From: Ilias Lazaridis <ilias@...>
Date: 2011-06-05T14:15:26+09:00
Subject: Re: CORE - Altering Behaviour of "each do" (default param "item")

On 4 Éïŭí, 21:02, Robert Klemme <shortcut...@googlemail.com> wrote:
> On 04.06.2011 16:19, Michael Sokol wrote:
>
> > Actually it would be possible, although very ugly.
>
> > You need to evaluate in the context of the current instance
> > (instance_eval) - not yield - the block you pass to the each method
> > of your object (an array in this case).
>
> > Then you can define a print method that behaves like a coroutine
> > (check out ruby fibers) and returns each element of your array.
>
> There is another approach which does not suffer from the side effect of
> changing self which can be disastrous: create a thread local which holds
> a stack of values and make item peek at the top element of it:
>
> class Object
>    def new_each
>      stack = (Thread.current[:__item__] ||= [])
>
>      each do |x|
>        stack.push x
>        begin
>          yield
>        ensure
>          stack.pop
>        end
>      end
>
>      self
>    end
>
> private
>    def item
>      stack = Thread.current[:__item__] or
>        raise "Not in new_each"
>      stack.last
>    end
> end
>
> a = %w{a b c}
> b = [1, 2, 3]
>
> a.new_each do
>    printf "%-20s: %p\n", 'outer 1', item
>
>    b.new_each do
>      printf "%-20s: %p\n", 'inner', item
>    end
>
>    printf "%-20s: %p\n", 'outer 2', item
> end
>
> This approach
> - does not change 'self'
> - is thread safe
> - is nesting safe
>
> Kind regards
>
>         robert
>
> --
> remember.guy do |as, often| as.you_can - without endhttp://blog.rubybestpractices.com/