[racket-dev] FFI question; using _or-null around output ptr gives binding error.

Here's an ffi object I'd like to construct:

(define do-fit-int
    (get-ffi-obj "do_fit" libfit
      (_fun (func      : (_fun _int _pointer -> _double))
            (val-num   : _int = (length x-values))
            (x-values  : (_list i _double*))
            (y-values  : (_list i _double*))
            (z-values  : (_list i _double*))
            (errors    : (_list i _double*))
            (param-num : _int = (length params))
            (params    : (_list i _double*))
            -> (_or-null (_list o _double* param-num)))))

My addition is the (_or-null ...) that's wrapped around the output list, and this signals the error:

expand: unbound identifier in module in: param-num

It looks like I can work around this error by doing the work explicitly:

(define do-fit-int
    (get-ffi-obj "do_fit" libfit
      (_fun (func      : (_fun _int _pointer -> _double))
            (val-num   : _int = (length x-values))
            (x-values  : (_list i _double*))
            (y-values  : (_list i _double*))
            (z-values  : (_list i _double*))
            (errors    : (_list i _double*))
            (param-num : _int = (length params))
            (params    : (_list i _double*))
            -> (result : _pointer)
            -> (and result
                    (cblock->list result _double* param-num)))))

...which compiles, though I can't say anything yet about whether it runs correctly (principally because I haven't yet found a set of inputs on which the fit library doesn't choke).

My question: should or-null handle this?


John

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 4669 bytes
Desc: not available
URL: <http://lists.racket-lang.org/dev/archive/attachments/20101105/42ec5cad/attachment.p7s>