From: Simen <toalett@...>
Date: 2006-03-26T21:34:23+09:00
Subject: Re: deciphering poignant guide chapter 4 example

unknown wrote:

> The poinant guide says it's just "words that look just like
> variables". And "Symbols are lightweight strings." But that
> doesn't help me much. The PickAxe 2nd ed, in chapter 3 says,
> "The construct :artist is an expression that returns a Symbol
> object corresponding to artist.", so, I can understand that
> (i.e. that there's some Symbol class and we're getting an
> instance of it by using that notation), but I'm still not
> getting the point... why not just use strings? What does
> having "Symbols" buy the programmer?

Symbols are not merely lightweight strings, they represent names in the
Ruby interpreter. Check this sample out to see the difference:

  $ irb
  irb(main):001:0> "Hello".object_id
  => -605430718
  irb(main):002:0> "Hello".object_id
  => -605437098
  irb(main):003:0> :hello.object_id
  => 3985678
  irb(main):004:0> :hello.object_id
  => 3985678
  irb(main):005:0> :hello.object_id == :hello.object_id
  => true
  irb(main):006:0> "hello".object_id == "hello".object_id
  => false
  irb(main):007:0>

Symbols with the same values are the same objects, that is not the case
with strings. Symbols represents identifiers, strings values. For
example

  def method_missing( id, *args )
    puts id.class
  end
  asdf # undefined method

Would output "Symbol", since the method identifier is indeed an
identifier. Symbols are simply names of stuff. Use them when you need to
name something whose value may change, but not whose semantics (ie
ary[:greeting] may be "Hello world" or "Hello johnny" but it's allways a
greeting. You shouldn't use Symbols for stuff you're going to output to
some io, for example, there's no point in doing

  puts :"Hello world"

instead of

  puts "Hello world"

(Maybe some Ruby gurus could explain this better than me).

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