SICP 問題 2.23

(define (for-each proc items)
  (cond
   ((null? items) 'done)
   (else
    (proc (car items))
    (for-each proc (cdr items)))))

gosh> (for-each (lambda (x) (newline) (display x))
                (list 57 321 88))

57
321
88done