sicp-ex-2.31

Pretty nice.

  
 (define (tree-map proc tree) 
   (define nil '()) 
   (map (lambda (subtree) 
          (cond ((null? subtree) nil) 
                ((not (pair? subtree)) (proc subtree)) 
                (else (tree-map proc subtree)))) 
        tree)) 
  
 ;; Usage: 
 (define my-tree (list (list 1 2) 3 (list 4 5))) 
 (tree-map square my-tree) 
 (tree-map (lambda (x) (+ x 1)) my-tree) 
  

my way:

  
 (define (tree-map proc tree) 
   (cond ((null? tree) nil) 
         ((pair? tree)  
          (cons  
           (tree-map proc (car tree))  
           (tree-map proc (cdr tree)))) 
         (else (proc tree)))) 

my way. For me this is more succinct than using cond and there's no need to check for nil explicitly (or define it):

  
 (define (tree-map proc tree) 
   (map (lambda (sub-tree) 
          (if (pair? sub-tree) 
            (tree-map proc sub-tree) 
            (proc sub-tree))) 
        tree))