Expression eval in Racket

expr_eval.rkt

; Small interpreter
; (Const (Add 1 (Add-v 2 3))) = 6

(define (Const? x) (eq? x `Const))
(define (Add? x) (eq? x `Add))

(define (eval_expr exp)
  (cond [(Const? (car exp))
      (if (number? (cdr exp))
          (cdr exp)
          (eval_expr (cdr exp)))]
      [(Add? (car exp))
          (if (number? (car (cdr exp)))
              (if (number? (cdr (cdr exp)))
                  (+ (car (cdr exp)) (cdr (cdr exp)))
                  (+ (car (cdr exp)) (eval_expr (cdr (cdr exp)))))
              (if (number? (cdr (cdr exp)))
                  (+ (eval_expr (car (cdr exp))) (cdr (cdr exp)))
                  (+ (eval_expr (car (cdr exp))) (eval_expr (cdr (cdr exp))))))]))