SICP 問題 2.78
(define (attach-tag type-tag contents) (if (eq? type-tag 'scheme-number) contents (cons type-tag contents))) (define (type-tag datum) (cond ((number? (car datum)) 'scheme-number) ((pair? datum) (car datum)) (else (error "Bad tagged datum -- TYPE-TAG" datum)))) (define (contents datum) (cond ((number? (car datum)) 'scheme-number) ((pair? datum) (cdr datum)) (else (error "Bad tagged datum -- CONTENTS" datum))))