racket interpeter

;mlet - (struct mlet (var e body) #:transparent)
        [(mlet? e)
         (letrec ([v (eval-under-env (mlet-e e) env)]
               [new-env (cons (cons (mlet-var e) v) env)])
           (eval-under-env (mlet-body e) new-env))]
        
        ;fun -(struct fun  (nameopt formal body) #:transparent)
        [(fun? e)
         (if (and (string? (fun-nameopt e)) (string? (fun-formal e)))
             (letrec ([new-env (cons (cons (fun-nameopt  e) e) env)])
               (closure new-env e))
             (if (and (false? (fun-nameopt e)) (string? (fun-formal e)))
                 (closure env e)
                 (error "bad parameters passed to function")))]
        
        ;closure - (struct closure (env fun) #:transparent) 
        [(closure? e) e]