Using WCTs as `out' parameters

[Ludovic Courtès]

Hi,

I'm wondering: why can't one use a WCT as an out parameter?  This is
forbidden in `c-types.scm' by the `check-typespec-options' method for
`<gw-wct>' objects.

Consider the following scenario.  On the C side we have:

  /* Allocate and initialize a new chbouib based on SEED.  Return
     ERR_INVALID_ARG if SEED is out of range and ENOMEM if allocation
     failed.  */
  errcode_t allocate_and_initialize_a_chbouib (int seed,
                                               chbouib_t **the_new_chbouib);

which one would want to specify like this:

  (wrap-as-wct! ws
                #:name '<chbouib>
                #:c-type-name "chbouib_t *"
                #:c-const-type-name "const chbouib_t *"
                #:destroy-value-function-name "destroy_and_dealloc_a_chbouib")

  (wrap-function! ws
                  #:name 'make-chbouib
                  #:c-name "allocate_and_initialize_a_chbouib"
                  #:returns '<errcode>
                  #:arguments '((int seed)
                                ((<chbouib> out) the-new-chbouib)))

I looked at how GnuCash deals with this [1] and it seems that all the
`*-create' functions have a WCT as their return type and never use an
`out' WCT.  This means that the caller can only know whether these
function fail, not why.

Thanks,
Ludovic.

[1] 
https://cvs.gnucash.org/cgi-bin/cvsweb.cgi/gnucash/src/business/business-core/gw-business-core-spec.scm?rev=1.33