Trouble with returning parameters passed in to C primitives.

Trouble with returning parameters passed in to C primitives.

[Richard Shann]

In GNU/Denemo we have been calling functions written in C from scheme by
defining procedures via scm_c_define_gsubr. Recently I have tried
returning one of the parameters passed (more exactly, the scm_list_ref
of a scm_car() of one of the parameters) and I have run into trouble.
If I pass 'a in as a parameter I get something back which prints on the
console as (quote a) but which is not eq? to 'a

I can pass in a string and get an equal? string back. But what I really
want to do is pass in a procedure and execute it if I get it back, thus

(set! choice (d-PopupMenu (list '("Offset Position" . do-offset))))
(if choice
 (choice))

where do-offset is defined as (define (do-offset) (display "hello world"))
and d-PopupMenu is my C-primitive which gets a choice from the user. 

If the user cancels choice is #f and all is well, if he accepts the one
option on the menu, do-offset is returned but I get the error message

(#f Wrong type to apply: ~S (do-offset) #f)

(BTW I can't find a blow by blow explanation of this error message
anywhere, I see some discussion of earlier versions of it on the net...)

Anyone able to help?

Richard Shann

Re: Trouble with returning parameters passed in to C primitives.

[Daniel Hartwig]

[-- Attachment #1: Type: text/plain, Size: 944 bytes --]

On Dec 7, 2012 5:51 AM, "Richard Shann" <richard.shann@virgin.net> wrote:
> If I pass 'a in as a parameter I get something back which prints on the
> console as (quote a) but which is not eq? to 'a

You seem to be double quoting the value.  No need for ' the second time.

> I can pass in a string and get an equal? string back. But what I really
> want to do is pass in a procedure and execute it if I get it back, thus
>
> (set! choice (d-PopupMenu (list '("Offset Position" . do-offset))))

Here you pass the symbol "do-offset" rather than the value.  read up on
quote and quasiquote syntax.  Use unquote to insert the /value/ of
do-offset:

(set! choice (d-PopupMenu (list `("Offset Position" . ,do-offset))))

or:

(set! choice (d-PopupMenu (list (cons "Offset Position" do-offset))))

> (#f Wrong type to apply: ~S (do-offset) #f)
>

You are trying to apply a list contain a symbol, when it should just be a
procedure: no list or symbol.

[-- Attachment #2: Type: text/html, Size: 1197 bytes --]

Re: Trouble with returning parameters passed in to C primitives.

[Richard Shann]

Thank you! I am much clearer on the meaning of quote now.
Richard Shann
> On Dec 7, 2012 5:51 AM, "Richard Shann" <address@hidden> wrote:
> > If I pass 'a in as a parameter I get something back which prints on
> the
> > console as (quote a) but which is not eq? to 'a
> 
> You seem to be double quoting the value.  No need for ' the second
> time.
> 
> > I can pass in a string and get an equal? string back. But what I
> really
> > want to do is pass in a procedure and execute it if I get it back,
> thus
> >
> > (set! choice (d-PopupMenu (list '("Offset Position" . do-offset))))
> 
> Here you pass the symbol "do-offset" rather than the value.  read up
> on quote and quasiquote syntax.  Use unquote to insert the /value/ of
> do-offset:
> 
> (set! choice (d-PopupMenu (list `("Offset Position" . ,do-offset))))
> 
> or:
> 
> (set! choice (d-PopupMenu (list (cons "Offset Position" do-offset))))
> 
> > (#f Wrong type to apply: ~S (do-offset) #f)
> >
> 
> You are trying to apply a list contain a symbol, when it should just
> be a procedure: no list or symbol.
>