2.29.tex

\documentclass[a4paper,12pt]{article}
\usepackage{listings}
\lstset{language=Lisp}

\begin{document}
\begin{lstlisting}
(define (make-mobile left right)
  (list left right))
(define (make-branch length structure)
  (list length structure))
(define (left-branch m) (car m))
(define (right-branch m) (cadr m))
(define (branch-length b) (car b))
(define (branch-structure b) (cadr b))

(define (branch-weight b)
  (let ((s (branch-structure b)))
    (if (number? s)
        s
        (total-weight s))))

(define (total-weight m)
  (+ (branch-weight (left-branch m))
     (branch-weight (right-branch m))))

(define (balanced? m)
  (define (torque b)
    (* (branch-length b) (branch-weight b)))
  (= (torque (left-branch m))
     (torque (right-branch m))))
\end{lstlisting}
If we change the representation of mobiles, we only need to redefine
the selectors as follows,
\begin{lstlisting}
(define (make-mobile left right)
  (cons left right))
(define (make-branch length structure)
  (cons length structure))
(define (left-branch m) (car m))
(define (right-branch m) (cdr m))
(define (branch-length b) (car b))
(define (branch-structure b) (cdr b))
\end{lstlisting}
\end{document}