Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!ucsd!swrinde!zaphod.mps.ohio-state.edu!rpi!sci.ccny.cuny.edu!phri!marob!cowan From: cowan@marob.masa.com (John Cowan) Newsgroups: comp.lang.lisp Subject: Re: FALSE vs empty list Message-ID: <26E3F167.2314@marob.masa.com> Date: 4 Sep 90 18:24:38 GMT References: <24305@uflorida.cis.ufl.EDU> Distribution: usa Organization: ESCC, New York City Lines: 34 In article , stephen@estragon.uchicago.edu (Stephen P Spackman) writes: >Myself I think of it this way: all the world is typed. Every type >contains a "no, forget it" value. IF is polymorphic; it tests for this >distinguished value of ANY type. Finally, Lisp doesn't mention types >(Lisp types aren't my types because I they aren't prior) so it >collapses all functionally identical atoms into single objects - >effectively using "pre-casting", if you think about the arrows. That >way I can fit most programming languages into the same model. Smalltalk models 'nil' as the sole object of the type UndefinedObject. 'True' and 'false' are likewise the sole objects of types True and False. Nil is not false is not an empty collection. >Myself, I find polymorphism in IF and OR (and AND if it works on >lvalues! - in C syntax if not semantics <<(a && b) = c>>) >stupendously useful and entirely formalisable, though I like, as I >say, prior typing (the whole idea of equality without type hurts my >mind. I mean how do you compare two THINGS for equality when for all >you know one is a yak and the other is a superintelligent shade of the >colour blue? I mean, you can't even pick them up until you know their >types! No flames, this is just a personal failing. :-). You pick either object and ask it, "Are you equal to the other object?" It can answer yes, no, or search-me. If it answers search-me, a meta-protocol for equality asks the opposite question of the other object. If it, too, answers search-me, the meta-protocol replies false. >Is anyone any clearer on this? > >stephen p spackman stephen@estragon.uchicago.edu 312.702.3982 -- cowan@marob.masa.com (aka ...!hombre!marob!cowan) e'osai ko sarji la lojban