From: Bill Kelly <billk@...>
Date: 2002-12-01T04:05:36+09:00
Subject: Re: Ruby Document

Hi Shannon,

From: "Shannon Fang" <xrfang@hotmail.com>

> >pigeon% ruby -e 'p :PropFind'
> >:PropFind
> >pigeon%
> >
> >pigeon% ruby -e 'p PropFind'
> >-e:1: uninitialized constant PropFind (NameError)
> >pigeon%
> >
> This only show the difference between symbol and name. But my key problem is 
> WHY we need symbol. What is it used FOR?

I don't know how authoritative an answer I can present here, but:
I think symbols are neat. :)  Whenever you type a specific symbol
in your program, say, :some_symbol, you get back a single corresponding
instance of class Symbol.  Take a look at the object id's in the 
following example:

irb> :some_symbol.id
3309838
irb> :some_symbol.id
3309838
irb> "some_string".id
22399992
irb> "some_string".id
22394712
irb> "some_symbol".intern
:some_symbol
irb> "some_symbol".intern.id
3309838


Hopefully the above illustrates clearly enough that when creating a
Symbol, there is a one-to-one mapping between the name of the symbol,
and the instance of class Symbol existing in Ruby to represent a 
symbol of that name. . . . . 

As far as what they're used FOR, I've only arrived at that through
induction.  A typical use of symbols in Ruby is to represent the
name of a method.  For ex: "string".respond_to? :intern #=> true.

Or, as you've noted, method-defining methods such as attr_reader,
etc... It's convenient notation to represent the name of the method
with a symbol... You could make a method like attr_reader that
worked with strings as arguments instead of symbols, if you wanted
to... but attr_reader "myvar" just doesn't look as nifty to me as
attr_reader :myvar..... <grin>  (I suspect, also, that the 
representation of method names as symbols is connected to the
mechanism of method-name lookup internally in Ruby classes, but I
have not perused that part of the Ruby source to verify this...)

In my own code, for what it's worth, I tend to use symbols in
places where I might have in other languages used an integer
constant to represent a (certain kind of) recurring named type-
value.  Such as:

  NOUN = 0
  VERB = 1
  ADVERB = 2
  ADJECTIVE = 3

Instead, in Ruby, I'll just go:  if word.type == :noun ....


Hope this helps,

:bill      # :)