation K captured by ‘call/cc´ does not
trigger any unwind handler, one still needs the legacy ‘dynamic/wind’
as long as the legacy ‘call/cc’ exists. One could amend SRFI 248 by
integrating ‘call/cc’ fully by demanding that instantiating K raises a
condition object, which leads to triggering the unwind handlers. This
is less conservative but probably The Right Thing.

Marc

Am Do., 1. Jan. 2026 um 04:24 Uhr schrieb Eleanor Bartle
<xxxxxx@eleanor-nb.com>:
>
> Apologies for the noise. I’m a passive nerd, attempting to understand Scheme internals, enamoured by delimited continuations.
>
> One thing I don’t understand in this SRFI is how it would be able to dispense with dynamic-wind, were it not for call/cc. How does that come about? More precisely, why does the unwinding/rewinding of a delimited continuation not necessitate dynamic-wind, and why does that reason not apply to call/cc?
>
> Eleanor