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Practical example: Length of an improper list
The length of an improper list is undefined

We chose to return the symbol improper-list if list-length encounters an improper list

This example is similar to the catch and throw example shown earlier in this section


(define (list-length l)
   (lambda (do-exit)
     (letrec ((list-length1
                (lambda (l)
                   (cond ((null? l) 0)
                         ((pair? l) (+ 1 (list-length1 (cdr l))))
                         (else (do-exit 'improper-list))))))
       (list-length1 l)))  ))