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Re: [Scheme-reports] multiple returns from map functions?
- To: "scheme-reports@x" <scheme-reports@x>
- Subject: Re: [Scheme-reports] multiple returns from map functions?
- From: "Aaron W. Hsu" <arcfide@x>
- Date: Thu, 05 May 2011 15:18:52 -0400
- In-reply-to: <email@example.com>
- Organization: Indiana University
- References: <firstname.lastname@example.org> <B5B1129E42584497AC01C33006AE770C@SIXFOUR> <op.vu0rchkz0p3ku8@localhost> <email@example.com>
On Thu, 05 May 2011 14:48:40 -0400, Andy Wingo <wingo@x> wrote:
> On Thu 05 May 2011 19:39, "Aaron W. Hsu" <arcfide@x> writes:
>> On Thu, 05 May 2011 11:43:53 -0400, <xacc.ide@x> wrote:
>>> The brings up the question whether list control procedures can accept
>>> Should the following be valid?
>> This is interesting, normally, `map' is specifically not allowed to take
>> circular lists and you get an error when trying to deal with them.
>> However, that is partly because of the restriction prior to this that
>> arguments should be of the same length. Now, I think, there is an
>> opportunity to consider `map' in different terms. Whether we should do
>> this or not is still up to question.
> I don't know either, but do see srfi-1.
Here's a naive one argument map form that actually does handle circular
lists. It's probably not very clean, but it works.
> (define (map f lst)
(let ([circular #f])
(define (loop t r)
[(null? t) '()]
(cons (f (car t))
(loop (cdr t) r))]
[(null? (cdr r))
(cons (f (car t))
(loop (cdr t) '()))]
[(eq? t r)
(let ([res (list (f (car t)) (f (cadr t)))])
(set! circular (cdr res))
[else (cons (f (car t)) (loop (cdr t) (cddr r)))]))
[(null? lst) '()]
[(null? (cdr lst)) (list (f (car lst)))]
(let ([res (loop lst (cddr lst))])
(when circular (set-cdr! circular res))
> (let ([x (iota 10)])
(set-cdr! (list-tail x 9) x)
(map (lambda (x) (1+ x)) x))
Warning in pretty-print: cycle detected; proceeding with (print-graph #t)
#0=(1 2 3 4 5 6 7 8 9 10 . #0#)
Aaron W. Hsu
Programming is just another word for the lost art of thinking.
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