At Thu, 23 Feb 2017 10:04:16 +0100, Jens Axel Søgaard wrote:
> A technical issue: How do you plan to implement continuation marks?
>
> I see several options:
>
> i) adding continuation marks to Chez (tricky)
> ii) pass continuation marks along each function call using an extra
> argument (expensive)
> iii) code transformation (expensive?)
>
> Since Chez is "friendly" (provides TCO) is it possible to use a code
> transformation
> without a performance hit?
Here's a sketch of how a `call/cm` function could work, relying on the
way that `eq?` works for detecting equivalent continuations in Chez:
(define (call/cm key val proc)
;; For this sketch, assumes `key` is always the same
(call/cc
(lambda (k)
(if (and (pair? stack)
(eq? k (caar stack)))
(begin
(set-cdr! (car stack) (cons key val))
(proc))
(begin0
(call/cc
(lambda (k)
(set! stack (cons (cons k (cons key val))
stack))
(proc)))
(set! stack (cdr stack)))))))
This approach causes the continuations that appear at the Racket level
to *not* be `eq?` when they're the same, because a continuation mark
can insert an extra frame (but only one per frame that would exist
otherwise). That loss seems ok, because Racket doesn't currently
provide an equality check on continuations.
Besides making `with-continuation-mark` expand to call `call/cc`, all
the other control operators, like `call/cc` and `dynamic-wind`, need to
be wrapped to cooperate with it. Those wrappers are going to happen,
anyway, to implement delimited control.
My tests suggest that this approach runs just a little slower than
continuation marks in Racket (which are deeply built in), and that's
good enough to get started. Longer term, I think something like this
approach could work and perform better as built into Chez; I've
experimented with that, but not conclusively.
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