[racket] Typed Racket - Parameterized Structures Part Duex

From: Ray Racine (ray.racine at gmail.com)
Date: Fri Mar 16 06:15:38 EDT 2012

Thanks for the previous TR assist.
I tried manually annotating stuff inside the 'get' procedure below but
can't seem to get it to type check.
Thanks again in advance.

(struct: (T) Try ())

(struct: (T) Failure Try ([exception : exn]))

(struct: (T) Success Try ([result : T]))

(: failed (All (T) (Try T) -> Boolean))
(define (failed try)
  (Failure? try))

(: succeded (All (T) (Try T) -> Boolean))
(define (succeded try)
  (Success? try))

(: get (All (T) (Try T) -> T))
(define (get try)
   ((Failure? try) (raise (Failure-exception try)))
   ((Success? try) (Success-result try))
   (else (error "Can I not be 'sealed'"))))

(: tmap (All (T U) (Try T) (T -> U) -> (Try U)))
(define (tmap try fn)
   ((Success? try)
    (Try (fn (Success-result try))))
   ((Failure? try) try)
   (else (error "hmmmm"))))

(: attempt (Try String))
(define attempt (Success "Good to go"))

(: test  (-> String))
(define (test)
  (if (succeded attempt)
      (get attempt)
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