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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: CL: Processing more than one element of a sequence at a time?
Date: Sat, 31 Aug 2024 05:09:24 -0000 (UTC)
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Frode V. Fjeld wrote:

> Raffaele Ricciardi <rfflrccrd@gmail.com> writes:
> 
> > in CL, is there an idiomatic way to process more than one element of a
> > sequence at a time?
> 
> Yes: (loop for (a b) on list by #'cddr collect (cons a b))
> 
> Or without LOOP, I'd do:
> 
>   (do ((a (pop list) (pop list))
>        (b (pop list) (pop list))
>        (result nil))
>       ((null list) (reverse result))
>     (push (cons a b) result))

Testing:

(setq lst '(p 2 q 3 r 4 s 5))

(do ((a (pop lst) (pop lst))
     (b (pop lst) (pop lst))
     (result nil))
    ((null lst) (reverse result))
  (push (cons a b) result))

((P . 2) (Q . 3) (R . 4))

The last pair is missing.


Gauche Scheme

(define lst '(p 2 q 3 r 4 s 5))

(do@ ((:for a (pop! lst))
      (:for b (pop! lst))
      (result '()))
  ((begin (push! result (cons a b))  (null? lst))
   (reverse result)))

((p . 2) (q . 3) (r . 4) (s . 5))


Given:

Use ":for" when the same expression is to be assigned
to the variable every time.

(define-syntax do@-aux
  (syntax-rules ()
    [(do* (inits ...) ((var update) ...) (test expr ...) stuff ...)
     (let* (inits ...)
       (if test
         (begin expr ...)
         (begin
           (begin stuff ...)
           (let loop ()
             (begin (set! var update) ...)
             (if test
               (begin expr ...)
               (begin  stuff ...
                 (loop)))))))]))

(define-syntax do@
  (syntax-rules (:for  !!!)
    [(do@ !!! (inits ...) (updates ...)
       ((:for var expr) more ...) until body ...)
     (do@ !!! (inits ... (var expr)) (updates ... (var expr))
       (more ...) until body ...)]
    [(do@ !!! (inits ...) (updates ...)
       ((var init update) more ...) until body ...)
     (do@ !!! (inits ... (var init)) (updates ... (var update))
       (more ...) until body ...)]
    [(do@ !!! (inits ...) (updates ...)
       ((var init) more ...) until body ...)
     (do@ !!! (inits ... (var init)) (updates ... )
       (more ...) until body ...)]
    [(do@ !!! inits updates () until body ...)
     (do@-aux inits updates until body ...)]
    [(do@ inits-updates until stuff ...)
     (do@ !!! () () inits-updates until stuff ...)]))