As I've continued to experiment with this, I've run into something that I don't
really understand. I've managed to come up with this snippet of code.
(define (do prompt-tag)
(define (loop element continue)
(if continue
(+ element (call-with-continuation-prompt continue prompt-tag loop))
element))
(call-with-continuation-prompt
(thunk
(for/fold () ([i (in-range 10)])
(let/cc continue
(abort-current-continuation prompt-tag (let () i) continue)))
(abort-current-continuation prompt-tag 0 #f))
prompt-tag loop))
(do (default-continuation-prompt-tag))
(do (make-continuation-prompt-tag))
The first of the two calls at the end returns 45, but the second returns 0. Why
does using a non-default prompt tag change the behavior if I'm installing my
own prompt anyway?
> On May 19, 2015, at 19:24, Alexis King <lexi.lam...@gmail.com> wrote:
>
> I'm trying to implement a for/stream loop using for/fold/derived that will
> return a lazy stream, as would be expected. One way to do this is by using
> delimited control, which is what I'm currently trying. If there's an easier
> way, let me know, but I'd still like to figure this out as a pedagogical
> exercise.
>
> Right now, I'm just trying to play with for/fold and some control operators
> to get a feel for how a solution should work. Using racket/control, I've
> managed to get this working snippet:
>
> (define result-stream
> (let ()
> (define (stream-loop element continue)
> (stream-cons element
> (call-with-values
> (thunk (call/prompt continue))
> stream-loop)))
> (call-with-values
> (thunk
> (prompt
> (for/fold ()
> ([i (in-naturals)])
> (let/cc continue
> (abort i continue)))))
> stream-loop)))
>
> This will create an infinite, lazy stream bound to ‘result-stream’ containing
> all the natural numbers. It works, which is cool, but it also seems pretty
> overcomplicated.
>
> Is there a better approach to this sort of thing that I'm missing? My
> intuition for working with control operators isn't the best, so I wouldn't be
> surprised if I was overlooking something.
>
> Thanks,
> Alexis
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