Re: typechecking

[Dirk Herrmann]

Han-Wen Nienhuys wrote:

> address@hidden writes:
>  
>
> > Code like the following:
> >
> > void f (SCM obj)
> > {
> >  if (obj) {
> >    /* do something */
> >  }
> > }
> >
> > is almost always wrong, since what the user typically wants to check is, 
> > whether obj is SCM_BOOL_F or not:
> >    
>
> It would be interesting to see if we could map the Scheme semantics
> (true = !SCM_BOOL_F) to C. That would require mapping SCM_BOOL_F to
> (void*)0x0. Is this desirable, and does anyone see a possibility for this?
SCM values, where the 3 least significant bits are zero indicate 
non-immediates. That is, the SCM value can without any modification be 
used as a pointer to a scm_t_cell object on the heap. This works, since 
scm_t_cell has at least 8 bytes and thus the heap is constructed such 
that all scm_t_cell objects are placed on an address that is a multiple 
of 8.

SCM_BOOL_F on the contrary is an immediate value. To make SCM_BOOL_F == 
0x00 would require either to introduce it as a special case and leave 
otherwise the definitions of non-immediates the same, or to change the 
type system such that non-immediates are characterised differently. Both 
solutions, however, would require to perform some operation on every 
access to the heap. On the contrary, they would make all comparisons 
against SCM_BOOL_F very simple and also help to avoid some errors.

The effects of such a change are difficult to predict. It may be that it 
is not too much effort to try it out (at least the special case 
solution), if everywhere the correct access macros are used (SCM_IMP, 
SCM_FALSEP, ...).  Then, for a try you would only have to change those.

Best regards
Dirk