sicp-ex-2.23

  
 ;; for-each 
  
  
 ;; The below didn't work ... basically, I needed some kind of block 
 ;; structure, since if has the form (if (test) true-branch 
 ;; false-branch).  I needed to have true-branch execute the proc, then 
 ;; call the next iteration of for-each, and the only way I knew how to 
 ;; do that was with brackets ... but of course that doesn't work, as 
 ;; the interpreter tries to apply the result of the first proc call as 
 ;; a function to the rest. 
 (define (for-each proc items) 
   (if (not (null? items)) 
       ((proc (car items)) 
       (for-each proc (cdr items))))) 
  
 (for-each (lambda (x) (newline) (display x)) (list 1 2 3 4)) 
  
  
 ;; This one works. 
 ;; Moral: cond is better for multi-line branches. 
 (define (for-each proc items) 
   (cond ((not (null? items)) 
          (proc (car items)) 
          (for-each proc (cdr items)))))