sample of how instance variables and accessors behave

foo.rb

~ (master)(*)$ irb
>> 
>> class Foo
>>   attr_accessor :read_write
>>   attr_reader :readonly
>>   attr_writer :writeable
>> 
?>   def initialize(read_write, readonly, writeable)
>>     @read_write = read_write
>>     @readonly = readonly
>>     @writeable = writeable
>>   end
>> end
nil
>> foo = Foo.new(1, 2, 3)
#<Foo:0x7fe0831770d0
    attr_accessor :read_write = 1,
    attr_reader :readonly = 2,
    attr_writer :writeable = 3
>
>> foo.read_write
1
>> foo.readonly
2
>> foo.writeable
NoMethodError: undefined method `writeable' for #<Foo:0x007fe0831770d0 @read_write=1, @readonly=2, @writeable=3>
	from (irb):15
	from /opt/github/rbenv/versions/1.9.3-p194/bin/irb:12:in `<main>'
>> foo.writeable = 25
25
>> foo
#<Foo:0x7fe0831770d0
    attr_accessor :read_write = 1,
    attr_reader :readonly = 2,
    attr_writer :writeable = 25
>
>> foo.readonly = 100
NoMethodError: undefined method `readonly=' for #<Foo:0x007fe0831770d0 @read_write=1, @readonly=2, @writeable=25>
	from (irb):18
	from /opt/github/rbenv/versions/1.9.3-p194/bin/irb:12:in `<main>'
>> foo.read_write = 1000
1000
>> foo
#<Foo:0x7fe0831770d0
    attr_accessor :read_write = 1000,
    attr_reader :readonly = 2,
    attr_writer :writeable = 25
>
>> foo.instance_eval('@readonly = 25')
25
>> foo
#<Foo:0x7fe0831770d0
    attr_accessor :read_write = 1000,
    attr_reader :readonly = 25,
    attr_writer :writeable = 25
>
>> foo.instance_eval('@some_new_instance = 3000')
3000
>> foo
#<Foo:0x7fe0831770d0
    @some_new_instance = 3000,
    attr_accessor :read_write = 1000,
    attr_reader :readonly = 25,
    attr_writer :writeable = 25
>
>>