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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Inverting alists -- *NOT* a homework assignment!!
Date: Thu, 19 Sep 2024 12:30:54 -0000 (UTC)
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Christopher N. Vogt wrote:

> > ;; Hi.  I would like to invert an alist of the form ;; ;; ((reference-1
> > referent-a referent-b...) (reference-2 referent-c ...)...) ;; ;; to
> > another alist of the form ;; ;; ((referent-a reference-1) (referent-b
> > reference-1) ;;  (referent-c reference-2) ...) ;; ;; I have gotten
> > something to work but it's so stupefyingly *UGLY* that I ;; just know
> > someone out there can demonstrate a more better beautiful ;; elegant
> > means of achieving the same result. ;; ;; [ I'm working in emacs-lisp
> > with the 'cl' package, so the code below ;; ought to be reasonably close
> > to Common Lisp...] ;; ;; ;; Mind you, I am teaching myself lisp and THIS

.....

> I'd do it something like this:
> (defun invert-alist (alist)
>    (loop for list in test
>          for reference = (first list)
>          appending (loop for referent in (cdr list)
>                          collect (list referent reference))))

Gauche Scheme

(define (invert-alist alist)
  (@ append-map xs :
    (map (cut  list <> (car xs)) (cdr xs)) :
    alist))

Given:

(define-syntax @-aux
  (syntax-rules (:)
    ;; No parameters given; use "cut".
    [(_ func () ((e0 e ...)) : stuff ...)
     (func (cut e0 e ...) stuff ...)]
    [(_ func vars (e0 e ...) : stuff ...)
     (func (lambda vars e0 e ...) stuff ...)]
    [(_ func vars (e0 e ...) expr more ...)
     (@-aux func vars (e0 e ... expr) more ...)]
    [(_ func vars () : expr more ...)
     (@-aux func vars (expr) more ...)]
    [(_ func (vars ...) () var more ...)
     (@-aux func (vars ... var) () more ...)]
))

(define-syntax @
  (syntax-rules ()
    [(_ func stuff ...)
     (@-aux func () () stuff ...)]))