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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: The LOOP macro
Date: Sat, 31 Aug 2024 20:02:56 -0000 (UTC)
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> Or if the predicate functions FOO-P and BAR-P are sufficiently 
> expensive that you don't want to compute more often than absolutely 
> necessary: 
> 
>   (loop for x in things 
>         for foo-p = (foo-p x) 
>         for bar-p = (bar-p x) 
>         when foo-p collect x into foos 
>         when bar-p collect x into bars 
>         when (and foo-p bar-p) collect x into both 
>         finally (return (values foos bars both)))

Gauche Scheme

(define things '(2 9 33 -44 0 5 -27 88 6 99 -7))

(rlet1 al '()
  (dolist (x things)
    (let ((odd-p (odd? x))  (neg-p (negative? x)))
      (if odd-p (apush! al 'odd x))
      (if neg-p (apush! al 'neg x))
      (and odd-p neg-p (apush! al 'both x)))))

((both -7 -27) (neg -7 -27 -44) (odd -7 99 -27 5 33 9))

Given:

(define-syntax ainc!
  (syntax-rules ()
    [(_ alist key val func default)
     (let ((pair (assoc key alist)))
       (if pair
         (set-cdr! pair (func val (cdr pair)))
         (set! alist (cons (cons key (func val default)) alist))))]
    [(_ alist key val func)
     (ainc! alist key val func 0)]
    [(_ alist key val)
     (ainc! alist key val +)]
    [(_ alist key)
     (ainc! alist key 1)]))

(define-syntax apush!
  (syntax-rules ()
    [(_ alist key val)  (ainc! alist key val cons '())]))