[plt-scheme] typed scheme problem

Hi,

I'm getting a type error I can't resolve, and my best attempt to
resolve it has resulted in a nonsensical error message.


In one module I have:

=====

#lang typed-scheme
(require scheme/promise)

;; Lazy streams
(define-type-alias (Stream A) (Promise (U StreamNil (StreamPair A))))
(define-struct: StreamNil ())
(define-struct: (A) StreamPair ((x : A) (xs : (Stream A))))

(provide StreamNil StreamPair Stream)

=====

And in another:

=====

#lang typed-scheme
(require scheme/match)
(require scheme/promise)
(require "stream.ss")

;; Parser Combinators
(define-struct: (A T) Ok ((ans : A)
                          (tks : (Stream T))
                          (ncur : Integer)
                          (nmax : Integer)))
(define-struct: Fail ((nmax : Integer)))
(define-type-alias (ParserResult A T) (U (Ok A T) Fail))

(define-type-alias (Parser A T) ((Stream T) Integer Integer ->
(ParserResult A T)))


(: any (All (A) (-> (Parser A A))))
(define (any)
  (λ (tks ncur nmax)
    (match (force tks)
      ((struct StreamNil ())
       (make-Fail nmax))
      ((struct StreamPair (x xs))
       (let* ((ncur (+ ncur 1))
              (nmax (if (> ncur nmax) ncur nmax)))
         (make-Ok x xs ncur nmax))))))

=====

'any' fails to typecheck with the message:

typecheck: Polymorphic function force could not be applied to arguments:
Domain: #(struct:Promise (a))
Arguments: #(struct:Promise ((U StreamNil16 (StreamPair18 A))))
 in: (force tks)


So, fair enough: maybe I have to instantiate force's type variable.  So, I try:

(match (#{force @ (U StreamNil (StreamPair A))} tks)

... and get the error:

typecheck: Wrong function argument type, expected #(struct:Promise ((U
StreamNil16 (StreamPair18 A)))), got #(struct:Promise ((U StreamNil16
(StreamPair18 A)))) for argument 1 in: tks

Unfortunately, there's no way to tell from that error message why the
argument type is wrong.  In fact, it looks to be correct.  Any
thoughts?

-jaz