[racket] specifying types for fields in non-polymorphic structures

No, there's no 'and' for types. Adding that would significantly increase
the complexity of the type checker, and so it's not on the horizon for
Typed Racket. Bounded polymorphism, as I mentioned earlier, is more likely
to be added.

Sam
On Apr 25, 2014 11:06 PM, "Alexander D. Knauth" <alexander at knauth.org>
wrote:

> Then is there something like and/c for types? (like U is like or/c for
> types) That could probably solve it.
>
> It seems like typed racket is already doing this in some cases, for
> example:
> #lang typed/racket
>
> (define positive-real? (make-predicate Positive-Real))
>
> (define (f x)
>   (if (positive-real? x)
>       (if (exact-integer? x)
>           x
>           (error "error"))
>       (error "error")))
>
> > f
> - : (Any -> Positive-Integer)
> #<procedure:f>
> In this, typed racket figures out that f produces a Positive-Integer, when
> it was only given that it produces something that is both a Positive-Real
> and an Integer.
>
> But I couldn’t find it in the docs.
>
> On Apr 25, 2014, at 10:48 PM, Sam Tobin-Hochstadt <samth at cs.indiana.edu>
> wrote:
>
> What you're looking for is called bounded polymorphism. Sadly, Typed
> Racket doesn't support this, so I'd try one of the options David suggests.
>
> Sam
> On Apr 25, 2014 10:46 PM, "Alexander D. Knauth" <alexander at knauth.org>
> wrote:
>
>> That’s what I tried at first, but my actual struct is a lot more
>> complicated than posn, and it kept giving be type-check errors, and trying
>> to enforce the types of the fields myself just ended up with completely
>> unreadable code and even more type errors, so I gave up on making it
>> polymorphic, but I still wanted to be able to specify specific cases of it
>> as types that would only contain that specific case of it.
>>
>> On Apr 25, 2014, at 10:25 PM, David Van Horn <dvanhorn at cs.umd.edu> wrote:
>>
>> > On 4/25/14, 9:57 PM, Alexander D. Knauth wrote:
>> >> But then the posn constructor doesn’t enforce that it’s arguments have
>> to be Reals, and the posn? predicate doesn’t check it, and the accessors
>> don’t say that they always produce Reals.
>> >
>> > Maybe I'm not seeing the big picture, but that's what the Posn type is
>> > for.  If you apply posn to something other than reals, you won't get a
>> > Posn.  If you have a Posn and apply posn-x, you get a real.
>> >
>> > (define: (f [p : Posn]) : Real
>> >  (+ (posn-x p) (posn-y p)))
>> >
>> > David
>> >
>> >
>> >> On Apr 25, 2014, at 9:49 PM, David Van Horn <dvanhorn at cs.umd.edu>
>> wrote:
>> >>
>> >>> How about this?
>> >>>
>> >>> (struct: (x y) posn ([x : x] [y : y]))
>> >>> (define-type Posn (posn Real Real))
>> >>> (define-type Origin (posn Zero Zero))
>> >>>
>> >>>
>> >
>>
>>
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>>
>
>
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