I read docs, played with examples, and (re-)discovered, that the position
of #:break or #:final in the iterator clauses (of course) matters. So this,
e.g.
(for/fold ([j-found? #f] [pos 0]) ([ch ch-stream] [ch-idx (in-naturals 1)]
#:break j-found?)
solves my problem.
On Tuesday, December 13, 2022 at 8:40:06 AM UTC+1 [email protected] wrote:
> I haven't asked any questions here for a long time, so hope it's OK to
> cross-post here:
>
>
>https://racket.discourse.group/
>
> (which I just discovered.)
>
>
> Cheers,
>
> Tim
>
> On Monday, December 12, 2022 at 10:28:22 PM UTC+1 [email protected] wrote:
>
>> hi, I'm trying to do something slightly harder than what the attached
>> example tries to do, but this example suffices to show that I, evidently,
>> am missing something about how for/fold and/or #:break work together.
>>
>> (I have used both before, but have gotten out of practice with racket of
>> late and am stuck longer on this than seems reasonable.)
>>
>> I expect both tests to pass, but both fail (the "j" for jewel in "xxjxx")
>> is seen but not "found", as it were.
>>
>> I'd be grateful for a bug fix and/or explanation.
>>
>> Cheers,
>>
>> Tim Hanson
>>
>>
>>
>> #lang racket
>>
>> (require rackunit)
>>
>> (define (port->char-stream aport)
>> (printf "(port->char-stream aport)~n")
>> (define (process-next-ch-iter)
>> (let ([ch (read-char aport)])
>> (printf "p->cs ch: ~a~n" ch)
>> (if (eof-object? ch)
>> empty-stream
>> (stream-cons
>>ch
>>(process-next-ch-iter)
>>
>> (process-next-ch-iter))
>>
>>
>> (let ([my-port
>>(open-input-string "xxjxx")])
>>
>> (let ([ch-stream (port->char-stream my-port)])
>> (let-values ([(pos-found? pos)
>> (for/fold ([j-found? #f]
>> [pos 0])
>> (#:break j-found?
>> ;#:final j-found?
>> [ch ch-stream]
>> [ch-idx (in-naturals 1)])
>> (begin
>> (printf "ch: ~a~n" ch)
>> (printf "ch-idx: ~a~n" ch-idx)
>> (printf "(eq? ch #\\j): ~a~n" (eq? ch #\j))
>> (values
>>(eq? ch #\j)
>>ch-idx)))])
>>
>> (check-equal? pos-found? #t)
>>
>> (check-equal? pos 3)
>>
>> (close-input-port my-port
>>
>>
>>
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