From: dblack@...
Date: 2006-03-26T22:59:10+09:00
Subject: Re: deciphering poignant guide chapter 4 example

Hi --

On Sun, 26 Mar 2006, Simen wrote:

> 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.

There's no constraint or direct cause-and-effect operating on Ruby
here, though.  For example, when you do:

   Array.instance_methods

you get a list of strings, corresponding to method names.  These
strings identify the methods, as much as :asdf does in your example.

> 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"

I'd add that a lot of Ruby methods take strings or symbols: attr and
friends, send, define_method, etc.  So there's also no point doing:

   send(meth_name.to_sym)

which one sees occasionally.


David

-- 
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

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