I can't see why the second code doesn't work like the first:
(use-modules (srfi srfi-1))
(define D '(((3 4))))
(let ((r 1))
(set! D (append D '(())))
(display D)(newline)
(set-car! (drop D r) (car (drop D (- r 1)))))
(let ((r 2))
(set! D (append D '(())))
(display D)(newline)
(set-car! (drop D r) (car (drop D (- r 1)))))
output:
(((3 4)) ())
(((3 4)) ((3 4)) ())
(use-modules (srfi srfi-1))
(define D '(((3 4))))
(map (lambda (r)
(set! D (append D '(())))
(display D)(newline)
(set-car! (drop D r) (car (drop D (- r 1)))))
'(1 2))
output:
(((3 4)) ())
(((3 4)) ((3 4)) ((3 4)))
guile (GNU Guile) 2.0.11
i686-pc-linux-gnu
