Evaluating Expression Trees

• If the expression is a constant, evaluate it directly

• Otherwise, look up the appropriate operator procedure and apply it to
  • The result of evaluating the left operand, and

  • The result of evaluating the right operand

     (define evaluate
       (lambda (expr)
         (cond ((constant? expr) expr)
               (else
                ((look-up-value (operator expr))
                 (evaluate (left-operand expr))
                 (evaluate (right-operand expr)))))))