; A pipe is
;  (make-pipe num sym)
(define-struct pipe (len kind))

;(define (func-for-pipe p)
;  ... (pipe-len p) ... (pipe-kind p) ...)
  
; A pipeline is either
;  - empty
;  - (cons pipe pipeline)

;(define (func-for-pipeline pl)
;  (cond
;    [(empty? pl) ...]
;    [(cons? pl)
;     ... (func-for-pipe (first pl))
;     ... (func-for-pipeline (rest pl))]))

; total-length : pipeline -> num
(define (total-length pl)
  (cond
    [(empty? pl) 0]
    [(cons? pl) (+ (pipe-len (first pl))
                   (total-length (rest pl)))]))
(total-length empty) "should be" 0
(total-length (cons (make-pipe 10 'copper)
                    (cons (make-pipe 20 'lead)
                          empty)))
"should be" 30

; modernize : pipeline -> pipeline
(define (modernize pl)
  (cond
    [(empty? pl) empty]
    [(cons? pl) (cons (modern-pipe (first pl))
                      (modernize (rest pl)))]))

; modern-pipe : pipe -> pipe
(define (modern-pipe p)
  (cond
    [(symbol=? (pipe-kind p) 'lead)
     (make-pipe (pipe-len p) 'copper)]
    [else p]))
(modern-pipe (make-pipe 10 'plastic)) "should be" (make-pipe 10 'plastic)
(modern-pipe (make-pipe 20 'lead)) "should be" (make-pipe 20 'copper)

(modernize empty) "should be" empty
(modernize (cons (make-pipe 10 'copper)
                 (cons (make-pipe 20 'lead)
                       empty)))
"should be" (cons (make-pipe 10 'copper)
                  (cons (make-pipe 20 'copper)
                        empty))