From: Robert Klemme <shortcutter@...>
Date: 2008-03-21T20:09:56+09:00
Subject: Re: including instance methods and setting an instance varia

On 21.03.2008 11:15, Leon Bogaert wrote:
> Can I "connect" with the instance if it get's instanciated? Because I 
> wanted to do something like:
> 
> if not self.respond_to? :each
>   return false
> end

The bit above can be shortened to

return false unless respond_to? :each


There are at least these three options:

1. do not check at all

This is my preferred solution, as it is the simplest and works well.

irb(main):001:0> class Foo
irb(main):002:1> include Enumerable
irb(main):003:1> end
=> Foo
irb(main):004:0> f=Foo.new
=> #<Foo:0x7ff8bff4>
irb(main):005:0> f.map {|x| x+1}
NoMethodError: undefined method `each' for #<Foo:0x7ff8bff4>
         from (irb):5:in `map'
         from (irb):5
         from :0

2. check on the class level, this should generally be sufficient

irb(main):001:0> module Checker
irb(main):002:1> def self.included(cl)
irb(main):003:2> cl.instance_method :each
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> class Baz
irb(main):007:1> include Checker
irb(main):008:1> end
NameError: undefined method `each' for class `Baz'
         from (irb):3:in `instance_method'
         from (irb):3:in `included'
         from (irb):7:in `include'
         from (irb):7
         from :0
irb(main):009:0> Baz
=> Baz
irb(main):010:0> Baz.ancestors
=> [Baz, Checker, Object, Kernel]
irb(main):011:0>

Note that even the exception does not prevent inclusion.  But you can 
react on this and define the method for example.


3. on instantiation, in this case you need to be in the inheritance chain

irb(main):001:0> module Checker
irb(main):002:1> def initialize(*a,&b)
irb(main):003:2> method :each
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> class Foo
irb(main):007:1> include Checker
irb(main):008:1> def initialize
irb(main):009:2> super
irb(main):010:2> end
irb(main):011:1> end
=> nil
irb(main):012:0> Foo.new
NameError: undefined method `each' for class `Foo'
         from (irb):3:in `method'
         from (irb):3:in `initialize'
         from (irb):9:in `initialize'
         from (irb):12:in `new'
         from (irb):12
         from :0
irb(main):013:0>


4. check when individual objects are extended

irb(main):001:0> module Checker
irb(main):002:1> def self.extended(o)
irb(main):003:2> o.method :each
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> x = Object.new
=> #<Object:0x7ff89f4c>
irb(main):007:0> x.extend Checker
NameError: undefined method `each' for class `Object'
         from (irb):3:in `method'
         from (irb):3:in `extended'
         from (irb):7:in `extend'
         from (irb):7
         from :0
irb(main):008:0>


Kind regards

	robert