(defun eval-expr (expr env)
  (cond
    ((numberp expr) expr)
    ((symbolp expr) (lookup-symbol-value expr env))
    ((listp expr)
     (case (first expr)
       ('+ (eval-sum expr env))
       ('- (eval-difference expr env))
       ('* (eval-product expr env))
       ('/ (eval-quotient expr env))
       ('def (eval-def expr env))
       ('if (eval-if expr env))
       (t (error "Unknown operator: ~a" (first expr)))))))

(defun eval-sum (expr env)
  (reduce #'+ (mapcar #'(lambda (x) (eval-expr x env)) (rest expr))))

(defun eval-difference (expr env)
  (reduce #'- (mapcar #'(lambda (x) (eval-expr x env)) (rest expr))))

(defun eval-product (expr env)
  (reduce #'* (mapcar #'(lambda (x) (eval-expr x env)) (rest expr))))

(defun eval-quotient (expr env)
  (reduce #'/ (mapcar #'(lambda (x) (eval-expr x env)) (rest expr))))

(defun eval-def (expr env)
  (let ((var (second expr))
        (value (eval-expr (third expr) env)))
    (push (cons var value) env)
    value))

(defun eval-if (expr env)
  (if (eval-expr (second expr) env)
      (eval-expr (third expr) env)
      (eval-expr (fourth expr) env)))

(defun lookup-symbol-value (symbol env)
  (let ((binding (assoc symbol env)))
    (if binding
        (cdr binding)
        (error "Undefined symbol: ~a" symbol))))

(defun repl ()
  (let ((env '()))
    (loop
       (print (eval-expr (read) env)))))

(repl)