Re: another nit.

[Han-Wen Nienhuys <hanwen at cs dot uu dot nl>]

hjstein@bfr.co.il writes:
> Why are you talking about using assq directly given that you have a
> get_property function wrapping it?  What's the point of that?

Sorry for spreading such confusion... there are two separate issues
that we're confusing, which is

* a possible (non-)improvement to ass{q|oc|v}-ref

* what kind of functions I write in my programs.

As for the 2nd, I have a recurring pattern in LilyPond that looks like

   /* pseudo codish */
   SCM s = assq (key, alist));
   if (s != SCM_BOOL_F &&  gh_SOMETYPE_p (gh_cdr (s));
      {
	SOMETYPE t = gh_scm2SOMETYPE (gh_cdr (s));
		 [..]
      }

if assq-ref were to return some kind of null-value in the case of a
non-existing key (null-value = an object that fails all useful
type-predicates), I can condense the code a little:


       SCM s = my_assq_ref (key, alist);
       if (gh_SOMETYPE_p (s))
	  {
		SOMETYPE t= gh_scm2SOMETYPE (s); 
	  }

In C I can easily implement my own assq_ref that returns
SCM_UNDEFINED if a key is not there:

This seemed useful to me, and my suggestion was to extend assq-ref
extend itself in this way. However, I forgot that there are--- by
definition---no Scheme values that are `nothing', so my suggestion
would not be possible for a Scheme callable function.

Dirk suggested that I could work my way around this by defining my own
`undefined-key' smob type, and use that in stead of SCM_UNDEFINED.

On second thought however, the idea of returning a valid Scheme value
has its merits. My latest insight is that '() would be the right
return value for my application, because I also have lots of code that
does conses values onto assoc-entries, ie.

     alist = 
	scm_assq_set (list_key,  gh_cons (put_on_head, scm_assq_ref (list_key, alist)), alist)


If assq_ref returns '() for undefined keys, I don't have to initialize
entries for lists (like the entry associated with list_key in the
above example)  to SCM_EOL.

(note that in my C-program, I use the convention that #t is the only
boolean that is true)

-- 

Han-Wen Nienhuys   |   hanwen@cs.uu.nl    | http://www.cs.uu/~hanwen/