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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