From: Rick DeNatale <rick.denatale@...>
Date: 2006-08-26T02:32:24+09:00
Subject: Re: That's Ruby, Virginia!

On 8/24/06, Morton Goldberg <m_goldberg@ameritech.net> wrote:
> Your code only works in irb. You get an error, as you should, if you
> run it 'real' Ruby.
> In irb  'array_101' gets added to Array. That's the only reason
> 'a.array_101' doesn't produce an error. In "real" Ruby, the following
> produces your result, as it should.
>
> <code>
> #! /usr/bin/ruby -w
>
> class Array
>     def array_101
>        for i in [0...length]
>           p self[i] ### prints the array because i = 0...length, not 0.
>           puts "#{i} => #{self[i]}"
>        end
>     end
> end
>
> a = %w[a s d]
>
> a.array_101
> </code>
>
> <result>
> ["a", "s", "d"]
> 0...3 => asd
> </result>
>
> The only thing I don't understand is why 'array_101' gets added to
> Array in irb.

It doesn't it gets added to the Kernel module, just like in real Ruby.

The difference is that in irb, it's a public method and in real Ruby
it's private, I think that this came up in another context in the past
few days.

== file array_101.rb ==
def array_101
        for i in [0..self.length]
                puts "#{i} => #{self[i]}"
        end
end

a = ['a', 's', 'd']
p a
p a.array_101

class X
end

p X.new.array_101
=== end of file array101.rb===
$ ruby array101.rb
["a", "s", "d"]
array101.rb:9: private method `array_101' called for ["a", "s",
"d"]:Array (NoMethodError)

but change the file to wrap the method in the Kernel module
module Kernel
    def array_101
       ...
    end
end

$ ruby array101.rb
["a", "s", "d"]
0..3 => asd
[0..3]
array101.rb:3:in `array_101': undefined method `length' for
#<X:0xb7e1f7b4> (NoMethodError)
        from array101.rb:16

Note that we are getting through a.array_101, but now we blow up on
X.new.array_101, but INSIDE the array_101 method, because since it's
in Kernel, and class Object includes Kernel, makes it available to ALL
objects.

-- 
Rick DeNatale

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