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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Is there a better way to read multiple values from a string
Date: Wed, 2 Jul 2025 15:48:19 -0000 (UTC)
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B. Pym wrote:

> Pascal J. Bourguignon wrote:
> 
> > > I have a strings like "0  2  0  1".  The number of values may vary, but
> > > they all can be read using READ
> > > 
> > > Right now I do the following
> > > (loop 
> > >      :for (x pos) = (multiple-value-list 
> > > 		     (read-from-string first-line nil nil :start (or pos 0)))
> > >      :while x
> > >      :collect x)
> > > 
> > > 
> > > Is there a more idiomatic way to read all the tokens in the string and
> > > return them as a list?
> > 
> >     (with-input-from-string (in string)
> >       (loop :for n = (read in nil  nil)
> >             :while n :collect n))
> 
> If we are allowed to use a collector, then the loop
> becomes much shorter.  Compare:
> 
> (loop :for n = (read in nil  nil) :while n :collect n))
> (until (eof-object? (bag (read))))
> 
> Gauche Scheme:
> 
> (define bag (mlistbag))
> 
> (with-input-from-string "0 4 2 5"
>   (lambda()
>     (until (eof-object? (bag (read))))))
> 
> ;; Remove the eof-object.
> (drop-right (bag) 1)
>   ===>
> (0 4 2 5)
> 
> Given:


A better way.


;; Remove the eof-object.
(bag 1 'drop)
  ===>
(0 4 2 5)

Given:

(define (mbag init func :optional (pass-through #f))
  (let ((val init) (func func) (pass-through pass-through))
    (lambda args
      (if (null? args)
        val
        (let ((x (car args)))
          (if (null? (cdr args))
            (begin
              (set! val (func x val))
              (if pass-through
                x
                val))
            (let ((kons (cadr args)))
              (if (or (eq? 'drop kons) (eq? 'DROP kons))
                (do ((n x (- n 1)))
                  ((zero? n)  (if (eq? 'drop kons) (reverse val) val))
                  (set! val (cdr val)))
                (begin
                  (set! val (kons x val))
                  (if pass-through
                    x
                    val))))))))))

(define (mlistbag :optional (raw #f) (pass-through #t))
  (let ((bag (mbag '() cons pass-through)))
    (lambda args
      (if (null? args)
        (if raw
          (bag)
          (reverse (bag)))
        (apply bag args)))))