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From: "B. Pym" <No_spamming@noWhere_7073.org>
Newsgroups: comp.lang.lisp
Subject: .Re: simple lisp function
Date: Thu, 6 Jun 2024 10:38:54 -0000 (UTC)
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Pascal Bourguignon wrote:

> > this simple function i'm trying to write is giving me headaches!
> > basically i want to do something that does:
> > (variations 'x '(y z)) -> ((x y z) (y x z) (y z x))
> >
> > i come from a procedural programming background and find functional
> > programming very confusing (especially recursion).  can someone give
> > me some hints?  my attempts at this make no sense so pasting them here
> > would only confirm my newbish forray into LSIP.  thanks for any help!
> 
> (defun variations (item list)
>   (if (null list)
>     (list (list item))
>     (cons (cons item list)
>           (mapcar (lambda (rest) (cons (car list) rest))
>                   (variations item (cdr list))))))

Gauche Scheme:

(use srfi-1)  ;; split-at
(use srfi-42) ;; list-ec

(define (variations x lst)
  (list-ec (: i (+ 1 (length lst)))
    (receive (a b) (split-at lst i)
      `(,@a ,x ,@b))))

(variations '- '(a b c))
  ===>
((- a b c) (a - b c) (a b - c) (a b c -))

(variations '- '(a))
  ===>
((- a) (a -))

(variations '- '())
  ===>
((-))