Return a pair - syntax error

问题

I'm using pl in racket: https://pl.barzilay.org/ The download can be found here: http://pl.barzilay.org/pl.plt

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

Error:

   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")

What I did wrong and how can I fix it?

回答1:

The #lang pl language that I use in my class is a variant of Typed Racket. One of the changes it has is that cons has a more restricted type (as you've seen) which allows it to only construct proper lists.

(As a sidenote, the reason there is no formal documentation is that this language is intended to be used in the class, and as such it's something that is subject to random pedagogic needs and not as something to be used for random Racket code... So my class notes are the main place that "documents" the language.)

来源：https://stackoverflow.com/questions/49465474/return-a-pair-syntax-error