From: Aryk Grosz <tennisbum2002@...>
Date: 2009-12-01T13:07:53+09:00
Subject: Re: Date class method `once` wraps original call in Array. Why?

They wrap it in an array so you that they can cache "nil" and "false" 
return values:

result ||= expensive_computation


will still run the expensive computation if it returned nil or false, 
however, if you did:

(result ||= [expensive_computation])[0],

you could cache nil as a return value and it wouldn't actually run the 
expensive computation.

Matt Haley wrote:
> I'm reading the Pickaxe and on page 391, there is a code listing for
> the class method `once` from the Date module in Ruby:
> 
>   def once(*ids) #:nodoc:
>     for id in ids
>       module_eval <<-"end:"
>         alias_method :__#{id.to_i}__, :#{id.to_s}
>         private :__#{id.to_i}__
>         def #{id.to_s}(*args, &block)
>           puts "Inside once: \#{args.inspect}"
>           (@__#{id.to_i}__ ||= [__#{id.to_i}__(*args, &block)])[0]
>         end
>       end:
>     end
>   end
> 
> I was wondering why the original method called is wrapped in an Array.
> So I started digging the the source history hoping to find an
> explanation. I found the commit that changed it to use the Array but
> no explanation for the change. The commit where it was modified is
> here:
> 
> <http://svn.ruby-lang.org/cgi-bin/viewvc.cgi/trunk/lib/date2.rb?r1=710&r2=709&pathrev=710>
> 
> I created a simple class and use the code with and without the Array
> and it seems to work the same for my simple test. Just curious if I'm
> not seeing something obvious.

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