Scheme \ DrRacket foldr

Question

我试图确定一个符号列表是否有重复项，如果是这样，我希望foldr给我一个新列表，每个符号只出现一次

这个想法是这样的：

(define schema2 '(aa bb cc))

(define (schema? schema)
  (foldr (lambda (obj lst)
           (if (contains? lst obj)
               (lst)
               ((cons obj lst)))
           ) (list) schema))

(define (contains? list obj)
  (cond
    ((empty? list) #f)
    ((eq? (car list) obj) #t) 
    (else (contains? (cdr list) obj)))
  )

(schema? schema2)

但出于某种原因，我收到错误

application: not a procedure;
 expected a procedure that can be applied to arguments
  given: '(aa)
  arguments...: [none]