From: joswig@...
Date: 2007-03-10T01:10:07+09:00
Subject: Re: Paul Graham explains Ruby symbols

On Mar 9, 3:13 pm, "Rick DeNatale" <rick.denat...@gmail.com> wrote:
> On 3/8/07, jos...@corporate-world.lisp.de
>
>
>
> <jos...@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.

Lisp 1.5 is not really in use anymore.

>
> 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 byjoswig, 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.
>

Symbols in Ruby and Smalltalk are like keyword symbols in Common Lisp.
Keyword symbols are symbols in the package KEYWORD and they
have themselves as value and they are unique.



> --
> Rick DeNatale
>
> My blog on Rubyhttp://talklikeaduck.denhaven2.com/