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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Emacs Lisp's "mapconcat" in Common Lisp?
Date: Fri, 30 Aug 2024 23:21:11 -0000 (UTC)
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Pascal Bourguignon wrote:

> Teemu Likonen <tlikonen@iki.fi> writes:
> 
> > It's a Common Lisp newbie here; I'm more experienced in Emacs Lisp. I
> > wonder if there is similar function in CL like Emacs Lisp's "mapconcat":
> >
> >     (mapconcat 'identity '("one" "two" "three") "-")
> >     => "one-two-three"
> >
> > I can do the same in CL with this:
> >
> >     (let ((list '("one" "two" "three")))
> >       (format nil "~{~a-~}~a" (butlast list) (car (last list))))
> >
> > But I have a feeling that there could be a more elegant way. Is there?
> 
> Yes, write:       (mapconcat 'identity '("one" "two" "three") "-") ; elegant!
> 
> 
> Of course, with:
> 
> 
> (defun mapconcat (fun list sep)
>    (when list
>       (let ((~sep (with-output-to-string (*standard-output*)
>                      (map nil (lambda (ch) (princ (if (char= #\~ ch) "~~" ch)))
> sep))))
>        (format nil (format nil "~~A~~{~A~~A~~}" ~sep)
>                (funcall fun (first list))
>                (mapcar fun (rest list))))))
> 
> 
> 
> (mapconcat 'identity '("one" "two" "three") "-")
> --> "one-two-three"
> 
> (mapconcat (lambda (x) (concatenate 'string "[" x "]")) '("one" "two" "three") "
> ~")
> --> "[one]~[two]~[three]"

Gauche Scheme:

(define (mapconcat fun lst sep)
  (reduce-right
    (^(a b)
      (apply string-append (map x->string (list a sep b))))
    ""
    (map fun lst)))

(mapconcat values '(one "two" three) '-)
  ===>
"one-two-three"

(mapconcat square '(2 3 4) '---)
  ===>
"4---9---16"