[racket] typed racket, filters, and polymorphism

Is it possible to have a struct that does certain things according to the guard?
#lang typed/racket

(struct (a) foo ([a : a]) #:transparent
  #:guard (lambda (a _)
            (unless (exact-integer? a)
              (error 'foo "expected Integer, given ~v" a))
            a))

(ann (foo (ann 1 Any)) (foo Integer))

(: x : (foo Any))
(define x (foo 1))

(ann (foo-a x) Integer)

;. Type Checker: Polymorphic function `foo1' could not be applied to arguments:
;Argument 1:
;  Expected: a
;  Given:    Any
;
;Result type:     (foo a)
;Expected result: (foo Integer)
; in: (foo (ann 1 Any))
;. Type Checker: Polymorphic function `foo-a' could not be applied to arguments:
;Argument 1:
;  Expected: (foo a)
;  Given:    (foo Any)
;
;Result type:     (a : ....)
;Expected result: Integer
; in: (foo-a x)

On Sep 25, 2014, at 9:42 PM, Alexander D. Knauth <alexander at knauth.org> wrote:

> What I’m trying to accomplish is something more like this:
> #lang typed/racket
> 
> (require "dimensions.rkt")
> 
> (struct (d) unit ([name : Any] [scalar : Positive-Real] [dimension : d]) #:transparent
>   #:guard (lambda (name scalar dimension _)
>             (unless (dimension? dimension)
>               (error 'unit "expected Dimension, given ~v" dimension))
>             (values name scalar dimension)))
> 
> (define-type (Unitof d) (unit d))
> 
> (define-type Unit (Unitof Dimension))
> 
> (define Unit? (make-predicate Unit))
> 
> (define-type Unitish
>   (U (Unitof Any)
>      Dimension
>      Positive-Real))
> 
> (: ->unit : (All (d) (case-> [(Unitof d) -> (Unitof d)]
>                              [Unitish -> Unit])))
> (define (->unit u)
>   (cond [(unit? u)
>          (unless (Unit? u) ; this should never happen anyway because of the guard
>            (error '->unit "expected (Unitof Dimension), given ~v" u))
>          u]
>         [(dimension? u) (unit u 1 u)]
>         [(positive-real? u) (unit u u dimensionless-dimension)]))
> 
> 
> On Sep 25, 2014, at 6:19 PM, Sam Tobin-Hochstadt <samth at cs.indiana.edu> wrote:
> 
>> No, I don't think you can do this. Can you say more about what you're
>> trying to accomplish?
>> 
>> Sam
>> 
>> On Thu, Sep 25, 2014 at 6:15 PM, Alexander D. Knauth
>> <alexander at knauth.org> wrote:
>>> Do any of you have any advice for getting a function like this to
>>> type-check?
>>> #lang typed/racket
>>> 
>>> (: check-int : (All (a) (case-> [a -> a]
>>>                                [Any -> Integer])))
>>> (define (check-int int)
>>>  (unless (exact-integer? int)
>>>    (error 'check-int "expected Integer, given ~v" int))
>>>  int)
>>> 
>>> ;. Type Checker: type mismatch
>>> ;  expected: a
>>> ;  given: Integer in: int
>>> 
>>> 
>>> 
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> 
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