From: Rick DeNatale <rick.denatale@...>
Date: 2007-03-09T06:11:43+09:00
Subject: Re: Paul Graham explains Ruby symbols

On 3/8/07, Chad Perrin <perrin@apotheon.com> wrote:
> On Thu, Mar 08, 2007 at 02:17:13AM +0900, Rick DeNatale wrote:
> > On 3/6/07, Luciano Ramalho <ramalho@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 in Lisp, but what he says
> > >applies equally well to Ruby and Smalltalk.
> >
> > I find this a little too implementation-centric a description.
>
> Unfortunately, there's no way to differentiate a string literal from a
> symbol in a definitive manner without brushing up against
> implementation.  At least, I haven't seen such a thing yet.

lets see

def is_symbol?(arg)
   arg.object_id == arg.to_sym.object_id rescue false
end
>
>
> >
> > 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.
>
> Ahh . . . but think about how they're "equal".  They're equal because of
> the manner in which they're implemented.  If you want to separate the
> concept of symbols from the implementation to some degree, you might
> explain as little of implementation as possible while still getting the
> point across, then say that "this could change so that symbols still
> behave the same way but are implemented somewhat differently, but this
> is how it's done right now".
>
> How exactly, other than the difference between : and '', do you
> differentiate a string literal from a symbol without discussing
> implementation?  I don't much see a way to do it.
>
> Obviously, a symbol is different in easily explained ways from string
> variables, without having to drag implementation into it.  Things aren't
> quite so clear-cut between symbols and string *literals*, though.
>
>
> >
> > Not long ago Matz experimented with the idea of making Symbol a
> > subclass of String in 1.9, but this appears to have been dropped in
> > more recent versions.
>
> That's interesting -- I didn't know that.  Thanks for mentioning it.
>
> --
> CCD CopyWrite Chad Perrin [ http://ccd.apotheon.org ]
> "It's just incredible that a trillion-synapse computer could actually
> spend Saturday afternoon watching a football game." - Marvin Minsky
>
>


-- 
Rick DeNatale

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