XPost: comp.lang.scheme

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

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