SICP 习题2.21 square-list

print-list的时候写了三个if，是个愚蠢的地方

(define (map-a proc items)  (if (null? items)    '()    (cons (proc (car items))      (map-a proc (cdr items)))))(define nl newline)(define print-list  (lambda (s)    (if (not (null? s)) (display (car s)))    (if (not (null? s)) (display " "))    (if (not (null? s)) (print-list (cdr s)))))(nl)(print-list (map-a abs '(-10 2.5 -11.6 17)))(define square  (lambda (x) (* x x)))(define (square-list-a items)  (if (null? items)    '()    (cons (square (car items)) (square-list-a (cdr items)))))(define (square-list-b items)  (map-a square items)) (nl)(print-list (square-list-a '(1 2 3 4)))(nl)(print-list (square-list-b '(1 2 3 4)))