From: Rick DeNatale <rick.denatale@...>
Date: 2007-03-09T23:13:18+09:00
Subject: Re: Paul Graham explains Ruby symbols

On 3/8/07, joswig@corporate-world.lisp.de
<joswig@corporate-world.lisp.de> wrote:
> On Mar 7, 6:17 pm, "Rick DeNatale" <rick.denat...@gmail.com> wrote:
> > On 3/6/07, Luciano Ramalho <rama...@gmail.com> wrote:
> >
> > > Paul Graham offers this excellent explanation for the symbol type:
> >
> > > "Symbols are effectively pointers to strings stored in a hash table.
> > > So you can test equality by comparing a pointer, instead of comparing
> > > each character." [1]
> >
> > > Of course, he's talking about symbols inLisp, but what he says
> > > applies equally well to Ruby and Smalltalk.
> >
> > I find this a little too implementation-centric a description.
> >
> > The key aspect of symbols is that two symbols are identical if they are equal.
> > The fact that there may (or may not be) a hash table used to ensure
> > this is irelevant.
> >
> > And that description really misses the boat as far asLispis
> > concerned.Lispsymbols aren't strings, they are really names to which
> > three (separate) things can be bound, a value (which can be anyLisp
> > object), a function, and a property list. Actually the name is also
> > one of the slots in a symbol.
>
> Actually it's five in Common Lisp: Value, Function, Name, Property
> List, Package.
>
> > Note that inLispthe value of a Symbol is separate from it's name,
> > and two Symbols can have the same value, they just can't have the same
> > name.
>
> No, that's wrong. Two different symbols can have the same name in
> Common Lisp.
>
> CL-USER 30 > (eq '#:foo '#:foo)
> NIL
>
> CL-USER 31 > (symbolp '#:foo)
> T
>
> CL-USER 32 > (symbol-name '#:foo)
> "FOO"
>
> >
> > So inLispa symbol is more like an entry in the table of global names.
>
> No. In Lisp a symbol is a data structure with above five (virtual)
> slots.
> Symbols can exist without a 'table of names'. These 'table of names'
> are called packages. A symbol can belong to a package. You can then
> lookup the symbol via the package by its name. Again a symbol
> can exist without being interned in a package

Well, I never claimed to be a Lisp expert, the last time I used Lisp
seriously it was Lisp 1.5.

On the other hand, the author of the originally referenced
'explanation of symbols' seemed to be basing his description on his
knowledge of Lisp.

Given all the other differences pointed out by joswig, I think that my
original contention that:

> > Symbols in Ruby and Smalltalk are more alike than Symbols inLisp.

seems to be more true than ever.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/