返回一对-语法错误

我正在使用 pl 球拍内: https://pl.barzilay.org/ 可在此处找到下载: http://pl.barzilay.org/pl.plt

( : f1 : -> (Pairof Symbol String))
(define (f1)
  (cons 'wwww "aaa"))

错误:

   Type Checker: Polymorphic function `cons' could not be applied to arguments:
Argument 1:
  Expected: a
  Given:    'wwww
Argument 2:
  Expected: (Listof a)
  Given:    String

Result type:     (Listof a)
Expected result: (Pairof Symbol String)
 in: (cons (quote wwww) "aaa")

我做错了什么,我该如何纠正?