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From: sfk@otter.hpl.hp.com (Steve Knight)
Newsgroups: comp.lang.lisp
Subject: Re: FALSE vs empty list
Message-ID: <1350029@otter.hpl.hp.com>
Date: 6 Sep 90 15:41:42 GMT
References: <24305@uflorida.cis.ufl.EDU>
Organization: Hewlett-Packard Laboratories, Bristol, UK.
Lines: 31

Ahhh, the old arguments are the best arguments ....

The type-distinctness of empty-list and false in a Lisp or Lisp-like
language is an aesthetic consideration.  Many folks, and I'd include myself,
believe that making them distinct is more convenient and makes programs
more readable.  Ultimately, our arguments must boil down to "I don't think
of an empty list and false in the same way.  I don't think that applying
boolean operators to lists makes good programming practice." and so on.

These are preferences that cannot be grounded logically.  You might argue
that making them distinct or the same confers some special advantage.  
Review of debates shows that any advantages or disadvantages are less than
uniformly persuasive.  A lot of folks go away unconvinced that enough 
benefit/penalty has been shown to change their views.

If you are happy applying boolean operators to lists and list operators to
booleans, then you will have little inclination to change.  You may even
enjoy the conciseness of idioms such as
    (car (or (member x L1) (member x L2) '(())))
which exploit the conflation of lists and booleans.

On the other hand, you may be inclined to wonder why only lists and booleans
should enjoy the benefits of conflation.  For example, we could map 0 and
NIL onto one another & perhaps 1 onto TRUE.  No, they are kept distinct
because it is generally felt to be more comfortable that way.  Distinct
concepts are typically given distinct, non-overlapping data-types.

It's not a big deal.  It's just one of those many features that makes 
Lisp feel a bit dated.

Steve