From: Bill Kelly <billk@...>
Date: 2002-12-01T06:47:06+09:00
Subject: Re: Ruby Document

Hi Shannon,

> On Sun, 1 Dec 2002 04:05:36 +0900
> "Bill Kelly" <billk@cts.com> wrote:
> 
> > irb> "some_symbol".intern
> > :some_symbol
> This is what I don't understand. "some_symobol" is a instant value,
> without a name. why the system assign a symbole :some_symbol to it?
> So if I have a string "This is a test" then the intern will be
> :This_is_a_test? That's sort of arbitrary...

irb> "This is a test".intern
:This is a test

It's not arbitrary, it's just that, using String#intern, we can 
create symbols that we can't represent in the source code using
the :name syntax.  (Even though inspect will print them out that
way.)

> Relating to your explaination, and previous messages, I still have some vital
> question about understanding symbol:
> 
> 1. I tested using IRB, it seems that a symbol is not related to the name.
> For example,
> 
> irb(main):001:0> p :a.id
> 3602702
> nil
> irb(main):002:0> a=3
> 3
> irb(main):003:0> p a.id
> 7
> nil
> irb(main):004:0> p :a.id
> 3602702
> nil
> irb(main):005:0>
> 
> It seems that I can create a symbol at any time, and it is not related
> to the name. For example, after a=3, :a still point to the same place.
> It seems that I have no way to use :a to reference to a? I am really
> confused... :((

The trouble here would seem to be a confusion of the functionality of
the Symbol class, with actual symbols in the source code.  They aren't
related that way..  All the Symbol class provides is essentially
similar to:

irb> class SymbolLike
irb>   @@syms = {}
irb>   def initialize(str)
irb>     @name = str
irb>   end
irb>   def to_s
irb>     @name
irb>   end
irb>   def SymbolLike.lookup(str)
irb>     @@syms[str] ||= SymbolLike.new(str)
irb>   end
irb> end
nil
irb> a = SymbolLike.lookup("sym1")
#<SymbolLike:0x2aa5770 @name="sym1">
irb> b = SymbolLike.lookup("sym1")
#<SymbolLike:0x2aa5770 @name="sym1">
irb> a.id
22358968
irb> b.id
22358968
irb> a.to_s
"sym1"

...Just think of it as an invisible hash table behind the scenes
that ensures a single object instance of Symbol maps to any input
string of identical characters.  It hasn't any direct relation
(that I'm aware of) to variable names or any other identifiers in
the source code, except that the :name syntax is a short cut for
"name".intern.

> 2. I guessed some usage of symbol in your explanation. Please have a
> look at the following javascript code:
> 
> <html>
> <body>
> <script>
> a=3
> b=6
> c=prompt("which var do you want?","a")
> alert(c)
> alert(eval(c))
> </script>
> </body>
> </html>
> 
> In the first alert, it will give out the name of the var you choose, but
> the second will give out the value of the var you choose. I vaguely
> guessed the use of symbol is related to this purpose. Can you implement
> the above JavaScript code in Ruby? Is it related to the use of symbol?

Well....... In Ruby I doubt whether I would see a use for symbols in
this example...  

A variation might be:

irb> class Test
irb>   def a; "3"; end
irb>   def b; "6"; end
irb> end
nil

irb> t = Test.new
#<Test:0x2abbbf8>

irb> prompt = "'which method do you want?'", "t.send(:a)"
["'which method do you want?'", "t.send(:a)"]

irb> puts prompt.join(" ")
'which method do you want?' t.send(:a)
nil

irb> puts prompt.map{|s| eval s}.join(" ")
which method do you want? 3
nil


More simply, without the eval, we could say:

irb> method_you_want = :a
:a
irb> puts "which method do you want? " + t.send(method_you_want)
which method do you want? 3
nil


Hope this helps,

Bill