From: MenTaLguY <mental@...>
Date: 2008-02-23T02:41:26+09:00
Subject: Re: ||= [] idiom

On Fri, 22 Feb 2008 21:33:22 +0900, Thufir <hawat.thufir@gmail.com> wrote:
>> Often it can make sense to factor the ||= portion into a separate
>> method:
>>
>>  def category_for(tag)
>>    @categories[tag] ||= []
>>  end
>>
>>  # ...
>>
>>    category_for(resource.tag) << resource
> 
> What's the real-world application of this?

There are two reasons to do it:

 1. If you're doing this in non-trivial code, you're likely to end up
    with multiple @categories[something] ||= [] scattered around; it
    only makes sense to factor them into a single method at that point. 
    This is an advantage for maintainability.

 2. "category_for" has a more direct meaning in the problem domain:
    category_for(tag) will simply give you the category array for a
    particular tag, without needing to expose on the spot the details
    of how that category array is obtained.  This is an advantage for
    readability.

Also, regarding maintainability again, let's say that you decide to
start using objects of class Category instead of arrays; assuming
there is a Category#<< for adding a resource to a category, you can
simply change this:

 def category_for(tag)
   @categories[tag] ||= []
 end

to this:

 def category_for(tag)
   @categories[tag] ||= Category.new
 end

It's the DRY principle:  Don't Repeat Yourself.

If there is repetition in your code, it is harder to read (because
you have to mentally filter out the repeated parts to see the
differences -- which are the important part), and you make more work
for yourself (because you must then edit many different places to
effect a single change, and are more likely to make mistakes).

-mental