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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Ex. 3.5 in ACL
Date: Wed, 9 Jul 2025 14:45:44 -0000 (UTC)
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Zachary Beane wrote:

> > I am just trying to solve Ex 3.5 in Graham's ANSI Common Lisp book. I am
> > reading it on my own and not as part of a university course. The task is:
> >
> > define a function pos+, that takes a list as param and returns a list
> > that adds the position of each element of the list to the element's value.
> > Thus:
> > (pos+ '(7 5 1 4))
> > returns:
> > (7 6 3 7)

> ...but it's also easy with DO:
> 
>    (defun do-pos+ (orig-list)
>      (do ((i 0 (1+ i))
>           (list orig-list (cdr list))
>           (new-list nil (cons (+ (car list) i) new-list)))
>          ((endp list) (nreverse new-list))))


(define (do-pos+ orig-list)
  (do_ ((i 0 (+ 1 i))
        (x :in orig-list)
        (result :collect (+ i x)))
    (#f  @ result)))

(do-pos+ '(200 300 400))
  ===>
(200 301 402)

Given:

(define-syntax do_-aux
  (syntax-rules ( <> @  :in :collect )
    [ (do_-aux ((x what <>) more ...) (seen ...) stuff ...)
      (do_-aux (more ...) (seen ... (x what what)) stuff ...) ]
    [ (do_-aux ((x :in seq) more ...) seen (lets ...) (bool z ...) stuff ...)
      (do_-aux ((x (and (pair? the-list) (car the-list)) <>) more ...)
        seen
        (lets ... (the-list seq))
        ((or (null? the-list) (begin (pop! the-list) #f) bool) z ...)
        stuff ...) ]
    [ (do_-aux ((accum :collect x) more ...)   stuff ...)
      (do_-aux ((accum '() (cons x accum)) more ...) stuff ...) ]
    [ (do_-aux (spec more ...) (seen ...) stuff ...)
      (do_-aux (more ...) (seen ... spec) stuff ...) ]
    [ (do_-aux () seen lets (bool y ... @ result) stuff ...)
      (do_-aux () seen lets (bool y ... (reverse result)) stuff ...) ]
    [ (do_-aux () seen (lets ...) more ...)
      (let (lets ...)
        (do seen more ...))
    ] ))
(define-syntax do_
  (syntax-rules ()
    [ (do_ specs more ...)
      (do_-aux specs () () more ...) ] ))