---------- Forwarded message ----------
From: "enami" <xxxxxx@github.com>
Date: Jan 27, 2016 9:29 PM
Subject: [srfi-121] generator->list etc consumes extra item when number of items is specified (#5)
To: "scheme-requests-for-implementation/srfi-121" <srfi-121@noreply.github.com>
Cc:

The generator->list etc consumes extra item when number of items is specified. It is troublesome when user want to process each n items but don't want to lose any items.

Here is a example to illustrate the problem (I used gauche but override generator->list with reference implementation version):

$ gosh
gosh> (use gauche.generator)
#<undef>
gosh> (define generator->list
        (case-lambda ((gen) (generator->list gen +inf.0))
                     ((gen n)
                      (let ((next (gen)))
                           (if (or (eof-object? next)
                                   (= 0 n))
                               '()
                               (cons next (generator->list gen (- n 1))))))))
generator->list
gosh> (define g (list->generator '(1 2 3 4 5 6)))
g
gosh> (generator->list g 2)
(1 2)
gosh> (generator->list g 2)
(4 5)
gosh> (generator->list g 2)
()
gosh>

Note that item 3 and 6 are lost. generator->vector! also loses an item when vector is exhausted first.

$ gosh
gosh> (use gauche.generator)
#<undef>
gosh> (define (generator->vector! vector at gen)
  (let loop ((value (gen)) (count 0) (at at))
    (cond
      ((eof-object? value) count)
      ((>= at (vector-length vector)) count)
      (else (begin
              (vector-set! vector at value)
              (loop (gen) (+ count 1) (+ at 1)))))))
generator->vector!
gosh> (define v (make-vector 3))
v
gosh> (define g (list->generator '(1 2 3 4 5)))
g
gosh> (generator->vector! v 0 g)
3
gosh> (g)
5
gosh> v
#(1 2 3)
gosh>

generator->vector! returns 3 but actually four items are consumed and item 4 is lost.

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