From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2010-07-06T16:30:49+09:00
Subject: Re: extending a core class cleanly [noobish alert]

On Mon, Jul 5, 2010 at 11:22 PM, David Masover <ninja@slaphack.com> wrote:
> On Monday, July 05, 2010 03:30:44 pm Fearless Fool wrote:
>> Assume I'd like to create a statistics package designed to work on
>> Arrays [but see P.S.].  One way to do this would be to create functions
>> that take an array as an argument:
>>
>>   def mean(array)
>>     return NaN unless array && array.size > 0
>>     array.reduce(0.0) {|accum, val| accum + val} / array.size
>>   end
>>
>> ... but that's so old school.  What I'd really like is to *cleanly*
>>
>> extend the Array class so you can operate on it directly, as in:
>>   >> [1, 2, 3].mean
>>
>>   => 2.0
>>
>> I'm sure that it's considered bad style to simply open the Array class
>> and extend it, since that can lead to unexpected user surprises:
>>
>>   class Array
>>     def mean
>>       return NaN unless self.size > 0
>>       self.reduce(0.0) {|accum, val| accum + val} / self.size
>>     end
>>   end
>
> What kind of surprises?
>
>> My hunch is that the stats functions should be wrapped in a module and
>> then included (extended?) when desired.  But despite reading the
>> tutorials and docs, I'm not sure what's considered the stylistically
>> correct way (and the syntax) to package that up.
>
> Simple enough:
>
> module MyModule
>  def mean
>    ...
>  end
> end
>
> Then, you can always do this:
>
> [].extend MyModule

I agree, this is the less dangerous thing to do: just extend the
specific array instances with your functionality. That way, all other
arrays are left intact.

Jesus.