;; 8.3  Expression Trees

(define make-constant
  (lambda (x) x))

(define constant? number?)

(define make-expr
  (lambda (operator operands)
    (cons  operator operands)))

(define operator car)

(define operands cdr)

(define operators  ;; from the original version of exprs with left and right
  (lambda (expr)
    (if (constant? expr)
        '()
        (cons (operator expr)
              (append (operators (left-operand expr))
                      (operators (right-operand expr)))))))

(define evaluate
  (lambda (expr)
    (cond ((constant? expr) expr)
          (else (apply (look-up-value (operator expr))
                       (map evaluate (operands expr)))))))

(define look-up-value  ;; back to the version in the book -- sorry Josh
  (lambda (name)
    (cond ((equal? name '+) +)
          ((equal? name '*) *)
          ((equal? name '-) -)
          ((equal? name '/) /)
          (else (error "Unrecognized name" name)))))