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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Multiple arguments to mapcar?
Date: Wed, 28 Aug 2024 02:46:16 -0000 (UTC)
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Pascal Costanza wrote:

> Johan wrote:
> > I want to map over a list, but to treat the first object differently
> > from the rest. My previous experience with Lisp was Interlisp, which
> > has dynamic binding. In Interlisp I could code my problem similar to
> > this:
> >
> > (defun foo ()
> >   (let ((first-time t))
> >     (mapcar #'bar '(1 2))))
> >
> > (defun bar (n)
> >   (cond
> >     (first-time
> >      (setf first-time nil)
> >      (+ n 1))
> >     (t (+ n 2))))
> >
> > and (foo) would return (2 4). This doesn't work with lexical binding,
> > and I don't want to make first-time a global variable. What is the
> > best design pattern in Common Lisp?
> 
> (loop for n in '(1 2)
>        for x = (+ n 1) then (+ n 2)
>        collect x)

Gauche Scheme

(use gauche.parameter)

(define first-time (make-parameter 'unset))

(define (foo)
  (parameterize ((first-time #t))
    (map bar '(400 500 600))))

(define (bar n)
  (cond
    ((first-time)
     (first-time #f)
     (+ n 1))
    (#t (+ n 2))))

gosh> (foo)
(401 502 602)
gosh> (first-time)
unset