[racket] Typed Racket struct with more permissive constructor

On 05/31/2012 09:50 AM, Sam Tobin-Hochstadt wrote:
> On Thu, May 31, 2012 at 11:42 AM, Neil Toronto<neil.toronto at gmail.com>  wrote:
>> Currently, the floating-point bitmap type is defined so that its size fields
>> are Integer, and a guard assures that they're really Nonnegative-Fixnum:
>>
>> (struct: flomap ([values : FlVector]
>>                  [components : Integer]
>>                  [width : Integer]
>>                  [height : Integer])
>>   #:transparent
>>   #:guard
>>   (λ (vs c w h name)
>>     (with-asserts ([c  nonnegative-fixnum?]
>>                    [w  nonnegative-fixnum?]
>>                    [h  nonnegative-fixnum?])
>>       [...] ; elided length check on vs
>>       (values vs c w h))))
>>
>>
>> What I'd *really* like, though, is to have the fields be Nonnegative-Fixnum
>> in the first place, and simply have a more permissive constructor. Is there
>> a way to do that? As it is, `flomap-components' always returns an Integer,
>> but no flomap instance can actually have an integer-valued `components'
>> field.
>
> Could you just structure the program this way?
>
> (struct: flomap ([values : FlVector]
>                  [components : Nonnegative-Fixnum]
>                  [width : Integer]
>                  [height : Integer])
>   #:transparent)
>
> (: make-flomap : FlVector Integer Integer Integer ->  flomap)
> (define (make-flomap v c w h)
>    (with-asserts ... (flomap v c w h)))

I could, but `make-flomap' is already defined and used extensively (it's 
an analogue of make-flvector). Also, not using the structure name to 
create instances from their field values now makes me feel dirty. :D

Neil ⊥