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From: HenHanna <HenHanna@dev.null>
Newsgroups: comp.lang.lisp
Subject: Re: .Re: simple lisp function
Date: Thu, 6 Jun 2024 20:21:06 +0000
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B. Pym wrote:

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


I remember writing this in Lisp and Python.  a few years ago.


Is Scheme (or Gauche) used outside of the Academia?  

   What kind of people (other than Grad students, Researchers in
Prog.Lang design)
               would know about  Split-at  and  List-ec  and  Receive 
?