a for/fold #:break question, I think

[tbh...@gmail.com]

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))))

[tbh...@gmail.com]

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

[tbh...@gmail.com]

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.