[plt-scheme] ffi equivalent of array in struct

This is cool. I was going to suggest such a thing myself but Im not good 
enough with macros yet to pull it off myself.

Danny Yoo wrote:

>>> You can use `make-cstruct-type' with a list of 10 _int values, that 
>>> will create a struct type of the desired size.
>>>
>> Do I have to list them all? (make-cstruct-type _int _int _int ...) ? 
>> The real problem is a struct with 65536 chars in it. I dont want to 
>> have to type all those..
>>
>> Or I suppose I could make cstruct's which contain other cstructs and 
>> balloon upto the size I wanted anyway:
>> (define 4bytes _int)
>> (define 8bytes (make-cstruct-type 4bytes 4bytes))
>> (define 16bytes (make-cstruct-type 8bytes 8bytes))
>> (define 32bytes (make-cstruct-type 16bytes 16bytes))
>> Almost there!
>> (define 65536bytes (make-struct-type 32000bytes 32000bytes))
>
>
>
> Hi Jon,
>
> This seems repetitive, so maybe a macro will help.
>
> Here's one that I just cooked up (starting to learn about 
> quasiquotation now...):
>
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> (module n-bytes mzscheme
>   (require (lib "foreign.ss"))
>   (require-for-syntax (lib "stx.ss" "syntax"))
>
>   (define 4bytes _int)
>   (define 8bytes (make-cstruct-type (list 4bytes 4bytes)))
>
>   (define-for-syntax (multiple-of-four? n)
>     (and (number? n) (= 0 (remainder n 4))))
>
>   (define-for-syntax (build-int-stx-list stx n)
>     (let loop ([n n]
>                [acc #'()])
>       (cond
>         [(= n 0) acc]
>         [else
>          (loop (- n 4)
>                (quasisyntax/loc stx
>                  (_int . #,acc)))])))
>
>   (define-syntax (n-bytes stx)
>     (syntax-case stx ()
>       [(_ n)
>        (cond
>          [(multiple-of-four? (syntax-e #'n))
>           (quasisyntax/loc stx
>             (make-cstruct-type
>              (list #,@(build-int-stx-list stx (syntax-e #'n)))))]
>          [else
>           (raise-syntax-error
>            #f
>            "expected numeric literal divisible by four"
>            stx)])])))
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>
> n-bytes requires a literal number to work.  But if we have this, then 
> we can say:
>
>     (define 65536bytes (n-bytes 65536))
>
> without too many problems.
>
>
> Good luck!
>
>