Re: another nit.

[Jonathan Bartlett <johnnyb at wolfram dot com>]

I think the difference is that if you aren't sure, you use assq.  If you
are sure, you use assq-ref.  Why would someone use assq-ref instead of 
assq when they don't know if the key exists?  assq-ref is for known keys,
just as vector-ref is for known indices.

Jon

On 6 Jul 2000, Harvey J. Stein wrote:

> Jonathan Bartlett <johnnyb@wolfram.com> writes:
> 
>  > What does vector-ref return when someone accesses it beyond a valid
>  > index?  Whatever this behavior is, should also be the behavior of
>  > assq-ref.
> 
> I thought of that, but I don't think it's the appropriate analogy.  An
> array has a predetermined starting & ending index, so the set of
> "keys" (the indices) is known in advance. In an alist, the
> keys are arbitrary and vary when insertions are done.  Alists should
> have the same interface as hashtables (and other dictionaries), not as
> vectors.  A more appropriate analogy btw vectors and alists not having
> a particular key would be referencing an index in a vector before that
> element has been set to something.
> 
> In fact, it's *extremely* desireable for these fcns to have the same
> interface because then one can easily replace alists by hash tables
> when necessary.  Even better would be a set of dictionary fcns using
> GOOPS which act appropriately both on alists & hash table objects.
> 
> As for making assq-ref's return value unspecified when the key doesn't
> exist, it could be reasonable, but I think it's a bad idea.  The
> return value being unspecified means not that it's always
> #<unspecified> but that it could be anything.  It could just as easily
> be nil or #f or 3 or the last item in the list or ...
> 
> -- 
> Harvey Stein
> Bloomberg LP
> hjstein@bfr.co.il
>