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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Python syntax in Lisp and Scheme
Date: Fri, 13 Sep 2024 19:22:53 -0000 (UTC)
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Pascal Costanza wrote:

> What about dealing with an arbitrary number of filters?
> 
> (defmacro predicate-collect (list &body predicates)
>    (let ((collectors (mapcar (lambda (predicate)
>                                 (declare (ignore predicate))
>                                 (gensym "COLLECT"))
>                              predicates))
>          (collect-t (gensym "COLLECT")))
>       `(with-collectors (,@collectors ,collect-t)
>           (dolist (l ,list)
>             (cond ,@(mapcar (lambda (predicate collector)
>                               `((funcall ,predicate l) (,collector l)))
>                              predicates collectors)
>                   (t (,collect-t l)))))))
> 
> An example:
> 
>  > (predicate-collect '(-5 -4 -3 -2 -1 0 1 2 3 4 5)
>      (function evenp)
>      (lambda (n) (< n 0))
>      (lambda (n) (> n 3)))
> (-4 -2 0 2 4)
> (-5 -3 -1)
> (5)
> (1 3)
> 
> 
> I use the list collector macros by Tim Bradshaw here - see
> http://www.tfeb.org/lisp/hax.html#COLLECTING

Gauche Scheme

(define (multi-partition the-list . predicates)
  (if (null? predicates)
    (list the-list)
    (receive (yes no)
      (partition (car predicates) the-list)
      (cons yes (apply multi-partition no (cdr predicates))))))

(multi-partition '(-5 -4 -3 -2 -1 0 1 2 3 4 5)
  even?
  (is < 0)
  (is > 3))

((-4 -2 0 2 4) (-5 -3 -1) (5) (1 3))

Given:

(define-syntax is
  (syntax-rules ()
    [(is x)
     (lambda (y) (equal? y x))]
    [(is compare x)
     (lambda (y) (compare y x))]
    [(is key compare x)
     (lambda (y) (compare (key y) x))]))