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