From: Rick DeNatale <rick.denatale@...>
Date: 2009-02-05T06:04:36+09:00
Subject: Re: Making object methods available externally

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On Wed, Feb 4, 2009 at 3:06 PM, Rob Biedenharn
<Rob@agileconsultingllc.com>wrote:

>
> When you need it, yes. However, it isn't typical that you'd do something
> like this.  Either the method belongs on the instance or it belongs on the
> class itself.  Occasionally, for convenience, you want a class method to be
> called on an instance and then you'd likely do it this way. Your example is
> simple (and, being an example, somewhat contrived), but illustrates a point.
>  You can call #say_it with no parameter because the instance variable is
> there so an instance *knows* what to say, but to call .say_it on the class,
> you need to pass the parameter.
>

Ruby does have a method Module#module_function which can be used in cases
similar to this. For a rather silly example:

module Test
  def foo
    puts "Foo"
  end

  module_function :foo
end

Test.foo

begin
  foo
rescue Exception => ex
  puts "oops! #{ex}"
end

self.extend Test
foo

when run produces:

Foo
oops! undefined local variable or method `foo' for main:Object
Foo

What module_function does is to copy one or more instance methods and make
them singleton methods of the module.  It only works for modules not
classes.

The canonical use case is in the Math module so that you can either write:

Math.sin(.2)

or

class X
   include Math

   def compute(val)
        2 + sin(val)
   end
end
-- 
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale

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