From: Anton Ivanov <forall.x@...>
Date: 2008-09-11T03:45:50+09:00
Subject: Re: Redefe each to include each_with_index and arity=2

Dan Diebolt wrote:
> Is is possible to redefine each so that each_with_index behavior can be 
> acheived with an extra parameter to the block each is called with? In 
> other words, how do you redefine each so that instead of using this 
> construct:

> ======
> This is my attempt to redefine Array#each but it does not work:
>  
> class Array
>   alias :old_each :each
>   def each(&block)
>     puts "arity=#{block.arity}"
>     case block.arity
>     when 1
>       old_each &block
>     when 2
>       each_with_index &block
>     end
>   end
> end
>  
> a=[1,2,3]
>  

I'm not exactly sure why your code does not work.  It seems that 
each_with_index and each depend on each other in ways which you are not 
capturing with your redefined each.  But you can roll your own index 
tracking:

  def each(&block)
    i = 0
    arity = block.arity
    old_each do |curr|
      case arity
        when 1: yield curr
        else yield curr, i
      end
      i+=1
    end
  end

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