From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2009-11-20T03:17:03+09:00
Subject: Re: Re-opening an existing module and changing a method

On Thu, Nov 19, 2009 at 6:27 PM, Aldric Giacomoni <aldric@trevoke.net> wrote:
> "I hear and I forget; I see and I remember; I do and I understand."
> Wouldn't it be nice if it were that simple - clearly they did not know
> about the subtle art of debugging.
>
> So, I'm trying to hack at the math module.
> Expected:
>>> Math.sqrt(2)
> => sqrt(2)
> Actual result, mileage does not vary:
>>> Math.sqrt(2)
> => 1.4142135623731
>
> require 'mathn'
> module Math
>  alias :old_sqrt :sqrt
>  def sqrt x
>    result = old_sqrt x
>    if result.is_a? Float
>      "sqrt(#{x})"
>    else
>      result
>    end
>  end
> end
>
> puts Math.sqrt(2)
>
> I had originally not attempted an alias, I just did "result = super x"
> but it didn't really amount to much, either.
> Where am I thinking about this wrong?

The problem is that mathn is using the module_function method to
convert sqrt into an method of the Math module.
According to http://ruby-doc.org/core/classes/Module.src/M001642.html:

"Module functions are copies of the original, and so may be changed
independently"

So I think you are not redefining the module function, but the
original, which doesn't have any effect when you call Math.sqrt (this
is calling the version created by module_function). Doing this:

irb(main):042:0> module Math
irb(main):043:1> def sqrt x
irb(main):044:2> result = super x
irb(main):045:2> p [result, result.class]
irb(main):046:2> result
irb(main):047:2> end
irb(main):048:1> module_function :sqrt
irb(main):049:1> end
=> Math
irb(main):050:0> Math.sqrt 2
NoMethodError: super: no superclass method `sqrt'
	from (irb):44:in `sqr

allows you to actually override the version created by
module_function, but I don't know how to then call the original, since
neither the original alias you had nor super are working. But maybe
this points you in the right direction.

Jesus.