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

2009/11/19 Jesús Gabriel y Galán <jgabrielygalan@gmail.com>:
> 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.

Got it:

irb(main):001:0> require 'mathn'
=> true
irb(main):002:0> module Math
irb(main):003:1> class << self
irb(main):004:2> alias :old_sqrt :sqrt
irb(main):005:2> end
irb(main):006:1> def sqrt x
irb(main):007:2> result = old_sqrt x
irb(main):008:2> p [result, result.class]
irb(main):009:2> result
irb(main):010:2> end
irb(main):011:1> module_function :sqrt
irb(main):012:1> end
=> Math
irb(main):013:0> Math.sqrt 2
[1.4142135623731, Float]
=> 1.4142135623731

Jesus.