From: Steve Tu <lists@...>
Date: 2012-12-01T00:30:04+09:00
Subject: Novice: Understanding Constants - Definition and Redefinition

Looking at the code from
here:http://rubylearning.com/satishtalim/ruby_constants.html

I copied the Const class into my test module, and bolted it inside a
method. This then caused me a smallish problem, as I then received an
error when trying to set the constants from within a method.

After googling a bit, I found that I could set the constants from within
a method by using the 'class.const_set' method - as:

 class Const
    def get_const
      CONST
    end
    CONST = OUTER_CONST + 1
 end


def constants_test
    STDOUT.puts "Constants Test (Const Is) : "+defined? Const
    STDOUT.puts "Constants Test (Const.new.get_const):
"+Const.new.get_const.to_s
    STDOUT.puts "Constants Test (Const::CONST): "+Const::CONST.to_s
    STDOUT.puts "Constants Test (::OUTER_CONST): "+::OUTER_CONST.to_s
    # Odd, I can set a constant (or even create a new one) like this
line
    Const.const_set("NEW_CONST", 234)
    # But the following line throws a constant redef error
    #STDOUT.puts "Constants Test : (Const::NEW_CONST = 345
)"+(Const::NEW_CONST = 345).to_s
    STDOUT.puts "Constants Test :
(Const::NEW_CONST)"+Const::NEW_CONST.to_s
 end

What I'm then confused about is really two things:
1) Why is it allowable to in effect modify a class? I can follow
extending a class and adding additional features etc (although I haven't
got that far yet) - but I don't quite grasp the concept of
Const::NEW_CONST = value.

2) I thought everything was in a method anyway - but why is it not
allowed to do something like Const::NEW_CONST = value from within a
method, but then allow Const.const_set("CONSTANT", value)?

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