Re: Another scheme/dsssl question

[Norman Walsh <norm@xxxxxxxxxxxxx>]

> > Is it possible to test for the existance of a definition?
> > 
> > Suppose I want to make it possible for a derived stylesheet to
> > override a value.  I'd like to write something like this:
> > 
> > (define default-foo '("x" "y" "z"))
> > ...
> > (if (some-predicate? foo)
> >     foo
> >     default-foo)
> > 
> > Is that possible?
> 
> You can test for procedure-ness with (procedure?).  It's not clear
> from 8.5.10.1 that this will *only* return #t for things actually
> (define)d, since the Standard defines data types in terms of
> predicates that return #t for them, but it's worth a shot.

I think I wasn't as clear as I could have been.  The difficulty
I'm having is that, for example,

  (list? foo)

produces an error message if "foo" is entirely undefined.  I'm
looking for a predicate (or some other mechanism) for testing if
a given "thing" is defined at all.

--norm




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